This report has been prepared

for information and record

purposes and is not to be referenced

in any publication,

NATIONAL BUREAU OF STANDARDS REPORT

8147

EXPERIMENTAL FIRES IN ENCLOSURES

by

D. Gross

and

A. F. Robertson

II. S. DEPARTMENT OF COMMERCE

NATIONAL OUREAU OF STANDARDS

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1002-11-10122 December 12, 1963

NBS REPORT

8147

EXPERIMENTAL FIRES IN ENCLOSURES

by

D. Gross

and

A. F. Robertson

IMPORTANT NOTICE NATIONAL BUREAU OF ! for use within the Governmen

and review. For this reason, whole or in part, is not autl Bureau of Standards, Washin

the Report has been specifics

ress accounting documents intended Approved for public release by the is subjected to additional evaluation Director of the National Institute of f

r ® ll

(!!' ng of

. thls ^

epo

.rt ' e ' th. er the Office of the Director, National Standards and Technology (NIST) by the Government agency for which on October 9, 201 5. c°P ies for its own use -

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

EXPERIMENTAL FIRES IN ENCLOSURES

by

D. Gross

and

A. F. Robertson

Abstrac t

Results are presented of experimental

measurements of the mass rates of burning,

temperatures and gas compositions in model

enclosures of three sizes. Interest was

confined to the fully-developed period of

burning in which the burning rate of a combustible fiberboard crib was limited by the

size and shape of the ventilation opening.

The burning rate was found to be generally

proportional to A/h ( A=area, h-height of

window opening), but a characteristic transition region was found for each enclosure

which resulted in an abrupt shift in the data

line. Burning rate data were correlated in

terms of the ventilation parameter A/h and

the exposed surface area of the crib.

The overall process involved in enclosure

fires appears to be a combined gravitycontrolled fluid dynamic regime and a radiationcontrolled thermal regifne. The dimensionless

Froude group is the criterion for similarity

in a gravity- (buoyancy-) controlled regime, and

from the limited agreement found among the data

for the two larger enclosures, this group

appears to include the essential parameters in

the scaling of burning rates of fires in geometrically similar enclosures.

EXPERIMENTAL FIRES IN ENCLOSURES

by

D. Gross

and

A. F. Robertson

1. Introduction

The analysis of fires in rooms, buildings and full-size

structures is admittedly complex due to the interaction of

mechanical, diffusional, thermal and chemical processes.

Although the same interactions complicate experimentation

and analysis on small-scale models, the use of models permits

a great many more experiments to be performed under more

controllable conditions.

As part of a program to develop basic information on the

use of models in fire research, exploratory experimental

measurements were made of the mass rates of burning, temperatures and gas compositions in model enclosures. Interest

was mainly confined to the fully-developed stages of fires

in which the ventilation opening limited the burning rate

of a reproducible crib-type combustible load [l]. The data

supplement other published Information on fire behavior in

both model [2, 3? 4] and full-scale [5, 6] enclosures and

permit consideration of appropriate scaling relationships.

One ultimate objective is the use of models to evaluate the

degree of hazard when flames spread along the surfaces of

flammable interior finish materials in enclosures.

2. Test Procedures and Equipment

In this series, experiments were limited to geometrically

similar ( 1: 1: 2) box-type enclosures of three sizes containing

(usually) only a single rectangular ventilation opening.

Several tests with dual windows were also made. Size and

construction details are summarized In Table 1. Fig. 1 is a

schematic diagram of the experimental apparatus. A celluiosebase fiberboard was used as the crib material because of its

rapid flaming characteristics, and the cribs were constructed

so that the spacing between sticks was three times the stick

width. The sticks were of square cross-section and, except

for selected tests, were 2.5 cm thick for the Scale I and II

model enclosures and 7.5 cm thick for the Scale III model

enclosure. Except as noted later, the crib for the two smaller

enclosures were of very nearly identical weight and construction

and averaged approximately 270 g and 3200 g, respectively,

for the Scale I and Scale II enclosure tests.
- 2 -

The crib was supported from 1 to 3 stick widths above the

enclosure floor and was located between the center and rear

of the box. The test was started by igniting kerosene-soaked

fiberboard wicks placed in the openings of the bottom row of

sticks. Temperatures were measured at several locations

within the box by means of chrome 1 -a lumel thermocouples made

of 0.051 cm dia. wires with asbestos and glass, siliconeimpregnated insulation and bare, unshielded beads. Temperatures were automatically recorded on a multipoint potentiometer

recorder.

Gas composition measurements were made using an uncooled

probe made of 1/4 in. o.d. copper tubing placed at the top

front of the burning crib. All gas compositions were assumed

"dry." Moisture and other condensable combustion products

were removed prior to measurement by passing the sample through

the following, in order: (1) a glass fiber filter, tar and

resin trap, (2) an ice-cooled moisture and liquid trap, ( 3 ) two

air line filters in series prior to the vacuum pump, and (4)

one air line filter after the vacuum pump. All instruments

were operated above room temperature. The gas sampling arrangement is shown schematically in Fig. 2.

The analyzers used for CO ^nd COp were Model ^00 "Lira"

Infrared Analyzers, manufactured by Mine Safety Appliances

Co. These instruments analyze an unknown 'gas by comparing its

infrared absorption quality with the constant infrared absorption quality of a known gas. This is accomplished by alternate

interruption of infrared radiation beams through the unknown

sample gas and the known comparison gas causing a differential

response by a single sealed-in detector gas which is related

to the volume fraction of the desired component. These

instruments had full-scale ranges of 0-l4 percent CO and 0-20

percent C0-, had a response speed of 5 seconds (to 90% of

final ^ reading), and a rated accuracy of ± 2 percent of

full scale. For oxygen determinations a Model F3 Oxygen Analyzer

manufactured by Beckman Instruments, Inc., was used. Here the

very high magnetic susceptibility of oxygen is employed to

rotate a quartz fiber suspension system and the rebalance

potential is a direct and proportional measure of the oxygen

content of the sample gas. The instrument had a full-scale

range of 0-25 percent oxygen.

All instruments were standardized at their zero points

with commercial high-purity nitrogen and at or close to their

span points with instrument calibration mixtures. Electrical

output signals from all instruments were recorded continuously

on balancing potentiometer-type recorders.
- 3 -

3. Discussion of Results

Rate of Burning . The rate of burning of the crib, r,

was taken as the mean rate of weight loss from 80 percent to

30 percent of the original crib weight and expressed in g/sec.

As illustrated by the typical weight-time curve in Fig. 3, a fairly uniform rate of burning occurs during this period which

corresponds to the fully developed stage of burning. These

average burning rates have been plotted in terms of a ventilation parameter, A -/h, where A and h are the area and height of

the window opening, respectively. Considering that the velocity

of non-viscous flow through an orifice is proportional to the

square root of its height, the ventilation parameter may be

taken to represent the total induced air flow through the

opening. (See Fig. 4) Test results for all three enclosures

are summarized in Table 2 and plotted in Fig. 5* It may) be

observed that for each of the two smaller enclosures and the

corresponding crib weight, a limiting value of A */h was

reached beyond which the window no longer limited, and the

rate of burning had a constant value equal (very nearly) to

the rate of burning of the crib In the open.

For each enclosure, the date for the vertical and horizontal windows are in close agreement. This means that a

rectangular vertical window will permit burning at a greater

rate than a horizontal window of equal area. However, a

slight variation in this trend was noticed above the transition

zone for the Scale II enclosure and this may be noted from the data

points inFig. Results with two horizontal windows located

at top and bottom are also included and suggest that the

effective height for multiple windows extends from the bottom

of the lowest low-level opening to the top of the highest

high-level opening with an increased effective velocity.

A characteristic transition region was found for each

enclosure in which the data line for small window openings was

shifted upward. The data for this line has been extended to

include extermely small openings where an essentially pyrolytic reaction was occurring with the absence of visible flaming.

In such cases, repeated and prolonged ignition was required to

elevate the enclosure temperature to that required for sustaining the pyrolysis. This transition region appears to be associated with (1) a characteristic change in the position of

flaming from predominantly within the crib to regions of more

ready access of oxygen, and (2) a corresponding change in the

fraction of the window opening used for inducing air Into the

enclosure.
For the Scale II and Scale III enclosures^ the mean value

of the ratio between r and a </h is approximately 0.0015 g/sec

5/2

cnr in fair agreement with the overall average value of

0,001 g/sec cm^^ (6 kg/min summarized by Thomas [ 2 J for fires in model and full-size enclosures burning wood fuel.

Surface Area . As indicated under Test Procedure, cribs

of identical weight, construction and exposed surface area

were used for the bulk of the tests in each of the Scale I and Scale II enclosures, whereas some weight (and surface area)

variations were used in the Scale III enclosure. Additional

tests were subsequently performed in the Scale II enclosures

using cribs of varying weights and surface areas, and in the

Scale III enclosure in which equal weight cribs^ but of different surface areas were used (at the same A >/h) . The

results indicated that, for a ventilation-limited enclosure,

the mean rate of burning could be related to the exposed

surface area of the crib, as shown in Figs. 6a, b, and c. The data for the Scale III enclosure (Fig. 6c) group together

nicely when surface area variations are thus accounted for.

The visually estimated ’"best fit" lines for the three

enclosures have been redrawn in Fig. 7, from which it may be

noted that, despite a definite pattern' of differences between

the scales, a general overall concordance was found to exist.

The lines for the Scale II and Scale Il.I enclosures are of

unit slope in conformance with a direct proportionality,

whereas the data for the Scale I enclosure indicates a power

relationship. The maximum burning rate was approximately

‘ p 0.0007 g/sec cm for all scale sizes, and the location of the

transition region corresponded to a value for A /h/i of

1/2 ' £

approximately 0.1 cm

The effect of stick spacing on the rate of burning was

only explored for a single ventilation condition in the

Scale II enclosure and with a different fiberboard. The comparative results are given in Table 3* Although these tests

should not be considered conclusive, it appears as if the

spacing chosen as standard for the overall test series (i.e. a spacing of 3 stick widths) may not result in the maximum rate

of burning such as would be the case for burning in the open [ 7

It should be emphasized that these results may apply only

to the enclosure configuration and construction and location of

crib studied, and that caution should be exercised in trying to

apply the data to other situations.
- 5 -

Temperature . Thermocouple temperature measurements near

the upper edge of the crib were averaged over the time period

corresponding to the 80 to 30 percent weight loss and are

plotted in Fig. 8. For each enclosure, the average temperature

increased with k>/hA^ , but tended to level cut for the largest

window openings. * ' There was an upward shift in temperature

from the smallest to the largest size enclosure.

It is realized that a bare, unshielded thermocouple placed

in a chamber in which the gases and walls are not at the same

temperature will not give a true indication of the gas

temperature. The magnitude of the radiation error introduced

depends upon the temperatures, the surface heat transfer coefficient, the size (surface area) of the thermocouple bead,

and Its emissivity. Under the test conditions and temperatures,

the radiation errors [.'8 ] are generally less than about 5 percent of the absolute temperature (or less than 80°C) and no

effect on the conclusions drawn are expected by such variations.

No radiation measurements were made.

Gas Compo s ition . Gas composition measurements at the top

front of the burning crib were assumed representative of the

gases generated by the combustion with a minimum of dilution

and mixing effects. Check traverses which indicate the approximate changes in gas composition due to flow patterns and mixing are shown in Fig. 9 for the Scale II enclosure.

The mole percent gas composition measurements for the

Scale II and Scale III enclosures were also averaged over the

time period corresponding to the 80 to 30 percent weight loss.

As shown in Fig. 10, the general trends were similar and shifts

corresponding to the transition regions were noticed, particularly for the Seale III enclosure. The percentages of CO and C0

p the Scale III enclosure were slightly lower (and more variable)

than the corresponding values in the Scale II enclosure. This

may have resulted from a different pattern of flow and mixing

in the two enclosures, since care was taken to locate the

sampling tubes in geometrically similar positions.

The CO percentage increased with the ventilation parameter

for both the Scale II and Scale III enclosures whereas §2 Goncentratlon decreased and generally remained at or below 1 percent for window openings for which active flaming was observed,

kAh/k = 0.06 cum' 2 approximately. At this ventilation also,

the m)0| concentration increased and remained above 18 percent

for air larger window openings in the Scale II enclosure and

generally above 16 percent in the Scale III enclosure. The data

for the two largest ventilation openings in. the Scale III enclosure appear to be anomolcus (presumably due to insufficient

load for the burning to be considered ventilation-limited ) and have not been plotted In Fig. 10.
- 6 -

Neutral Zone and Velocity Measurements . In order to

determine the volumetric flow rate of exhaust products,

attempts were made to measure the position of the neutral

zone and the velocity profile across the window opening as

described by Van Arsdale [:9lThe optical distortion technique used for neutral zone

determinations depends upon the difference in the densities

and hence the refractive indices of the cool and hot gases

at the neutral zone boundary. By placing a grid of white

lines on a black background behind the plane of the enclosure

window, the displacement or distortion of the grid lines by

the hot gases and flames may be photographically recorded.

Based upon a limited number of tests with the Scale II enclosure,

it was concluded (l) that the boundary height shifted periodically during each test, with the frequency of oscillation

related inversely to A vni, whereas the amplitude of oscillation

was greatest at A a/K values corresponding to the transition

region, and (2) that, except for the transition region, the

mean height of the neutral zone boundary Increased with A i/h.

For measuring velocities of the hot exiting gases, a flow visualization technique involving interrupted streak

photography of low density perlite particles was used. As

shown schematically in Fig. 1.1, the perlite particles (preferably those in the 75 to 150 micron size range) were injected

along the optical axis of the camera, the direction In which

the flowing gases have no velocity component. Illumination

was provided by four large flash bulbs' located in a hooded

parabolic trough beneath the enclosure opening

? which projected

a narrow (2-in.) band of light upward. Sufficient illumination

intensity was required to show up the particles against the

Intensity of the flame and against the incandescence of the

particles themselves. The flash bulbs (Sylvania type 2A)

were triggered in sequence in order to extend the Illuminated

period to about 0.2 sec. Light reflected from the particles

was periodically interrupted by a rotating flat-bladed fan,

with two 30"degree blades, driven by an loOO rpm constant

speed motor, providing a light period of 1/72 second followed

by a dark period of 1/360 second. Difficulties with clogging

of the perlite in the dispensing tubes (despite the use of

vibrators and air pressure) did not permit a reliable constant flow dispersion of perlite particles to be achieved.

Nevertheless, the interrupted streak technique for velocity

measurement still appears feasible, and would be extremely

useful if combined with simultaneous photographic recording

of the neutral zone boundary.
- 7

-

4. Analysis

Combustion Reactions and Products . The decomposition

of a complex carbohydrate fuel into elementary fuels -- carbon, gaseous hydrocarbons, carbon monoxide and hydrogen

-- is considered to occur prior to the start of actual

combustion. Analysis of basic combustion reactions may

therefore be an aid in the interpretation of cellulose

burning and the associated heat and mass flow balances.

If we consider that the normal product of combustion

of cellulose burning is C0

?

and that part of it is reduced

to CO, the extent of this ^secondary reaction will be

dependent upon the relative amounts of cellulose (carbon)

and oxygen, the time of contact between the initially formed

CO^, the reactivity of the (carbon) surface and the temperature [10].

Many theories have been postulated for the type and

sequence of reactions involved in the combustion of carbon

and of cellulose. However, whatever the theory, the net heat

liberated by combustion is determined by the ratio of CO to

C0_ in the products [11]. In fact, a measure of the effectiveness of glow retardants is the resultant increase in the C0:C0p

ratio of the products and minimization of the heat available

for propagating the oxidation.

The following analysis Is similar to that outlined extensively by Kawagoe [5] but is based upon the measured exhaust

gas compositions and has been carried through on a molar basis.

Assuming the fuel to consist of pure dry cellulose, C^H-,qO^,

a simplified equation for the combustion reaction would be? ^

c 6H

io°5

+ ^+3x+f ) o

2 +

where x -

2q (3 +3x+f)N2 -> 6xC0 2+6(l-x)C0+5H20+f02

+ (3+3x+f)N

is the fraction of complete combustion,

2

I.e., the fraction of the fuel carbon which is converted to

C0

2 (expressed in terms of the mole volumes of the products),

and f, the uncombined oxygen factor Is equal to the moles of

oxygen in the products. The value of f may be determined from

the relation:
- 8 -

[o

2

] f

6+5+f+

2 i (3 +3x+f)

The combustion reaction equation assumes that all fuel carbon

is converted to CO and COp (none is dispersed as soot) and

that all the fuel hydrogen appears as water vapor. Each mole

of cellulose produces a total of six moles of CO and COp

irrespective of the relative percentages of each.

With complete combustion, the theoretical minimum anount

of air required would be 6 moles, so that the quantity

n - 3+ax+f

n “ 6

is defined as the excess air factor and has a value of 1 when

x = 1 and f = o, corresponding to complete combustion with no

excess air.

It is evident that for a carbon fuel, the combined CO

plus Op is 21$ by volume (on a dry basis) and that a COp d

percentage of 21 represents no excess air while a smaller COp

percentage represents an excess of air. As the hydrogen content

of the fuel increases, the volume percent of COp plus Op decreases. Likewise, the presence of Op (f>0) represents excess

air and its absence no excess air. The two conditions

should occur simultaneously; however, the CO percentage depends

upon both temperature (especially with excess air) and the

availability of oxygen (i.e. without excess air). The presence

of CO together with Op at low temperatures (small openings) is

not unusual —it actually represents complete combustion for the

existing temperature.

At any given temperature (and pressure), a reaction will

not proceed completely in either direction, although equilibrium may usually be very far in the direction of complete

combustion. At ordinary temperatures (< 1500°K) for example,

the reaction 2C0 + Op 2C0p goes to completion with no Op

or CO in equilibrium. In the presence of carbon (charcoal)

and absence of oxygen, however, the governing reaction probably approaches C + COp—^ 2C0, for which the temperatureequilibrium composition [10, 12] as shown in Fig. 12.
- 9 -

For comparison, the equilibrium composition for the gasphase reactions CO + H0

Q «;. .>C 0 o + H0 and 2 CO + 0 o ——

2 C0 o f

re also shown. d ' d d d d

s can be seen, the measured C0/C02 ratios for the Scale II

enclosure tend to increase slightly with the measured temperature at the probe inlet. However, the agreement with the

predicted equilibrium mixture in the presence of carbon is

not particularly good, and extreme deviations occur at

temperatures above 675°C. It is evident that in an open

system such as this with the possibility of continuing reactions occurring in the sampling tube (where carbon is no

longer present), the assumption of equilibrium conditions

and a single simple reaction, is Inappropriate.

Since the overall reaction: Cellulose + O^— CO^ -+ CO + HpO is very rapid at the high temperatures involved in

flaming gas phase combustion, but is relatively slow below

200°C, cooling of the products should produce a "frozen

equilibrium" condition in which very little change should

occur in the proportions of the different gases. As previously

stated, gas composition measurements for this exploratory

study were made using an uncooled probe. The sampling rate

was maintained at 2.700 ec/min to satisfy the requirements

of the gas analysis instruments. In a check measurement, a

water-cooled probe was located in close proximity to the

uncooled probe and alternate samplings were made from each

probe. Gas samples were also withdrawn and analyzed by

means of a mass spectrometer. The average values of the two

sets of measurements, listed in Table 4t show significant

composition changes with and without, probe cooling. However,,

the CO/CCh ratio was not affected to anywhere near the same

extent. This data is also shown in Fig. 12.

In a study of sampling of hot combustion gases from a

controlled Bunsen-type methane flame, It ms found that both

rate of sampling and probe size had a small but noticeable

effect on sample composition Lljl. It was also determined

that the water-cooled probes used were unable to quench the

reactions completely, but that the lower the temperature of

the gas being sampled, the more effective was the quenching.

The "apparent quenching temperature'* (2200-2700 K) based upon

CO/CO^ and ratios was found to be considerably below

the calculated flame temperature (2800 K) for one methane

mixture used. Extrapolation of data for several mixtures led

to the conclusion that the "apparent quenched temperature"
10

would appear to coincide with the calculated flame temperature

at about 1600°K and thus at this temperature, a probe should

function perfectly and the composition of the cooled gas should

be the same as the composition of the flame gas. Due to dilution and mixing, the maximum measured temperatures in the model

enclosure tests never exceeded 1600°K.

Variations in the value of x, the fraction of complete

combustion, is shown for the Scale II and III enclosures in

Fig. 13-

Similarity Considerations . In order to enhance the usefulness of the data presented, an attempt was made toward

understanding and establishing appropriate similarity criteria

for combustion reactions in restricted ventilation enclosures.

From the basic chemistry of cellulose burning and consideration of fluid flow through an opening, Kawagoe [5]

showed that the fuel combustion rate r could be expressed as

a function of the coefficient of discharge C, the opening

area A, the effective height of the opening h, the temperature difference between the enclosure and the surroundings,

and the volume of exhaust gas per unit weight of fuel G, as

follows:

r °<

AT

To A/h

G

G is a function of the temperature T in the box, the

fraction of complete combustion, x, and the amount of excess

air, n. It may be noted that, using an appropriate density

(p), the quantity —

pGA

less fuel to air ratio. The effective height of the opening

is determined by the location of a neutral zone, which is

the horizontal level at which no pressure difference and hence

no flow exists. This is shown schematically in Fig. 4.

The significance of the neutral zone in building ventilation was described by Emswiler [14] who considered multiple

openings in a structure and illustrated the computation of

air flow in terms of temperature difference and Bernoulli's

equation. Recently, Brown and Solvason [15] presented a

generalized theory of natural convection across single and
- 11

multiple openings in vertical partitions. For ventilation

problems, it may be assumed that density differences are

small compared with the mean density, and that vertical

temperature gradients do not seriously alter the flow

conditions. However, for flows involving flames and hot

combustion products, the consideration of the effects of

large temp, gradients is necessary.

It appears as if the overall process is a combined

gravity-controlled fluid dynamic regime and a radiationcontrolled thermal regime. In the latter regime, for

geometrically similar systems of different sizes, the

radiation group (bulk transport/radiation) and the Peclet

group (bulk transport to conduction) are the conditions for

similarity and are generally incompatible. However, since

conduction is important only insofar as it affects the loss

of heat through the boundary walls, similarity with respect

to radiation and bulk transport may be attained. The

similarity conditions for systems at the same temperature

require that both the fluid velocities and the conducted

heat loss per unit area of wall be made equal In model and

prototype. For steady-state conditions, the latter is

satisfied if the walls are made of the same material of

the same thickness (or have the same ratio of thickness to

thermal conductivity) and are subjected to the same temperature difference.

As with burning cross-piles of wood in the open, the

dimensionless Froude group is the criterion for similarity

in a gravity (buoyancy) controlled regime [ 7 ]• The

modified Froude number based on buoyancy is defined as

where p Qand p^ are the densities of the cold air and the

hot product gases respectively, v is the effective gas

velocity, and h is the characteristic height determining

buoyancy (here taken as the window height). The velocity

may be determined from the following relation between the

volume of products formed per unit fuel weight G, the

effective exhaust area (assumed constant and equal to A/2),

and the mean rate of burning r:

• 2 Gr

A

v
12

The modified Froude number may therefore be written:

N

Fii Po"Pl /

( 2Gr \ LZI2

gh

T

o

p 2 2

j iLx„

T -T P

1 o

j A^h

The volume of gases produced (G) will depend upon the

temperature, the amount of excess air and the extent to

which combustion is complete. The value of G for cellulose

containing 13 percent moisture is given both graphically and

in tabular form by Kawagoe [ 5 ] or, for pure dry cellulose,

may be computed from the relation

G - (1.07 + 3 . 98n -0.b06x) . 103 cm3 /g

For those tests in the Scale II and Scale III enclosures

for which gas composition and temperature measurements were

available, the values of G and N have been computed and

these are listed in Table 2. Figure lb indicates the change

in Froude number as a function of hfh. Despite considerable

scatter, it may be seen that there is the general tendency for

Npp to decrease with increased A/h/A s

. A Froude number of 1

indicates equal inertia and buoyancy forces., so that buoyancy

tends to predominate at higher values of pJh/p^ . The characteristic shift at the transition zone is not nearly as pronounced as in the burning rate curves, perhaps an indication

of the appropriateness of the Froude number grouping in accounting

for the observed chemical and physical changes in the burning

behavior

.

Heat Balance . Considering that 5 moles of water vapor are

formed from the burning of 1 mole of cellulose, the measured CO,

CO^, and 0^ volume percentages were converted from a dry to a wet basis and an estimation made of the heat content (enthalpy)

of the product gases. The mole fraction of water vapor was

obtained from the ratio -—

~nk • The heat content

6-''-5+f+2T (3 +3x+f)

values for the gases CO^, CO, 0 o , H^O, and at the measured

temperatures [16] are listed for a typical case in Table 5*
13

Since the mole ratio of exhaust products to cellulose is equal to

6 + 5 + f + 79/21 (3 + 3 x + f) = 31 * 8 , the net heat content of

the exhaust products is equal to 31*8 (9.18 - 2.14) = 224 kcal/mole

of cellulose. The corresponding net heat of combustion, from the

relation 219*4 + 405*6 x, is 581 kcal/mole of cellulose, yielding

a value of O .39 for the ratio of net heat content to net heat of

combustion. It may be seen from Fig. 15 that, except for a single

point at high iwh,from 15 to 50 percent of the total lower heating

value (3820 cal/g for complete combustion) of cellulose appears

in the gaseous products. For a ventilation-limited enclosure,

therefore, the ma^or portion of the generated heat is transferred

by radiation from the flames and glowing fuel. For a laboratory

crib fire burning in the open, an overall heat balance showed

that convective heat accounts for over 60 percent and radiative

heat approximately 15 to 20 percent of the total heat evolved [17]

5. Summary

Results are presented of experimental measurements of the

mass rates of burning, temperatues and gas compositions in model

enclosures of three sizes. Interest was confined to the fullydeveloped stages of fires in which a single (or multiple) ventilation opening limited the burning rate of a reproducible crib-type

cellulose-base combustible load.

The burning rate was found to be generally proportional to

A*/h (A = area, h = height of window opening), but a characteristic

transition region was found for each enclosure which resulted in

a shift in the data line. The transition region is associated

with a characteristic change in the position of flaming from predominantly within the crib to regions of more ready access of

•oxygen accompanied by a change in the fraction of the window

opening used as inlet and exhaust ports. Burning rate data from

the three sizes of enclosures were correlated in terms of the

ventilation parameter PtJh and the exposed surface area of the

crib, A s. Despite a definite pattern of differences between the

scales, a general concordance was found to exist. The maximum

burning rate was found to be 7 x lO”1

*' g/sec per cm2 of crib

surface area for all scale sizes, and the location of the transition region corresponded to a value for As/h/Ag of approximately

0.1 cml/2 .
For ©ach enclosure, the average temperature measured

by thermocouples increased with A/h/ \ but tended to level

out for window openings greater than that at the transition

region. There was a slight upward shift in temperature

from the smallest to the largest size enclosure.

Continuous exhaust gas composition measurements were

made by means of a sampling tube placed at the top front

of the burning crib, using infrared absorption analyzers

for CO and C0 2 and a magnetic susceptibility analyzer for 02-

At this sampling position, the CO percentage increased with

A/h for the two largest enclosures. The 02 concentration

decreased with A/h and remained at or below 1 percent for

window openings for which active flaming was observed, while

the COp concentration increased and remained above 16 to 18

percent.

Attempts were made to employ flow visualization techniques

to measure the position of the neutral zone and the velocity

profile across the window opening. The feasibility of simultaneous photographic recording of the neutral zone (from

optical distortion of a background grid) and the velocity

pattern of the hot exiting gases (from interrupted streak

photography of low density particles) was examined but could

not be perfected soon enough to be used throughout this test

series.

Analysis of the combustion reaction in terms of the

exhaust products was made. The measured C0/C02 ratios were

found to Increase slightly with the measured temperature

at the probe inlet, and comparisons were made with temperatureequilibrium compositions of simple carbon combustion reactions.

The overall process involved in enclosure fires appears

to be a combined gravity-controlled fluid dynamic regime and

a radiation-controlled thermal regime. ^Although the similarity

conditions are generally incompatible, partial modeling appears

to be feasible if heat conduction through the boundary walls

is the same in model and prototype. For a gravity- (buoyancy-)

controlled regime « the dimensionless Froude group is the

criterion for similarity,. and, from the limited agreement

found among the data for two enclosures, this group appears

to include the essential parameters in {he scaling of burning

rates of fires in geometrically similar enclosures.
- 15 -

The heat content of the sampled product gases was

calculated and its fraction of the total lower heat of

combustion of cellulose was found to vary with pJh. it

may be concluded that for a ventilation-limited enclosure,

the major portion of the generated heat is transferred by

radiation from the flames and glowing fuel.

6. Acknowledgments

The data summarized in this report were assembled

through the combined efforts of permanent members of the

Fire Research Section and a succession of college students

serving 3-or 6-month training periods. It is a pleasure

to acknowledge the determined efforts of the following

student trainees*. M. Heinemann, R. Kalin, J. Klein,

D. Newman, P. Rauen, J. Sharlet, VJ. Snively, J. Stokley and

G. Van Arsdale.

USCOMM-NBS-DC
- 16 -

7. References

[1] D. Gross, ’’Preliminary Results of Experimental Fires

in Enclosures" NBS Tech. Report No. 7327, Aug 1961.

[2] P. H. Thomas, "Studies of Fires in Buildings Using

Models," Research IT . pp. 69-77, 87-93, I960.

[3] P- H. Thomas and J. M. Hesolden, "Behavior of Fully

Developed Fire in an Enclosure, " Combustion and

Flame, 6, pp. 133 ~5, 1962.

[4] D. Hird and C. F. Fischl, "Fire Hazard of Internal

Linings" Department of Scientific and Industrial

Research and Fire Offices' Committee, Special Report

No. 22, H.M.S.O., London, 1954.

[5] K. Kawagoe, "Fire Eehavior in Rooms," Building Research

Institute Report No. 27, Mln.ls.tr y of Construction, • Tokyo,

Japan, 1958. ’ 5

[6] H. Do Bruce, "Experiments! Dwelling-Room Fires,"

Report No. D194l, Forest Products Laboratory, USDA,

June 1953*

[7] D. Gross, "Experiments on the Burning of Cross Piles

of Wood," NBS J. Researcii 66C, pp. 99-105, 1962.

[8] W. M. Rohsenow, "A Graphical De^ermina tion of UnshieldedThermocouple Thermal Correction, " Trans . ACME, 68 . pp. 195-8, 1946.

[9] G. D. Van Arsdale, "A11 Investigation of the FireInduced Flow Regime at the Single Ventilation

Opening in a Model Room Enclosure," NBS Tech. Report

No. 7358, Oct 1961.

[10] R. T. Haslam and R. P. Russell, "Fuels and Their

Combustion, " McGraw-Hill Book Co., New 'fork, N. 1 ., 1926.

[11] F. L. - owne, "Theories of the Combustion of Wood and

Its Control-A Survey of the Literature," Report

No. 2136, Forest Products Laboratory, USDA, Dec 1958.

[12] D. D. Wagman, et al., "Heats, Free Energies and

Equilibrium Constants of Some Reactions Involving

Og, H^, H^O, G ’ C0 ’ C0

2

anc^ • NBS J* of Research

34, pp. 143-161 (RP 1634) Feb 1945-
- 17 -

[13] C. Halpern, and F. W. Ruegg, "A Study of Sampling of

Flame Gases," NBS J. Research 60, pp. 29-37, 1958-

[14] J. E. Emswiler, "The Neutral Zone in Ventilation,"

Trans. ASHVE, 32, pp. 59-7S 1926.

[15] W. G. Brown, and K. R. Solvason, "Natural Convection

Theory to rectangular Openings in Partitions, Part I.

Vertical Partitions," Intern' 1 J. of Heat and Mass

Transfer, pp. 859-868, 1962.

[16] F. D. Rossini, et al., "Selected Values of Properties

of Hydrocarbons," NBS Circular ^61, 1917-

[171 W. L. Fons, et al., "Project Fire Model," Summary

Progress Report I (I960) and II (1962), Forest Service,

USDA.

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Table 2. Suaury of Test Result*

A-/TT Gas Composition, Dry

Horizontal Window

103 g/« 1

24 27.5

23 41.3

22 55.1

21 63.3

18 68.9

19 74.4

20 79.9

11 82.6

15 90.9

17 96.4

16 102

13 110

55 do 56 do

58 do

62 do

54 do 6U do

60 do 59 do

63 do

12 124

6 138

8 207

3 270

1.70 35.9

2.55 66.4 3.40 102

3.91 125 4.25 142 4.59 159 4.93 177 5.10 187

5.61 216

5.95 236

6.29 256

6.80 287

do do

7.65 343 8.^0 402

12.8 740 16.7 1100

.0377 .0697

.107

.131 .149

.167 .186

.196

.227

.248

.269

.302

.173

.211 .232

.259

.301

.775 .600

.591 1.25

.360

.422 .777 1.22

.0582

.0833

.101 .100

.0945 .0953

.123 .174

.189 .196

.277 .353

.174

.212

.272 .280

.340

.219

.302

.363

.234 .484

.590

.658

.636

.0609

.0872

.106

.105 .0997

.0998

.129

.183

.198 .207

.291

.371 .105 .156

.219 .252

.357

.592

.631 .750 1.02

.508

.620

.691

.667

500

275 400 420

405

375 450

445

475

425 500

420

535

525

550

475

425 470

655 475 545 540

645

Dual Horizontal Windows*

48 7.3 49 17.0

40 23.0

42 43.9

43 52.0

44 60.6

45 68.4 46 76.8

47 104

39 147

0.45 44.3

1.05 103

1.42 137

2.71 257

3.21 302 3.74 350

4.22 391 4.74 435

6.42 570

9.10 770

.0466

.108 .144

.270

.316 .366

.409

.456

.598

.809

.0953

.0975

.113

.232 .404

.446

.461

.471 .714

.665

.100

.102

.118

.243 .423 .468

.484

.495

.748

.697

248 410

450

525 550

525 560

525 550

Vertical Window

31 14.2 16.7

33 17.0 do

32 19.9 do 30 22.7 do

34 28.4 do

35 34.1 do

28 42.6 do

36 48.3 do

29 56.8 do

37 65.3 do

27 71.0 do

26 99.4 do

38 114 do

25 142 do

* "Effective" A-/h values

SCALE 11

58.0 .0609 69.6 .0731 81.2 .0853 92.9 .0975 116 .122 139 .146 174 .183 197 .207 232 .244 266 .279 290 .304 406 .426 464 .487 596 .625 are listed. Horizontal Window.0688 .0724 230

.0991 .104 300

.132 .139 325

.189 .198 320

.207 .217 400

.154 .162 485

.129 .135 575

.152 .160 525

.189 .198 655 .257 .271 550

.522 .548 525

.603 .632 600 .636 .667 .683 .717 650

18A 109

6 121 17A 174

16A 218

7 A 260

6A 329 10A 385

7 430

3 471 23A 545 9D 654

7B do

8B do

2B do

12 B do 11B do 10B do

1A do

4 747 14A 835 13A 1020

9 1320

12A 1640

31A 2180

2.35 167

2.59 195 3.76 338 4.70 473 5.60 616 7.10 876 8.30 1110

9.2 1300

10.1 1500

11.8 1870

14.1 2450

do do

do do

do do

16.0 3000

18.0 3540

22.0 4790

28.2 7000

35.2 9720

46.7 14900

.0143 .0923

.0168 .176 .0290 .505 .0406 .765 .0529 1.02

.0752 1.71

.0953 1.84

.117 2.04

.129 2.22

.161 2.35

.839 2.42

.352 3.03

.280 3.33

.210 3.62

.176 3.34

.150 2.75 .104 2.64

.211 3.83

.258 5.25

.304 6.10

.411 7.76

.601 8.10

.835 9.38 1.28 9.15

.00773 128

.0151 150

.0438 215

.0653 270

.0876 305

.147 310

.158 485

.175 600

.191

.202 645

.829 635

.436

.380 670

.311

. 240 665 .169 635 .112 550

.329 610

.451 675

.524 540

.607 605

.696 750

.806 550

.786

12 3.2

1.0 7.0

0.1 6.5

1.5 10

0.1 13.7

19

19

0.53 .65

0.38 .73 0.32 .75

9.53 .66

0.72 .58

7.8 2.1

.24 0.90

.02 0.88

.31 0.88

.02 0.78

14,200 1.63

13,900 1.02

14,900 1.29

14,700 0.49

Vertical Window

33 48

37 72

31 100

30 114

29 144

34 188

51 204

15 217

14 260

25 305 46 362

44 505

28 578

45 658

32 725 48 797

47 878

38 940

49 1080

43 1120

50 1740

36 2230

47.7 330

do 500

do 700

do 800

do 1000

do 1300

do 1400

do 1500

do 1800

do 2100

do 2500

do 3500

do 4000

do 4550

do 5000

do 5500

do 6000

do 6500

do 7500

do 7750

do 12000

do 17400

.0283 .369 .0430 .515

.0601 1.29

.0687 1.41

.0859 2.26

.117 2.67

.120 2.73

.129 2.45

.155 2.50

.180 2.89

.215 2.84

.301 3.45

.344 4.48

.391 4.92

.430 5.57

.472 6.13

.515 6.71

.558 6.68

.644 6.96

.666 7.43 1.03 8.71 1.49 8.48

.0314 220 7.8

,0443 200

.111 350 1.2

.121 355 0.5

.194 355 0.8

.229 375 0.5

.235 400 0.2

.210 480

.215 520 0.8

.248 565 0.2

.244 575 <0.1

.296 645 0 .383 645 0.5

.423 660 0.2

.478 640 0.2

.527 680 <0.1

.576 675 <0.1

.574 655 0.5

.598 675 0.3

.638 665 0.5

.748 685 0.5

.728 750 0.5

4.3 9.9 0.43

6.3 18 0.35

6.3 19 0.33

7.0 18.5 0.38

7.2 19.2 0.38

7.7 18.7 0.41 8.4 19.7 0.43

7.3 19.5 0.37

9.5 >20 <0.48

11.2 >19.7 <0.57

9.5 19.5 0.49 >11.5 18.8 >0.61

>12.3 19.4 >0.63 >13.9 18.7 >0.74

>11.7. >20 >0.58

>14.0 19.2 >0.73

>14.0 19.3 >0.73

>13.0 19 >0.68

>14 19.5 >0.72

>12 17.7 >0.68

>14 18.6 >0.75

.70 11,500 1.26

.74

.75

.73

.73

.71 .70

.73

.68 .64

.67 .62

.61 .57

.63 .58

.58

.59

.58

.60

.57

0.3 0.92

.12 0.90

.19 0.90

.11 0.88

.05 0.86

.18 0.90

.04 0.85

.02 0.82

.00 0.84

.10 0.83

.04 0.81

.04 0.79

.02 0.82

.02 0.79

.09 0.80

.06 0.80

.09 0.80

.10 0.82

.09 0.80

10,100 1.31 9,940 1.14 9,940 1.88 10.100 1.51 10.300 1.33

12,600 0.76

12.800 .71 12,600 .46 13,900 .37 13.800 .48 13,800 .43 13.300 .44 14,200 .47 13,800 .45 13.700 .39 13,900 .31 13,800 -33 14,200 .20 15.100 .09

SCALE III Horizontal Window

6 1130

10 1340

7 1510

11 2190

8 2600

12 2970

9 3910

13 6020

7.80 3180

9.24 4080

10.4 4900

15.1 8560

17.8 11000

20.5 13500

27.0 20500 41.6 29200

.0335 4.01

.0431 9.75

.0517 11.6

.0815 22.8

.105 13.9

.117 20.6

.162 40.4

.280 60.0

.0422 232

.103 410

.122 550

.216

.132 668

.178 785

.319 874

.428 1020

10.3 3.7

7.9 5.6

0.3 5.9

0.1 5.3

3.9 3.1 2.9 3.0

0.9 5.7

0.1 8.6

8.3

10.1 18.0

19.3

14.7 15.4

16.8

16.6

.45 .69

.55 .65 .33 .75 .28 .79

.21 .83 .20 .84

.34 .75

.52 .65

5.1 1.7 3.0 1.1 0.08 0.89 0.24 0.92

1.3 1.1 0.96 1.3 0.24 0.89

0.02 0.94

14.100 1.89 13.000 1.08 13.000 2.22

17,900 0.98

23.200 2.03 18.000 1.82 21.300 1.31

Vertical Window

14 320

1 565

15 736

17 1660

18 1660

21 1660

2 2610

3 4130

4 8180

16 8180

19 8180

5 12260

20 21200

145 3850

do 6900

do 8850

do 19700

do 20000

do 31900

do 50400

do 100,000

do 150,000

do '256,000

.0406 10.3

.0412 13.7

.102 13.8

.148 31.0

.0827 29.5

.152 19.9

.336 46.7

.302 64.0 1.056 50.0

.598 103

.487 161 .898 93.1 1.095 122

.108 .0818

.159

.233

.124

.151 .514

.383

.527 .617 .786

.556

.521

350

550

668

745 749 710

875 920 850

1039

971 950 1090

8.3

3.7

0.1 1.0 1.0

0.0

0.8

0.5 5.5 11.8

5.5

3.8

2.3 8.0

6.9

13.5

8.8

8.1 2.2

0.3

10.5

15.1 18.0

18.5

17.7

17.1

16.4

16.4

13.7

7.5

.52 .65

.25 .80

.13 .89

.43 .70

.39 .72

.79 .56

.54 .65

.49 .67 .16 .86

.04 .96

3.1 1.1 1.2 1.1 0.03 0.95

0.23 0.89 0.24 0.90

0.0 0.78

0.20 0.86

0.12 0.86

2.1 1.3

9.1 2.5

11,800 <.80 15.700 2.28 15.500 1.12 16.000 1.08

In, 400 1.03

17,500 0.63

20,200 0.52

19,100 1.23

26.900 -37 53,300 .74

Table 3. Effect of Crib Construction on Rate of Burning

Scale II Enclosure

r- 5/2

AVh = 2500 cm

Test

No.

Spacing

(in stick widths)

Initial

Crib

Weight

Mean Rat

of Burni

g g/sec

62 1 2730 3.05

61 2 2730 2.98

60 3 27^0 3. 3^

63 5 2780 4 A3

Table 4. Gas Composition Measurements Using Cooled and lincooled

Probes.

Composition, mole percent __ _ Mass Spectrometer " continuous TnTf’Wf'eT’WiSTy SeT

Gas

Cooled Probe Uncooled Probe Cooled Probe Uncooled

CO 1.2 4.1 1.9 5.4

CO

2

1.5 7.4 4.1 15.0

°2 20.4- 12.8

N

2

68.2 7k.

Ar © o

i

—0„ 9

H2° 7.8 —

C0/C0

2

0.8 o.55 0.46 0.36

T = 580°C

pf- CM® co o

CDw II 0O Ch

p> m. CO ITS 3 IN- 0 • 5 O

(a 1! 03 X

CDG iW> 3 W to 0 00 in S Pi- rn o H

X?p 1! «H EH o

j. P a G o oo oo p irs 00 <rs W o

CM 0

3i II P

«hO 1 G r* o Os

•Hp P 0 CO rH 0 3 EH o

rH0 H o HH 0

r—

0H

f* 0

oa Si

ITn0

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CM o rH j- IN PO pH o m nr • © e • s O o o rH O

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G • • • 9 • O

CM W

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FIG.

ISCHEMATIC

DIAGRAM

OF

EXPERIMENTAL.

APPARATUS

o4=. h-

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SKETCH

OF

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SAMPLING

ARRANG

Weight

.Groms

FIG.4-

IDEALIZED

FLOW

THROUGH

WINDOW

OPENING

001

ceo

D

G

n J

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4

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Factor,

£

crrAj

FIG.

5-

BURNING

RATE

AND

VENTILATION

FACTOR

FOR

THREE

ENCLOSURES

FIG.6A- SPECIFIC BURNING RATES AND VENTILATION FACTORS

FOR SCALE I ENCLOSURE

FOR SCALE IL ENCLOSURE

FIG.6C- SPECIFIC BURNING RATES AND VENTILATION FACTORS

FOR SCALE HE ENCLOSURE

Specific

Burning

Rate,—

7—

g/sec

cm

An/TT '/ Specific Ventilation Factor, ~z— cm'2

A s FIG.7- SPECIFIC BURNING RATES AND VENTILATION FACTORS

FOR THREE ENCLOSURES , "BEST FIT'

1 LINES

1200

o

o

v*

— U

o ^

CL to

o

o o o o o

o o o o o

o 00 o Vi" C\J

0>0

4 ojr.43j3dujoi

FIG.

8

-TIME-AVERAGED

TEMPERATURES

FOR

THREE

ENCLOSURES


:
o

COo

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Ll)

[=1 c> "uo

00 oo L - co 8 o’> Lu > O “ oa

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co .i. £ x oou- a

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Flood-Lighted

Grid

Pattern

/Test

Enclosure

Product

FIG. 12- TEMPERATURE -EQUILIBRIUM COMPOSITIONS FOr? SIMPLE

COMBUSTION REACTIONS, THEORETICAL AND EXPERIMENTAL

o o

C—co

CO£ ij

Froude

Number,

50

ec

IA.

s.o

0.1

o A

A

A

A AD

1

A a

A A *

Scale Vertical Horizontal

XX A A mm

A

A

0.01

Specific Ventilation Factor,

A^T '/ cm 2

FIG. 14 - FROUDE NUMBER AND VENTILATION FACTOR FOR TWO ENCLOSURE

!' * u

I

o.

to

to

r

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: •:

j

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i