NUREG/CR-5275

SAND85-1264

R3

FLAME Facility

The Effect of Obstacles and Transverse Venting on

Flame Acceleration and Transition to Detonation for

Hydrogen-Air Mixtures at Large Scale

Manuscript Completed: March 1989

Date Published: April 1989

Prepared by

M. P. Sherman, S. R. Tieszen, W. B. Benedick

Sandia National Laboratories

Albuquerque, NM 87185

Prepared for

Division of Systems Research

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555

NRC FIN A1246
We dedicate this report to our coauthor, colleague, and

friend, William."Bill" Benedick (1926-1989). All who worked

with him were impressed with the breadth and depth of his

knowledge. There was no one with whom it was more pleasant

to work. He will be greatly missed by those who knew him at

Sandia and around the world.
ABSTRACT

This report describes research on flame acceleration and

deflagration-to-detonation transition (DDT) for hydrogen-air

mixtures carried out in the FLAME facility, and describes

its relevance to nuclear reactor safety. Flame acceleration

and DDT can generate high peak pressures that may cause

failure of containment. FLAME is a large rectangular

channel 30.5 m long, 2.44 m high, and 1.83 m wide. It is

closed on the ignition end and open on the far end. The

three test variables were hydrogen mole fraction (12 - 30%),

degree of transverse venting (by moving steel top plates -

0%, 13%, and 50%), and the absence or presence of certain

obstacles in the channel (zero or 33% blockage ratio). The

most important variable was the hydrogen mole fraction. The

presence of the obstacles tested greatly increased the flame

speeds, overpressures, and tendency for DDT compared to

similar tests without obstacles. Different obstacle

configurations could have greater or lesser effects on flame

acceleration and DDT. Large degrees of transverse venting.

reduced the flame speeds, overpressures, and possibility of

DDT. For small degrees of transverse venting (13% top

venting), the flame speeds and overpressures were higher

than for no transverse venting with reactive mixtures

(> 18% H ), but they were lower with leaner mixtures. The

effect ol the turbulence generated by the flow out the vents

on increasing flame speed can be larger than the effect of

venting gas out of the channel and hence reducing the

overpressure. With no obstacles and 50% top venting, the

flame speeds and overpressures were low, and there was no

DDT. For all other cases, DDT was observed above some

threshold hydrogen concentration. DDT was obtained at 15%

H2 with obstacles and no transverse venting.

-ii /iv-

CONTENTS

Page

Executive Summary ..................................... 1

1. Introduction ........................................... 5

2. The FLAME facility . ............................... 12

3. Instrumentation ................................... 25

4. Data Acquisition System ........................... 33

5. Data Processing ................................... 40

6. Test Results ...................................... 47

7. Conclusions ........................................ 116

8 . References ........................................ 120

Appendix A. Position of Structures and Instrumentation 127

Appendix B. Application to Hydrogen-Air-Steam Mixtures 138
LIST OF FIGURES

Figure

1.1

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

_'2.9

3.1

3.2

4.1

56.2

5.3

•6.2

Burning Velocity and Flame Speed Definitions

FLAME Channel and Its Surroundings ............

Map of FLAME Facility at Site 9920, Area Y ....

Schematic of FLAME Facility With Simple

Obstacles Installed, 33% Blockage Ratio ........

Interior of the FLAME Channel With Simple

Plywood Baffles Installed .....................

Interior of the FLAME Channel With Plywood

Boxes Simulating Half-Scale Model of the Ice

Condenser Upper Plenum Region .................

Interior of the FLAME Channel After Test F-26

Damage to the FLAME Structure at the

Ignition End After Test F-14 ..................

Exterior View of the Ignition End of the

FLAME Channel Showing the Steel Frame and

Sacrificial Plywood Sheet .......................

Prior to a Test, a Polyethylene Bag Has Been

Taped to the End of the FLAME Channel and

Attached to the Guidelines ....................

Deflagration Time-of-Arrival ..................

Deflagration and Detonation Time-of-Axrivals

Schematic of Data Acquisition System Showing

Main Flow of Information ......................

Combustion Front Trajectories and Profiles ....

Graphical Illustration of Equivalent Planar

Flam e Speed ...................................

Test F-22, 15% H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Test F-10, 12.3% H2 . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .

Test F-10, 12.3% H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

P age

9

13

14

15

16

17

18

20

21

23

26

28

35

42

44

46

49

50

-vi -
LIST OF FIGURES

(continued)

Figure

6.3

6.4

6.5

6.6

6.7

6.8

6.9

6.10

6.11

6.12

6.13

6.14

6.15

6.16

6.17

6.18

6.19

6.20

6.21

6.22

6.23

6.24

Test F-8, 18.4% H2

Test F-8, 18.4% H 2

Test F-12, 24.7% H2

Test F-12, 24.7% H2

Test F-14, 30.0% H.2

Comparison of Flame Sy

No Top venting and No

Test F-22, 15.0% H2

Test F-22, 15.0% H2

Tests F-22 and F-23

Tests F-22 and F-23

Test F-23, 14.5% H2

Test F-23, 14.5% H12

Test F-21, 10-15% H12

•eeds for Tests With

Obstacles .. . . . . . .

.o.........°...............

. . . . . .° . . . . . . . . . . . . . . . . . . ..

.........................

Page

52

53

54

55

58

59

61

62

64

66

68

69

72

Comparison of Flame Speeds for Tests With

No Top Venting and Obstacles .......... ........

Test F-4, 28.0% H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Test F-4, 28.0% H12

Test F-2, 19.7% H12 ...........................

Test F-6, 15.0% H12............................

Test F-5, 12.6% H2 ................. ............

Comparison of Flame Speeds for Tests With

50% Top Venting and No Obstacles ..............

Test F-26, 28.5% H12 ...........................

Test F-26, 28.5% H2 ... ............................

73

74

76

78

79

80

81

83

84

-vii-
LIST OF FIGURES

(continued)

Figure

6.25

6.26

6.27

6.28

6.29

6.30

6.31

6.32

6.33

6.34

6.35

6.36

6.37

6.38

6.39

6.40

6.41

6.42

6.43

6.44

6.45

6.46

Test F-24, 15.5% 12

Test F-29, 18.5% H-2

Test F-29, 18.5% 12

Test F-25, 19.7% H12

Test F-25, 19.7% 112

Comparison of Flame

50% Top Venting and

Test F-15, 15.4% H2

Test F-15, 15.4% 11

Test F-16, 17.6% 11

Test F-16, 17.6% 113

Test F-18, 18.1% H2

Test F-18, 18.1% H2

Test F-20, 20.7% 11

Test F-20, 20.7% H2

Test F-19, 24.8% H11.

Test F-19, 24.8% 112

Comparison of Flame

13% Top Venting and

.. . . . . . . . . . . . . . . . . .. . . . . . . .

Speeds for Tests With

Obstacles....................

.o...,......................

.,.........,..o....,..,.......

.,.......................,...

............ •..,..,.......,..

Speeds for Tests With

No Obstacles .................

86

88

89

91

92

94

96

97

98

99

100

101

102

103

105

106

107

108

109

111

112

113

The Effect of Transverse Venting on Tests

With 25% H12 and No Obstacles ..................

Test F-9, 6.9% H 2.............................

FLAME DDT Results .............................

FLAME Velocity Results ........................

FLAME Overpressure Results ....................

-viii-
ACKNOWrLEDGMENTS

.The authors would like to acknowledge the contributions of

Dr. J. Fisk. He designed the FLAME facility and played a large

role, in carrying out the earlier tests. Dr. Fisk has written

sections on the structural design of FLAME and on details of the

site for this report. These specialized sections were not

included in the report but remain on file. The FLAME facility

was constructed by the Cardenas Construction Company,

Albuquerque, NM. The initial computer programs for operating the

data acquisition system were developed by 0. Lindquist of Santa

Fe Energy Research Corporation, Santa Fe, NM. Dr. J. Barry,

while a graduate student on a mentorship, developed the second

generation data acquisition computer programs and otherwise

assisted in the research. S. Winters operated and further

developed the DAASY data acquisition system. I. Lenz was in

charge of the high-speed cinematography. Dr. M. Carcassi from

the University of Pisa played an important role in the data

reduction while at Sandia as a Visiting Scientist. He has been

working on modelling the FLAME tests. The advice of Professors

J. H. S. Lee and R. Knystautas of McGill University was highly

valuable in this research. The authors would like to thank the

NRC Contract Managers J. Larkins and P. Worthington. We wish to

thank M. Berman, supervisor of Division 6427, for obtaining the

support for this research, and for continuing technical advice.

The authors also appreciate the technical support of C. Daniel,

D. Montgomery, M. Oliver, D. Beeker, D. Weigand, and B. Heideman.

I

EXECUTIVE SUMMARY

The purpose of this report is to describe research on-flame

acceleration and deflagration-to-detonation transition (DDT) of

hydrogen-air mixtures carried out in the FLAME facility, and to

explain its relevance to nuclear reactor safety. In addition to

the quasi-static pressures generated by the slow combustion of

hydrogen in containments during hypothetical severe accidents,

rapid combustion, i.e. accelerated flames and detonations, can

generate strong pressure waves, which might threaten containment

and lead to a release of radioactivity.

FLAME is a large (30.5-m long) rectangular channel designed

and built for the U.S. Nuclear Regulatory Commission. It was

built as a half-scale model of the upper plenum volume in ice

condenser pressurized water reactor (PWR) containments. The

effects of three variables known to be important in small-scale

tests and in a relatively small number of large-scale tests were

investigated: hydrogen concentration, obstacles in the path of

the combustion front, and the degree of transverse venting. The

results of this research help define the threat from flame

acceleration and detonation in ice condenser and other

containments. A methodology of estimating the likelihood of DDT

based on FLAME and other results was developed.

During hypothetical severe nuclear accidents, large

quantities of hydrogen :an be generated by the oxidation of the

zirconium fuel cladding and from other mechanisms. As the

hydrogen escapes into containment, it may continuously burn. The

combustion in such a "diffusion flame" is limited by the rate of

laminar or turbulent mixing of hydrogen and oxygen. The' pressure

rise will be small, but there can be severe thermal loads on

structures near the flame. If the hydrogen does not immediately

burn, a premixed volume of hydrogen, oxygen, and other diluent

gases will form. These can burn as a deflagration or detonation,

with much higher pressure rises than for a diffusion flame.

Deflagrations (flames) are combustion fronts traveling at

subsonic speeds relative to the unburned combustible gas.

Ordinary deflagrations travel at speeds much less than sonic. For

these deflagrations the pressure will be nearly uniform

throughout containment, and the peak pressure will be bounded by

the adiabatic isochoric (constant volume) complete combustion

pressure (AICC pressure). The AICC pressure can be computed to

high accuracy by thermodynamic calculations. At most, the AICC

pressure for hydrogen-air, or hydrogen-air-steam mixtures is

eight times the precombustion pressure. If the deflagration

speed is accelerated to more than about 100 m/s, shock waves will

be generated, and the peak instantaneous pressure can be higher

-1-
than the AICC pressure. If the deflagration is fast enough, a

transition to a detonation may occur. Detonations are combustion

fronts traveling at supersonic speed relative to the unburned

gas. For detonations the peak reflected pressures will be

considerably above the AICC pressure, up to 35 times the

precombustion pressure. For highly accelerated flames or

detonations, dynamic loads will be imposed on-the containment in

addition to more slowly changing quasi-static loads.

In our FLAME experiments, the hydrogen mole fraction was

varied between 12% and 30% hydrogen. At 12% hydrogen there was

negligible flame acceleration, regardless of the degree of

transverse venting or the presence of obstacles. The most

reactive hydrogen-air mixtures are near stoichiometric (two

hydrogen molecules per one oxygen molecule), i.e. about 30%

hydrogen. The flame speeds and overpressures Varied by over two

orders of magnitude over the range of hydrogen mole fractions

tested: velocities of from less than 10 m/s to 1800 m/s and

overpressures of from less than a few kPa to over 2000 kPa. The

reactivity of the mixture as determined by the hydrogen

concentration, was the most important variable.

The presence of obstacles in the path of the expanding flame

is known to promote flame acceleration in small-scale experiments

by.:enlarging the burning surface and increasing the local burning

rate. The FLAME channel was built with provision for attaching

obstacles to the walls and floor. We carried out tests with no

obstacles in the channel (other than two mixing fans and

thermocouple rakes), with baffles that obstructed 1/3 of the

channel cross section, and with half-scale models of the air

handlers in the ice condenser upper plenum region. It should be

remembered that our observations of the effects of obstacles were

based on a limited set of obstacle configurations. Other

obstacle configurations would give different quantitative

results. The presence of obstacles causes a dramatic increase in

flame speed and overpressure and a lowering of the minimum

hydrogen mole fraction for DDT. For example, with a large degree

of transverse venting (50% top venting) and no obstacles, DDT was

not observed at 28% hydrogen, and the maxiumum speed of the flame

down the channel was 126 m/s. With obstacles, DDT was observed

at 20% hydrogen. With obstacles present, the distinction between

detonation and highly accelerated flames blurs. Even without a

detonation, deflagrations are accelerated to speeds of 500 - 700

m/s, and high pressure pulses are observed. A DDT was observed

at 15% hydrogen in a test with obstacles and no top venting.

This is below the old value of lean "detonation limit" of 18%

hydrogen, still sometimes quoted in the combustion literature.

.The effect of venting is complex, being tied to combustible

mixture reactivity, chamber geometry, and scale. In small-scale

experiments, the presence of venting transverse to the path of

-2-
the deflagration inhibits flame acceleration. FLAME was built,

with the top covered by movable steel plates. Tests were carried

out without top venting, with a large degree of top venting

(50%), and a small degree of top venting (13%).

The presence of large degrees of transverse venting does

inhibit flame acceleration, as in small-scale tests. Without

obstacles, low flame speeds, low overpressures, and no

detonations were observed even with nearly stoichiometric

mixtures. With.obstacles, DDT was obtained at 20% hydrogen. In

these large-scale tests, the effects of small degrees of

transverse venting differ from small-scale tests. There are two

opposing phenomena. The loss of gas in the channel tends to

reduce the pressure and hence flame acceleration. The turbulence

created by flow through the vents, and possible reignition in the

channel by rapid burning above the vents, tends to increase flame

speed. We found that for less reactive mixtures (below 18%

hydrogen) the small degree of transverse venting did inhibit

flame acceleration somewhat. For more reactive mixtures, the,

combustion was more violent with the 13% top venting than withino

top venting.

The conclusions, from these tests are here summarized:

1. The reactivity of the mixture as determined by the hydrogen

concentration is the most important variable. For very lean

mixtures no significant flame acceleration and no transition

to detonation was observed.

2. The presence of obstacles in the path of the flame greatlyincreases flame speeds and overpressures, and reduces the

lean limit for transition to detonation.

3. Large degrees of transverse venting reduce flame speeds and

overpressures.

4. Small degrees of transverse venting reduce flame speeds and

overpressures for less reactive mixtures, but increase them

for more reactive mixtures.

An application of the DDT results from FLAME to nuclear

reactor safety is that of Sherman and Berman in which a

methodology for estimating the likelihood of detonation in

containment during a severe accident was presented. A short

presentation of this methodology is given in Appendix B. This

work was applied to hypothetical accidents at the Bellefonte

nuclear power plant with a large dry containment. The

methodology has been used in the NUREG-1150 containment loads

expert panel evaluation for the Sequoyah nuclear power plant with

-3-
an ice condenser containment and for the Grand Gulf Mark 3 BWR

plant. The results of applying the methodology ranged from the

Bellefonte studies, in which detonation threats appear remote for

the accidents considered, to the Sequoyah studies in which

certain accident scenarios lead to predictions of local

detonations in the ice condenser., In a hypothetical accident

within an ice condenser containment with the return air fans not

operating, a highly reactive mixture is formed in the ice

condenser. Since the ice condenser has a geometry which can

promote flame acceleration, the methodology developed predicts a

DDT is highly likely there.

-4-
1. INTRODUCTION

1.1 Purpose of FLAME facility

The FLAME facility was designed and constructed for the U. S.

Nuclear Regulatory Commission to study hydrogen combustion

problems relevant to nuclear reactor safety: accelerated flames,

transition to detonation, combustion in simulated reactor

containment geometries, etc. The main concerns with hydrogen

combustion are that the resultant overpressure may threaten

containment and lead to a release of radioactivity, and that

safety-related equipment may be damaged by thermal or mechanical

loads, leading to a breach of containment. The possible

importance of hydrogen combustion to overall risk has long been

understood. [1,2] Less well understood were conditions in which

potentiallly destructive detonations and highly accelerated

deflagrations can occur. The experimentation in FLAME has been

mainly directed to furnish data from a large-scale facility to

help clarify these phenomena. We studied the effects of

variation in hydrogen concentration, degree of venting transverse

to the channel axis, and the presence of particular obstacle

types.

1.2 History of Hydrogen Concern

During the Three Mile Island Unit 2 (TMI-2) accident, a

considerable amount of hydrogen was generated due to the

oxidation of zirconium by steam in the reactor core. The

hydrogen was released into containment, forming a combustible

mixture estimated to be about 8% hydrogen in air. [3] After

nearly ten hours into the accident, a deflagration took place

with a peak overpressure greater than 192 kPa (28 psig). It has

been estimated that the time to burn the hydrogen in TMI-2 was 12

seconds. [3] It was an ordinary deflagration. We will use the

words "flame" and "deflagration" interchangeably. A hydrogen

deflagration can be considered "ordinary" if its speed is much

less than the speed of sound relative to the adjacent combustible

mixture. For an ordinary hydrogen deflagration in containment,

the pressure will be nearly spatially uniform, and the resultant

pressure loads on a containment structure will be quasi-static.

Quasi-static pressure loads are those in which the characteristic

time for pressure change is long compared to the natural period

of the structure. Containment structure natural periods relevant

to hydrogen burns are of the order of 10 to 200 milliseconds.

[4,5] In contrast to the quasi-steady loads caused by an

ordinary deflagration, a highly accelerated deflagration or a

detonation will cause dynamic loads on structures in addition to

static loads. The pressure loads at TMI-2 did not threaten the

strong containment structure. However, the pressure rise would

have been higher and the combustion even more rapid if the

hydrogen concentration had been higher. This might occur in

-5-
smaller sized containments, if more hydrogen had been generated,

or if the released hydrogen was more concentrated and not mixed

throughout containment.

In the Chernobyl accident, it was reported that there were

two or more explosions. [6] The first is generally believed to

have been a steam explosion. The second and possibly later

explosions may have been due to hydrogen combustion. At the time

this report is being written, the initial conditions of the

possible hydrogen combustions are not known.

1.3 Adiabatic Isochoric Complete Combustion Pressure

In the combustion of a premixed hydrogen-air or hydrogen-airsteam mixture, we can distinguish three regimes: ordinary

deflagrations, highly accelerated deflagrations, and detonations.

If a hydrogen-air mixture is far from stoichiometric (n 30%

hydrogen), either very lean or very rich, its deflagration speed

will be small compared to the speed of sound. The pressure in

all accessible volumes will be spatially uniform. The pressure

will rise to a peak in a period of seconds, and then decay as the

gas cools by heat transfer to the surroundings and possibly to

water sprays. The peak pressure is bounded by the Adiabatic

Isochoric Complete Combustion Pressure (AICC pressure), often

called the Constant Volume Explosion Pressure. The AICC pressure

can be determined to high accuracy because its determination

requires only thermodynamic property data of several simple

chemical species which are accurately known. In particular, for

hydrogen combustion one needs the heats of formation, species

enthalpies, and either equilibrium constants for the dissociation

reactions or free energies for the species H2 , 02, N2 1, H20, OH,

NO, H, and 0. Other species such as N, NO , HO 0 ana H2 O2 are

present in small amounts and could be includej'. Rhe AICC

pressure can be accurately computed using standard computer codes

[7] or estimated from graphical results. [8] For combustion of a

homogeneous mixture in a fixed volume, the peak pressure can be

below the AICC pressure because of incomplete burning and because

of heat transfer from the hot gases to the cooler surroundings

during the combustion.

1.4 Flammability Limits

The flammability limits of a combustible mixture, at a given

temperature and pressure, are defined as the limiting

concentrations of fuel which will propagate a deflagration

indefinitely. [0-11] For a given fuel-air mixture, there will be

a lean and a rich limit. The flammability limits are assumed to

be independent of the method of ignition, provided it is

sufficiently strong to ignite a flame. It is also independent of

the size of the enclosure, provided it is much larger than the

-6-
quenching distance. The flammability limits depend on the. i

direction of flame propagation because of buoyancy effects. The

lean and rich limits span a wider range of concentration for

upward than downward propagation.

Hydrogen-air-steam flammability limits have been measured-by

several researchers [12-21] at temperatures below 2000C and for

pressures from atmospheric up to 7 atmospheres. In the

combustion of premixedhydrogen-air-steam mixtures, the presence

of steam acts as a diluent, reducing the combustion temperature.

The effect of increasing amounts of steam on hydrogen-air-steam

mixtures, is to narrow the combustible range of hydrogen-to-air .between the lean and rich limits. The lean and rich limits meet

with the addition of. sufficient steam, and the mixture is said to

be inerted by the steam. Approximately 55% steam mole fraction

will inert hydrogen-air-steam mixtures, although the measured

mole fraction of steam for inerting has varied between about 52%

and 63%. [12-21]

Moderate degrees of turbulence have no significant effect on

the flammability limits. [21] There is some information on

flammability limits of.hydrogen-air lean mixtures at higher

temperatures [22] and of hydrogen-oxygen-nitrogen mixtures. [23]

These studies show the flammability limits widen with increasing

temperature. There is still a lack of higher temperature

hydrogen-air-steam flammability limit data.

1.5 Combustion Completeness

None of the flammability-limit papers carried out a

systematic study of the combustion completeness at just inside

the flammability limits; however, several of the papers did

include some measurements of post-combustion composition. [20,21]

The completeness of combustion for flammable hydrogen-air and

hydrogen-air-steam mixtures was investigated at large scale [24-

28] and intermediate scale. [29-33] For initially quiescent lean

mixtures, these studies show the combustion completeness varies

from low fractions of hydrogen burned at the upward flammability

limits to complete burning of hydrogen near the downward

flammability limits. For mixtures which showed low combustion

completeness in the quiescent case, initial turbulent motion in

the chamber greatly increases the combustion completeness.

Combustion completeness increases with increasing container

volume.

1.6 Definitions of Burning Velocity and Flame Speed

The "burning velocity" is the normal component of velocity of

a deflagration relative to the unburned gas ahead of the front.

Unless the unburned gas is stationary, the speed of propagation

-7-
of a flame relative to a stationary observer will not be the,

burning velocity. If the burned gas is stationary, the "flame

speed" will be the burning velocity multiplied by the ratio of

unburned gas to burned gas densities, as shown in Figure 1.1.

The volumetric rate of burning is the surface integral of the

local rate of burning. If the flame front is highly convoluted,

the volumetric rate of burning can be high even if the local

burning rate is not high. The burning velocity is the

fundamental quantity for studying the combustion front structure.

It is most often reported -in the combustion literature. For

studies of pressure buildup, venting, transition to detonation,

etc., the volumetric rate-of-burning, or some average flame speed

is more useful. In this report we will use two quantities, the

speed of propagation of the flame and an equivalent planar-flame

speed. They are defined in Section 5.

There exists a wide range of combustion front speeds

beginning with laminar deflagrations, wrinkled laminar

deflagrations, turbulent deflagrations, highly accelerated

turbulent deflagrations, quasi-detonations, and detonations.

We will briefly discuss, them in the following paragraphs.

1.7 Laminar Deflagrations

The minimum burning velocity is the laminar burning velocity.

There are several experimental studies of hydrogen-air laminar

burning velocities, e.g., References 34 and 35, which are in

reasonably good agreement with each other and with theoretical

calculations of laminar burning/velocity. [36] For H2-air

mixtures at room temperature, the peak laminar burning velocity

is about 3. m/s for a rich mixture of about 40% H2, 2 m/s at

stoichiometric conditions (L 30% H2 ), and progressively less for

leaner mixtures. Liu and MacFarlane [37] carried out

experimental measu rements at higher temperatures for hydrogen-air

and hydrogen-air-steam mixtures. At 250'C the peak burning

velocity for H_-air was measured to be about 9 m/s. The presence

of steam reduces the laminar burning velocity. The laminar flame

speed will be about six times the laminar burning velocity. For

hydrogen-air mixtures at ambient conditions, it will be below 20

m/s. Compared to the speed of sound, • 300 m/s, this is

negligible.

For lean H2-air mixtures the laminar flame front is not

stable but deforms into. a cellular structure. This will increase

the flame area and increase the apparent laminar flame speed

somewhat.

1.8 Ordinary Turbulent Deflagrations

In practical accident situations the flame will be turbulent.

With increasing turbulent intensity, turbulent flame speeds

-8-
u2 ul = burning velocity

burned gas unburned gas

STATIONARY FLAME

U 0 V =U2 -Ul

,•U

burned gas unburned gas

STATIONARY BURNED GAS

Flame speed to stationary observer, U, Is

U = U2 (Pu/Pb)Ul (-6 ul for H2-alr)

v = u1 - Ul = (Pu/Pb - 1)ul = unburned gas velocity

Pu = unburned gas density

Pb = burned gas density

Figure I.I. Burning Velocity & Flame Speed Definitions

-9-
increase, reach a maximum, decrease, and then quench. [39,40]

The the ratio of maximum turbulent flame speed to laminar flame'

speed is largest for nearly stoichiometric mixtures. For

hydrogen-air mixtures at room temperature and pressure, the

maximum ratio is above 16. [40] Hence for 2-air mixtures the

turbulent burning velocity might be as high as 35 m/s, and the

turbulent flame speed as high as 200 m/s for high degrees of

turbulence. However, much higher effective flame speeds can

occur in accidents even with moderate degrees of turbulence

through the phenomenon of flame acceleration discussed in the

next section.

Since many of the studies of combustion completeness and some

of flammability limits included a time history of the pressure

rise, one can infer flame speeds. Benedick, Cummings, and

Prassinos [29,30] used an array of thermocouples to measure flame

time-of-arrival, and hence flame speeds. They found upward flame

speeds up to about 15 m/s for 15% hydrogen. Kumar, Tamm, and

Harrison [38] measured burning velocities only up to 20 m/s in a

small 2.3-m diameter sphere, even though they used highly

reactive H -air concentrations, and passed some of the flames

through a turbulence producing grid.

1.9 Flame Acceleration - Highly Accelerated Deflagrations

In contrast to "ordinary deflagrations" it is possible to

have deflagrations moving at hundreds of meters per second. In

such cases we say there is considerable "flame acceleration."

Strong shock waves can be generated, giving highly nonuniform

pressures in the volume. The local peak pressure can greatly

exceed the AICC pressure during the pressure pulses. The strong

shock waves preheat unburned gas, and the flows generated in the

unburned gas create great turbulence. Deflagration-to-detonation

transition (DDT) may be observed between the deflagration and

the leading shock waves.

It is known that when flames pass through an obstacle field,

the effective deflagration speed can be greatly increased. [41-

44] This is due both to the increase in flame front area by

flame folding and to the local increase in burning velocity by

increased turbulence. In large volumes, the effect of

hydrodynamic-combustion instabilities can also greatly increase

flame speed. [45,46] We define highly accelerated flames as

turbulent deflagrations in which the flame speeds are

sufficiently high so that compressibility effects become

important, i.e., the Mach number of the flame front relative to

the speed of sound of the unburned gas, is not small compared to

unity.

-10-
1.10 Deflagration-to-Detonation Transition

Although transition to detonation has been studied for over

50 years, [47753] 'there is still considerable uncertainty whether

transition will occur in practical accident situations.

Transition may be more likely to occur than previously believed.

[54] The "Gas Dynamic" explanation for DDT is essentially a onedimensional explanation. The volume expansion of the hot burned

gases generate shock waves moving into the unburned gas. The

shock waves preheat the unburned gas, increasing the burning

rate, which leads to generation of further shock waves. Some of

the shock waves merge into a strong enough waves to that there is

a local ".explosion" that transforms into a steady detonation.

Oppenheim [55]- has shown that this cannot be the entire story.

Transition often begins with a local explosion in region of high

turbulence, even though the shock wave compression heating of the

gas is not sufficient to cause a local explosion.

Analyses of postulated severe accidents in ice condenser

containments predict that detonable mixtures can be formed under,

certain conditions. [56] The possibility of forming local

detonable mixtures and undergoing transition to detonation in a

large dry PWR containment, the Bellefonte nuclear power plant,

has been investigated. [57] The FLAME facility has supplied

experimental data which have been used to understand the

possibilities of accelerated flames and detonations. In

particular, its data were used in the Bellefonte study, [57] and

in the NUREG-.il50 Containment Loads expert panel analysis.

-11-
2. THE FLAME FACILITY

FLAME (Flame Acceleration Measurements and Experiments) is a

large horizontal rectangular channel made of heavily-reinforced

concrete. [58-60] The dimensions were selected to be half-scale

of the upper plenum region of an ice condenser PWR containment.

This section will discuss the configuration of the FLAME channel.

The instrumentation used in FLAME is discussed:in Section 3.:The

data acquisition system is discussed in Section 4.

The upper plenum of the ice condenser PWR containment is an

annular region of rectangular cross section extending about 350

degrees around the containment. It contains large obstructions

in the form of air handler units mounted on the walls, and

considerable wall roughness due to the presence of pipes, wires,

etc. FLAME was designed so that its interior width, 1.83 m (6.0

ft), and height, 2.44 m (8.0 ft) are half the corresponding

dimensions of the upper plenum. It length, 30.5 m (100.0 ft) is

just short of half the length of the upper plenum. FLAME also

differs from the upper plenum in being straight instead of

curved, and in having a solid concrete floor instead of doors

which cover the ice condenser.

FLAME was designed in 1981-2 and built in 1983 at the Sandia

National Laboratories Explosive Test Site 9920. A photograph of

FLAME and its surroundings is shown in Figure 2.1. A map of the

area is shown in Figure 2.2. The channel is oriented from the

southeast to the northwest. A schematic diagram oA the FLAME

structure is shown in Figure 2.3. The main structure consists of

a heavily reinforced concrete "U" shaped structure., Each wall has

five viewports, 20.3 cm (8 inches) in diameter, anj 44

penetrations consisting of 2.54-cm (1 inch) black ixron pipe. The

penetrations are used for the entry of instrumentation lines,

hydrogen supply lines, ignition cables, and compressed air lines

used to power mixing fans. The walls and floor are covered with

1105 structural embeds (bolt holes) which are used as strong

attachment points for obstacles to be placed in the flow path and

for attachment of devices such as the mixing fans to the walls.

A photographic view of the interior of the FLAME channel with

simple plywood baffles attached to the side walls is shown in

Figure 2.4. The blockage ratio was 33%, i.e., obstacles blocked

one-third of the cross section of the channel. A photographic

view of the half-scale model of the ice condenser upper plenum

region of PWR containments with ice condensers is shown in Figure

2.5 prior to test F-26. The boxes simulated the air handlers

present in this region, and the angles bolted to the-side walls

simulated some of the wall roughness present. The blockage ratio

was 11%. The photograph in Figure 2.6 was taken after test F-26

in which there was transition to detonation. The violent loads

resulted in complete destruction of the plywood boxes. The

-12-
Figure 2.1. Flame Channel and Its Surroundings. Heated Detonation Tube in the

foreground.
N

it

'-A

F 50 feet

I 20 meters

H, UNE

TV TOWER

Figure 2.2. Map of Flame Facility at Site 9920, Area Y
STEEL LATTICE FRAME

Y ,PLYWOOD END DOOR

PLYWOOD.

OBSTACLE

REINFORCED

CONCRETE

VIEWPORT THERMOCOUPLE

RAKE

J

ý0-1.83m --I,' [I .0.30m 1*

0.74m

1.83m

'--A

1= 30.48m -I

STEEL

STEEL CHANNEL

TOP

PLATE

2.44m

Figure 2.3. Schematic of FLAME Facility With Simple Obstacles Installed, 33% Blockage

Ratio.
I-.

Figure 2.4. Interior of the FLAME Channel With Simple Plywood Baffles Installed. The

blockage ratio is 33%. Two thermocouple rakes on the midplane are visible.
Figure422.5.1 Interior of FLAME Channel With Plywood:Boxes Simulating Half-Scale Model of

the Ice Condenser Upper Plenum Region. Blockage ratio is 11%.
I

Figure 2.6. Interior of FLAME Channel After Test F-26. Detonation has destroyed boxes

shown in Figure 2.5.
FLAME channel structure was designed to withstand loads resulting

from detonations. To monitor some of the loads, strain gauges

were bonded on some of the reinforcingbars in one of the walls.

The top of FLAME can be partly or whol:ly covered with up to

thirty eight 5.1-cm-thick (2.0 inch) steel plates, each 0.80 m

(31.5 inch) wide. Each plate is reinforced with three 20.3-cm (8

inch) channels. The plates and reinforcing channels are bolted

to the top of FLAME using embeds placed every II.4 cm (4.5 inch)

along the length of the structure. Different degrees of

transverse venting can be achieved by various arrangements of the

top plates. Tests have been conducted with the top fully closed

by the plates (0% top venting), with every other plate removed

(50% top venting), and with the plates separated by one bolt hole

(13% top venting). The designer of FLAME suggested that tests

not be conducted withý more than 50% top venting to prevent damage

to the side walls. Without the top plates, the side-walls would

lose simple support on the top and become cantilever beams. Some

of the top plates and their reinforcing channels have been

permanently deformed by detonation loads. However, they still

fulfill-their function.

A special wheeled carriage device resting on the top of the

FLAME walls hbs been used to move the top plates`. The plates

each weigh over a ton. Changing the degree of top venting

usually takesitwo technicians several days work.

To act asI a gas seal, the top of FLAME was covered with a

0.15 mm (6 mil) virgin polyethylene sheet. The sheet was taped

to'painted metal angles-on the outer top edges of the concrete

walls, and to metal angles on the ends of the channel. The,-.

polyethylene sheet does offer--some small restraint to gas

expansion before it melts or is torn off. Methods of tearing the

sheet just prior to ignition of the gas in the channel were

considered but not pursued..

The southeast- (ignition) end of FLAME is closed by a rigid

plate. The hydrogen-air mixture is normally ignited near the

center of this plate. In the first fourteen tests the rigid

plate was a 5.1-cm-thick (2 inch) steel plate, reinforced with

25.4-cm (10 inch) steel channels.- The plate was bolted to the

FLAME structure using 19 embeds, supported on the two vertical

sides and the bottom. This plate was permanently deformed'and

the concrete near the top embeds was damaged as a result of loads

from a detonation in test F-14, as shown in Figure 2.7. The

damaged embeds were replaced, and the concrete at the channel end

was repaired. To avoid damage to the FLAME structure, we

replaced the steel plate with 1/2 inch (1.27 cm) plywood sheets

held in place by a strong steel lattice frame as shown in Figure

2.8. The plywood panels were designed to withstand loads

expected from ordinary deflagrations, but to blow out at

-19-
a. Exterior Showing Concrete Cracking and Bent Steel

I

b. Closeup of. the .TOp c. Interior Showing Cracked

Concrete Walls

Figure 2.7 Damage to the FLAME Structure at the Ignition End

After Test F-14

-20-
Figure 2.8. Exterior View of the Ignition End of the FLAME Channel Showing the Steel

Frame and Sacrificial Plywood Sheet. These replaced a steel end door that

was bent in test F-14.
overpressures of about 350 kPa (50 psig). In tests conducted

since its use, this system has worked. The plywood survived

tests with ordinary deflagrations, and was destroyed in tests

with detonations and highly accelerated deflagrations. There has

been no damage to the steel lattice frame or the concrete in 16

additional tests. The only deviationfrom the design is that the

bursting pressure of the plywood may be below the design

overpressure.-

Prior, to a test, the.northwest end of FLAME was closed with a

0.10-mm (4 mil) polyethylene bag, as shown in Figure 2.9. The

bags were cylindrical in shape, with one end open. The open end

of the bag was taped to the end of FLAME to form a gas-tight

seal. One small openingýin the taped surface was left. Four

wire-rope guide lines surrounded the bag running parallel to the

channel axis. Each guide line'had clips which could freely move

down the length of the line.- The bags were manufactured with

loops made of strapping tape .on its sides. The side of the bag

was supported when these'loops were attached to the clips on the

guide lines. Just prior to a -test, the bag was compressed until

it was flush with the end of the channel. The remaining opening

in the bag-to-channel tape seal was then closed. At a later time

after all personnel had retired to safe areas, hydrogen was

allowed to enter the channel. The side supports permitted the

bag to expand parallel to the channel axis while largely

preventing motion perpendicular to the axis. This was needed to

counter loads due to buoyancy and wind. The bag lengths were

designed so that the bags would be almost fully inflated when the

desired amount of hydrogen had entered the channel., Hence the

richer the mixture, the longer the bag used. The longest bag

used was over 10 m long. For additional protection from wind, a

surrounding wind break was used. After a canvas wind break was

destroyed by detonation loads in test F-12, plastic sheet was

used. Our procedure was to test with a wind speed of no more

than 8 kmph (5 mph).:The possibility of 'wind damage to the bag or

the top sheet was the main environmental constraint on testing.

In particular, testing in the sprin'gwas discouraged because

strong winds often develop.during the day in the spring season.

The hydrogen used in the tests is obtained from a tube-type

semitrailer of 33,300-fta (943-ms) capacity. Prior to a test,

one or two tubes in the trailer were prepared with .the desired

amount of hydrogen by filling them to a desired pressure. After

all personnel retired to the concrete buildings, the hydrogen

from the prepared tubes was emptied into the FLAME channel. The

hydrogen entered the FLAME channel through three penetrations

near the floor, one at either end of the channel, and one in the

middle. The gas was mixed using two air-driven mixing fans of

2100 ft /min (59 m /min) capacity. One fan was placed near the

ignition end and one near the exit, hence leaving the main

section of FLAME free of their obstructi on. After the gas had

-22-
Figure 2.9. Prior to a Test, the Polyethylene Bag Has Been Taped- to the End of the FLAME

Channel and Attached to the Guidelines. The plastic windbreak can be seen on

the extreme left and right.
been mixed, five gas samples in five different locations in the

channel were obtained. The gas sampling systems each consisted of a line through a wall penetration, a solenoid valve, a 50-cm8

evacuated gas sample bottle, and a larger vacuum bottle. The

solenoid valves were opened, gas samples pulled into the sample

bottle, and the solenoids valves were closed. After the test,

manual valves on both ends of the sample bottle were closed. The

sample bottle was removed from the gas sampling system and sent

to the analysis laboratory at Sandia National Laboratories.

Initially, gas composition was measured by gas chromatography.

Later in the testing program, mass spectroscopy was used, which

was deemed more reliable. The reported hydrogen mole fractions

were an average of the reported values for the five sample

bottles, excluding outliers. Based onthe variation in hydrogen

mole fraction for the different bottles, we estimate the

uncertainty in hydrogen mole fraction was below ± 1/2.

The ignition system was capable of three independent methods

of ignition, a bridgewire, a spark plug, and a glow plug. All

three systems have independent cabling. All tests to date have

been conducted using a single point bridgewire ignition. A

capacitive firing set was used to provide high-amplitude current

to vaporize the bridgewire.

-24-
8. INSTRUMENTATION

3.1 Introduction

The instrumentation used in FLAME is discussed in this

section. The 'discussion of test results:in Section 6 is based on

combustion front time-of-arrival measurements and on pressure

measurem-nts. Combustion front time-of-arrival has been measured

by a variety of instruments: thermocouples, photodiodes, lithiumniobate gauges, and high-speed cinematograhy.

When FLAME was constructed, strain gauges were mounted on'-''

selected reinforcing bars that were buried in the concrete walls.

Although some of these gauges failed soon after the concrete was

poured, data were obtained from the other gauges. This data will

not be covered in this report. In the effort to model the

behavior of the FLAME tests, it was stated that it would be

useful to measure the flow velocity in FLAME, even if only at one

point, the center of the exit plane. Several efforts were made

to measure flow velocity, but they were not successful because of

the wide range of speeds found in the tests and the fast -

transient response required.

3.2 Time-of-arrival measurement

The main tool for measuring flame time-of-arrival was

chromel-alumel (type K) thermocouples mounted on vertical "rakes"

along the midplane of the channel, about equally spaced axially.

The positions of these rakes and of *the other instruments for the

various tests are given in Appendix A. Two thermocouple rakes

were used in the first test; four rakes were used in tests F-2 to

F-12; five vertical rakes were used in the later tests. A

horizontal rake near the exit was included from test F-16 to

F-19. From test F-20 onward the horizontal rake was removed and

several thermocouples were mounted on one of the side walls near

each rake. The thermocouple wire used is 0.25 mm (0.01 inch)

diameter with a roughly spherical welded junction of

approximately 1.0 mm (0.04 inch) diameter. Later butt welded

thermocouple junctions not much larger than the wire diameter

were used.

The thermocouples gave a clear indication of flame time-ofarrival. Prior to the flame arrival, the measured temperature

was either constant with some noise present or slowly rising.

There was a clear break in the slope of the temperature-time

curve at the flame time-of-arrival. In addition, the

thermocouples indicated detonation time-of-arrival. The results

from one thermocouple taken in a test with a lean hydrogen

mixture and a slow flame (test F-9, 6.9% H2 ) are shown in Figure

3.1. Figure. 3.1b is a sufficient enlargement of the

-25-
120

110

100

90

•,80

w 70

cc

!R 60

cc

w 50

w240

30.

20

10

0

40

35

,•30 'U

a.

IF25

20

0 5 10

TIME (sec)

15 20 16.80 16.85 16.90

TIME (sec)

16.95 17

Figure 3.1. Deflagration Time-of-Arrival
time-of-arrival region in Figure 3.1a to show individual

digitally recorded sample temperature measurements. As in most

tests with lean burns, there was usually little noise in the

thermocouple signal. The selected time for the flame time-ofarrival in Figure 3.1b is uncertain to no more than a single

sample period, Iclose to the limit that can be expected from

digital recording. For the tests with higher flame speeds, the

noise in the thermocouple signals was usually higher, and the

uncertainty in the time-of-arrival greater than one sample

interval, typically. ± 3 sample intervals. For a typical sampling

speed of 2 kHz, this corresponds to + 1.5 ms. In Figure 3.2

results are shown from a thermocouple near the ignition end of a

test that had a transition to detonation. Figure 3.2a shows the

overall thermocouple record, while Figures 3.2b and 3.2c show

enlargements of the two interesting periods. The time-of-arrival

estimate of the flame is shown in Figure 3.2b and the estimate

for the returning shock wave is shown in Figure 3.2c.

In tests where there was transition to datonation, it appears

that the thermocouples responded to the arrival of the detonation

or shock wave in two ways, as illustrated in Figure 3.2c. The

slope of the temperature-time curve was steeper after the

arrival, as expected by higher heat transfer rates to the

thermocouple from a higher temperature faster moving gas. Even

more dramatically, there is usually a strong oscillation in the

signal near the time-of-arrival, often with continued increased

noise levels. This oscillation cannot be response to temperature

changes in the gas. We speculate that since very hot combustion

gases are partly ionized, the increased noise level in higher

temperature combustion and the nonthermal behavior with "

detonations or shock waves may be due to magneto-hydrodynamic

effects. However, we have not made any study to back up this

hypothesis. If the onset of this behavior is considered the

detonation time-of-arrival, then the results are in good,

agreement with the detonation time-of-arrival of the other

instrumentation to be discussed shortly: germanium photodiodes,

pressure transducers and lithium-niobate gauges. This

corroboration gives confidence that we are correctly measuring

detonation/shock wave time-of-arrival with thermocouples.

We did. not attempt to use the thermocouples to accurately

measure gas temperature, only time-of-arrival. Because of the

comparatively large size of the thermocouple bead and hence

finite thermal capacitance, the thermocouple reading lags the

change in the surrounding gas temperature. There are also errors

due to radiation losses from the bead to the cold channel walls,

and conduction losses down the thermocouple wires. Methods exist

for compensating these effects. [61] Using these methods, better

estimates of the gas-temperatures could be obtained from the

data, but we have not.done so.

-27-
900

800

700

uLJ

0.:7M

'LJ

c.

'F" -

600

500

400

300

200

100

0

0 50 100 150 200 250

TIME (ms)

300 350 400

Figure 3.2. Deflagration and Detonation Time-of-Arrivals. From

temperature history test F-14, Thermocouple TA7.

30.0% H2 . No top venting. No obstacles.

-28-
40 600

. APN PERuIOD•,oV-SAPIG P/ 550 - SAMPUNQPEI

35 o500

S4500

w w

30 400

cc: w w

CL ~350-

25- 300-,

250- - E- ST. FLAME tIME-OF-ARRIVAL

20 ,, ,','j , ,', ,1. . . .•" '" ' , " ' '200 •'-

25 26 27 28 29 30 3132333435 115 120 11

TIME (ins) TIME (ms)

Figure 3.2. (continued)
In addition to thermocouples, we. have also used two other

instruments to measure flame time-of-arrival, germanium

photodiodes, and lithium niobate gauges., These were "backup"

detectors should the thermocouples fail to give good results. We

continued to us'e them after the thermocouples had proven I

satisfactory but with no incentive to increase their number.

Germanium photodiodes (GPDs) are sensitive to the infrared

radiation of hot steam. We used seven GPDs, five mounted in the

optical ports, and two mounted in access pipes near the exit.

This measurement differed from the thermocouple measurement in

that it is along "line of sight" rather than at a point. The

photodiode results become more useful in tests with more intense

combustion. In tests with slow deflagrations, the signal' showed

a gradual rise. It was difficult to select a particular "timeof-•arrival." For detonations, the signal rose abruptly. There

was a clear time-of-arrival that was in excellent agreement with

the results of pressure transducers and thermocouples.

Lithium-niobate gauges produce a large-amplitude output

voltage when heated. We mounted two of these gauges flush on the

channel walls. Initially, it was hoped that, because of their

large output signal, the signal from these gauges would not

require amplification but could be directly digitized. This was

attractive because we had more digitizer channels than

amplifiers. However, we have found that some amplification was

required. Of more importance, the lithium-niobate gauge produced

results in which the flame time-of-arrival was less distinct than

the thermocouple data.

As the test series progressed, more high-speed

cinematography was used. The cameras were run at speeds between

2000-4000 frames/second. The edge of the films contained timing

marks every millisecond to determine the framing rate. The most

useful view was at the channel exit perpendicular to the channel

axis. In the field of view was a vertical pole with marks every

foot. The position of luminous combustion fronts could be

determined frame-by-frame. -This was useful in determining if the

front was moving at detonation speed. Also of great value was an

axial view into the channel. To protect the camera from debris

thrown from the channel, we placed the camera to the side of the

channel and used a large front-surfaced mirror. Several mirrors

were destroyed in the test series. Of less value were views

through the ports in the side of FLAME. Without a means of

establishing the time-after-ignition, and with the limited field

of view, not much of value was seen in the port views. These

views did show the presence of reverse flows during part of the

test. A video camera mounted on a tower was used monitor the

region for safety purposes. However,, during the test the video

camera was viewing the top of FLAME, and the results recorded on

a VCR. With the comparatively low speed of ordinary video (30

frames/s) and low resolution, the video results were of limited

-30-
use., However,-:they did show that the plastic top cover was late

in releasing during test F-5.. From test F-26 onward, high speed

motion pictures cameras were mounted above .the channel.and used

to photograph the channel from above.

3.3 Pressure Measurement

The number, type, and location of pressure:s, transducers used

in FLAME tests is given in the Appendix. The principle type used

was the Kulite model XTS-1-190. It is a quartz membrane

Wheatstone bridge resistive gauge with resonant period of 10 #s.

A porous metal shield, Feltmetal type 1102, was placed in front

these pressure transducers to minimize the effect.of "flash

temperature" *rise on the accuracy of the reading. An

experimental study [62] has shown that pressure transducers can

accurately measure hydrogen deflagration pressures using a porous

metal shield. This confirmed simple theoretical, results which

indicate that the pressure drop through the porous metal shield

is negligible for characteristic pressure rise times of the order

of several milliseconds. The shield will have a detrimental

effect on response to much faster pressure rises due to

detonations and/or shock waves.

It is of interest to estimate the uncertainty in the

pressure measurements.- All the Kulite XTS-1-190 pressure

transducers were initially calibrated in the Sandia Laboratory

calibration laboratory, and occasionally recalibrated. The

calibrations verified the factory calibrations on gauge

sensitivity, zero offset and linearity. The discrepancies and

nonlinearities detected were negligible. After a time some of

.the gauges failed due to water ingress, or due to overranging in

violent tests. The failed gauges could easily be detected from

their behavior. What-we did not have was the capability to check

the behavior of the gauges to transient pressures. The response

of the gauges to rapid pressure changes (above a fewikHz) was

hindered by the use of felt-metal shielding previously discussed,.

and by the limited frequency of the digital sampling. The gains

of the amplifiers used were calibrated by. inputing DC voltage '

standards. In summary,- static calibrations indicate that,

uncertainty in measuring slowly changing pressures was low, below

a few percent. However, there was no testing of the response to

more rapidlychanging pressures.

Attempts were-made to use Kistler and PCB quartz

piezoelectric pressure transducers. These dynamic gauges can

measure pressures between a low frequency limit, typically a few

Hz, and an upper limit, typically about 100 kHz. These gauges

gave useful information in some tests, however, they obviously

were not functioning correctly in many other tests. There may

have been two reasons for their failures. One reason may have

been that they were operated without thermal protection to avoid

degrading their fast response. There may have been flash heating

effects on the response of the transducers during tests and/or

-31-
gradual thermal degradation of the transducers during the series

of tests. A second reason was that after testing was completed

it was recognized that the signal from these gauges to the 7612

digitizers should not have been "AC coupled" as they were in

later tests. This not only filtered out the DC (0 Hz) response,

but partly filtered out AC response up to about 30 Hz. Hence the

failure to use DC coupling invalidated the results from these

gauges in tests with slow deflagrations. We have relied on the

response of dynamic pressure transducers for determining peak

pressure in only two of the tests with transition to detonation:

test F-14, where they were the only pressure transducers

operating, and test F-12. The failure to be able to use the

response of these dynamic pressure transducers in other tests

with strong pressure spikes doubtless led to underestimates of

the peak pressure for those tests.

3.4 Strain gauges

The locations of the 32 strain gauges on reinforcing bars in

the FLAME east wall are given in Appendix A. Thesemeasurements

could be used to measure the axial and bending strains of the

reinforcing bars. During the pouring of concrete and imbedding

of the strain gauges, several of the gauges failed. As the

testing period continued, additional gauges failed. However, at

the end of the testing period many of the gauges still continued

to operate.

Only a cursory look at the strain gauge data was made.

These brief examinations showed significant oscillations of the

walls in tests with detonations and highly accelerated'flames and

negligible signals for tests with lower loads.

3.5 Exit velocity

For modeling purposes, it would be useful to know the flow

velocities in FLAME, at least the velocity at the exit plane. We

attempted to measure the exit flow velocity at the center of the

exit plane of the channel. One approach'was the construction of

a pitot tube with a miniature pressure transducer. The pressure

transducer was enclosed in the pitot tube to minimize the

internal gas volume, and hence improve the transient response.

Because of the very wide range in flow velocity encountered, we

either had insufficient dynamic pressure to record or we

overranged and damaged the fragile transducers. In the test

series with obstacles, the debris from the obstacles destroyed

the pitot tube. We discontinued using it.

Other simpler systems were used to measure velocity.

balloons hung from strings pendulum fashion have been

photographed during tests. The motion of the spheres is an

indication of flow velocity. A light-weight foamed plastic "popcorn" and soap solution bubbles were introduced in the flow,

but the spatial resolution of the high speed cinematography was

not sufficient to enable them to be visible.

-32-
4. DATA ACQUISITION SYSTEM

4.1 Introduction

The data acquisition systems for FLAME were locatedlin •

building 9920. The transducers in the FLAME channel were"

connected to building 9920 via roughly 100 meters of buried

cables. Most of the signals were transmitted over 20 pair or 9g

pair twisted shielded cable. The germanium photodiodes and thel

piezoelectric pressure transducer signals were transmitted viaý_

coaxial cables. All the analog signals were converted to'digital

form and recorded in various transient digitizer memories.

Digital recording of data is much cheaper and more convenient'

than analog tape recording. However, for recording events with:

poorly known amplitudes and timing, as in flame acceleration, -.-

there are difficulties with digital recordings. This is

discussed in the following paragraphs.

The primary data acquisition system was based on

commercially available CAMAC (Computer Aided Measurement And

Control - IEEE 583 standard) modules' with a capacity of over 100

data channels. As a backup, about 40 incoming signals were

simultaneously recorded on a custom built system termed DAASY:

In addition, six-data channels were available, on Tektronix 7612

digitizers. The main advantage of having a backup data

acquisition system was that different amplifier gains and

sampling rates could be used. Usually one system had a fast

sampling rate and low amplifier gain to accommodate a vigorous

combustion, while the other system had a slower sampling rate and

higher amplifier gain.

Throughout the test series there was considerable difficulty

in selecting the proper amplifier gains and sampling rates. It

was difficult to predict the peak pressure to be expected.

Pressures varied greatly from test to test (10 to 3000 kPa). If

too high an amplifier gain was selected, .pressure peaks could be

clipped, as in test F-19. If too low an amplifier gain was

selected, much of the signal could be in the noise level, as in

tests F-3 and F-5. Since flame speeds varied greatly from tesýi

to test, it was also difficult to predict the length of the test

(80 ms to 27 s). If too fast a sampling rate was selected, theO-.

important events could occur after the digitizers filled their =:ý

memories, as in test F-13. If too slow a sampling rate was

selected, the important events could be recorded with low

temporal resolution. This is the problem of finding the correct

time "window" to record the test. Aside from' the use of a backup

data acquisition system, we were able to ameliorate the sampling

rate problem by buying more digitizer memories and by using

variable sampling rates with the programmable clocks. The

favorite strategy used was to estimate the period of the test and

-33-
select the corresponding sampling rate to fill about 85%:of the

digitizer memory. The last 15% of memory was filled at a much

lower sampling rate. This greatly lengthened the recording

window and prevented losing the event. If the event occurred at

theý slower sampling rate, it would be recorded with lower time

resolution. However, if the event did happen that late, often

the lower resolution was adequate because the flame speeds were

not high.

4.2• Primary data acquisition system

The primary data acquisition system is shown schematically

in'Figure 4.1. Only the flow of information from the transducers

is ,shown. For clarity, the figure ignores the flow of

information from the computer to the crate controller required to

arm the devices, the possible flow of information from the

programmable clocks to the computer, and the role of the computer

terminals. The arrows with the thicker lines and larger heads

represent the flow of information taking place "real-time" during

the test; the arrows with the thinner lines and smaller heads

represent the flow of information from the volatile -digitizer

memory to storage and/or hard copy after the test is completed.

The flow' of transducer information in "real-time" was from the

transducer, to the amplifier, to the digitizer, to the digitizer

memory, as shown in Figure 4.1. The digitizers were triggered by

signals from trigger generators. The digitizer sampling rate was

under the control of either an internal clock, or an external,

programmable clock. Just after a test, the data resided in.the

digitizer volatile memory. Within a few minutes after each test,

we'began to store the data more permanently. This second data

flow was from the digitizer memory, to the digitizer, to the

CAMAC crate controll~er, to' the computer-, to hard disk storage.

After this had been accomplished, data was then copied to floppy

disks for archival storage..'

The main data acquisition system was under-the control of a

Digital Equipment Corporation LSI 11/23 (later upgraded to an LSI

11./73) computer. Attached to the computer were two terminals,

either of which could be used, but not both simultaneously. The

Tektronix 4010 graphics terminal was used to run. the data

acquisition programs written in Tektronix SPS BASIC and plot the

results. A hard copy unit was attached to the Tektronix 4010 to

obtain graphical results. The second terminal was a Datamedia

DT/80. It was used for editing programs, formatting floppy

disks, listing programs on a printer attached to it, etc. The

computer "booted up" in the RTI1 operating system. SPS BASIC was

run only on the Tektronix 4010 terminal, but RT11 operations

could be run on either terminal.

-34-
TWISTED-SHIELDED

CABLE OR

COAXIAL CABLE To DAASY system

01

HARD COPY OUT

LEGEND

REAL-TIME

POST-TEST

Figure 4.1. Schematic, Diagram of Data Acquisition System Showing.Main Flo'w of Inf bor'mati'n
All transducer signals requiring amplification passed through

Trigtek model 205A instrumentation amplifiers. These amplifiers

were in the form of fourteen rack-mounted banks of seven

independent amplifiers, for a total of 98 channels. The Trigtek

amplifiers gains were set by manually turning a switch on the

front face of the amplifier. The possible gains ranged from 0.1

to 1000 with possible setting of (0.1,0.2,0.5,1,2,5,10,20,50

100,200,500,1000). Each amplifier had an adjustment screw so

that the gain and zero could be calibrated. The Trigtek 205A

amplifiers were rated from 0 Hz to 100 kHz (down 3 db).

From the amplifiers, the signal went to a transient digitizer

module in a CAMAC crate. CAMAC is a standardized system

consisting of CAMAC crates, crate controllers, and working

modules. The crate provides electric power, mechanical support,

signal and control lines to the CAMAC. modules., There are 25

"slots" in each CAMAC crate. Modules may occupy one, two, or

more slots, depending on the module width. Each crate must have

a.crate controller which is the only module which communicates

w~ith the computer. Data from working modules must flow through

the crate controller to go to the computer. Likewise •data from

the computer to the working modules passes through the crate

controller. Our primary data acquisition system contained two

CAMAC crates with LeCroy 8901 GPIB interface crate controllers.

The connection between the crate controllers and the LSI computer

was over a General Purpose Interface Bus (GPIB - IEEE 488)

highway. A GPIB connection was selected because it permitted us

to also attach other noil-CAMAC GPIB instruments 1on the highway at

the price of a lower maximum data transfer rate than dedicated

CAMAC data transfer highways. The working modules consisted of

six BiRa 908 transient digitizers, their BiRa associated 903

memories, LeCroy 8610 trigger generators, and. LeCroy 8501

programmable clocks.

4.2.1 The BiRa 908 Digitzers

The BiRa 908 transient digitizers are a three-slot-wide CAMAC

module that can be programmed to simultaneously digitize data

using either 32, 16, 8, or 4 active channels with a 12 bit

resolution. The maximum sampling rate and the memory allocated

per channel increase as the number of active channels decreases

as shown in the following table. The memory per channel shown in

the table is for a single 903 memory module. If two 903 modules

are used per digitizer, the memory per channel would be double

the values given. Up to 12 memories can be attached per

digitizer.

-36-
Table 4.1

BiRa Transient Digitizer Performance

Number of; Maximum Sampling Memory per Channel,

Active Channels Rate, kHz per memory module

32 5 1K (1024)

16 10 2K

8 20 4K

.4 40 8K

Most of the data was recorded with 16 active channels at

speeds of I to 5 kHz. Up to test F-16 a single 903 memory was

used with each digitizer. The value of having more memory became

obvious. We purchased six additional memories to increase the

number of data points per channel. Three memories were used to

store pressure data on a single digitizer because of the need for

high resolution pressure data over a length of the test. Twomemories were used on each of four digitizers used to store

thermocouple, germanium photodiode, and lithium niobate gauge

data. A single memory was retained on the digitizer assigned to

strain gauge data.

If under control of the internal clock, the sampling rate was

constant during a test; if under the external clock, the sampling

rate could be varied during a test. The digitizers and the

programmable clocks were triggered by TTL level trigger pulses

coming from LeCroy model TG8610B trigger generators. The trigger

generators would receive trigger signals from the ignition signal

or from a transducer and convert these signals into the TTL

trigger pulse.

4.3 Backup DAASY system

The backup data acquisition system, the DAASY system, was

limited to 40 data channels. The memory per channel was fixed at

4K, and the sampling rate was required to be constant during a

test. The maximum sampling rate was somewhat higher than the

CAMAC system, 80 kHz. It was run under the control of a separate

Digital LSI 11/23 (later upgraded to LSI 11/73) computer. The

DAASY system used a Tektronix 4010 graphics terminal for all

terminal functions.

4.4 Tektronix 7612 digitizers

We made limited use of six data channels available using

Tektronix 7612 transient digitizers. These devices included two

-37-
independent data channels, each with built-in amplifiers,

programmable clocks, 8 bit resolution transient digitizers with

sampling rates up to 200 Msamples/s, and 2K of memory per

channel. The very fast sampling rates were of little value in

flame acceleration studies, while the limited memory and limited

resolution were problems. In contrast, the 7612s were the

primary data acquisition system for detonation studies where the

phenomena are faster and more predictable.

4.5 Computer language and programming

For various reasons, Tektronix's SPS BASIC was the

programming language chosen for all the data acquisition systems,

CAMAC, DAASY, and 7612s. The initial set of programs for arming

the CAMAC devices, storing the data on hard disk, and obtaining

plots of the data required inputing all parameters by keyboard

each time the devices were used. The plots were of "raw data",

i.e., digital level versus sample number. A second generation of

programs was developed that would arm CAMAC devices using

parameters from a setup file created days before the test. These

programs permitted plots of "engineering data", i.e., quantities

such as temperature or pressure versus time. To obtain pressure,

temperature, etc., data on. gauge sensitivity, amplifier gain, and

signal offset were stored in a gauge file usually created before

the test. See Section 5 for details of the generation of

engineering units.

,Two useful features.-of-our programs deserve special mention.

All our plotting routines permit- creation of plots from a

selected portion of a data array, as well as the entire array.

This permitted detailed plots of the data in periods of interest.

A second useful feature is the Graphics Input mode of theý

Tektronix 4010 terminal. A cross hair can be placed at a given

location on a plot such as the flame time-of-arrival or a I

pressure peak, and the values of the coordinates at that point

printed on the plot. This saved a great deal of time measuring

the coordinates of the desired points.

At the start of this research, the DAASY system programs for

data acquisition were not documented, were difficult to learn to

ujse, and lacked many convenience features,, such as the ability to

plot a portion of a data array. During the course of the

research, the DAASY programs-were improved in convenience,,but

they were never documented or made "user friendly."

4.6 Disk storage

The data acquired by the CAMAC system was stored in a DSD 880

"Winchester" hard disk. Later this hard disk was replaced with

two Digital RX02 hard disks of greater storage capacity.

-38-
Eventually, the hard disks became full. Archival storage of data

was therefore done on 8-inch floppy disks. Two floppy disk-,

copies of data were made. With the abandonment of the use of 8-

inch floppy disks in newer systems, we are converting all our

data storage to the newer-5-1/4 inch floppy disks. The DAASY

system had no hard disk storage and relied solely on 8-inch

floppy disks.

-39-
S. DATA PROCESSING

5.1 Introduction

This section discusses what was done with the data obtained

in the FLAME tests to give the results shown in section 6. The

first step was to convert "raw data" into "engineering

variables." Raw data is in the form of an array of digitized'

levels. The digitized levels were converted to temperatures,

pressures, voltages, etc., by the linear relation,

iEV = (DL - DATUM)*[DS*GF/AG] + OFFSET, (5.1)

where EV = engineering value, pressure, temperature, etc.

DL = digital level in the data.

DATUM = digital level selected as datum.

DS = digitizer sensitivity in volts/level.

GF = gauge factor, e.g. kPa/volt, Kelvin/volt, etc.

AG = amplifier gain.

OFFSET = engineering value at the datum.

The datum was obtained by averaging a given number of the first

data points, typically the first 75 points. Using the first test

data points to give a datum, rather than using pretest values,

minimizesthe problem of zero drift of transducers. Averaging a

significant number of points to obtain the datum was used to

minimize error due to noise in the signal. The digitizer

sensitivity of the BiRa 509 transient digitizers wa~s fixed at 800

levels/volt throughout the testing period. The response of all

the transducers used could be expressed by a linear sensitivity,

the gauge factor, GF. For pressure measurements, the offset was

set at zero. Hence the results are in terms of "gauge pressure."

Absolute pressure can be obtained by adding the local barometric

pressure to the gauge pressure. Typically barometric pressure

was about 84 kPa (12.2 psia). Values of ambient pressure,

temperature and humidity were recorded for the tests. Because

there was no cold junction compensation for the thermocouples,

and accurate values of temperature were not needed for time-ofarrival measurement, an approximate datum was used in the data

reduction. Typically the datums used were 250C in warmer

weather, and 100C in cold weather. Hence the absolute value of

temperature measured could be off by ± 50C because of errors in

the datum. See section 3 for a discussion of other temperature

measurement errors. The data from the germanium diodes and the

lithium-niobate gauges was~expressed in volts.

The sample number of the data was converted into time

relative to the ignition signal as zero time. For a constant

sample rate and post-trigger mode,

-40-
Time =, (Sample Number - 1)/Sampling. Frequency .(5.2)

For constant sampling frequency in the pretrigger .mode,,

Time = (Sample Number - Pretrigger points). (5.3)

Sampling Frequency

The uncertainty in the time ,a sample was taken relative to the.:

ignition trigger could,be up to one sample period.

In the case where two or three sampling frequencies were

used, the above equations were used, for the period in which the

first frequency was operative, and for the second frequency,

Time = [Time at end of. first frequency period]

+ [Sample. number - Freq. 1 samples] , (5.4.)

Sampling Frequency 2

and similarly for a third frequency.,

5.,2 Combustion front time-of-arrival.

The main tool for determining the displacement of the

combustion front as a function of time, and-the corre sponding

velocities and accelerations, are the time-of-arrival data from

thermocouples mounted on vertical rakes along the channel

,midplane. The combustion front time-of-arrival is therefore

known at a discrete number of elevations (7 to 12) and axial

positions. ITn cases where data was missing at one of these

elevations fqr a.given thermocouple.*rake, time-of-arrival data

from adjacent elevations of that rake were linearly interpolated.

The time-of-arrival of the combustion fronts at axial

positions other than those occupied by the thermocouple rakes,

was estimated by linearly interpolating the thermocouple data for

each thermocouple elevation. A plot of-time-of-arrival versus

axial distance for each elevation we call "combus~tion 'front

trajectories." Figure 5.la shows such combustion.front..

trajectories for test F-14.. Thermocouple data was supplemented

with data from other. transducers in tracking the retonation wave.

The combustion front position as a function of time, isochrones,

were estimated bycross plotting the axial-positions of'these

curves at a given set of times. Combustion front profiles for,

the test F-14 are shown in Figure 5.1b.. The horizontal speed of

propagation and acceleration of the combustion front are

estimated by finite differences of the thermocouple.time-ofarrival data-at a given elevation.

-41-
130

120

110 .. I. I I I I % I I s I i ' .

DEFLAGRATION

DETONATION AND/OR SHOCK WAVE

.J

U0

w

100

90

80

70

60

50

40

baI

2

0

15

0.5

0

30

20

10

.0 0 5 10 15. 20 25 30

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

DISTANCE FROM IGNITION END (m)

Figure 5.1. Combustion Front Trajectories and Profiles. Test F-14. 30.0% H2 . No top

venting. No obstacles.
To obtain-a measure of the propagation speed of the flame

averaged over the channel cross section, we define the equivalent

planar flame speed as the volumetric burning rate divided by the

channel cross sectional area,

1 dV

veq - A' ' (5.5)

c

where v eq is .: the equivalent planar flame speed, A the channel

cross sectional area, and V the volume of burnedegas in the

channel. Since we used time-of-arrival data for the flame only

on the channel midplane -we estimate vq by a finite difference

approximation to the following equation.

1 dA

eq h dt,

where h is the-channel height and A ,::is the burned gas: area on the

midplane. In general equation 5.6 will not give the same result

as equation 5.5. However, in some cases it will give-the same

results, and in many cases will give a reasonable approximation

to the results of equation 5.5. If the flame shape translates

along the channel axis with constant speed, the results of both

equations will be identical.If *the flame front speed parallel

to the channel'length is a function of time and elevation,: y, but

not of the transverse direction, z, i.e., v= v(y,t), then'the

results of both equations will be identical. However, if~there

is a change' un flame fropt shape in the z direction as the flame

propagates down the.channel, then the results of equation 5.6

will not be thelsame as the 'results of equation .5.5-

The proceidure used to compute the equivalent planar flame

speeds is a finite difference approximation to equation 5.6. The

explanation is presented in graphical terms as shown in Figure

5.2, although the procedure is performed numerically. At time t,

the combustion front is approximated by linear interpolation of

estimates at the thermocouple elevations. Let the area on the,

midplane behind the combustion front be"A(t). The small regions

below the lowest thermocouple and above the highest thermocouple

are excluded. The corresponding mean axial position of the

combustion front,

X(t) = A(t)/h, (5.7)

where h is the height of the channel, minus the distance below

the lowest thermocouple and above the highest thermocouple. At a

later time (t + At), the area behind the combustion front is

A(t+At), and the mean axial position of. the front is X(t+At).

The equivalent planar flame speed is then estimated by

-43-
I I I - I I .. I , . I [ /,

Z 2.0 01.5

1.0

>0.5 wO .0 ..J

uJ

X(75 ms) X(85 mh)

-- . I ""

0 5 10

DISTANCE FROM IGNITION, END ,(m)

15

Veq = [A(85 ms) - A(75 ms)]/[h*10 ms]

= [X(85 ms) - X(75 ms)]/[10 ins]

X(85 ms) = A(85 ms)/h; X(75 ms) A(75 ms)/h

V.q Is the equivalent planar flame speed at

Time = 80 ms, Position = [X(85 ma) + X(75 ms)]/2

Figure 5.2. Graphical Illustration of Equivalent Planar Flame

Speed

-44-
v.. = [A(t+At) - A(t)]/(hAt) = [X(t+At)- X(t)]/At (5.8) eq

The value of equivalent planar flame speed thus computed is

assumed to correspond to a mean axial position between X(t) and

X(t+At), and a mean time between t and t+At. The graphical

illustration in Figure 5.2 was taken from test F-14. For the

ýexample shown, t = 75 ms, At = 10 ms, A(t) is the-shaded area in

the top part of the figure, A(t+At) is the shaded area in the

middle section, and X(t) and X(t+At) are shown in the lower

section of the figure.

For tests ýin which'there was tran'sition to detonation, the

time-of-arrival of the detonation/shock waves.can be followed

with the response of the pressure transducers,-thermocouples,

germanium diodes, and Lithium Niobate gauges. Within the spatial

and temporal resolution of our data acquisition, detonations and

shock waves appear to be normal to the axis of the channel,

independent of elevation or width across'the channel.

5.3 Pressure Measurement

The major problem with analyzing the pressure measurements is

to determine which responses are valid, and which are artifacts

of noise and/or transducer failure. Because there is always a

period of zero gauge pressure at the start of a test, the

"noisiness" of a pressure transducers signal is evident at that

period. The noise may be random or consist of periodic spikes,-

for example, Figure 5.3 shows a noisy signal with spikes. To

help'judge whlch pressure signals were valid, the digitized pressure data was taken from site 9920 on floppy disks, and

installed on the Area 5 VAX computer. For a given test, plots

were made of all the pressure transducer responses using the same

time interval and pressure range. This permitted overlaying the

data. Although in most cases, the failure of a transducer was

clear from the peculiar response, there were some cases which

were not obvious.' Methods of judging invalid results were

developed. For example, -there are several tests in which there

was a slow pressure buildup followed by a large pressure spike.

The peak value of the spike given by different transducers often

varied considerably. Were these different values representative

of large variations in peak pressure at different locations, or

were they in error? If, when overlayed, the response of a

transducer to the slow buildup was consistent with the results of

other transducers, this was taken as an indication that the

response of the transducer probably was valid. If the response

of one transducer to the slow buildup was inconsistent with the

results of several other transducers, this was taken as an

indication that the transducer response was probably invalid, and

it was discarded. All the pressure histories shown in section 6

were judged valid.

-45-
1

0.9

0.8

0.7

CO

CO

CL,

0.6

0.5

0.4

0.3

0.2

0.1

0

-0.1

"250 300 350 400

TIME (ms)

Figure 5.3. Test F-22, 15% -H2

-46-
8. EXPERIMENTAL RESULTS

6.1. Introduction

A summary of the tests is given in Table 6.1. Pressure is''

given to two significant figures. In the following sections, the

results of tests are considered in groups with similar experimental conditions. We begin in section 6.2 with tests F-7'to F-14,

tests with no obstacles in the channel and no transverse venting.

TABLE 6.1

Summary of the Test Parameters and Some Test Results

TESTS WITH NO OBSTACLES

Test

No.

F1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Top

Vent.

s0

50

50

50

50

50

0

0

0

0

0

0

0

13

13

13

13

13

13

H Mole Peak

Fraction, Overpress.,

%__ kPa

12.4 a

19.7 2.8

20.8 - a

28.0.. 20

12.6 0.9

15.5 3.4

12,.0: 1.2

18.4 26

6.9. a

12.3 2.6

12.9 4.5 e

24.7 95/1100',

12.0 d

30.0 250/2100'

15.4 3.1

17.6 10

14.9,.

18.1 36

24.8

20.7 78

Peak Equivalent

Planar Flame

Speed,.

m/s . Comments

7

54

65

•126b

4(12)

19

16

170

1 .2c

17

30'

374

932'

50

75

136

160g

483"

Top sheet restraint.

Limited burn.

DDT near exit.

All data lost.

DDT near exit.

.. S6me' data lost.

DDT at 1/3 length.

TESTS WITH

21

22

23

24

25

26

27

28

29

0

0

0

50

50

5 0 ".

50

50

50

10-15%

15.0

14.5

15.5

19.7

28.5

13; 1

14.9

18.5

OBSTACLES

650

3100

1200

a

1500

2000

9

9

23

580

700

540

46

890

1860

15

33.4

130

No mixing fans

DDT near exit

DDT near exit

Box obstacles,. DDT

DDT = Deflogration-to-Detonation Transition

a) Indicates pressure signal within the noise level.

b) Plastic top sheet restraint gave faster value early in test.

c) Indicates horizontal propagation velocity of thin layer below roof."

d) First pressure value refers to deflagration, the second to detonation.

e) Based on dynamic pressure transducers, somewhat uncertain.

f) Peak planar equivalent flame speed is highly uncertain because

it was increasing rapidly near the channel exit.

g) Peak planar eq. flame speed highly uncertain due to early DDT.

-47-
6.2 Tests With No Obstacles and No Top Venting

Tests F-7 to F-14 were carried out with no top venting. The

top of the channel was completely closed with steel plates.

There were no obstacles in the channel other than instrumentation

and mixing fans. Test F-9 was different from all the others and

will be considered separately. In this section we will consider

several of these tests in order of increasing hydrogen

concentration, F-10, F-8, F-12, and F-14, with hydrogen mole

fractions of 12.3%, 18.4%, 21.7% and 30.0%, respectively.

The combustion front trajectories for test F-10, Figure 6.1,

were generated by linear interpolation of the thermocouple timeof-arrival data on the channel midplane. The corresponding

combustion front profiles shown in Figure 6.1b are cross plots of

the data from Figure 6.1a. They represent the flame front

profiles on the midplane from 0.60 seconds after ignition to 2.60

seconds in equal time increments of 0.25 seconds. In the lower

plot of Figure 6.1b the vertical scale is exaggerated compared to

the axial length scale for clarity. In the smaller upper plot of

Figure 6.1b, marked "TO SCALE", the two length scales are in

proportion. We will use similar combustion front trajectory and

combustion front profile figures in discussing many of the other

tests. The slight concave downward curvature of the lines in

Figure 6.1a and the small increase in distance between the

combustion front profiles in Figure 6.lb indicate some flame

acceleration was occurring, but it was not dramatic. The peak

propagation velocity observed was 19.3 m/s, where propagation

velocity is taken as the distance between two thermocouple rakes

divided by the difference in time-of-arrival for thermocouples of

the same elevation. The deflagration front was initially convex

relative to the unburned gas. It gradually transitioned into an

unsymmetrical shape concave in the lower half of the channel,

inclined forward in, the middle section, and nearly vertical near

the top of the channel. This was a partial conversion to the

"tulip" shape seen in many small-scale experiments of flame

propagation in tubes. [63,64]

Pressure histories from a transducer near the ignition end

and near the exit end in test F-10 are shown in Figure 6.2.

These were typical of the ten pressure transducer records for

this test. All pressure histories shown in this report are

relative to ambient pressure, typically 84 kPa (12.2 psi). To

the extent practical, all pressure history plots for a given test

use the same horizontal and vertical scales so that comparisons

are made easier. The Cartesian coordinates of the transducers

are noted in the figures. The X coordinate is the distance from

the ignition end; the Y coordinate the distance upward from the

floor; the Z coordinate the distance from the channel midplane.

See the Appendix for the figure which illustrates the coordinate

-48-
3

2.5-

0 -

0 -

0.5-

.. . . I . . . . I

O 0.018

n 0.0043

O 0.094

c .0.221

0 0.526

09 0.830'

0 1.135

E 1.440

EB 1.745

V 2.050

O 2.354

* 2.418

I I I I I I I I j I I

I

I -

2

0

>1

w

w

0-

TO SCALE

0 -4. ISIS

I I . . . I . . I

0 5

DISTANCE

' 1 . . . I . . I I ' ' '

10 15 ..20 25

FROM IGNITION END

'11

30

(m)

0 5 10 15 20 25

DISTANCE FROM IGNITION END (m)

30

a. b.

Figure 6.1. Test F-IO, 12.3% IH
2

1.5

1

a.

CO,

w

a.

I

c~n C;

0.5

0

-0.5

-1.5i

a.

w

co

CO)

a.

4,

4-

3.5

3

2.5

2

1.5

1_

0.5

0

-0.5

-1

-1.5

-2

-2.5

-3

-3.5 •

0o

-2

0 1 2 3

TIME (sec)

1 2 3 4

TIME (sec)

a. b.

Figure 6.2. Test F-10, 12.3% H2
system, and for the table of pressure transducer locations. The

factors which could influence the accuracy of the pressure

measurements are discussed in Section 3.

The pressure histories in Figure 6.2 shows oscillations

around ambient pressure of nearly equal positive and negative

amplitudes. The pressures peaks were roughly 1.4 kPaý, (0.2 psi)

near the ignition end and about 3.4 kPa"'(0.5 psi) near the exit

end of the channel. Up to the time that the deflagration exited

the channel, the period of the oscillations was about 300 ms.

Just after the deflagration left the channel, at about 2.7

seconds, there was a renewed burst of oscillations with about a

200 ms period. The oscillations faded away after about 3.4

seconds. The acoustic wave travel time down the length of the

channel and back was about 200 ms traveling through ambient

temperature gas, and less traveling through higher temperature

combustion products. Hence the axial acoustic wave travel times

seem a bit less than the observed pressure oscillation periods.

In test F-8 with 18.4% hydrogen, compared to test F-10 with

12.3% hydrogen, the deflagration speeds were much higher, as

shown in Figure 6.3, and the peak pressures were much higher, as

shown in Figure-6.4. There was clear evidence of flame

acceleration. The peak deflagration propagation speed observed

was 205 m/s, and the peak equivalent planar flame speed was 170

m/s. The initially convex combustion front became inclined

forward and slightly concave. The two pressure histories shown in

Figure 6.4 ari typical. There was one oscillation with a much

larger positive than negative phase. The amplitude of the oscillations did not change much along the length of the channel.

In test F-12 with 24.7% hydrogen there was a transition to

detonation near the exit. Figure 6.5 shows considerable flame

acceleration. The highest propagation speed observed was 508 m/s.

As in tests with lower hydrogen concentrations, the initially

convex flame front gradually became somewhat concave. There was

only a slight forward inclination of the front. Four pressure

transducer histories for this test are shown in Figure 6.6. All

the pressure histories show the same pattern, a gradual pressure

rise out to about 130 ms, a second pressure rise and slight fall,

and then a pressure spike. The second pressure rise occurred

during the period of most rapid flame acceleration. That the

spike was due to a retonation wave moving toward the ignition end

is confirmed by the agreement of time-of-arrival data from

pressure transducers, germanium photodiodes, thermocouples, and

lithium niobate gauges. Indeed, the reflection of the retonation

wave off the ignition end wall is clearly visible in Figure 6.6

and can be detected in many of the other pressure histories. The

peak pressures reported were measured using Kistler piezoelectric

gauges. They reported peak pressures higher than those reported

by the Kulite gauges.

-51-
400 400 I III I ý I I I . I I .I

0

0D

-- I

cc:

U0

w

LLI

350'

300

250-

200-

150-

100-

50-

I I ~

I

cn

Tg Elev.. m

0] 0.018

A 0.043 -

o 0.094

0 0.221

* 0.526

0 0.830

a 1.135

@ 1.440 -

w 1.745

V 2.060

o 2.354

+ 2.418

I . . . . I . .1 .

S2

1.5

0

w

0.5

:0

0 . I I I I I I I I I Ii Il

0

I'll'

5

' 1 . . ... Ij . I . . . . I 0II 10 15 20 25 30 5 10 15 20 .25 30

DISTANCE FROM IGNITION END (m)

DISTANCE FROM IGNITION END (m)

Figure 6.3. Test F-8, 18.4% H2
3 0 ... . . .. . 3 0 .. . . .. . . . .. . .. ... . . .. . . .. . . . . . . .. . . .. . TRANSDUCER "ANIE

POSITION POSITION

25 X -4.04 m X - 21.450

Y - 2.067 m Y - 2.121 m

20 Z - 0.914 m 20 Z " 0.914 m

Cc~~ 15: "LC

,w

Lu 5,,,. 5- 7

0 -0

-10 07.

0-- 50 100 150 200 250 300 350 400 0 50 10.0 150 20'0 250 300 350 400

TIME (ms) TIME (ms)

Figure .6.4. Test F-8, 18.4% H2
0

0

-J

4.:

200

190

180

170

160

150

140

130

120

110

100

90

80

70

60

50

•I DID D L L I ,L

DEFLAGRATION

DETONATION AND/OR SHOCK WAVE

I

X 2

PRESSURE0

*THERMOCOUPLES 0

0PHOTODIODES

• +71 4:R EFLAGRATION L 1 I o,=, El,.R^o. m•

1r 0o 0.016 U!

/ I0. ..22 21 

080 0.53

7 2.143

m 2.418

0

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

Figure 6.5. Test F-12, 24.7% H2
oI C,

I

cis

CL

w

0.

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

00-

w

CO)

CO) w

cc

0.6-

0.5-

0.4-

0.3-

0.2-

0.1-

0.0-

TRANSDUCER POSITION

X - 14.586 m

Y- 0.297 m

Z - -0.914 m

-0.1 -i

0 50 100 150 200

TIME (sec)

250 300 0 . . .I. . .I . . . .1 I ' 'l j 1 1 1 1 1

50 100 150 200 250 300

TIME (sec)

b.

Figure 6.6. Test F-12, 24.7% H2
owCL

co

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

CL)

cf)

wL

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

01

0 50 100 150 200 250 300

TIME (ms)

0 50 100 150 200 250 300

TIME (ms)

C. d.

Figure 6.6. (Continued)
Of great interest in this test was the location and condition

at the transition to detonation. Unfortunately, the

instrumentation was not set up to carefully investigate

conditions near the exit, and the exact location of the

transition is unknown. The transition may have been triggered by

the last thermocouple rake or the mixing fan near the exit.

In test F-14 with 30.0% hydrogen none of the Kulite pressure

transducers functioned. Only piezoelectric pressure transducers

were operating.

The thermocouple results shown in Figure 6.7 show more.'

pronounced flame acceleration than in test F-12. The

deflagration speeds just prior to transition were large. The

initially convex flame front became deeply concave. However, the

location of the transition was also near the exit end of the I

channel as in test F-12. High-speed cinematography shows the

detonation leaving the channel.

A comparison of equivalent planar flame speed versus axial

position in tests F-7 toF-14 is shown-in Figure 6.8. The

equivalent planar flame speed Was defined in Section 5. For lean

mixtures with hydrogen mole fractions around 12%, the speeds are

low and do not increase significantly with increasing axial

distance. Flame acceleration is evident in the three tests with

hydrogen mole fractions of from 18% to 30%.

In summary, for-.tests. with no obstacles and, no top ventingi:

1. The flame speed and peak pressure increased with

increasing hydrogen concentration.

2. Flame acceleration is evident for hydrogen mole

fractions of 18%'and above, but not at 12%.

3. DDT first occurred at hydrogen mole fractions betweený

18.4 and 24.7%, near the exit.

4. The initially convex flame shape became slightly-tostrongly concave.

6.3 Tests With No Top Venting and Obstacles

It should be remembered that all the obstacle tests but one

were conducted with a single obstacle configuration, simple

baffles on the side walls with 33% blockage ratio. Changes in&.

obstacle size, shape, spacingI and blockage ratio might alter the

results and conclusions we draw from our obstacle tests.

The presence of obstacles dramatically increased the flameý-•

speeds, overpressures, and probability of DDT for tests

-57-
,-I

cc

E

U- 0

130

120

110

100

90

80

70

60

50

40

30

20

10

0

. . . I . . . . TO Scw•AI3Ei . . . . . . .. I I.' I

DEFLAORATION

DETONATION AND/OR SHOCK WAVE

I

01

00

2

""1.5

0.5

0

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

0 5 10 15 20 25

DISTANCE FROM IGNITION END (m)

30

Figure 6.7. Test F-14, 30.0% H2
ww

w

U.

z

5-

w

1000

900

.800

700

600

500

400

300

200

100

0

0 5 10 15' 20 25 30

DISTANCE FROM IGNITION END (i)

Figure 6.8. Comparison of Flame Speeds .for Tests With No Top

Venting and No Obstacles

-59-
with comparable hydrogen mole fractions. This section discusses

the results of tests with obstacles and no top venting, tests F21 to F-23. These obstacle tests used planar baffles attached to

the side walls with a blockage ratio of 33%, as previously shown

in Figure 2.3. We will present evidence that a DDT did occur in

test F-22 with 15.0% hydrogen. To the knowledge of the authors,

15% hydrogen mole fraction is the leanest hydrogen-air mixture

for which a DDT has been reported. Peraldi, Knystaustas and Lee

[65] did observe DDT at about 16% hydrogen in a 30-cm-diameter

tube with obstacles for flame acceleration and a large

length/diameter ratio. The evidence for DDT in test F-22 is not

as convincing as that for DDTs in tests with higher hydrogen

concentrations. For test F-23 with 14.5% hydrogen, the

evidence is that a DDT did not occur. A difference of 0.5%

hydrogen mole fraction is within the uncertainty of the hydrogen

mole fraction measurement. Nevertheless, whether we have

bracketed the minimum hydrogen concentration for DDT in this

geometry, or if the results indicate the stoichastic nature of

DDT at the minimum hydrogen mole fraction boundary, we have found

t4ýe boundary for this geometry. In test F-21, the mixing fans

f'a-iled to operate. Stratification of hydrogen was observed.

The combustion front trajectories and combustion front

profiles for test F-22 are shown in Figure 6.9. The large

curvature of the lines in Figure 6.9, and the large increase in

spacing between the combustion front profiles in Figure 6.9,

illustrate the large flame acceleration produced by the presence

of obstacles. The combustion front trajectories indicate two

regions, the first ten meters and the last twenty meters. In

approximately the first 10 meters, initially convex deflagration

front takes 300 ms to accelerate to a planar front traveling at

about 760 m/s. The planar front continues to move at this speed

for the last 20 meters of the channel. This speed corresponds to

choked conditions in the burned gas. The deflagration is moving

at a speed near the lower Chapman-Jouguet point relative to the

unburned gas ahead of it. In one-dimensional flow, this is the

fastest speed a deflagration can propagate.

Four representative pressure histories for Test F-22 are

shown in Figure 6.10. Near the ignition end, Figure 6.10, the

pressure rise is gradual. Although there are numerous small

pressure spikes, the peak pressure is small, 0.22 MPa, (32 psi).

Further from the ignition end, a dominant pressure spike develops

that becomes large in magnitude and more abrupt in onset. Near

the exit, Figure 6.10, the spike has a peak of 3.12 MPa (452

psi). In Figure. 6.11, the peak pressures indicated by the

pressure transducers in tests F-22 and F-23 are plotted as a

function of distance from the ignition end. The reader is

cautioned that the accuracy of individual peak pressure values

can be significantly in error due to noise and lack of temporal

resolution. However, the trend shown should be correct. The

-60-
350

.300

co

S250

LL

0

aj 200

150

100

1 ~~?I6 L6.... ICA II~

2

E

0

w

0.5

0

0 5 10 15. 20 25 30

DISTANCE FROM IGNITION END (m)

0 5 10 15 20 25

DISTANCE FROM IGNITION END (m)

30

'Figure 6..9. Test F-22, 15.,O% HI
1 1

0.9

0.8

0.7

0%

co

CI)

C/)

ccC.

0.6

0.5

0.4

0.3

0.2

co

w

cc

CL

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

-0.1

0.1

0

-0.1

250 300 350 400 250 300 350 400

TIME (ms) TIME (ms)

Figure 6.10. Test F-22, 15.0% HI2
1 - .. . . . . ... 3.2 ..... ........ 

3

0.9 2.8

0.8- 2.6 _ _

TRANSDUCER 2.4 TRANSDUCER

0.7 POSmON 2. PO8ON

X - 21.450 m X = 25.591 m

' 0.- ~ Y 2.121 m 2 Y~ - 2.122 m

0. 6 Z- 0.914 m C- Z =-0.914 m

~0.5 1.

1.6

o0.4 1.4

co 20) 1.2 wO3 i0 -

0.2 - 0.6 0. - 0.1.43 1 -

0L 0.2

0.20.2- 0. 0.4-Z

-0.1 ' .. . .- 0.2

250 300 350 400 325 327 329 331 333 335

TIME (ms) TIME (ms)

Figure 6.10. (Continued)
10 I . . .. .

• F-22

• o F-23

nm

w

D 0 (/) 1,

Co•

0

0~0

0 0

* 8

0

.0.1

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (i)

Figure 6.11. Tests F-22 and F-23. Measured peak pressures.

-64-
increase in peak pressure with increasing distance from the

ignition end is clearly shown to be dramatic. For Figure 6.10,

the peak pressure/initial pressure ratio of the spike is about 38

(ambient pressure = 0.084 MPa). This corresponds to a shock wave

of Mach number of 5.7, a value in the range of detonation wave

numbers. This is one piece of evidence that a DDT occurred.

However, there is no evidence of a retonation wave. At the time

the combustion wave exited the channel, many of the obstacles had

been broken off the walls of the channel. The obstruction of the

obstacles remaining on the walls, and of the debris flying

through the channel, may be the reason a returning retonation

wave was not seený.

The detonation cell width, X, for a 15.0% hydrogen-air

mixture is about 360 mm. Hence the ratio of gap size between the

obstacles, d = 1219 mm (4 ft), to detonation cell width, was d/X

= 3.4. DDT has been observed for ratios of d/X that low, with

the observed combustion front being a "quasi-detonation". Quasidetonations are highly disturbed detonations traveling

significantly below the detonation C-J speed. The ratio of the

unobstructed channel width, W, to X was W/X = 5.0. Benedick,

Knystautas, and Lee [66] have shown that a detonation could.,,

propagate from a rectangular opening to an open space when W/X

9 for this FLAME channel aspect ratio. The presence of the

ground under the bag will tend to support the shock wave on that

plane and prevent the extinguishment ofja detonation after it

exits the channel,. Hence, the mixture used in test F-22 could

undergo DDT in the channel and is marginally able to continue to

propagate outsidethe channel. This discussion should be kept in

mind when the photographic data is discussed in the next

paragraph.

The evidence for a DDT in test F-22 rests on two pieces' Of

data, the high pressure seen near the exit previously discussed,

and the speed of a luminous wave seen exiting the channel. -There

was no evidence of a combustion wave moving at detonation speed

inside the channel from the thermocouples, pressure transducers,

or germanium photodiodes. The DDT must have occurred near or at

the channel exit. High-speed cinematography (3854 frames/s)

taken perpendicular to the channel axis shows the motion of a

moderately to weakly luminous wave exiting the channel. The

luminous front was observed on seven frames. The speed of the

wave was determined by comparing the position of the wave on

consecutive frames of film. In particular, the position of the

wave on each frame was determined by observing the position of

the sharp break in slope of the plastic bag at the top and bottom

positions due to lateral expansion behind a compressive -wave.

The wave position was measured on a digital position imaging

screen. Shown in'all'frames was a vertical pole with reference

stripes each 0.305 m (1 foot) apart. The distances between these

stripes on the imaging screen was used as a distance standard.

-65-
3 ........... 3 1~

w

0

z

co

2.5-

2-

1.5-

1-

0.5-

0

U BOTTOM

TOP

/0

U

/0

/ U

0/ U

w

0

z

I2.5-

2-

1.5-

0.5-

0

0

REFLECTION

FROM

GROUND

STRIKES x

iI iI

0

'I * F-22

0/ss

F-23

,-E

iI

0 -U U

0 0.5 1 1.5 2 0 0.5 1 1.5 2

TIME (ms) TIME (ms)

Figure 6.12. Tests F-22 and F-23. Trajectory of Wave Exiting Flame Channel.
Since the pole was closer to the camera than the FLAME exit, a

correction for parallax was made, increasing the measured

distances by 18%. The results are shown on Figure 6.12. The

luminous front is seen to move at a nearly constant speed of 1686

m/s relative to the ground. This is above the C-J detonation

velocity of 1516 m/s. The difference can be due to the motion of

the unburned gas out of the channel. High-speed cinematography

looking into the channel showed explosive growth of bright

luminous fronts from the obstacles and the last rake, but did not

clearly show these fronts covering the entire cross section as

would be expected by a detonation. In summary, it appears that

there was a deflagration-to-detonation transition in test F-22.

If a self-sustaining detonation was not achieved, the pressures

in the channel and wave speed outside the channel were not less

than those expected of a detonation.

The experimental results for test F-23 are sho*n in Figures

6.13 and 6.14. The combustion front trajectories are similar to

those for test F-22, except the times-of-arrival are somewhat

later, corresponding to lower flame speeds. The propagation

speed down the last 10 meters of channel was 620 m/s. The peak

pressures observed for test F-23 are shown with those of test F22 on Figure 6.11. They show a similar trend, increasing with

distance from the ignition end, but the values near the channel

exit are lower. The largest peak pressure/initial pressure ratio

seen was 14, corresponding to a shock'wave Mach number of 3.5,

below that required for a detonation. High-speed cinematography

(4675 frames/s) at the channel exit showed the motion of a veryweakly luminous wave exiting: the channel. Using the same

technique as in evaluating the data from test F-22, the position

of the wave at the top the the confining bag is shown in Figure

6.12. The points do not lie on a straight line. The large

change in slope near the sixth point coincides with the arrival

of a reflected wave from the ground. Consequently, we believe

the slope given by the first points is representative of the

speed of the wave coming from the channel into an open space.

This velocity is about 935 m/s, below the C-J detonation speed of

1496 m/s. High-speed cinematography looking into the channel

show no evidence of detonation or even strong local explosions.

In summary, the evidence shows that a DDT did not occur in test

F-23. It appears that the twotestsF-22 and F-23 have closely

bounded the conditions for DDT.

Test F-21 was interesting because of an error made in

preparation of the test. The air supply to the air-driven mixing

fans was cut off. Analysis of gas samples taken at the-upper

locations indicated a hydrogen mole fraction of 15%, while gas

samples at the lower locations at the lower locations indicated a

hydrogen mole fraction of 10%. During the filling of the FLAME

channel, hydrogen enters the channel through three widely

separated ports low in the far wall. It appears the jets lost

-67-
400-

350 - TO' SCAD I

E

0

1=

300

250

200

2

E;.5

0

I-l -J

0.5

0

150

100

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

0 5 10 15 20 25

DISTANCE FROM IGNITION END, m

30

Figure 6.13. Test F-23, 14.5% H12
0.7 0.7

S

w

CO,

w

CL

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.6

0.5

0.%

w

CO) CO

w

ccCL

0.4

0.3

0.2

I

0.1

0.0

-0.1 -0.1

250 300 350 400 450 500

TIME (ms)

250 300 350 400 450 500

TIME. (ms)

Figure 6.14. Test F-23, 14.5% H2 . No top venting. 33% blockage ratio. Pressure

histories.
0.7 1.4

0

C,)

2,

w

cc

0.6

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

aw

Cri

w

a:

0.8

0.6

0.4

1.2

1

0.2

0

-0.2

250 300 350 400 450 500 389 391 393 395 397 399

TIME (ms) TIME (ms)

Figure 6.14. (Continued)
their momentum rapidly because of the low mass density of

hydrogen and drifted upward to the ceiling. Calculations done

with the HMS code by J. Travis confirmed this explanation. [67].

The deflagration front trajectories and fronts shown in Figure

6.15 show that the flame travelled slower near the bottom than

the top in the first ten meters. After that axial distance, the

front tended to be vertical and planar.

The equivalent planar flame speed versus axial distance for

tests F-21 to F-23 is shown in Figure 6.16. The lines

representing the results for the three tests are nearly the same

for the first twenty meters. The equivalent planar flame speeds

tend toward asymptotic values in the last section of the channel,

with the richer mixtures giving a higher speed.

In summary, for tests with obstacles in the channel and no

top venting:

1. The flame speeds and pressures are much higher than in

comparable tests without obstacles.

2. The flame accelerates toward speeds corresponding to

choked conditions in the burned gas, = 500 - 700 m/s.

3. Transition to detonation was seen at 15% hydrogen.

6.4 Tests With 50% Top Venting and No Obstacles

Tests F-i to F-6 had 50% of the top covered with steel

plates. Prior to each test, the entire top was covered with a

thin plastic sheet. The expectation was that the sheet would

quickly be destroyed causing minimum restraint to gas flow. The

results of tests F-1 - F-4, and F-6 follow a consistent pattern.

Test F-5 had some anomalous results. In test F-5, the top sheet

was observed not to fail until the flame had progressed part way

down the channel. We will consider tests F-2, F-4, and F-6 in

detail. The pressure rises for these tests were low. In two

tests, F-i and F-3, the pressure signals Were in the noise level.

We will examine the pressure response only for the test with the

highest hydrogen mole fraction tested, test F-4 with 28.0%

hydrogen. In none of these tests was there a detonation. We

will consider these tests in order of decreasing hydrogen mole

fraction.

For test F-4 with 28.0% hydrogen, the deflagration front

trajectories and profiles are shown in Figure 6.17. In Figure

6.17a, for each elevation, the lines of time-of-arrival versus

distance from the ignition end are nearly straight and parallel.

The time-of-arrival at any given axial distance monotonically

decreased with increasing elevation from the floor. As shown in

Figure 6.17b, this means the deflagration propagated down the

-71-
co

E

-J

IL0

wb

450

400

350

300

250

200

150

100

,1, TO,1, ISCALIE, 1

. TO SCALE

2

~1.5

0

.-J

0.5

0

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

0 .. 5 10 ý 15 20 25

DISTANCE FROM IGNITION END (m)

30

Figure 6.15. Test F-21, 10-15% H2. No top venting. 33% blockage ratio. Combustion front

trajectories and profiles.
1000

E

Co

0.n

co

.U. w

w

-J

LL

z

-J

<Iw

I--

D

w

900

800

700

600

500

400

300

200

100

0

0 5

DISTANCE

10 15 20 25 30

FROM IGNITION END (m)

Figure 6.16. Comparison of Flame Speeds for ,Tests With No Top

Venting and Obstacles

-73-
300

275

250

ca 225

.. 200

c 175

< 150

0125

I--

75

50

25

0

' 'TO I S IC . TO SCALE

2

0.:5

0

>1R

0.5

0

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

a:..

0 5 10 15 20 25

DISTANCE FROM IGNITION END (m)

30

b.

Figure 6.17. Test F-4, 28.0% H'2
channel at nearly constant speed, 126 m/s. The flame profile was

inclined forward from the vertical by about 70 degrees. There

was no flame acceleration.

Pressure histories from four transducers used in test F-4 are

shown in Figure 6'.18. The initial smooth pressure rise up to 40

ms in Figure 6.18a may correspond to the time before the top

plastic sheet fails. This period of smooth pressure rise

diminished further from the ignition end. The smooth pressure

rise was followed by a period of rapid pressure oscillations,

with periods typically about 2 ms. Periods this'short can only

be associated with venting out the top and pressure waves

transverse to the' channel axis. The peak pressures shown in

Figures 6.18a and 6.18b occur at about 70 ms, when the flame

front has only moved a few meters from the ignition end. The

peak pressure occurred somewhat later in Figures 6.18c and 6.18d,

but still before the arrival time of the flame. It appears that

the peak pressures were associated with the transient processes

soon after ignition, and that after the steadily propagating

flame was achieved, the resultant overpressures were less than 10

kPa.

Figure 6.19 shows the combustion front trajectories and

profiles for test F-2 with 19.7% hydrogen; Figure 6.20 shows the

combustion front trajectories and profilesjfor test F-6 with

15.5% hydrogen. The flame speeds decrease as the hydrogen mole

fraction decreases. Aside from the lower speed, the pattern of

steeply inclined flame fronts with uniform propagation speed down

the channel is-the same for all three tests considered, and for

F-i and F-3.

The results of test F-5 shown in Figure 6.21 are clearly

different from the other five tests. The comparatively high

initial flame velocities for test F-5 are inconsistent with those

of the other tests. This is evident in Figure 6.22. However,

further from the ignition end, the behavior changes. The

equivalent planar flame speed drops to levels comparable to test

F-i with about the same hydrogen mole fraction. Video pictures

of this test ýshow that the plastic top sheet did not fail early

in test F-5 as in the other five tests. We believe this is the

cause of the-anomalous behavior.

In summary, .for the tests,.with 50% top venting and no

obstacles:

1. There was no flame acceleration and no DDT, even for

nearly stoichi•ometric mixtures.

2. The flame front was planar and steeply inclined from the

vertical with the top leading. %

-75-
01"

(L

R:

CO)

Cr)

LUCC

A.

25

20

15

10

5

0

-5

S

0.

LU

cc

COCO)

Lu

cc0.

25

20

15

10

5

0

-5

-10 -10

TIME (ms)

a.

0 50 100 150 200 250 300 350 400

TIME (ms)

Figure'6.18. Test'F-4, 28.0% H2 5.0% top''venting. No obstacles. Pressure histories.
25 ..... ...... . .. . . . . . . . . . 25

20-

15-

TRANSDUCER

POSITION

X - 14.586 m

Y " 0.297m

Z - -0.914 m

20

I 15

0-%

coCL wý

%NO

10-

5-

0-

-10

'1.1 ii 4 , iii I lJdl, - i,

* co

- w

10

5

0

1i . 1 1 1 1 I

-5

C 50 100 150 200 250 300 350 400

-10

TIME (ms)

C.

0 50 100 150 200 250 300 350 400

TIME (ms)

d.

Figure 6.18. (Continued)
800

700 .... I SCA

TO SCALE

I

'600

X500

< 400 n-'

0

L 300

200

100

0

2

z1.5

0

,-J

0.5

0

0 5 10 15 20 25

DISTANCE FROM IGNITION END (m)

30

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

Figure 6.19. Test F-2, 19.7% H2 . 50% top venting. No obstacles. Combustion front

trajectories and profiles.
2

co

-J

0

1.75-

1.50

1.25-

1

0.75-

0.50-

0.25'

0-

(

TO SCALE

2

Z1.0

0

>!R w

-JwU

0.5

0

I I I

) 5 10 15 20 25 30

5 10 15 20 25

DISTANCE FROM IGNITION END (m)

DISTANCE FROM IGNITION END (m)

Figure 6.20. Test F-6, 15.5% H2 . 50% top venting. No obstacles. Combustion front

trajectories and profiles.
4

E

-J

LL

9

I3.5

3

2.5

2

1.5

1

0.5

TO SCALE

000I

1

2

•1.5

z.

.Iw-JLUl

0.5

0

0 0 .. 5 10 15 20 25 30

0 5 10 15, 20 25 30 DISTANCE FROM IGNITION END (m)l

DISTANCE FROM IGNITION END (m)

Figure 6.21. Test F-5, 12.6% H2. 50% top venting. No obstacles. Combustion front

trajectories and profiles.
1vAn I I I. I 1 .1 I I I H I I* I I I I

120-

TJ

7

C0,

z

4:

rn110±

100±

90

TEST H_2•,_

E F-I 12.4

A F-2 19.7

* F-3 20.8

V F-4 28.0

o F-5 12.6

* F-6 15.5

80 ±

70+

60+

50±

40±

30±

20±

W W w -

10±

I I

0

0 5 10 15 20 25 30

DISTANCE FROM -IGNITION END (m)

Figure 6.22. Comparison of Flame Speeds for Tests with 50% Top

Venting and No Obstacles

-81-
3. The flame propagation speed increased with increasing

hydrogen mole fraction.

4. The overpressures were low and the peaks were associated

with initial transient effects and not the steady flame

propagation.

6.5 Tests With 50% Top Venting and Obstacles

Tests F-24 to F-29 were carried out with 50% top venting and

obstacles. Test F-26 used obstacles that simulated the geometry

of the upper plenum region of an ice condenser pressurized water

reactor containment. At one time members of the reactor safety

community expressed the opinion that DDT was impossible in a

geometry as open as that of test F-26. This test confirmed that

flame acceleration by obstacles, even with only 11%,blockage

ratio can cause a DDT in a vented geometry. We will: first

consider test F-26. All the'other tests considered here used the

the obstacle geometry with planar baffles of 33% blockage ratio

previously discussed. We will discuss those tests in order of

increasing hydrogen mole fraction.'

The combustion front trajectories and profiles for test F-26

are shown in Figure 6.23. From Figure 6.23a, it is clear that

the transition to detonation occurred about 7 meters from the

ignition end, between the second and third thermocouple rakes. A

rightward wave, the detonation, was observed moving at 2000 m/s.

A leftward wave, the retonation, was observed moving at about

2000 m/s. Test F-26 was the only test with obstacles in which a

retonation wave was observed.: The combustion front profile at 65

ms in Figure 6.23b is a crude estimate since there were no

thermocouple rakes near the DDT. Four representative pressure

histories of test F-26 are shown in Figure 6.24. Near the

ignition end, Figure 6.24a, the pressure history shows a gradual

pressure rise, fluctuations, the retonation wave, and more

fluctuations. Through most of the channel the pressure

transducer response looked like Figure 6.24c. There was a single

detonation wave with an abrupt rise to about 1.0 MPa (145 psi).

The higher narrow peak shown in Figure 6.24d of 1.5 MPa (220 psi)

was not observed by any other pressure transducer, even one at

the same axial distance from the ignition, but which was located

on the other side of the channel. Nevertheless, there is no

indication that thetransducer was not responding correctly.

The results from test F-24 with 15.5% hydrogen and obstacles

with 33% blockage ratio are shown in Figure 6.25. The

deflagration accelerated, but the highest propagation speed

observed was 82 m/s. The initially convex flame front became

concave relative to the unburned gas. The overpressures were

low, in the noise level.

-82-
U,-

-J

c:z I-- 0

80

70

60

50

40

30

20

10

0

......... .'

DEFLAGRATION _____ - DETONATION

2

0

w-J

0.5

0

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

a.

0 5 10 15 20 25

DISTANCE FROM IGNITION END (m)

30

b.

Figure 6.23. Test F-26, 28.5% H2 . 50% top venting. 11% blockage ratio. Combustion front

trajectories and profiles.
0.4- . 16.

1.5

1.4T

POSITION

0.3 ___1.3 X - 14.586 m

TRANSDUCER 1.2 Y = 0.297 m

POSITION Z -0.914 m

X = 4.804 m 1.1

Y-2.067m 1

. 0.2 z= 0.914 m 

C 0

a .0.9

2 -_ 0.7

0.1 o0.6 Ww

~0.7

0O O.1 c0.4

0.0cIl 0.5 0.2

0.1

-0 • .1 .. ." . ' ' ' ' ' I ' ' ' 0 .21 l . . I ' ' l . . -'-7' 

0

50 60 70 80 90 100 70 75 80 85 90 95

TIME (ms) TIME (ms)

a. b.

Figure 6.24. Test F-26, 28.5% R 2 . 50% top venting. 11% blockage ratio. Pressure

histories.
(LJ

CO)

CO) w

cc

1.6

1.5

1.4

1.3

1.2

1.11

0.9

0.8

0.7

.0.6

0.5

0.4

0.3

0.2

0.10

-0.1

c- w

cc

CO) w

0.

1.6

1.5

1.4

1.3

1.2

1.1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.10

00

01

75 80 85 90 95 100 75 80 85 90 95 100

TIME (ms) TIME (ms)

C . d.

Figure 6.24. (Continued)
1000

900

800

E 700

-J

!.I600

500

L.

0

? 400

F-- 3m

.111T SCALE

TO SCALE

2

E1.

o" .5

0

w

-J

0.5

0

300

200

100

0

0 5

DISTANCE

10 15 20 25

FROM IGNITION END

30

(m)

0 5 10 15 20 25

DISTANCE FROM IGNITION END (m)

30

Figure 6.25. Test F-24, 15.5% H2. 50% top venting. 33% blockage ratio. Combustion .front

trajectories and profiles.
The results from test F-29 with 18.5% hydrogen are shown in

Figures 6.26 and 6.27. The acceleration of the deflagration is

more pronounced than in test F-24. The highest observed

deflagration propagation speed was 100 m/s. The initially convex

flame shape becomes slightly concave. The peak pressures shown

in Figure 6.27 were associated with an oscillating burst that

appears to move down the channel with the flame. The magnitude

of the peak pressures was low, under 25 kPa.

The results of test F-25 with 19.7% hydrogen are shown in

Figures 6.28 and 6.29. Compared to test F-29, the flame speeds

are much faster, flame acceleration was more pronounced, and the

overpressures were much higher. Figures 6.29 shows as we move

down the channel, the pressure peaks increase and the onset of

the peak pressure becomes more abrupt. The pressure trace in

Figure 6.29d, at 25.5 meters from the ignition end, is similar to

that of a detonation. High-speed cinematography looking into the

channel shows the formation of a bright luminous front which soon

occupies the entire channel cross section. Highý-speed

cinematography (2896 frames/s) perpendicular to the channel exit

shows a luminous wave in five consecutive frames. A linear

regression of the axial position versus time of the luminous wave

shows it moved at a speed of 1616 m/s. The linear fit to the

data had little scatter. This compares to the C-J speed of 1690

m/s relative to the unburned gas for 19..7% hydrogen-air mixture.

The detonation cell width for such a mixture at 82 kPa, 300 K, is

about 60 mm. Hence d/X L 20, and W/X = 60. The physical

dimensionsare larger than critical for DDT and propagation of

the detonation from the channel into open space. The evidence

appears to indicate that a transition to a self-sustaining

detonation was achieved in test F-25. To the knowledge of the

authors,,this is the leanest hydrogen-air mixture in which a DDT

was seen in a geometry with transverse venting.

Figure 6.30 summarizes the variation in equivalent planar

flames speeds versus axial distance for these tests, excluding

test F-26. The equivalent planar flame speeds increase

monotonically down the channel and do not appear to reach a

plateau. At each axial position, the equivalent planar flame

speeds observed increase rapidly with increasing hydrogen mole

fraction.

In summary, for tests with 50% top venting, the presence of

obstacles

1. Greatly increased the flame speeds and overpressures

compared to similar tests without obstacles.

-87-
500

450

400

E 350

-J

* 300

cc

,250

0

Lb 200

'-150

100

50

0

TO SCALE

2

`15 z. 0

-J

0.5

0

0 5. 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

0 5 10

DISTANCE FROM

15 20

IGNITION END

25

(M)

30

Figure 6.26. Test F-29, 18.5%-H12. 50% top venting. 33% blockage ratio. Combustion front

trajectories and profiles.
25

20

15

I

0L

aCO

CO) wU

cc

CL

10-

5'

0-

-5-1

-10

a.

w

CO

CO)

.w

cc0.

25

20

15

10

5

:0

-5

-10

-15 -4 I-- -15 I . . I 2 I . .I I 4 0 I 0 100 200 300 400 500 0 100 200 300 400 500

TIME (ms) TIME (ms)

Figure 6.27. Test F-29, 18.5% H2. 50% top venting. 33% blockage ratio. Pressure

histories.
25

20

25

20

15

10

5-

0-

0a 10

a.

w

c 5

Uj 0 0

cc

CL

%wo

w

a:

C

-5

-10

-15

-5

-10

-15

0 100 200 300 400 500

TIME (ms)

0 100 200 300 400 500

TIME (ms)

Figure 6.27. (Continued)
E

CC

U.

0

Co

250

225

200

175

150

125

100

75

50

25

0

2

1.5

0.5

0

I .... .SCAL..TI 

TO SCALE

0 5

DISTANCE

10 15 20 25 30

FROM IGNITION END (M)

0 5 10 15 20 25

DISTANCE FROM IGNITION END (M)

30

Figure 6.28. Test F-25, 19.7% 112*. 50% top venting.

trajectories and profiles.

33% blockage ratio. Combustion front
CO)

CO)

0-

100

90

80

70

60

50

40

30

20

10

0

-10

-20

CO,

CO)

w

0.

100

90

80

70

60

50

40

30

20

10

0

-10

-20

tI

I

150 200 250

TIME (ms)

a.

300 350 150 200 250

TIME (ms)

300 350

b.

Figure 6.29. Test F-25, 19.7% H2. 50% top venting. 33% blockage ratio. Pressure

histories.
200

175

150

125

1200

1100

1000

900

800

a. 700

u-0600

:500

c'400

S. -n

w

cc

co

couJ

cc

CD

100

75

50

25

0

-25

200

100

0

-50 -100

200 250

TIME (ms)

300 240 250 260 270 280 290 300 310

TIME (ms)

C.• d.

Figure 6.29. (Continued)
1000- . .

TEST H2

90 go - F-24 15.5

A F-29 18.5

% 800 - F-25 19.7 0

LJ

uLJ 700-

C0

L 600 -

LL 500-

Z 400

0.

300-

z

LLI

,.J 200-

D 100-

LUJ

0 5 10 15 20 25 3(

DISTANCE FROM IGNITION END (n

Figure 6.30. Comparison of Flame Speeds for Tests With 50% Top

Venting and Obstacles

)

-94-
2. Showed flame acceleration continuing down the length of

the channel, whereas there was no flame acceleration

without obstacles.

3. Underwent DDT at 20% hydrogen, while there was no DDT

without obstacles even at 28% hydrogen.

6.6 Tests With 13% Top Venting and No Obstacles

In tests F-15 to F-20, all but 13% of the top of the channel

was covered with steel plates. The entire top was covered with a

thin plastic sheet to act as a gas seal. There were no obstacles

in the channel other than the mixing fans and the thermocouple

rakes. It was expected that the results would be intermediate

between those for tests with 50% and with no top vetting. This

was not what was observed. For the leaner mixtures, hydrogen

mole fractions below 18%, the hypothesis was true. For the more

reactive mixtures, the results were more violent than in tests

without top venting.

Test F-15 with 15.4% hydrogen is an example of the expected

behavior. The results of test F-15 are shown in Figures 6.31 and

6.32. Compared to a test with 50% top venting, F-6 with 15.5%

hydrogen, shown in Figure 6.20, the flame speeds are higher as

are the overpressures. However, compared to a test without top

venting and leaner mixture, F-10 with 12.3% hydrogen, the

overpressures are similar, as shown in Figure 6.2. As in next

three tests to be discussed, the combustion front profile changes

from initially a convex shape to a deeply concave one, with the

top end travelling faster than the bottom. Also as in the next

three tests, the pressure response near the ignition end, Figure

6.32a, shows a pressure peak at an early time, which must

correspond to transients just after ignition, and a late burst.

The further from the ignition end, the smaller the initial

pressure peak until it is unobservable midway down the channel.

The late time pressure burst moves down the channel from the exit

end to the ignition end. In test F-15 its magnitude at the exit

end was similar to that at the ignition end, but higher peaks

were seen at some of the transducers midway down the channel.

The shape of the combustion front of test F-16 with 17.6%

hydrogen, shown in Figure 6.33, is similar to that of test F-15

except that the speeds are higher and the faster propagation

speed of the flame near the top plates compared to the floor is

more pronounced. The pressure histories shown in Figures 6.34

for test F-16 are similar to that of test F-15, except that the

magnitude of the overpressures is higher. The results for test

F-18 with 18.1% hydrogen shown in Figures 6.35 and 6.36, and for

test F-20 with 20.7% hydrogen shown in Figures 6.37 and 6.38, are

again similar. The speeds and overpressures increase rapidly

with increasing hydrogen content, but the flame shapes, flame

acceleration, and pressure histories are similar. However, the

magnitude of the flame speeds and the peak overpressures become

higher than for comparable tests with no top venting.

-95-
,-I

L,- 0

1.1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

TO SCALE

.2

0

> 1

uJ

I.U

0.5

0

0

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (M)

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (W)

Figure 6.31. Test F-15, 15.4% H2 . 13% top venting. No obstacles. Combustion front

trajectories and profiles.
5 5

0--

w

Co

CO)

w

0-

0 CL)

CO) CO)

0.

0

-5 -5

0 0.25 0.50 0.75 1 1.25 0 0.25 0.50 0.75 1 1.25

TIME (sec) TIME (sec)

b. a.

Figure 6.32. Test F-15, 15.4% H 2- 13% top venting. No obstacles. Pressure histories.
600

550

500

450

400

TO SCALE

-J

CD00

<,-300_ . *L 115

c250 2

200

150 -J "

2 100 00 0.5

150 0

o- 0

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (M)

Figure 6.33. Test F-16, 17.6% H2. 13% top venting.

trajectories and profiles.

5 10 15 20 25

DISTANCE FROM IGNITION END (M)

30

No obstacles. Combustion front
10-

0--t

CI)

CO,

w

cc

5

0

aCo

Co

w

X5

0

-5

I

CD(0

-5

-10 -10

0 .100 200 300 400 500 600 700

TIME (ms)

0 100 200 300 400 500 600 700

TIME (ms)

Figure 6.34. Test F-16, 17.6% H2 . 13% top'venting. No obstacles. Pressure histories.
450

400

TO SCALE

-J

0

350

300

250

200

150

0

0

2

E1.

z.

0

w

-JLU

0.5

0

100 -1

50 

*

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

Figure 6.35. Test F-18, 18.1% H2 . 13% top

trajectories and profiles.

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (m)

venting. No obstacles. Combustion front
30-

25-

20-

40

35

TRANSDUCER

POITION

X - 0.787 m

Y - 0.276 m

Z - 0.914 m

0

ca

C,,

CL

15-

10-

5-

25

a .

Cl,

0.C,

20

15

0

9- 10

I II II

5

0-

0

-10

0 100 20

-5

' .... 30I 40I -10

I 300 400 -- v _ 500 0 100 200 300 400 500

TIME (ms) TIME (ms)

Figure 6.36. Test F-18, 18.1% H. 13% top venting. No obstacles. Pressure histories.
-J

IL

0

1

200

175

150

125

100

75

50

25

0

TO SCALE

0

2

E

z1.5

0

B>1.

LU

-J

0.5

0

0 5

DISTANCE

10 15 20 25 30

FROM IGNITION END (m)

0 5 10 15 20 25

DISTANCE FROM IGNITION END (m)

30

Figure 6.37. Test F-20, 20.7% H2. 13% top venting. No obstacles. Combustion front

trajectories and profiles.
80-

70

60-

80

70

60

%.0

w

CO

CO)

w

X~

50

40

30

20

w

q:

CO)

w

0.

50

40

30

20

0

10

0

10

0

-10 -10

0 50 100 150 200 250 300 0 50 "100 150 200 250 300

TIME (ms) TIME (ms)

Figure 6.38. Test F-20, 20.7% H2. 13% top venting. No obstacles. Pressure histories.
The results of test F-19 with 24.8% hydrogen are shown in

Figures 6.39 and 6.40. A DDT was observed at roughly 11 meters

from the ignition end. At the time, this result was so

unexpected that all but two of the pressure transducers were set

with too high an amplifier gain. The pressure peak was clipped

for these transducers, although their results are still useful

for determining detonation/retonation time-of-arrival. The

response of the two transducers that did not saturate is shown in

Figure 6.40. High speed cinematography taken along the channel

axis from the exit end show that the detonation began at the top

and progressed to cover the entire cross section. When the

detonation left the channel exit it was essentially planar and

perpendicular to the channel axis.

The variation of the equivalent planar flame speed versus

axial position is shown in Figure 6.41 for the tests with 13%

venting and no obstacles. DDT occurred so rapidly in test F-19,

that there were too little data to accurately determine the

deflagration speed prior to transition. Consequently, the curve

for F-19 is suspect. The results for tests F-20, F-18, and F-16,

show a flame acceleration leading to a first velocity peak

followed by a small decline in velocity and then a second

velocity increase. If we had carried out test F-20 prior to test

F-19, we would not have been so surprised that a DDT was

observed, and that it occurred about 11 meters from the ignition

end.

The conclusion that 13% top venting results are more severe

than no top venting results for sensitive mixtures and that both

are more severe than 50% top venting results is clearly seen in

Figure 6.42. We compare the time-of-arrival of the combustion

front midway between the floor and the ceiling for a test with no

top venting, F-12, a test with 50% top venting, F-4, and a test

with 13% top venting, F-19. The hydrogen mole fractions are

roughly comparable, 24.7%, 28.0%, and 24.8%, respectively. With

50% top venting the flame speed is slowest and there is no DDT,

even though the hydrogen mole fraction was higher than in the

other two tests. With no-top venting, the flame speeds were

higher, and DDT occurred near the exit. With 13% top venting,

the DDT occurred much near the ignition end. The flame speeds

for 13% top venting were as fast or faster than for no top

venting.

6.7 Test F-9

The primary objectives of this research were to observe flame

acceleration and DDT. Consequently, we planned on carrying out

tests with hydrogen mole fractions no lower than 12%. However,

due to a problem in carrying out test F-9, the expandable bag

could not be fully inflated, and we were forced to test with only

6.9% hydrogen. This hydrogen mole fraction is below the

-104-
90

85

80

75

7n

.. .- " ' ' I . . 1 . I . ' "

TO SCALE

DEFLAGRATION

DETONATION AND/OR SHOCK WAVE 0

EJ

0

I1

_ Elev.2 2 -

"65- 0 0.102 E

LL A 0.521 n1.5

Lb 60 c0 1.143 0

S55 V 1.448 1

0 1.753 ILI

50 0. 2.210 0.5

* 2.362

45 - 2.428

40-' 0

0 5 10 15 20' 25 30

DISTANCE FROM IGNITION END (M)

Figure 6.39. Test F-19, 24.8% H.2 13% top venting.

trajectories and profiles.

5 10 15 20 25

DISTANCE FROM IGNITION END (M)

30

33% blockage ratio. Combustion front
1

aCL

CO

LU

ccC.

I0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

-0.1

0-

w

CO

COwCr.

C.

0.9

0.8

0.7

0.6

0.5

0.4

0.3

.0.2

0.1

0

-0.1

50 75 100 125 150 50 75 100 125 150

TIME (ms) TIME (ms)

Figure 6.40. Test F-19, 24.8% H 2-

.13% top venting. No obstacles. Pressure histories.
500

E

ww

a.

co

UJ

a.

cI:

z

.-J

w

E.j

-J

n

w

450

400

350

300

250

200

150

100

50

0

0 5

DISTANCE

10 15 20

FROM IGNITION

25 30

END, m

Figure 6.41. Comparison of Flame Speeds for Tests With 13% Top

Venting and No Obstacles

-107-
300- 1

,0w 275-.

CO)

250

> 225

<200o~

O 175 R..A . --D-. I w ODET.

~ 150

_1 125 - _

z

Z 100

" S 75 -t --'---"--• RET. DDT DETONATION '- • "

50

25 t

0

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (i)

Figure 6.42. The Effect of Transverse Venting on Tests With -25%

H2 and No Obstacles

-108-
downward flammability limit of 9.0%, above the upward

flammability limit of 4.1%, and just above the horizontal

flammability limit of 6.0%. The results were interesting enough

to report, but they were different from the results of the other

tests. For this test, the channel had no top venting and no

obstacles.

After ignition, there were no observable effects seen on

video or audible from the blockhouse. We thought we had a

misfire. However, inspection of the data showed that there had

been a flame. A clear flame time-of-arrival was indicated on the

top two thermocouples for each rake. A faint indication of a

slight temperature rise was seen on the third thermocouple from

the top, presumably due to radiation from the flame. There was

no response on the lower thermocouples. The results are shown in

Figure 6.43. The near straightness of the line connecting the

time-of-arrivals indicates that a flame front moved down the

channel at a nearly constant speed of 1.2 m/s. The burned gas

was confined to the region between the ceiling, 2.438 m above the

floor, and a lower level that was above 2.050 m but below 2.354

meters above the floor. Hence the thickness of the burning layer

was between 0.084 and 0.388 meters (3.3 to 15.3 inches). It

appears there was a balance between buoyancy and burning giving

quasi-steady conditions.

6.8 Summary

Figures 6.44 through 6.46 summarize the salient results of

the FLAME tests. This section will be devoted to explaining

these figures. Because these figures condense a great deal of

information, they must be carefully scrutinized to be fully

understood.

Figure 6.44 shows the region of deflagration-to-detonation

transition as a function of the hydrogen concentration, top

venting fraction, and the presence or absence of obstacles. The

open circles indicate tests without obstacles in which DDT did

not occur; the shaded circles indicate tests without obstacles in

which DDT did occur. First considering the tests with no

obstacles, we see that for 50% top venting DDT does not occur

even near stoichiometric mixtures. For no top venting and 13%

top venting, the data points on the figure indicate the

borderline hydrogen concentration for DDT is below 25% hydrogen.

It is seen that there are not enough tests in the hydrogen mole

fraction range 20%-24% to indicate which of the two degrees of

top venting has a lower borderline hydrogen concentration.

Because the location of DDT in test F-19 with 13% top venting is

nearer the ignition end than in test F-12 with no top venting, we

believe low degrees of transverse venting promote DDT. The

increase in flame speed and overpressure due to the turbulence

created by flow out the vents, or possibly reignition of

combustible gas in the channel from rapidly burning gases above

-109-
30

25 2 A 2.418

Faint response

4) 20" 20 at 2.050 m,

none below.

-J

15

cc

LL

01 0

0 - . . I . . I . . . I . . . I . . I . . .

0 5 10 15 20 25 30

DISTANCE FROM IGNITION END (M)

Figure 6.43. Test F-9, 6.9% H.. No top venting. No obstacles.

Deflagration trajectories.

-110-
50- ZED

0

z

0

LL

0Iz

1-

Ill

LL,

a.P

40-

30-

20-

10

0-I

o DEFLAGRATION - NO OBSTACLES

* DETONATION - NO OBSTACLES

0 DEFLAGRATION - OBSTACLES

* DETONATION - OBSTACLES

I-A

D0) 0

w * 

1 • [ 1 I 1 I W

0 5 10 15 20 25 30

HYDROGEN MOLE FRACTION (%)

Figure 6.44. Flame DDT Results
E ~~speed~

LU 1000

( aric Sound speed

0)isobarI Co i

M = NO TOP VENTING

A = 13% TOP VENTING

-L 0 = 50% TOP VENTING '

f = INDICATES DDT

I NO OBSTACLES

100 OBSTACLES

I-

.j

w

0

5-

0 5 10 15 20 25 30

HYDROGEN MOLE FRACTION (%)

Figure 6.45. Flame Velocity Results

-112-
I ~ I I~ I I a irp e eut

loe o-3 0

. ...........

C)

Cl)

fwo

Ul cc

D

0

'•I

W

CL

1000-

100-

10:

d I . -7 , , I

u--I I I I I I I I I I I

0 5 10

' I

15 I I I T I I I I I F

20 25 30

HYDROGEN MOLE FRACTION (%)

Figure 6.46. Flame Overpressure Results

-113-
the plates, overcomes the pressure relief due to flow of gas out

of the channel. We expect that the borderline hydrogen

concentrations for DDT with small degrees of transverse venting

are lower than for no transverse venting, although there are not

enough data to show this. With larger degrees of transverse

venting, the data do show the increase in borderline hydrogen

concentration for DDT.

The open squares in Figure 6.44 indicate tests with obstacles

in which there was no DDT; the shaded squares indicate tests with

obstacles in which there was DDT. The most striking result of

the tests with obstacles is that the presence of obstacles

greatly reduces the required hydrogen concentration for DDT. DDT

is achieved with 50% top venting, but at a higher concentration

than with no top venting. There were no tests carried out with

obstacles and 13% top venting.

Figure 6.45 shows the maximum equivalent planar flame speed

as a function of hydrogen mole fraction for the five series of

tests, no top venting and no obstacles, no top venting and

obstacles, 50% top venting and no obstacles, 50% top venting and

obstacles, and 13% top venting with no obstacles. The legend box

indicates that tests with no top venting are indicated with

shaded squares; tests with 13% top venting are indicated with

open triangles, and tests with 50% top venting are indicated with

shaded circles. The tests with obstacles are distinguished from

those without because the data points are connected with a dashed

line instead of a solid line. Shown in chain-dashed lines at the

top of the graph are the isobaric sound speed for the burned gas,

and the Chapman-Jouguet detonation speed. For those tests in

which DDT did occur, an upward pointing arrow from the maximum

equivalent planar flame speed point indicates that the combustion

accelerates and approaches detonation speeds. However, since

detonation speeds were not accurately measured, no measured

detonation speed is shown.

Figure 6.46 shows the maximum overpressure as a function of

hydrogen concentration for the same five series of tests with the

same graphical conventions as in Figure 6.45. Shown in chaindash lines above are the adiabatic isochoric complete combustion

(AICC) pressure and the C-J detonation pressure. Since pressures

were measured near or after DDT, we do indicate separated points

for the maximum overpressures of the deflagrations and

detonations. Note that there were pressures measured above C-J

values, but below normally reflected C-J values from rigid

surfaces. The ratio of reflected C-J pressure to C-J pressure

for hydrogen-air mixtures is close to 2.3.

-114-
The vertical scale of Figures 6.45 and 6.46 are logarithmic.

This indicates that the flame speeds and overpresssures increase

rapidly over orders of magnitude range as the hydrogen

concentration is increased. The major effect of the presence of

obstacles is shown by lower hydrogen concentration required to

attain the same maximum equivalent flame speed or overpressure

compared to a similar test without obstacles. Examining Figure

6.25, we see that no DDT occurred below 400 m/s. With 50% top

venting and no obstacles present, this speed would not have been

attained even for a stoichiometric mixture. The inhibiting

effect of large degrees of transverse venting on the flame speed

and overpressures is evident. The complex behavior of small

degrees of transverse venting shows in the 13% top venting test

series. For lean mixtures below approximately 18% hydrogen mole

fraction, the flame speeds are lower and overpressures comparable

to similar tests without transverse venting. Above this hydrogen

concentration, the flame speeds and overpressures are higher than

in tests without transverse venting.

-115-
7. CONCLUSIONS

7.1 Introduction

The purpose of this section is to draw some general

conclusions from the results of the 29 tests discussed in

Section'6. In particular, the results are summarized in

Section 6.7 and Figures 6.44 through 6.46. The unique

aspect of the data is the large scale of the FLAME channel.

FLAME is a half-scale model of the upper plenum region of an

ice condenser containment. These studies of-flame

acceleration and transition to detonation of hydrogen-air

mixtures are applicable to nuclear reactor safety concerns

(see Section 7.3), and also relevant to other safety

problems.

The three variables in the tests were the hydrogen

concentration, the degree of transverse venting, andthe

presence or absence of obstacles. The hydrogen mole

fraction was varied between 12% and 30%. Leaner mixtures

were excluded (except for test F-9) because they would not

exhibit flame acceleration or deflagration-to-detonation

transition (DDT) in this facility. Tests with mixtures on

the rich side of stoichiometric would have been useful to

determine if stoichiometric mixtures are the most dangerous,

and how the decrease in reactivity on the rich side affects

flame acceleration and DDT. Except for test F-26, only one

obstacle configuration was used. More tests with other

obstacle configurations would have been useful. One must

keep in mind the limits of the testing program when

considering the applicability of the conclusions to other

geometries and mixtures.

7.2 Summaryýof Conclusions

The reactivity of the mixture is the most important

variable in-'flame acceleration and DDT. This is in accord

with the results of small-scale tests discussed in Section

1, and is expected. The maximum equivalent flame speeds in

Figure 6.45 and the maximum overpressures in Figure 6.46

vary by over two orders-of-magnitude as a function of

hydrogen concentration. Hence they are plotted

logarithmically while the hydrogen mole fraction on the

abscissa is plotted linearly. We found negligible flame

acceleration and consequently no DDT at the lean limit of

testing, 12% hydrogen. We did obtain DDT at 15% hydrogen

with obstacles present and no transverse venting. Peraldi,

Knystautas and Lee [67] found flame acceleration in

hydrogen-air mixtures down to 10% hydrogen, and DDT down to

16% hydrogen, in a long 30-cm diameter tube with many

-116-
annular rings to promote :flame acceleration. Although their

geometry was of small scale compared to FLAME, their

length/diameter ratio was much higher. Flame acceleration

might have been observed at leaner mixtures if the FLAME

channel were longer than 30.5 meters.

The presence of obstacles was the second most important

variable. The presence of obstacles in the channel greatly

increased the flame speeds, overpressures, and possibility

of DDT. The boundary between deflagration and DDT occurred

at a much lower hydrogen concentrations with obstacles

present, as was shown in Figure 6.44. Again this is in

accord with results found in small-scale tests.

In tests without obstacles and no transverse venting,

the main mechanism of flame acceleration appears to have

been hydrodynamic-combustion instabilities leading to a

concave flame front. This "tulip-shaped" flame has been

observed in small-scale tests [63,64] but not in large-scale

tests. If there were no obstacles present in FLAME, it was

easy to determine if there had been a DDT. The

overpressures were much higher and a distinct wave

(retonation) was seen,:moving back toward the ignition end.

The retonation is a shock wave passing through burned gases,

.that promotes burning of any residual unburned gas pockets

like a detonation.

It should be remembered that our tests with obstacles

present mostly used a single obstacle configuration, simple

baffles on the side walls with 33% blockage ratio. One test

was done simulating the geometry in the ice condenser upper

plenum region of PWRs with ice condenser containments.

Hence, the conclusions reported in the following paragraphs

for obstacle geometries were not verified over a range of

obstacle configurations.

In contrast to the results without obstacles, with

obstacles present it is difficult to tell if DDT has

occurred when the tested mixtures are at borderline6

concentrations. The presence of "quasi-detonations!', i.e.,

detonations whose structure is greatly perturbed by the

presence of the obstacles and which move at speeds

considerably below the detonation C-J speed, tends to blur

the strict separation between highly-accelerated

deflagrations and detonations. The peak overpressures can be

high even without DDT. From a safety point of view, it may

be unimportant to distinguish if a highly accelerated flame

has undergone transition to detonation because the pressure

loads be can similar in both cases. The flame speeds tended

to accelerate to the range 500-700 m/s, the choked flow

-117-
condition in the burned gas. This is seen in all tests with

obstacles except those with comparatively unreactive

mixtures -( ~ 12% hydrogen). The choked-flow condition

tended to be stable over roughly the latter half of the

channel. If DDT is defined as the formation of a self

sustaining detonation traveling at nearly C-J speed, then

cinematography of the wave leaving the channel becomes a

central means of verifying that DDT has occurred. However,

for the lean mixtures in the borderline region,

cinematographic analysis is complicated by the low

luminosity of the burned gas. In addition, no retonation

waves were seen. We hypothesize that this may be due to the

breakup of the returning wave by the obstacles and debris in

the channel. Of course, for mixtures which are much more

reactive than borderline mixtures, there is no problem

determining that DDT has occurred.

Peraldi, Knystautas, and Lee [67] found DDT possible if

the ratio of the obstacle inner diameter to the detonation

cell width was greater than one. In our tests DDT was

observed if the ratio of the gap width between the obstacles

to the detonation cell width was greater than about three,

provided the flame speed had reached the choked-flow

condition in the burned gas prior to the channel exit.

The third variable, transverse venting, also had a

strong influence on the flame speeds, overpressures, and

possibility of DDT. Large degrees of transverse venting

reduce flame speeds and overpressures. The tests without

obstacles and with a large degree of top venting (50%) show

that it is possible to suppress flame acceleration and DDT

even for highly reactive mixtures, i.e. nearly

stoichiometric hydrogen-air mixtures. With obstacles

present, DDT does occur if the mixture is sufficiently

reactive.

Venting has two opposing effects. The loss of gas out

the vents tends to lower the overpressure and slow the flame

speed. However, the venting tends to create turbulence and

increase the burning rate; i.e., the vents act as obstacles.

For small degrees of venting the second effect can dominate,

i.e., small degrees of venting can increase the possibility

of DDT. The flames speeds, overpressures, and tendency

toward DDT is increased by a small degree of transverse

venting for hydrogen-air mixtures with hydrogen mole

fractions above about 18%.

FLAME was built to study violent combustion effects at

larger scale than laboratory tests. These phenomena are

scale dependent, and get worse at larger scale. Our results

-118-
do show leaner hydrogen limits for DDT than at small scale.

Subsequent to this work, we have built small-scale models of

FLAME to observe'scale effects on flame acceleration and

DDT. This work will be reported in the future.

7.3 Application to Nuclear Reactor Safety

The results of the FLAME experimental program combined

with other relevant work was used to create a methodology to

predict the possibility of DDT in severe accident

conditions, particularly for treating hydrogen-air-steam

mixtures. [57] A brief summary of the methodology is

presented in Appendix B. The two input variables in the

methodology are the reactivity of a mixture and the flameacceleration potential of volume through which the

deflagration propagates. The output of the methodology is a

qualitative estimate of the likelihood of DDT.

The methodology was used in a study of the Bellefonte

nuclear power plant. [57] For the large dry PWR containment

at Bellefonte, the methodology predicted little danger of

detonation for the accidents considered. The methodology

was also used by the first author in the Containment Loads

Expert Panel of NUREG-i150 studies of the Sequoyah and Grand

Gulf plants. For the Sequoyah ice condenser containment, in

an analysis of many accident scenarios in which the large

mixing fans are inoperative, mixtures with 18-22% hydrogen

and little steam were predicted to occur in the ice

condenser. For these mixtures and in that geometry, the

formalism predicts that a flame is highly likely to undergo

DDT.

-119-
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Bellefonte Nuclear Power"Plant," Nuclear Technology,

321-347-, (April-1988).

58. J.W. Fisk et al., "Experimental Facilities to Study

Hydrogen Combustion and Detonation," Designing for

Hydrogen in Nuclear Power Plants, 'Ed. K. K. Niyogi and

M. Bernstein, pp. 25-30, .ASME, NY, 1984.

59. M.P. Sherman, "Hydrogen Flame Acceleration and

Detonation,:" Proceedings of'the Twelfth Water ReactorSafety Meeting,,•U.S. Nuclear Regulatory Commission,_

Washington, D.C., NUREG/CP-0058, October 1984.

60. 'M.P. Sherman et al., "The Effect of Transverse Venting

on Flame Acceleration and Transition to Detonation in a

Large Channel," Dynamics of Explosions, Vol. 106, Ed.

J.R. Bowen, J.C..Leyer, and R.I. Soloukhin, AIAA, NY,

1986.

61. T.G. Beckwith and N.L. Buck, Mechanical Measurements,

2nd Ed., Menlo Park, CA, (Addis'on-Wesley; 1969).

62. B.W. Marshall, Jr. and A.C. Ratzel, III, Pressure

Measurements in a Hydrogen Combustion Environment: An

Evaluation of Three Pressure Transducers, Sandia

National'Laboratories, Albuquerque, NM, NUREG/CR-3721/2

of 2, SAND83-2621/2 of 2, May 1984.

63. D. Dun n-Rankin,'P.K. Barr; and R.F. Sawyer, "Numerical

and Experimental Study of "Tulip" Flame' Formation in a

Closed Vessel," Twenty-First Symposium (International)

on Combustion, Combustion Institute, Pittsburg, PA,

1291-1301, 1986.

-125-
64. D.A. Rotman and A.K. Oppenheim, "Aerothermodynamic

Properties of Stretched Flames in Enclosures," TwentyFirst Symposium (International) on Combustion,

Combustion Institute, Pittsburg, PA, 1303-1312, 1986.

65. 0. Peraldi, R. Knystautas, and J.H. Lee, "Criteria for

Detonation in Tubes," Twenty-First Symposium

(International) on Combustion, Combustion Institute,

Pittsburg, PA, 1629-1637, 1986.

66. W.B. Benedick, R. Knystautas, and J.H. Lee, "Large-Scale

Experiments on the Transmission of Fuel-Air Detonations

From Two-Dimensional Channels," Dynamics of Shock Waves,

Explosions and Detonations, Vol. 94, Ed. J.R. Bowen, N.

Manson, A.K. Oppenheim, and R.I. Soloukhin, 546-555,

AIAA, NY, 1985.

67. J.R. Travis and T.G. Theofanous, "Some Examples of

Hydrogen Transport and Mixing," ANS Proceedings,

National Heat Transfer Conference, 182-193, 1987.

-126-
:-APPENDIX A

POSITION OF .STRUCTURES AND INSTRUMENTATION

The purpose of this appendix is to document the location of

the transducers used in the FLAME experiments.. Thelocations are

given using a Cartesiancoordinate system as shown-in the diagram

below. The X coordinate is-the axial, distance .from the inside of

the door at the ignition end; the Y coordinate is the elevation

from the floor; the Z coordinate is the distance from the

midplane of the. channel. The near wall was selected as the

direction of. positive Z.

FLAME TOP

SIDE VIEW•

0 FLAME FLOOR

-Z

FAR (SW) WALL

0 ---------- > X TOP VIEW

NEAR (NE) WALL

+Z

All dimensions are in meters, with a tolerance of + 0.005 m.

The coordinates of all devices in the walls are given at the

inner surface of the wall, except for the strain gauges.

The pressure transducers, lithium-niobate gauges, two

germanium photodiodes and the hydrogen inlets were each mounted

in one of the 88 access pipes, and five of the germanium

photodiodes were mounted in viewports. Their positions did not

change during the test series. The coordinates to the center of

the access pipes and viewports are quoted. In contrast, the

number and position of thermocouples used to measure combustion

front time-of-arrival changed several times during the test

series.

Overall channel dimensions

Channel length = 30.5 m (100 ft)

Channel width 1.83 m (6.0 ft)

Channel height 2.44 m (8.0 ft)

-127-
ACCESS PIPE LOCATIONS

PIPE X Y Z

1 0.787 0.276 0.914 Near

2 2.143' 0.297 0.914 Wall

3 3.569 0.283 0.914

4 4.890 0.305 0.914 Lower

5 .. 6.299 ............. 0.279 .............. 0.914

6 7.683 0.270 0.914

7 9.052 0.279 0.914

8 10.500 0.292 0.914

9 11.824 0.288 0.914

10 13.167 0.267 0.914

11 ........... 14.465 ............. 0.270 .... ....... 0.914

12 15.951 0.264 0.914

13 17.399 0.279 0.914

14 18.701 0.210 0.914

15 19.942 0.203 0.914

16 ........... 21.441 ............. 0.292 .......... 0.914

17 22.812 0.273 0.914

18 24.181' 0.264 0.914

19- 25.464 0.289 0.914

20, 27.000 0.286 0.914

21 28.321 0.286 0.914

22 29.816 0.311 0.914

23-ý 0.678 2.064 0.914 Near

24 2.089 2.075 0.914 Wall

25 3.550 2.080 0.914

26 4.804 2.067 0.914 Upper

27 6.236 2.111 0.914

28 ........... 7.658 ............. 2.125 .......... 0.914

29 9.027 2.130, 0.914

30 10.351 2.115 0.914

31 11.773 2.102 0.914

32 13.145 2.130 0.914

33 . ............ 14.516 .............. 2.100 ............ 0.914

34 15.929 2,146 0.914

35 17.304 2.134 0.914

36 18.669 2.134 0.914

37 20.063 2.134 0.914

38 21.450 2.121 0.914

39 ........... 22.790 ........... 2.111 .......... 0.914

40 24.194 2.121 0.914

41 25.432 2.151 0.914

42 26.988 2.140 0.914

43 28.343 2.137 0.914

44 29.804 2.102 0.914

-128-
45 0.827 0.262 -0.914 Far

46 2.188 0.260 -0.914 Wall

47 ,:• :•3.518 0.262 -0.914

48 4.937 0.295. -0.914 Lower

49 6.360 0.316 -0.914

50.. ........ 7.690 .............. 0.318 .. .... ...- 0.914

51 9.138 0.344. -0.914

52 10.554, 0.2917 -0.914

53 11.728 0.295 -0.914

54 13.252 0.289 -0.914

55 ........... 14.586 ............. 0.297 . ......... -0.914

56 15.977 0.283 -0.914

57 17.215 0.283 -0.914

58 18.599. 0.306 -0.914

59 20.047 0.287 -0.914

60 ........... 21.501 . ............ 0.264 .. ........ -0.914

61. 22.746 0.284 -0.914

62 24.149 0.273 -0.914..

63 25.622 0.313 -0.914

64 27.007 0.292 -0.914

65 28.308 0.271 -0.914

66 29.718 0.264 -0.914

67 0.708 2.172 -0.914 Par

68 2.102 2.164 -0.914 Wall

69 3.461 2.194 -0.914

70 4.991 2.189 -0.914

71 6.350 2.207 -0.914 Upper

72 ............. 7.677 .............. 2.122 . ......... -0.914

73 9.125 2.116 -0-914

74 10.478 2.135 -0.914

75 11.773 2.105 -0.914

76 13.145 2.113 -0.914

77 ... .......... 14.649 .............. 2.097 ............. -0.914

78 15.970 2.165 -0.914

79 17.183 2.161 -0.914

80 18.644 2.153 -0.914

81 20.060 2.146. -0.914

82 ............. 21.412 ............. 2.108. .......... -0.914

83 22.777 2.129 -0.914

84 24.124 2.124 -0.914

85 25.591 2.122 -0.914

86 26.994 2.130 -0.914

87 28.378 2.137 -0.914

88 29.686 2.132 -0.914

-129-
VIEWPORT LOCATIONS

Port

1

2

3

4

5

Near Wall (Z=0.914 m)

x y (m) -(m

3.124 1.213

9.500 1.213

14.961 1.210

20.485 1.219

26.911 1.222

Far Wall (Z=-0.914 m)

Port X Y

6

7

8

9

10

3.153

9.500

14.980

20.498

26.911

1.199

1.203

1.197

1.210

1.213

PRESSURE TRANSDUCER LOCATIONS

Transducer

PKU1

PKU2

PKU3

PKU4

PKU5

PKU6

PKU7

PKU8

PKU9

PKU1O

PDYK1

PDYK2

PDYK3

0.787

4.804

9.052

14.586

18.599

21.450

25.464

25.464

21.412

18.701

2.188

.22.746

29.816

Y~mL

0.276

2.067

0.279

0.297

0.306

2.121

0.289

2.122

2.108

0.210

0.260

0.284

0.311

Z (m)

0.914

0.914

0.914

-0.914

-0.914

0.914

0.914

-0.914

-0.914

0.914

-0.914

-0.914

0.914

Access Pipe

1

26

7

55

58

38

19

85

82

14

46

61

22

GERMANIUM PHOTODIODE LOCATIONS

GPD

1

2

3

4

5

6

7

3.124

9.500

14.980

20.498

26.994

26.911

27.007

1.213

1.213

1.197

1.123

2.130

1.213

0.294

-0.914

-0.914

-0.914

-0.914

-0.915

-0.914

-0.914

Viewport or

Access Pipe*

6

7

8

9

88*

10

64*

LITHIUM-NIOBATE GAUGES

Gauge #

1

2

icma

11.773

20.063

2.102

2.102

Z0(.)

0.914

0.914

Access Pip

-130-
Thermocouple Positions for test F-i

Rake S TC # XIZ.(m

1 9.747 0.064 0.000

2 9.747 0.089 0.000

3 9.747 0.140 0.000

4 9.747 0.267 0.000

5 9.747 0.546 0.000

6 9.747 0.876 0.000

7 9.747 1.181 0.000

8 9.747 1.486. 0.000

9 9.747 1.791 0.000

10 9.747 2.096 0.000

11 9.747 2.400 0.000

12 9.747 2.477 0.000

Rake N 1 20.714 0.064 0.000

2 20.714 0.089 0.000

3 20.714 0.140 0.000

4 20.714 0.267 0.000

5 20.714 0.546 0.000

6 20.714 0.876 0.000

7 20.714 1.181 0.000

8 20.714 1.486 0.000

9 20.714 1.791 0.000

10 20.714 2.096 0.000

11 20.714 2.400 0.000

12 20.714 2.477 0.000

Wall 1 21.441 0.292 -0.914

2 29.804 2.102 -0.914

3 4.890 0.305 -0.914

4 13.167 0.292 -0.914

5 11.773 2.102 -0.914

Thermocouple Positions for Tests F-2 to F-12

Rake A TC # X(m) Y(M) Z(m)

1 3.315 0.016 0.000

2 3.315 0.041 0.000

3 3.315 0.092 0.000

4 3.315 0.219 0.000

5 3.315 0.524 0.000

6 3.315 0.829 0.000

7 3.315 1.133 0.000

8 3.315 1.438 0.000

9 3.315 1.743 0.000

10 3.315 2.048 0.000

11 3.315 2.353 0.000

12 3.315 2.416 0.000

-131-
Rake 

B

Rake C

Rake D

123456789

10

11

12123456789

10

11

12123456789

10

11

12

11.088

11.088

11.088

11.088

11.088

11.088

11.088

11.088

11.088

11.088

11.088

11.088

19.304

19.304

19.304

19.304

19.304

19.304

19.304

19.304

19.304

19.304

19.304

19.304

27.077

27.077

27.077

27.077

27.077

27.077

27.077

27.077

27.077

27.077

27.077

27.077

0.010

0.035

0.086

0.213

0.518

0.822

1 .127

1.432

1.737

2.042

2.346

2.410

0.022

0.048

0.098

0.225

0.530

0.835

1 .140

1.445

1.749

2.054

2.359

2.423

0.022

0.048

0.098

0.225

0.530

0.835

1.140

1.445

1.749

2.054

2.359

2.423

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

-132-
Thermocouple Positions for tests F-18-F-21

Rake A TC# X(m) . (M) ZUm)

1 1.067 0.025 0.000

2 1.067 0.051 0.000

3 1.067 0.102 0.000

4 .1.067 0.229 0.000

5 1.067 0.521 0.000

6 1.067 0.838 0.000

7 1.067 1.143 0.000

8 1.067 1.448 0.000

9 1.067 1.753 0.000

10 1.067 2.210 0.000

11 1.067 2.362 0.000

12 1.067 2.426 0.000:"

Rake B 1 3.810 0.025 0.000

2 3.810 0.070 0.000

3 3.810 0.146 0.000

4 3.810 0.298 0.000

5 3.810 0.603 0.000

6 3.810 1.060 0.000

7 3.810 1.365 0.000

8 3.810 1.829 0.000

9 3.810 2.127 0.000

10 3.810 2.280 0.000

11 3.810 2.356 0.000

12 3.810 2.407 0.000

Rake C 1 11.582 0.025 0.000

2 11.582 0.051 0.000

3 11.582 0.102 0.000

4 11.582 0.229 0.000

5 11.582 0.527 0.000

6 11.582 0.832 0.000

7 11.582 1.137 0.000

8 11.582 1.441 0.000

9 11.582 1.746 0.000

10 11.582 2.051 0.000

11 11.582 2.356 0.000

12 11.582 2.419 0.000

-133-
Rake D TC # X(m) Y(m) Z

1 19.787 0.025 0.0

2 19.787 0.076 0.0

3 19.787 0.152 0.000

4 19.787 0.305 0.000

5 19.787 0.610 0.000

6 19.787 1.067 0.000

7 19.787 1.372 0.000

8 19.787 1.829 0.000

9 19.787 2.134 0.000

10 19.787 2.286 0.000

11 19.787 2.362 0.000

12 19.787 2.413 0.000

Rake E 1 27.559 0.025 0.000

2 27.559 0.076 0.000

3 27.559 0.152 0.000

4 27.559 0.305 0.000

5 27.559 0.610 0.000

6 27.559 1.067 0.000

7 27.559 1.372 0.000

8 27.559 1.829 0.000

9 27.559 2.134 0.000

10 27.559 2.286 0.000

11 27.559 2.362 0.000

12 27.559 2.413 0.000

Rake F 1 2979 1.22 -0.69

2 29.79 1.22 -0.48

3 29.79 1.22 -0.23

4 29.79 1.22 0.23

5 29.79 1.22 0.48

6 29.79 1.22 0.69

-134-
Thermocouple positions for Tests F-22 - F-29

Rake A 1 1.067 0.076 0.000

2 1.067 0.381 0.000

3 1.067 0.762 0.000

4 1.067 1.143 0.000

5 1.067 1.524 0.000

6 1.067 1.905 0.000

7 1.067 2.286 0.000

8 1.905 1.219 -0 610

9 1.930 1.219 -0.864

10 2.819 1.219 -0.889

Rake B 1 3.810 0.076 0.000

2 3.810 0.381 0.000

3 3.810 0.762 0.000

4 3.810 1.143 0.000

5 3.810 1.524 0.000

6 3*810 1.905 0.000

7 3.810 2.286 0.000

8 3.734 1.219 -0.610

9 3.810 1.219 -0.864

10 3.810 1.219 -0.889

Rake C 1 11.582 0.076 0.000

2 11.582 0.381 0.000

3 11.582 0.762 0.000

4 11.582 1.143 0.000

5 11.582 1.524 0.000

6 11.582 1.905 0.000

7 11.582 2.286 0.000

8 11.049 1.219 -0.610

9 11.074 1.219 -0.864

10 12.040 1.219 -0.889

Rake D 1 19.787 0.076 0.000

2 19.787 0.381 0.000

3 19.787 0.762 0.000

4 19.787 1.143 0.000

5 19.787 1.524 0.000

6 19.787 1.905 0.000

7 19.787 2.286 0.000

8 18.352 1.219 -0.610

9 18.377 1.219 -0.864

10 19.304 1.219 -0.889

Rake E 1 27.534 0.025 0.000

2 27.534 0.076 0.000

3 27.534 0.152 0.000

4 27.534 0.305 0.000

-135-
5

6

7

8

9

10

27.534

27.534

27.534

27.496

27.521

28.423

0.610

1.067

1.372

1.219

1.219

1.219

0.000

0.000

0.000

-0.610

-0.864

-0.889

Strain Gauge Locations

DEFINE:

Y: (+/-

Z'=Z-1 .283

TOLERANCES:

0.01 METERS)

Z'=0 AT CENTERLINE OF NEAR WALL

X: (+[- 0.05 METERS)

Z: (+/- 0.01 METERS)

STATION A

GAUGE

1

2

3

4

5

6

7

8

STATION B

GAUGE

1

2

3

4

5

6

7

8

STATION C

GAUGE

1

2

3

4

5

6

7

8

x

1.83

1.83

1.83

1.83

1.83

1.83

1.83

1.83

y

1.47

.1.47

1.47

1.47

0.05

0.05

0.05

,0.05

Z

-0.33

-0.31

0.31

0.33

-0.33

-0.30

0.30

0.33

x

8.53

8.53

8.53

8.53

8.53

8.53

8.53

8.53

y

1.47

1.47

1 .47

1.47

0.05

0.05

0.05

0.05

Z

-0.33

-0.31

0.31

0.33

-0.33

-0.30

0.30

0.33

x

15.24

15.24

15.24

15 .24

15.24

15.24

15.24

15.24

y

1.47 1.47

1.47

1.47

0.05

0.05

0.05

0.05

Z

-0.33

-0.31

0.31

0.33

-0.33

-0.30

0.30

-0.33

-136-
STATION D

GAUGE X Y Z)

1 21.95 1.47 -0.33

2 .21.95 1.47 -0.31

321.5 1.47 0.31

.4 21.95- 1.47 0.33

5 .21.95 0.05 -0.33

6 21.95, 0.05 -0.30

7 21.95 0.05 0.30

8- 21.95 0.05 0.33

STATION E

GAUGE2 X Y Z

1 28.65 1.47 -0.33

2 28.65 1.47 -0.31

3 28.65 1.47 0.31

4 28.65 1.47 0.33

5 28.65 0.05 -0.33

6 28.65 0.05 -0.30

7 28.65 0.05 0.30

8 28.65 0.05 0.33

-137-
APPENDIX B

APPLICATION TO HYDROGEN-AIR-STEAM MIXTURES

The research conducted in the FLAME facility on flame

acceleration and deflagration-to-detonation (DDT) at large scale

with hydrogen-air mixtures has a major limitation when

considering application to nuclear reactor safety. Many of the

containment atmospheres in postulated severe accidents contain

hydrogen-air-steam mixtures at somewhat elevated pressures and

temperatures. Consequently, there is a'need to relate our work

with hydrogen-air mixtures to these conditions. An approximate

methodology to handle this task was carried out by Sherman and

Berman. [57] In this appendix an abbreviated summary of this

methodology is presented. For a discussion of methods other than

DDT for initiating detonation, and of the limitations of the

evidence for the validity of the methodology see the original

paper.

The methodology is based on the following assumptions:

1. The likelihood of DDT can be expressed as a

function of two variables, one based on the

reactivity of the mixture, and a second based on

the flame acceleration potential of the volume

through which the deflagration propagates.

2. The reactivity of the mixture is represented by the

detonation cell width, X.

3. The qualitative flame acceleration potential of a

volume can be estimated from knowledge of its

geometric configuration and size by reference to

simple guidelines.

The significance of the second assumption is that the

detonability of a hydrogen-air-steam mixture, which has not

been measured, is assumed equal to that of a hydrogen-air

mixture at atmospheric pressure and room temperature having

the same detonation cell size, which presumably has been

measured or is believed known. The detonation cell width of

hydrogen-air-steam mixtures at a temperature of 1000C and an

air partial pressure corresponding to I atmosphere pressure

and 200C is shown in Figure B.I. The abscissa used is

equivalence ratio, the ratio of hydrogen to oxygen

concentration divided by that at stoichiometric conditions.

An equivalence ratio of unity means two hydrogen molecules

per oxygen molecule regardless of the steam concentration.

Equivalence ratios less than 1 are lean mixtures;

equivalence ratios greater than 1 are rich mixtures. In

Table B.1, mixtures are divided into five classes, depending

-138-
Constant-T, A•ir Density

366.<T(K)<375. , 40.4<AIr Denslty(mol./cu. m)<42.0

100

1

X-o

0

0

.-

0

0

0.1 1 Equivalence RatiO 10

Figure B.i. Detonation Cell Width of Hydrogen-Air-Steam Mixtures

-139-
on their detonation cell width from very dangerous class.17to:.

difficult-to-detonate, class 5., Shown are the corresponding,

hydrogen mole fractions for H2 -air mixtures on the lean and rich

sides.

Table B.1

Classification of Mixture Detonability

Example - Hydrogen-Air Mixtures At 200C and. i-atm Pressure

H2 Mole Cell H Mole

Mixture Fraction Equivalence Width Fraction Equivalence

Class (M) Ratio (mm) (M) Ratio

1 24 to 30 0.75 to 1.0 20 to 15 38 to 30 1.5 to 1

2 21 to 24 0.63 to 0.75 40 to 20 48 to 38 2.2 to 1.5

3 15 to 21 0.42 to 0.63 320 to 40 63 to 48 4.1 to 2.2

4 13.5 to 15 0.37 to 0 42 1200 to 320 70 to 63 5.6 to 4.1

5 <13.5 <0.37 > 1200 No data >5.6

The flame acceleration potential of a given volume is

classified into one of five geometric classes beginning with

geometric class 1, being the'most conducive to flamb

acceleration, to geometric class 5, being the least conducive.

A description of these classes follows'.

Geometric Class 1. Large geometries with. obstacles in the

path of the expanding unburned gases.< Partial confinement favors

gas expansion past the obstacles.;. A'large tube with numerous

obstacles, and with ignition going from a closed end to an open

end, is an example. Class ,1 geometries are the most favorable to

large flame acceleration.

Geometric Class 2. Geometries similar to class I but with some

feature which hinders, flame acceleration.' Examples Would be a

tube open on both ends or large amounts of transverse venting.

Geometric Class 3. Geometries that yield moderate flame

acceleration but are neutral to DDT. Examples are large tubes

without obstacles, and small tubes (several inch diameter) with

obstacles.

Geometric Class 4. Geometries unfavorable to flame acceleration.

Examples are large volumes with hardly any obstacles and large

amounts of venting transverse to the flame path, and small

volumes without obstacles. DDT will not usually occur in a class

4 geometry.

-140-
Geometric Class 5. .Geometries:so unfavorable to flame

acceleration that not even large volumes :of stoichiometric

hydrogen-air mixtures are likely, to detonate. The only examples

are totally unconfined geometry at large scale and a small

spherical geometry without obstacles and central ignition.

For a given mixture class and geometric class, the likelihood

of DDT is given in Table 2 as a results class. .The qualitative

likelihood of DDT for the results classes is given below.

Result Class 1. DDT is highly likely.---

Result Class 2. DDT is likely.

Result Class 3. DDT may occur.

Result Class 4. DDT is poSsible but unlikely.

Result Class 5. DDT is highly 'unlikely to impossible.

Table.B.2

Dependenco of Result Class on Mixture and Geometric Class

Geometric Mixture Classes

Classes, - - -

1 2 3 4 5

1 1 1 2- 3 4

2 1. .2 3 4 5

3 2 3 3 4 5

4 3 4 4 5 5

4 .5 1 5 5 5 5

The authors urge the users

original reference.

of this:methodology to read the

-141-
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M. Fleischman, NRC/RES

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NPC FORM 3351 U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER IAu'ndby rIri. odd Vd . N if.lyl Vo 

(2-41!

NACFoM 1102

NRCM 21.02 BIBLIOGRAPHIC DATA SHEET NUREG/CR-5275" 320,. 3202 SAND85-1264 ..

SEE INSTRUCTIONS ON THE REVERSE

2. TITLE AND SUBTITLE 3. LEAVE BLANK

FLAME Facility The Effect of Obstacles and Transverse

Venting on Flame Acceleration and Transition to Detonation for Hydrogen-Air Mixtures at Large Scale 4. DATE REPORT COMPLETED

MONTH 'YEAR

S. AUTHOR(S) August 1988

M. P. Sherman, S. R. Tieszen, W. B. Benedick MONTH 6. OATEREPORTISSUEO YEAR

April 1989

7. PERFORMING ORGANIZATION NAME AND MAILING ADDRESS 1I-cW*d ZP C.d*J 8. PROJECTITASK/WORK UNIT NUMBER

Sandia National Laboratories 9. FIN OR GRANT NUMBER

Albuquerque, NM 87185

A1246

10. SPONSORING ORGANIZATION NAME AND MAILING ADDRESS fIfcl.dZ•Z. Codel Its. TYPE OF REPORT

Division of Systems Research Final Report

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission b. PERIOD COVERED fI'MciusmdimI

Washington, DC 20555

12. SUPPLEMENTARY NOTES

13 ABSTRACT 000 wOld 014.1

This report describes research on flame acceleration and deflagration-to-detonation

transition (DDR) for hydrogen-air mixtures carried out in the FLAME facility, and

describes its relevance to nuclear reactor safety. Flame acceleration and DDT can

generate high peak pressures that may cause failure of containment. FLAME is a large

rectangular channel 30.5 m long, 2.44 m high, and 1.83 m wide. It is closed on the

ignition end and open on the far end. The three test variables were hydrogen mole fraction (12 - 30%), degree of transverse venting (by moving steel top plates - O, 13Z,

and 50%), and the absense or presence of certain obstacles in the channel (zero or

33% blockage ratio).• The most important variable was the hydrogen mole fraction.

The presence of the obstacles tested greatly increased the flame speeds, overpressures,

and tendency for DDT compared to similar tests without obstacles. Different obstacle

configurations could have greater or lesser effects on flame acceleration and DDT.

Large degrees of transverse venting reduced the flame speeds, overpressures, and

possibility of DDT. For small degrees of transverse venting (131 top venting), the

flame speeds and overpressures were higher than for no transverse venting with reactive

mixtures (>18% 112), but they were lower with leaner mixtures. The effect of the turbulence generated by the flow out the vents on increasing flame speed can be larger than

the effect of venting gas out of the channel and hence reducing the overpressure. With

no obstacles and 50% top venting, the flame speeds and overpressures were low# and there

was no DDT. For all other cases, DDT was observed above some threshold hydrogen concentration. DDT was obtained at 15Z H2 with obstacles and no transverse venting.

I4. DOCUMENT ANALYSIS - o. KEYWORDS/DESCRIPTORS . IS. AVAILABILITY

STATEMENT

FLAME facility experiments

Hydrogen-air combustion Unlimited

Flame acceleration 16. SECURITY CLASSIFICATION

Transition to detonation (Thi ,owl

b. IDENTIFIERS/OPEN-ENDED TERMS Unclassified

Unclassified

17. NUMBER OF PAGES

Is. PRICE

*U.S.GDVERNMENT PRINTING OFFICE:1989-24•q-90: 0043