AlllOE IMOBbT N

r^pUBLlCATIONSt

NBSIR 82-2611

Development of the Cone

Calorimeter -- A Bench-Scale

Heat Release Rate Apparatus

Based on Oxygen Consumption

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Fire Research

Washington, DC 20234

November 1982

1^0 U S DEPARTMENT OF COMMERCE

,U56

^2-2611

1982

NATIONAL BUREAU OF STANDARDS

NBSIR 82-2611

, / V DEVELOPMENT OF THE CONE

CALORIMETER - A BENCH-SCALE

HEAT RELEASE RATE APPARATUS

BASED ON OXYGEN CONSUMPTION

'jtTATIONAI. BtmEn’-T Qf STA.VDAWV8 UBiumv

DEC 1 198?

OCjV (XCCrO^-

CicC. \oO

Vytenis Babrauskas

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Fire Research

Washington. DC 20234

November 1982

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director
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TABLE OF CONTENTS

Page

LIST OF TABLES iv

LIST OF FIGURES v

NOMENCLATURE vli

Abstract 1

1. INTRODUCTION 1

2. DISCUSSION OF REQUIREMENTS 5

3. THE PRINCIPLE OF OXYGEN CONSUMPTION 8

4. APPARATUS DESIGN 8

4.1 Basic Apparatus Design 8

4.2 Oxygen Sampling System ^ 4.3 Cone Heater H

4.4 Hood System

4.5 Specimen Mounting

4.6 Load Cell

4.7 Physical Adjustment of Apparatus

4.8 Spark Ignition

4 . 9 Heat Flux Gage ^8

4.10

Calibration Burner ^8

5. ANALYSIS 18

5.1 Theory 18

5.2 Analysis of Data 18

5.2.1 Manual Analysis 18

5.2.2 Electric Analog Signal Conditioning .... 19

5.2.3 Digital Data Collection 19

6. TEST PROCEDURE 20

7. TEST LIMITATIONS 21

8. PERFORMANCE 22

8.1 General Observations 22

8.2 Exhaust System 23

8.3 Cone Heater 24

8 . 4 Oxygen Analyzer Response 24

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8.5 Calibration with Pure Gases

8.5.1 Oxygen/ Carbon Dioxide Balance 26

8.5.2 Other Gases 28

8.6 Measurements on Solid Materials 28

9. CONCLUSIONS 30

10. ACKNOWLEDGMENTS 30

11. REFERENCES 31

ill
LIST OF TABLES

Page

Table 1. List of major parts 33

Table 2. Exhaust system flows 34

Table 3. Stack temperature map 35

Table 4. Radiation from cone heater 36

Table 5. Effect of convective fluxes 37

Table 6. Flux deviation summary 38

Table 7. Linearity of response 39

Table 8. Burner height effect 40

Table 9. Effect of air flow rate on noise

Table 10. Oxygen/carbon dioxide balance ^2

Table 11. Rate of heat release for additional gases .... ^3

iv
LIST OF FIGURES

Page

Figure 1. Overall view of apparatus

Figure 2. Exploded view, horizontal orientation ^5

Figure 3. Exploded view, vertical orientation ^6

Figure 4. Elevation and side views, horizontal

orientation

Figure 5. Elevation and side views, vertical

orientation

Figure 6. Gas sampling ring probe

Figure 7. Gas analyzer Instrumentation

Figure 8. Thermoelectric cold trap

Figure 9. Soot filter

Figure 10. Inner and outer shells for cone heater

Figure 11. Base plate and brackets for cone heater ^4

Figure 12. Mandrel for winding cone heater

Figure 13. Sliding cone support brackets 56

Figure 14. Base plate and support pieces 57

Figure 15. Exhaust system 58

Figure 16. Exhaust duct 59

Figure 17. Horizontal specimen holder

Figure 18. Retainer frame for horizontal Ignltablllty

testing

Figure 19. Vertical specimen holder

Figure 20. Load cell attaching pieces ^5

Figure 21. Spark plug carrier

Figure 22. Power supply for spark plug ^5

Figure 23. Holder for Gardon gage

V
Page

Figure 24, Calibration burner 67

Figure 25. Analog signal module

Figure 26. PMMA (horizontal) rate of heat release and

heat of combustion at 50 kW/m^

irradlance

Figure 27. Red oak (horizontal) rate of heat release

and heat of combustion at 50 kW/m^

irradlance

Figure 28. PMMA (horizontal) rate of heat release at

several irradlances

Figure 29, PMMA (vertical) rate of heat release at

several irradlances

Figure 30, Red oak (horizontal) rate of heat release

at several Irradlances

Figure 31. Red oak (vertical) rate of heat release at

several Irradlances

Figure 32. PMMA (horizontal) variations during three

runs at 25 kW/m^ Irradlance

Vi
NOMENCLATURE

s

b

C

F

Ah

( M

•m

•n

AP

q

q“

t

At

Te

X

3

e

a

2 specimen exposed surface area (m )

stoichiometric factor ( —

)

orifice meter calibration constant (m^^^

view factor ( —

)

net heat of combustion (kJ/kg)

molecular weight (g/mol)

mass flow (kg/s)

mole flow (mol/s)

orifice meter pressure differential (p^)

heat rate (kW)

heat per unit area (kW/m )

stoichiometric oxygen/fuel mass ratio ( —

)

time (s)

time increment (s)

temperature for orifice meter (K)

mole fraction ( —

)

oxygen analyzer response parameter (s“^)

emissivity ( —

)

Stefan-Boltzmann constant (5.67 x 10”^^ kW m”^K

Subscripts

air - air

CO

2 - carbon dioxide

e - exhaust

f fuel

8 - gage

vii
h heater

©2 “ oxygen

p - pyrolysis

“ - ambient conditions

Superscript

° - baseline value (before test)

vlll
DEVELOPMENT OF THE CONE CALORIMETER — A BENCH-SCALE HEAT

RELEASE RATE APPARATUS BASED ON OXYGEN CONSUMPTION

Vytenls Babrauskas

National Bureau of Standards

Washington, D.C. 2023A

A new bench-scale rate of heat release calorimeter

utilizing the oxygen consumption principle has been

developed for use In fire testing and research. Specimens

may be of uniform or composite construction and may be

tested in a horizontal, face-up orientation, or, for ones which do not melt, also vertically oriented. An external

irradlance of zero to over 100 kW/m^ may be Imposed by means

of a temperature-controlled radiant heater. The rate of

heat release Is determined by measuring combustion product

gas flow and oxygen depletion, while the mass loss Is

simultaneously recorded directly. The instniment has been

designed to be capable of higher accuracy than existing

instruments and yet to be simple to operate and moderate In

construction cost. The Instrument is termed a "cone

calorimeter" because of the geometric arrangement of the

electric heater.

Keywords: Combustion of plastics; combustion properties;

fire tests; heat of combustion; ignltability; oxygen

consumption; rate of heat release.

1 . INTRODUCTION

In a building fire the rate of heat release from burning combustibles is

the quantity of most concern in predicting the course of the fire and its

effects. Measurement techniques for determining the heat release rate of an arbitrary, full-sized combustible assembly are difficult and are just now

beginning to be implemented. For instance, a technique [1] was recently

* Numbers in brackets refer to literature references at the end of this paper.

1
developed for measuring the rate of heat release of full-sized furniture

items. Even though full-scale measurement is possible in this case, it is

much more convenient to do studies in a bench-scale apparatus. The rate of

heat release of a full-scale item can conceptually be approximated by the use

of data from certain bench-scale measurements: (1)

Ignitabllity (time to ignition at a specified Irradiance)—This

effect is not important in a deterministic scheme for the first item to

ignite, but is important in a stochastic model and also for additional burning

items

,

(2) Surface Flame Spread Rate and Available Surface Area—The heat

released has to be proportional to the amount of material exposed to fire, for

which flame spread rates and corresponding burning areas are needed,

(3) Heat Release Rate per Unit Area—This number can be measured using a bench-scale test and approximates the heat release rate of a surface element

of the full-scale object.

For some classes of objects ignitabllity and flame spread effects are known to be not significant. In those cases we correctly expect that the

full-scale rate of heat release, q, is proportional to the bench-scale, q",

taken per unit area [2], The relationship depends on a proportionality, at

present empirical, which has to be established for a particular fire

scenario. This proportionality can be dependent on the thickness, area, and

mass of the full-scale items.

While the state of the art is just now emerging that will permit detailed

advantage to be taken of the rate of heat release values, their basic

Importance has long been recognized and measurements of this type were being

made as early as 1959 [3], At the National Bureau of Standards (NBS) we have

for some time felt the need for a small, bench-scale rate of heat release

calorimeter which would incorporate features not available together in

existing designs. We can first list the general attributes, commonly desired

for any laboratory test method:

2
Method to be quantitative

• Validity and accuracy

• Precision, repeatability, and reproducibility

• Ease of calibration

o Ease of specimen preparation

• Ease of data analysis and interpretation

• Low cost of apparatus

® Short preparation and testing time

• Ruggedness of apparatus

• Ease of physical installation and provision of utilities.

To this general list we can now add features specifically desired in a new

heat release rate apparatus:

• Use of the oxygen-consumption technique, rather than measuring

sensible heat

• Electric radiant heater (to avoid oxygen consumption from a fuelburning heater itself)

• Simple and affordable enough to be useful for testing laboratories

• Square specimens (for preparation convenience)

• Suitable for both uniform and composite materials

3
• Irradlance adjustable over zero to 100 kW/m^, to replicate a wide

range of fire conditions

• Specimens to be tested in horizontal or vertical orientation; those

which melt and drip to be tested horizontally only

• Irradlance to be easily calibratable before each test

• Irradlance uniformity

• Variable specimen thickness to be accommodated

• Specimen to be easily Insertable and removable

• Specimen field of view to consist primarily of

a) Temperature-controlled radiant heater, or

b) Water-cooled surfaces, or

c) Open air

• Specimen flames not to be appreciably deflected from natural flow

• Apparatus to also be usable for ignitability studies

o Electric spark ignition

• Sample mass loss measurement system (used for computing effective heat

of combustion)

• Calibration procedure to be simple (to be done daily)

p • Data presentation directly in kW/m

• Ease of maintainability and cleanability

.

4
2. DISCUSSION OF REQUIREMENTS

At NBS we have had direct experience with four rate of heat release

calorimeters. The original NBS calorimeter, "NBS-I," was conceived and

designed by A. F. Robertson, then developed and described by Parker and Long

in the early 1970's [4], Eventually a larger redesigned and improved version,

"NBS-II," was developed by Tordella [5]. More recently, feasibility of benchscale oxygen consumption techniques was investigated by Sensenlg [18], and a small developmental rate of heat release calorimeter based on oxygen

consumption was constructed by Lawson [6] for testing building materials in

the vertical orientation. Finally, we operated the Ohio State University

(OSU) apparatus [7] and explored in detail some of its calibration

difficulties [8].

The primary purpose of all these calorimeters is to measure the heat

release rate of products in order to estimate their contribution to a room

fire (as opposed to characterizing in detail their combustion chemistry

behaviors). Since the primary purpose in the present case is to be repetitive

product testing, certain features, such as ruggedness and modest cost, assume

a top priority, while others, which are only useful for research studies, such

as availability of redundant measurement paths, are considered of lower

importance. None of the calorimeters listed above were considered to possess

all the desired features; the most nearly adequate one, the NBS-II was also

extremely costly. Thus, a discussion of the reasoning why the listed features

are desired is appropriate.

Much of testing in existing instruments has been done on specimens in a vertical orientation, although horizontal specimen mounting has one distinct

advantage: no specimens fall down or melt and run away when tested

horizontally, but many do in the vertical position. While some arguments can even be made for testing exclusively in a horizontal orientation, it is clear

that the capability of testing horizontal specimens is at least as important

as testing vertical ones. Thus, the performance in the horizontal orientation

was considered crucial.

5
The apparatus is Intended for testing combustible materials used in

furnishings, wall linings and other applications in buildings. Almost

invariably these products are composite assemblies, rather than homogeneous

substances. While some fire contribution of individual component materials

can be recognized, the heat release behavior of a composite cannot be computed

from data on the individual materials. Thus, specimens should be useable

which are cut out through the thickness of manufactured goods. For

convenience a square shape is preferable.

Heat release rate can be measured simply by sensing the outflow enthalpy

directly by recording exhaust gas flow temperature rise. Experience indicates

that this is subject to large heat-storage errors. An improved technique

involves a substitution burner set to maintain a constant heat balance [4],

This is effective but costly and complex. To be preferred, instead is the

principle of oxygen consumption measurement [9], This is especially simple

since oxygen is neither stored nor lost along the apparatus flow path. Also,

packaged instruments for measuring concentrations are readily available

and, unlike heat sensing methods, do not have to be developed. At NBS, oxygen

consumption measurements in the NBS-II calorimeter were first made in 1978, A

prototype small vertical ©2 calorimeter was also built here during 1978-79.

Similar approaches have recently also been pursued in European laboratories

where Svensson and Ostman [10], Holmstedt [11], and Bluhme [12] have reported

recent applications.

Most existing calorimeters are subject to irradiance errors, especially

at higher irradlances or for larger samples, A burning specimen establishes a flame which can heat up internal apparatus surfaces. These heated surfaces

can then radiate back to the specimen and introduce an unexpected and unknown

increase in irradiance. Three techniques are available for minimizing this error. All internal apparatus surfaces which can irradiate the specimen

should be (1) temperature controlled, or (2) water-cooled, or (3) far away.

To make reasonable analysis of results possible, an electric pilot

ignitor arrangement should be used since it imposes no additional, localized

heat flux upon the specimen and does not consume oxygen. Electric spark

ignition was found, in addition, to be simpler and more reliable than a gas

pilot.

6
Some existing calorimeters show substantial variations In the Irradlance

across the specimen face from the heater. This Is undesirable both because

average values become difficult to determine and because unexpected combustion

changes can be Introduced by local hot spots. A uniformity of the order of

15% should be readily achievable with careful design.

Gas burners are undesirable If the specimen Irradlance Is to be smoothly

adjustable down to zero external flux. They are also undesirable In oxygenconsumption work because they Introduce extraneous oxygen depletion. Electric

heaters are not subject to these limitations. However, electric heaters, If

located far away from the specimen, may be sizeable and consequently to

consume excessive power. A suitable design requires a compact heater which

can be powered from available laboratory mains.

The design of the apparatus has to be such that a valid measurement Is

maintained for all types of responses. Known pitfalls In this area with other

equipment Include radiation errors Introduced when measuring specimens having

flames of high emlsslvlty and errors due to Instrument time response

problems. Use of oxygen consumption as a measurement technique Is highly

useful because It minimizes these difficulties.

For consistent operation. Instrument calibration should be re-checked

regularly. This suggests the usefulness of an arrangement where a calibration

burner can be operated simply, without the need for re-installing one each

time.

Experience suggests that ease of specimen insertion is an important

factor to be considered in design. Quick and simple insertion should be

possible in order to minimize starting time uncertainties.

The upper limit of specimen irradlance should be high enough to simulate

required full-scale heat fluxes, with special attention to the more fire

resistive materials. A value of at least 100 kW/m is desirable and

achievable.

7
Last but not least, for the instrument to be truly useful it has to be

rugged. Sufficiently heavy duty materials and construction should be used to ensure that operating conditions do not change, leaks do not occur, warpage

does not prevent a good fit, and similar problems do not cause erratic

operation.

3. THE PRINCIPLE OF OXYGEN CONSUMPTION

The net heats of combustion for common organic materials range from very

small up to about 50 MJ/kg. (Hydrogen, a special case, gives 120 MJ/kg). It

has occasionally been observed that the heat of combustion released per kg of

oxygen consumed is a nearly constant number. Huggett [9] has examined a wide

3 variety of fuels and concluded that Ah^/r^ “ 13.1 x 10 kJ/kg represents a value typical of most combustibles, including gases, liquids, and solids.

To implement this principle it would be necessary only to measure the

total mass flow of oxygen in the combustion products and to compare that to

the initial inflow, that is

q )

where the subscript » denotes baseline ambient condition prior to start of

test. From the form of this expression it can be seen that it does not matter

at what speed the products are exhausted or how much excess air is pulled

through. It is as if we were interested only in counting oxygen "holes".

4. APPARATUS DESIGN

4.1 Basic Apparatus Design (Figures 1 to 5, and Table 1)

Two features represented pivotal choices in the design of the

calorimeter. It was first decided that oxygen consumption would be the method

of measurement. In addition to the considerations given above, this has the

advantage that when an electrical radiant source Is used, no compensation is

required for changes in heater power; that is, oxygen measurements are unaffected when the specimen flames raises the temperature of the radiant

8
source, and the temperature controller, In turn, reduces power to maintain

constant heater temperature.

The second primary choice was the design of the radiation source. It was concluded that the simplest instrument design would involve an open

construction, not involving an air-tight box. This, again, is readily

possible only with the oxygen consumption technique (if sensible enthalpy were

measured, radiated heat would not be captured). The radiant heater itself,

for reasons of efficiency, should have the largest possible view factor and

the smallest surface area. In addition, the heater should not be so located

as to significantly disturb the flame flow pattern. Numerous alternatives

were considered; the design that seemed best suited was a modified version of

the ISO heater cone used in an ignitability test [13], For a horizontal

specimen this truncated cone shape appears to be the ideal configuration.

More flexibility as to heater shape is available when burning samples

vertically. However, for simplicity of operation it is easiest to use the same heater and merely to pivot it 90® into a vertical orientation.

The requirements for the exhaust/ sampling system may then be stated. The

hood must collect all the combustion products. The flow stream must be mixed

and straightened. Oxygen sampling should be done with a multi-point probe in

a region of well-developed flow. The probe should be arranged to minimize the

possibility of clogging by soot, A controllable adjustment of the flow is

desirable to optimize resolution. A provision is necessary, in any case, to

measure the flow velocity and temperature in order to determine the exhaust mass flow.

4.2 Oxygen Sampling System (Figures 6 to 8)

The unusual aspects of oxygen measurements in this application are that

only a small range, approximately 21 to 18% of oxygen concentration is used.

This contrasts sharply to instrumenting room fire tests for oxygen, where the

whole range of 21% to 0% can be expected. For calorimeter use, therefore,

requirements are placed on stability and low noise for the oxygen meter which

are quite strict. Instrument noise and short-term drift have to be kept to

below approximately ^ 0,005% ©2 to make the measurement feasible. Our

9
investigations at NBS led us to conclude that electrochemical or zirconium

oxide type oxygen meters would not give sufficient stability and that a paramagnetic oxygen analyzer is necessary for this application.

In addition to the need for a high stability O

2 meter, the other portions

of the gas sampling train have to be carefully designed and set up since any

drift or flow fluctuations can show up as pressure variations at the inlet or

the outlet of the O

2 meter and, therefore, appear as noise or error.

At the outlet end, changes in barometric pressure introduce an undesirable baseline shift, which is serious because of the narrow operating

range. This difficulty is solved by putting an absolute back-pressure

regulator on the outflow port of the oxygen analyzer. For proper operation

this regulator should be located in a constant temperature environment.

At the inlet end, the gas sample is first drawn in to the system through

a ring probe in the exhaust duct (Fig. 6). The probe has numerous holes

facing away from the incident flow direction. A forward-facing probe is not

viable due to soot clogging. Multiple holes are provided to permit the best

possible averaging over any local fluctuations.

From the probe the sample goes through a soot filter to a cold trap to remove moisture (Fig. 7). Several designs were tried for this. A

thermoelectric trap (Fig. 8) was designed since it has the advantage of

continuous operation and can readily be electrically adjusted to the proper

operating point. The cold trap outflow is equipped with a thermocouple, which

should register slightly below 0®C. For the design developed, typically 8 to

10 volts across the two thermoelectric modules is required. The water

condenses into the separation chamber and is removed at the drain valve. From

the cold trap the sample goes through the pump, at the exit of which is fitted

a small surge tank to minimize downstream pressure fluctuations. At this

point the major part of the flow goes through a relief pressure regulator and

is wasted. Such an arrangement helps minimize transit time. The remaining

flow passes through a drying cartridge, a CO

2 removal cartridge, a flow

regulator, a 7 pm filter and into the oxygen analyzer. The drying cartridge

is provided as back-up to the cold trap, in case of trap malfunction. The CO2

10
removal cartridge is used to permit simplified data analysis. The flow

regulator is necessary to reduce inlet pressure fluctuations with aging of the

cartridges. The 7 pm filter is recommended for the protection of the oxygen

analyzer cell.

4.3 Cone Heater (Figures 10 to 12)

In principle and in dimensions the cone heater follows closely the ISO

heater [13]. The heater is constructed in the following way. An outer shell

is welded to the top plate, which in turn is attached to the sliding hinged

support. An inner shell is then Inserted inside the outer shell, with a layer

of low density refractory fiber blanket separating the two. This provides a double-wall construction to minimize heat radiation to the far side. The

heater element (table 1) consists of a resistance heating wire packed in MgO

refractory and swaged in a high-temperature alloy sheath. The element is

wound into a cone shape using a winding mandrel (Fig. 12) attached to a lathe,

clamping one end to the retaining plate and winding slowly by hand. A total

of 7-1/2 tightly packed turns are wound, leaving about 45 mm free ends. The

heater is inserted in the shell assembly and held in with a base plate

(Fig. 11). The base plate also acts as a stop in setting the horizontal

position level and holds brackets for spark plug (bracket A in horizontal

testing, bracket B in vertical testing) and for calibration burner/heat flux

gauge. Both the calibration burner and the heat flux gauge use the square

opening in bracket A. The burner is oriented to discharge upwards.

The cone heater is mounted on a piano hinge to a sliding support bracket

(Fig. 13) which slides on polytetraf luoroethylene (PTFE) sleeves and is

secured with two thumbscrews to the main support rods (Fig. 14). A

supplementary lock bracket (Fig. 13) is used to retain the cone when in a vertical position.

The heater element is described in table 1. Power is controlled by an electronic temperature controller using three thermocouples wired in parallel,

located on the heater. Two different arrangements for thermocouples have been

tried—(1) thermocouples welded to the inside of the inner cone shell, and

(2) thermocouples looped around the heater element, with the bead touching the
element. The latter arrangement was found to give quicker and more stable

control over cone temperature, and therefore, irradiance when specimen flames

begin to impinge on the cone. The thermocouples should be of equal length and

0.16 to 0.32 mm diameter.

4.4 Hood System (Figures 15, 16)

The hood system is made from 24 gage stainless steel sheet for long life

but does not need to be Insulated because a heat balance is not made. A

heavy-duty, cast iron, high temperature blower is required since temperatures

up to about 340°C are obtained. The blower is equipped with a variable speed

DC motor. At full speed flows in excess of 32 A/s are possible. The normal

operating point provides for a flow of about 24 A/s. To provide proper mixing

at the oxygen sampling probe, the duct inlet is provided with a 1/2 D entrance

orifice (D = inside duct diameter). The ring probe is so located that at

least 6 D of straight run upstream and 2 D downstream are available for flow

straightening. The ring probe Itself (Fig. 6) is welded into a flange which

is then bolted between two flanges in the duct. This allows easy removal for

Inspection and cleaning. The hood system along with the blower and motor are mounted on rubber shock mounts to reduce the transfer of vibration to the rest

of the apparatus.

Velocity measurement is made by use of a sharp-edged orifice plate

located past the blower in the stack. The orifice is 1/2 D and is equipped

with ASME "flange taps" [14] for measuring differential pressure. Pressure is

measured with an electronic capacitative-type transducer. A thermocouple is

located in the center of the duct, ahead of the orifice, for determining gas

temperature and, thereby, density.

4.5 Specimen Mounting

(a) Horizontal Orientation (Figure 17)

Specimens are cut to 100 x 100 mm in size. Any thickness up to 50 mm can

be accommodated. It is emphasized, however, that thermally thin specimens

such as films, paints, etc., cannot be tested in any instrument in an

12
apparatus-independent way. These should be tested with the same substrate as will be used In their application. Adjustment for specimen thickness less

than 50 mm may be made by Inserting additional refractory pad layers, A

spacing of 25 mm Is maintained between the bottom of the cone and the top of

O the specimen, A minimum of 13 mm thickness of low-density (65 kg/m ) refractory fiber pad is used inside the specimen holder underneath the

specimen.

For rate of heat release testing specimen sides are protected with

aluminum foil—a single piece is cut to fit on the bottom and sides and be cut

flush with the top. This prevents spilling of any molten material and is

sufficient in most cases to restrict burning along the edges. Composite

specimens which involve a protective layer may have to be constructed by

extending the protective layer down along the edges to envelop the more

combustible layers within.

For ignltabllity testing there is sometimes the concern that the specimen

should not ignite by localized heating at the edges. To prevent this, an

outer frame (Fig, 18) is provided. This is placed on top of the specimen,

which should nonetheless be wrapped with alximinum foil to facilitate

cleaning. This frame weighs 415 g and can also serve to hold down any light

specimens that need to be slightly compressed,

(b) Vertical Orientation (Figure 19)

In the vertical orientation, the same size specimens, 100 x 100 mm, are

accommodated. If specimen thickness is less than 50 mm additional layers of

refractory padding are used to fill out the depth and permit the spring clip

to hold the specimen (Fig, 3), For 50 mm thick specimens a minimum of 13 mm

millboard backing is used. The specimen sides and back are wrapped in

aluminum foil. The vertical holder is provided with a small drip trough at

the bottom to collect and retain molten residue. It is, however,

inappropriate to test in the vertical orientation any materials which

predominantly melt and drip.

13
No additional provision needs to be made for Ignltablllty testing In the

vertical orientation since the specimen holder Is already equipped with a retaining edge around the front face.

4.6 Load Cell

A standard load cell Is used having a live load capacity of 500 g. The

particular Instrument selected for use here has, additionally, a mechanical

tare adjustment of 7 times Its live load range. This feature allowed rugged,

relatively heavy specimen holders to be designed without compromising mass

loss resolution. The mechanical tare adjustment Is set so that an empty

specimen holder reads just barely on-scale.

The load cell Is protected from heat by several techniques. Stainless

steel spacer rods exposed to convective cooling are used as supports for the

O specimen holder plate (Fig. 20). A 13 mm layer of medium density (735 kg/m"^)

refractory material is used as a spacer, and an additional layer of this same material Is used as a radiation shield for the top of the cell (Fig. 4).

Finally, In the horizontal orientation, where radiation to the cell is more

severe, an operating technique Is adopted of placing an empty specimen holder

on the unit in between tests and during warm-up to serve as a shield.

The load cell electronics is equipped with a direct digital readout

precise to 0.1 g.

4.7 Physical Adjustment of Apparatus

In the horizontal orientation the distance between the top of the

specimen and the bottom of the cone can be adjusted in the test apparatus. In

principle, raising the cone would decrease the available flux while increasing

flux uniformity. In practice, it was found that the more serious effect Is

due to fire plume wander. To capture the combustion products within the

confines of the cone it becomes advantageous to keep the cone as close as possible to the specimen. It was found that a spacing of about 25 mm gave the

best results. To maintain this constant distance the heater position is

adjusted when the specimen thickness is changed.

14
In the vertical orientation the cone position is adjusted so that the

center of the cone corresponds to the center of the specimen.

Similarly, an adjustment can be made to the distance between the top of

the specimen and the bottom of the exhaust hood. A large spacing can lead to

a failure to capture some of the combustion products, while a small spacing

can produce excessive impingement of the flames upon the exhaust duct. A

suitable spacing is Illustrated in figure 1.

4.8 Spark Ignition (Figures 21 and 22)

Earlier testing had Indicated that use of spark ignition was preferable

to a gas pilot since an electric spark does not Impose a localized heat flux

to the specimen surface and is easier to control and maintain. A commercial

furnace-type spark plug is used which has 75 mm long electrodes and an

approximately 27 mm insulator portion. This geometry permits the plug to be

mounted outside the fire plume while the spark gap is located directly over

the middle of the specimen. If a spark plug remains in the fire plume during

the whole combustion period, it was seen that large soot streamers accumulated

on the electrodes. Further, electrode life is reduced from prolonged

heating. Thus, a sliding withdrawal mechanism was designed. The cone is

equipped with two slotted brackets, one for horizontal and one for vertical

orientation. The spark plug (Table 1) is mounted on a PTFE slide which is

inserted into the appropriate bracket and lightly screwed down. The PTFE

slide also retains a 10 kS2 series resistor, inserted in the HV line to

minimize electromagnetic interference. The length of the bracket pemits the

spark plug to be fully withdrawn from the fire, using a handle, as Illustrated

in figures 2 and 3. The slide material was chosen to provide smooth travel,

withstand elevated temperatures and provide electrical insulation. Chassis

ground is not used, to minimize EMI. The spark power is supplied from a 10 kV

transformer. Spark plug height is 13 mm above the center of a horizontal

specimen and approximately 3 mm above the face plane of a vertical specimen.

Two operating modes are possible—continuous spark and intermittent spark.

The intermittent spark mode can be selected when a long time to ignition is

expected.

15
4,9 Heat Flux Gage

A 12.5 mm diameter commercial Garden gage of total heat flux type is used

to measure flux to the specimen. The water-cooled gage is mounted in a square

brass holder shown in figure 23. The holder fits into a square mounting

collar welded to the edge of the cone bottom (Figs. 2, 3), The same mounting

is used for both orientations. To change orientation the gage is merely

Inserted in the different orientation through the same square collar. The

heights have been set so that gage face corresponds to the center of the

specimen face plane in either orientation. The gage electrical output is read

on a mllllvoltmeter and is not electrically grounded to chassis. Hie gage is

removed prior to testing.

A calibration burner was constructed to permit easy calibration between

tests or as often as required. The burner is made of brass and has a square

screened opening, packed below with refractory fibers. For use, it is

Inserted in the same square collar as is used for the heat flux gage. The

burner orientation is chosen to discharge upwards, for either cone heater

orientation. Typical operating calibration Involves use of 99% pure methane

at a flow corresponding to 10 kW.

The equations necessary to Implement a practical realization of the

oxygen consumption principle have been presented by Parker [15], From the

basic theoretical relationship

an operating equation must be evolved. While it is not feasible to measure

oxygen mass-flow, oxygen volume-fraction is readily determinable.

Furthermore, depleted oxygen measurements must, perforce, be made on the

4,10 Calibration Burner (Figure 24)

5. ANALYSIS

5.1 Theory

16
exhaust stream. Since the mass flow in the exhaust stream Is different from

the mass flow In the Intake air stream by the amount of the gasified fuel

mass, an adjustment Is needed, based on stoichiometry. Finally, with most ©2

analyzers, water has to be trapped out of the sample stream. For simpler

analysis. It Is also convenient to trap out CO2

. Then from [15] It Is seen

that

^ ' \ir ®

[ 1 + (b - 1)X° ] - bX

2 ^2

where the denominator accounts for the difference In exhaust/Intake mass

flows. Also Included Is the assumption that CO, HCA, and other possible minor

combustion products are volumetrlcally negligible. Some typical values

Include:

Ah

Fuel b

[1

+ (b - Dxy (kJ/kg)

0

Methane (CH4 ) 1.5 1.105 12.54 X 103

Propane (C 3 H8 ) 1.4 1.084 12.78 X 103

Ethylene (C2 H4 ) 1.33 1.070 13.78 X 103

PMMA (nC5He02) 1.5 1.105 12.97 X 10 ^

setting X

°2

q =

= 0.2095 and b = 1.5 gives

[-0.2095 - Ah

32

28.97 “e

[

1 * 105 - 1.5 X^J

Huggett [9] recommends taking Ah^/r^ = 13.1 x 10 for arbitrary fuels; he also

tabulated specific values for a variety of fuels. Additional values are available in [16]. It can be seen for most common fuels Ah /r is within 5%

« c o ^ of 13.1 X 10 . Thus, in computations the value of Ah^/r^ Is set to 13.1 x 10

for unknown fuels, to 12.54 x 10^ for the calibration methane gas, and to its

actual value if the test material composition is known.

The mass flow rate through the system, m^, can be computed from the

orifice meter pressure drop and the temperature as

e

17
Then

(13.1 X lO^Kl.lO)

[0.2095 -

[1.105 - 1.5

and the rate of heat release per unit area is

- 4/A3

If a CO2 analyzer Is available then trapping of CO2 Is not necessary.

The rate of heat release expression, however, becomes more complex since CO

2 terms must be Included,

The equations above can be used directly by measuring the various

required variables. In practice, as Is discussed In section 8.5.1, the value

of C Is difficult to determine directly to better than ± 5%. Also, the value

of may not be exactly 0.2095, as assumed. Both of these potential sources

of error are eliminated If the apparatus Is, Instead, calibrated with a known

fixed methane burner flow and an effective C value computed from that.

5.2 Analysis of Data

5.2.1 Manual Analysis

Manual analysis of the data Is possible. This Is tedious for any but the

shortest tests. In applying the equation all the measurements should be at a consistent time. Temperature and pressure readings are nearly Instantaneous. Oxygen readings, however, are subject to a large delay due to transit

time through pipes and traps (see section 8.4). Thus oxygen data have to be

offset with respect to the others to obtain proper results.

18
5.2.2 Electric Analog Signal Conditioning

It Is possible to build an analog signal module to perform appropriate

mathematical operations; multiplication, division, subtraction, and square

roots. A circuit diagram for such a device Is shovm in figure 25. For a rigorous application of the equation, however, the oxygen signal would have to

be delayed with respect to the others. No simple, practical device is readily

available that could do this for the long delay times required. Without such

a delay, an error is introduced which is proportional to the rate of change

of q with time. Thus an analog device of this nature is limited in usefulness

to fairly steady heat release conditions.

5.2.3 Digital Data Collection

Most of the data were gathered on a digital data collection and reduction

system. Such a method can provide needed delay times of arbitrary length.

This arrangement also readily permits specimen mass loss rates to be computed

and an effective heat of combustion to be obtained as

Ah

c,eff

The operation of the system in general terms is as follows. Details are available in [17]. Data collection and analysis are done in separate

passes. In the data collection phase the following data channels are set up

to be scanned; real-time clock, oxygen concentration, AP, stack temperature,

load cell, and, optionally, CO2 and CO. A floppy disk is inserted and the

starting address selected. The data collection program first asks for test

title and heading. Next, either a single scan is recorded of values during a 10 kW gas burner burn, or else the previous calibration constant is entered.

The scan interval is set next (3 s for shorter tests, 6 s for longer).

Scanning is now started at regular intervals when the start button is

pressed. After the test is completed a final end-of-test trailer is written

on the disk and the procedure can be re-initiated for the next test.

In the data analysis phase, the proper test data are first read in from

the floppy disk into memory. The time channel is checked for consistency and

19
real-time values converted to teat time. At this point also an optional value

of Ah /r other than the standard one may be entered. Next, the gas analyzer CO o ^ data are all time-shifted by the delay time value (see section 8.4), Heat

release rate values are next determined and q" computed. These computations

are started after the last negative q" value (small noise component) is

passed. Also computed is the running total

q” » Z q" At

i

^

Next the mass loss curve is examined for outliers; the m curve is obtained

P

by making a least squares fit of a slope to the mass curve, taken over 10

points each side of the desired one. The heat of combustion is then computed

over a running 15 s time-base. This curve is again slightly smoothed by the use of a 3-polnt average. Air flow rate and CO2 and CO concentrations are simply determined from the appropriate input readings. Output is available in

three forms. (1) Tabular output of q", m", Ah^, m^ and CO2 and CO, at

appropriate time Intervals, (2) Plots of q", Ah^ or other variables of

Interest. (3) Tape file output of reduced data, to be used for further

plotting or analysis.

6. TEST PROCEDURE

Initially, the following calibrations were performed: the oxygen meter

response time setting was adjusted (not all oxygen meters have this

control). This control was adjusted by using the calibration burner and

observing the response when the gas flow is abruptly shut off. The response

speed was increased until an ©2 response undershoot was just barely

observed. Next, the heater temperature controller was calibrated against

Gardon gage readings, separately for the two orientations. Finally, a 10 kW

CH^ flow calibration was performed.

During routine testing the following procedure was used. The

thermoelectric trap was turned on about 30 minutes before test time. Any

accumulated water was drained out of the separation chamber. The drying and

CO2 removal traps were checked and replaced if needed. A new soot filter was inserted. The oxygen meter was spanned using room air and zeroed using N

2

»

20
An orientation was selected and the cone set for that orientation. For

horizontal specimens the height was set to be appropriate for specimen

thickness. The heater was turned on and set to a control temperature

corresponding to the desired flux. For horizontal testing, a blank specimen

holder was placed on the holder plate. The exhaust blower was turned on. After the control temperature was reached, a further 5 minutes was allowed for

full equilibration. The ignition spark plug was checked for condition and

cleaned if dirty. The load cell zero control was adjusted. The sample gas

pump was started.

The data collection system was initialized. Calibration values for 10 kW

CH^ flow were measured, if not previously done on that day. The specimen,

wrapped in aluminum foil, was placed in a cold holder. The empty holder was removed (horizontal only) and the specimen in its holder placed on the holder

plate. The data collection system was started. The last two operations could

be performed in well under 2 s. The spark plug holder was pushed in.

Sparking was started immediately for unknown specimens. With specimens whose

ignition times are roughly known, however, start of spark coiild be

appropriately delayed. Upon ignition, the spark plug was withdrawn. For

ignitabillty testing the time to Ignition was determined with a stopwatch to

0.2 s. Otherwise, a record of ignition time was available on the data

collection system, with a resolution of 1 scan (normally 3 s). After flaming

ceased, the specimen holder was removed. For horizontal testing the foil and

residue were discarded and the empty holder returned to position. If a specimen did not Ignite within 10 minutes, it was also removed and discarded.

7. TEST LIMITATIONS

In addition to requirements for specimen size and shape, the following

limitations emerged:

1) The specimen must not swell sufficiently to interfere with spark plug

operation or with the heater. This effectively limits swelling to 10 mm for

horizontal specimens and 20 mm for vertical ones. The use of the ignitabillty

frame for horizontal specimens may be beneficial in limiting swelling.

21
2) Specimens must not show explosive spalling or delamination. When this occurs Ignitablllty is difficult to determine and the concept of a rate of

heat release per unit area becomes dubious.

3) Any amount of melting can be tolerated for horizontal specimens,

including even liquid specimens. For the vertical orientation, however, only

very limited melting can be acceptable, i.e., only what can be contained in

the melt trough. It should be noted that with some materials melting tendency

is highly dependent on the irradiance.

8. PERFORMANCE

8.1 General Observations

One general requirement is that the hood system be able to collect all of

the combustion products. This can be visually verified—no spillage over the

side nor flow back-up is observed. Cross-draft conditions may result in a significant amount of plume wander. While an environmental enclosure, per se,

is not required, it is appropriate to only operate in laboratory spaces

without significant drafts or pressure fluctuations.

For realistic representation of the burning process, the apparatus should

not distort the pool-plume or wall-plume which would be exhibited by a freeburning specimen. In the horizontal configuration the cone heater is quite

close to the fire plume. Yet observations under a wide range of conditions do

not show any significant deflection of plume streamlines. The plxime can pass

out the cone top opening since it naturally necks down above the base.

Further, fresh air is entrained alongside the plume; thus the stream flowing

by the heater element has an outer layer of cooler air. Since the plume does

show turbulence, occasionally small flame packets will impinge upon the cone

top plate or bottom plate. These do not represent a significant portion of

-the total flow.

22
8,2 Exhaust System

For the orifice plate used, a relation was derived from formulas given in

[14] as

e

where C is a constant in the vicinity of 0.03, AP is measured in Pascals and

Tg in Kelvins. For an orifice to the exact ASME specifications, C = 0.031,

The orifice plate installation used does not fully conform to the intent of

the ASME calibrations, because sufficient flow straightening would be

difficult to provide in the available space. As a result, a calibration was made by using CO2 as a tracer gas. Under cold conditions CO

2 was metered into

the hood from a bottled supply. Gas readings were taken using the existing

gas instrumentation. The calibration showed C = 0,028 with an uncertainty of

± 5%, This is the value that is then used in checking the relative instrument

calibration results. It is further expected that this value can change

slightly with time as soot builds up on the orifice edge. The instrument

accuracy will not , however, change due to that since the operating calibration

procedure (section 6) takes into account any change in the value of C,

The relationship between mass and volume flows was explored at two

temperature conditions (ambient and Q

= 10 kW) and at 6 flow settings

(table 2). This shows that, as expected for fans in general, volume flow

rates remain nearly constant, even when temperatures and mass flows vary

substantially. It is also interesting to compute convective heat

flow. Only 20 to 41% of the total heat released was seen to be collected as convective duct flow. The rest is radiated out directly from the fire plume,

radiated from the exhaust system and convected away from the duct exterior.

Since this is an oxygen-consumption instrvnnent, no effort needs to be made to

increase this fraction.

For optimum response with a minimum of noise, the time constants on the

pressure transducer and stack thermocouple have to be adjusted to be slightly

faster than the ©2 meter. Time constants of 1 to 3 s were considered

appropriate. A stack temperature traverse is indicated in table 3.

23
8.3 Cone Heater

Table 4 shows the relationship between heater temperatures and measured

fluxes to a specimen. It Is noteworthy that horizontal and vertical

calibrations are nearly Identical, showing the limited role of convective

fluxes. A view factor, F, as such cannot be computed from basic geometry

since the cone edges do not drop abruptly from uniformly high temperature to

ambient, although If such a simplification Is made, F 0.79 Is computed. It

is readily possible, however, to compute

The values In table 4 show this combined emlsslvlty view factor product to be

around 0.85, which Includes also the convective contribution. The emlsslvlty

will vary slowly with time due to changes In the heater surface condition,

thus necessitating periodic calibration. Table 5 shows additional

calculations of the convective flux component, computed from measurements with

two Gardon gages, one of which had a window to eliminate convective heating.

In the horizontal orientation the convective flux Is negligibly small, but In

the vertical orientation small values, only mildly dependent on the total

flux, may be measured.

Incident flux uniformity Is Indicated In table 6. Measurements were

taken with a small Gardon gage on a nine-point grid (32 mm spacing). In the

horizontal orientation peak deviations are typically ±2%. In the vertical

orientations the deviations vary according to flux level. While the radiative

component Is roughly constant, the convective flux Increases towards the

top. Under all conditions, the center flux Is nearly Identical to the average

flux, although for more precise computation. It can be taken as 1.02 times the

average.

8.4 Oxygen Analyzer Response

The measuring system does not respond Instantaneously to oxygen level

changes due to two separate causes. (1) A finite time Is required to move the

sample through the lines, traps, pump, and Into the oxygen analyzer; and

24
(2) the oxygen analyzer Itself does not respond Instantaneously. Several

adjustments are available that affect the response time. The gas sampling

train (Fig. 7) uses a bypass waste arrangement with an adjustable bypass

relief pressure. This was set so that 0.25 il/mln flows through the oxygen

analyzer, 0.75 Jl/mln flows through the (optional) CO2 and CO analyzers, while

3.3 il/mln Is bypassed. The absolute back-pressure regulator was set for 1.05

standard atmospheres which Is substantially higher than recorded barometric

highs In our location.

Another adjustment Is an electrical O

2 analyzer response time setting,

available on the make of analyzer used here. This can be adjusted to Its

optimum value by establishing a steady burner fire and then abruptly shutting

off the gas flow. The oxygen response control Is speeded up until a barely

perceptible overshoot Is obtained during return to baseline.

It was found that the oxygen response to a unit step Input could be

described as

-3(t - t.)

s(t) =l-e t-t, >0 a

= 0 t - 1 , < 0

a —

Here t^ Is the transit delay time and 3 Is the time constant for the ©2

analyzer. The constants found were t^ = 30 s and 3 = 0.32 s . These values

were determined upon shutting burner flow off. Turn-on behavior cannot be

measured as readily since a pressure surge Is difficult to avoid at

Ignition. The constants did not vary with q.

8.5 Calibrations with Pure Gases

To be assured of correct operation, the apparatus must show linearity

when presented with calibration gases over a wide range of flows. Also, the

determination of the heat release rate should not be dependent on the relative

radiance of the flames or on stoichiometric requirements. Methane was chosen

for all the basic calibrations, supplemented by propane and ethylene as suitably different gases (section 8.5.2). Table 7 shows the linearity and

noise over a range of 1 - 12 kW. The measured response was determined ten

25
different times for each flow rate using the data acquisition/reduction

routines. The linearity is within 5% over the entire range and within 2% over

5 to 12 kW, The noise is at about ± 2% throughout.

The burner used (Fig. 24) necessarily has a fixed height and fixed throat

size. It was desired to determine whether burner size and location have a significant effect. For this purpose an additional burner was constructed

with an opening of 100 x 100 mm. Measurements were made at nine different

heights. Table 8 shows that there is no detectable Influence of burner

height, except when extremely close to the hood. The average difference in

response between the standard burner and the large burner is 3%,

The blower speed and the exhaust flow rate would be expected to have some effect on instrument performance. At very slow speeds it would be expected

that Incomplete combustion product collection would result, while at

excessively high speeds the change in oxygen concentration would be too small

for adequate resolution. Table 9 shows the results of measurements at two

different heat release levels, corresponding to slow burning and fast burning

specimens. Flows were varied over the total range available. The results

O show a very shallow minimum for noise around 0.018 to 0,024 m"^/s. The

conclusion, however, is that noise does not Indicate the selection of a specific exhaust flow: less than ± 2% noise can be obtained at any available

flow rate.

8,5,1 Oxygen/ Carbon Dioxide Balance

One available check on the performance of the instrument is the O

2 /CO2

balance. For complete combustion of a given fuel there is a fixed ratio of

oxygen consumed to carbon dioxide generated. For example, for methane

CH, + 20„ -* CO. + 2H„0

4 2 2 2

In other words, for each mole of CO

2 generated there should be two moles of ©2

consumed

2 .

26
where n denotes molar flows. The ratio is determinable from measured

quantities and as

2 ^2

The CO2 readings here refer to adjusted values, with the small ambient

concentration (~0.04%) subtracted out. Also, the O

2 readings here refer to a sample line where CO2 (and H2 O) has been scrubbed out. Table 10 shows the

results of these calibrations. The calculated values are quite close to the

expected 2.0 value.

The ratio of measured to supplied q values here refers not to a relative

gas burner calibration, but rather to a value calculated from using the mass

flow calibration as C = 0.028. The close agreement, to within less than 3%

difference, of the measured and the supplied values verifies that the tracer

gas method agrees well with the individual combustion calibrations.

In the same vein, calculations can be made of the absolute flows of O

2 and CO2

. At a given flow of methane

m- = Q/Ah

t c

Also, from stoichiometry, for complete combustion

is the flow of CO2 in the exhaust, based on measured m^ and gas analyzer

readings. Similarly, the change in oxygen flow in the exhaust is

The expected flows, meanwhile, are

27
• 6A • . # ^0^ T6 "f

‘ "f

The data in table 10 verify that the measured value of C - 0.028 Is to within

about 2% of the correct value.

From the above findings it can be seen that the absolute calibration has

about a 5% uncertainty. Comparison with the data in table 7 indicates,

however, that linearity to within 2% over a large portion of the range is

obtainable when a gas burner supplying methane at a q 10 kW level is used.

Thus, for operational purposes the gas burner calibration is to be preferred.

8.5.2 Other Gases

For a heat release apparatus to be satisfactory, the proper heat release

should be measured for any fuel. For gases, two additional fuels were chosen,

intended to exemplify gases more prone to sooting—propane and ethylene.

Table 11 shows the results. An average recovery of 1.00 for propane and 0.97

for ethylene is seen. It is reasonable to suppose that for ethylene the bulk

of the 3% not recovered goes to soot.

8.6 Heat Release Measurements on Solid Materials

Illustrative measurements are shown on two materials; black

polymethylmethacrylate (PMMA) and red oak. PMMA was chosen as exemplifying a thermoplastic material with minimum tendency to drip, while the red oak is

characteristic of woods and char-forming materials in general. The red oak

had a density of 778 kg/m^, conditioned at 50% R.H. and 23°C while the PMMA

O had a density of 1184 kg/m . Test thicknesses were 20 mm for red oak and

25 mm for PMMA. The PMMA used was black in order to minimize dlathermanous

heating effects.

Data for rate of heat release and effective heat of combustion under an irradlance of 50 kW/m^ are shown in figures 26 and 27. For PMMA the effective

heat of combustion should be constant. The data show this constancy except at

the start and the end where the m values change very fast. Under those

conditions an instantaneous Ah^ is not readily computable. The average Ah^

28
3 obtained from sixteen tests is (24,2 ± 1.1) x 10 kJ/kg. The effective net

heat of combustion fraction is 24.2/24.88 * 0,97. For red oak a constant heat

of combustion is not expected since there is not a unique degradation mecha3 nism. During steady burning Ah = 10 x 10 kJ/kg, but there is a peak at the

3 ^ 3 start of about 12 x 10 kJ/kg and second peak near the end 15 x 10 kJ/kg.

Negligible PMMA weight remains at the end of the test, while up to 20% of the

red oak mass remains when the fire goes out.

Figures 28 and 29 show the heat release rates under different Irradiance

conditions, 25, 50, and 75 kW/m^. Horizontal PMMA measurements (Fig. 28) show

a pre-ignition period, then a rapid rise, followed by slower equilibration to

nearly a steady state. Towards the end of the test the heat wave reaches the

back face and the burning rate increases. Just prior to burnout there is

usually a rapid increase in q". This can be physically seen to be due to

formation of globules—the material burns over an effective area greater than

the 100 X 100 mm projected area. Comparison of horizontal and vertical

conditions shows a significantly higher burning rate at the horizontal

orientation, amounting to about 80 kW/m^. This is not unexpected since the

heat flux to the fuel from the flames themselves is much higher in the

horizontal configuration than in the vertical.

The red oak specimens show a very different behavior (Figs. 30 and 31).

The horizontal q" values are again higher than the vertical. The second peak

arises because the specimen at that point is heated through to the rear face

and no longer behaves as if thermally thick. The bulk of the heat is released

during this later heating, after the first peak is passed. At the end of such

a test it can be observed that the original volume of the sample is almost

maintained in the form of the char structure, but that the weight associated

with it is very small.

Test repeatability with solid materials burning is somewhat harder to assess than with gases. Resolution with gases is limited due to turbulence in

the burning plume itself and due to noise in the measuring instrumentation.

With solids, however, a third factor is introduced, namely variability in the

pyrolysis rate. Real solids, even if subjected to uniform external heating

and flow fields, pyrolyze somewhat randomly and burn non-uniformly. Indeed,

29
one can sometimes see solid or molten particles being ejected out away from

the surface. Thus, there is a certain limit to the measurement precision due

to burning behavior irregularities. To Illustrate the response, however,

O figure 32 shows three tests on 25 mm PMMA samples heated at 25 kW/m . To assess repeatability numerically, it is most appropriate to consider the total

heat released during the entire combustion. When normalized by the specimen

mass this becomes simply the time-average heat of combustion. The figure

3

cited above is 24. 2 ± 1.1 x 10 kJ/kg, or a variation of ± 5%.

9. CONCLUSIONS

A bench-scale apparatus has been developed for measuring rates of heat

release of flat materials by use of the oxygen consumption principle. The

exclusive use of the oxygen consumption principle has allowed an efficient,

rugged, accurate Instrument to be designed which is neither physically large nor expensive to construct. The instrument shows a linearity generally to

within 5% and over the major operating region to within 2%. Noise is also

within 2%. Fuels of known combustion characteristics show recovery of known

heat of combustion to within the noise level of the apparatus. Proper control

of external heating, a problem with some heat release rate apparatuses has

been designed into the fundamental geometry. Mass loss and effective heat of

combustion measurements have been incorporated as part of the operation.

10. ACKNOWLEDGMENTS

David Swanson prepared shop drawings and constructed major portions of

the apparatus. Robin Breese conducted tests necessary for the development of

the apparatus. William Parker collaborated in the project at several

stages. Emil Braun wrote the data collection and analysis routines. The

Consumer Product Safety Commission provided partial support through its

Plastics Flammability Research Project.

30
11. REFERENCES

[1] V. Babrauskas, et al., Upholstered Furniture Heat Release Rates Measured

with a Furniture Calorimeter, Nat. Bur. Stand. (U.S.), NBSIR 82-

(1982).

[2] V. Babrauskas, Combustion of Mattresses Exposed to Flaming Ignition

Sources, Part II. Bench-Scale Tests and Recommended Standard Test, Nat.

Bur. Stand. (U.S.), NBSIR 80-2186 (1981).

[3] N. J. Thompson, E. W. Cousins, The FM Construction Materials

Calorimeter. NFPA Q. _51» 186-192 (Jan. 1959).

[4] W. J. Parker, M. E. Long, Development of a Heat Release Rate Calorimeter

at NBS, pp. 135-151 in Ignition, Heat Release and Noncombustibility of

Materials (STP 502). American Society for Testing and Materials,

Philadelphia (1972).

[5] J. P. Tordella, W. H. Twilley, Development of a Calorimeter for

Simultaneously Measuring Heat and Mass Loss, Nat. Bur. Stand. (U.S.), to

be published.

[6] J. R. Lawson, private communication.

[7] Proposed Test Method for Heat and Visible Smoke Release Rates for

Materials. American Society for Testing and Materials, Annual Book of

Standards, Part 18 (1981).

[8] V. Babrauskas, Performance of the OSU Rate of Heat Release Apparatus

Using PMMA and Gaseous Fuels, Fire Safety J. 5^, 9-20(1982).

[9] C. Huggett, Estimation of Rate of Heat Release by Means of Oxygen

Consumption Measurements. Fire and Materials. 61-65 (1980).

[10] G. Svensson, B. Dstman, Rate of Heat Release for Building Materials by

Oxygen Consumption. STFI Meddelande Serie A, nr. 761 (May 1982).

[11] G. S. Holmstedt, Rate of Heat Release Measurements with the Swedish Box

Test (SP-RAPP 1981:30). Statens Provningsanstalt , Boras, Sweden (1981).

[12] D. Bluhme, Nordtest Project 115-77, Part 2: ISO RHR Test Apparatus with

Oxygen Consumption Technique, Dansk Institut for Prj6vning og Justering,

Copenhagen (1982).

[13] Technical report ISO/TR 5657-1982 (E) , Fire Test-Reaction to Fire - Ignitability of Building Materials, International Organization for

Standardization (July 1982).

[14] Fluid Meters, Their Theory and Application. Report of ASME Research

Committee on Fluid Meters. American Society of Mechanical Engineers,

New York ( 1971)

.

[15] W. J. Parker, Calculations of the Heat Release Rate by Oxygen

Consumption for Various Applications, Nat. Bur. Stand. (U.S.),

NBSIR 81-2427 (1982).

31
[16] Fire Protection Handbook, tables in Chapter 4, National Fire Protection

Association, Quincy, MA (1981).

[17] Data Collection and Analysis Programs, written for Tektronix 4052. Copy

available from author.

[18] D. L. Sensenlg, An Oxygen Consumption Technique for Determining the

Contribution of Interior Nall Finishes to Room Fires, Nat. Bur. Stand.

(U.S.), Tech. Note 1128 (1980).

32
Table 1. List of Major Parts*

Mechanical

Heater element — Welman Co. "Calrod" TY 3272 (8 mm dla. , 3.38 m long 5000 W,

240 V max)

Temperature controller — Barber Colman 5624-12-035-330-0-00

Power controller — Robicon 313-223

Fan — Westinghouse cast Iron, high temperature type, model 504 (BAY9657-1)

Variable speed motor with controller (3/4 HP) — Dayton 2Z846

Heat flux gauge — Medtherm Co. GTW-7-32-485A

Pressure transducer - MKS Instruments 223AH-A-10

Spark plug — Eclipse Combustion 12568

Ignition transformer — Dongan A10-LA2

Load cell — Automatic Timing and Controls 6005C06EIXX (with external

demodulator)

Oxygen Sampling

Soot filter — Nuclepore 490605 (support only)

Pump — Thomas Industries 908CA18

Waste regulator — Norgren V06-221-NNAA

Supplemental drier — W. A. Hammond Co. "Drlerite"

CO 2 removal media — A. H. Thomas Co. "Ascarlte"

Flow regulator — Moore 63BDL

Final filter — Nupro SS-4FW-7

Oxygen analyzer — Beckman 755 (0-25% range)

Absolute back pressure regulator — Moore 43R

Thermoelectric cooling modules — Cambion Thermionic 806-1001-01

Certain commercial equipment, instruments, and materials are identified in

this publication in order to adequately specify the experimental procedure.

Such identification does not imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best for the purpose.

33
Table 2

Exhaust System Flows

Q

- 10 kW CH,

Fan

Setting

(Arbitrary)

AP

(Pa)

•V

(m^/s)

.m

e (kg/s)

T

(K)

AP

(Pa)

•V

(m^/s)

.m

e (kg/s)

^con

(kw:

50 90 0.0146 0.0173 503 48 0.0138 0.0097 2.00

60 156 0.0193 0.0229 498 80 0.0178 0.0126 2.53

70 241 0.0239 0.0284 496 124 0.0222 0.0158 3.14

80 334 0.0282 0.0335 481 . 194 0.0274 0.0201 3.70

90 428 0.0319 0.0379 453 249 0.0300 0.0234 3.65

100 448 0.0327 0.0388 468 274 0.0320 0.0241 4.12

T- - 297 K 00

Q

- 1.0 (T - TJm ^conv e

34
Table 3

Stack temperature map

Fraction of

radius

Temperature rise

(K)

0. ([center) 155

0.1 155

0.2 156

0.3 156

0.4 156

0.5 156

0.6 155

0.7 150

0.8 148

0.9 117

1.0 (edge) 86

Exhaust flow = 0 .023 m^/s

Me thane supply = 5 kW equivalent

35
Table 4

Radiation from Cone Heater

Horizontal Vertical

Irradlance

(kW/m^)

Heater

Temp.

(°C) (kW/m^)

eF

(-)

Heater

Temp.

(°c) (kW/m^)

eF

(--)

10 422 12.8 0.78 424 12.9 0.77

20 547 25.2 0.79 546 25.1 0.79

25 592 31.3 0.80 590 31.0 0.81

30 628 37.0 0.81 626 36.6 0.82

40 689 48.1 0.83 689 48.1 0.83

50 744 60.2 0.83 742 59.7 0.84

60 783 70.0 0.86 786 70.9 0.85

70 825 82.0 0.85 825 82.0 0.85

75 841 86.9 0.86 843 87.5 0.86

80 860 93.0 0.86 861 93.3 0.86

90 892 104.0 0.87 890 103.3 0.87

100 919 114.0 0.88 918 113.6 0.88

no 943 123.5 0.89 946 124.8 0.88
Table 5

Effect of Convective Fluxes

Horizontal Orientation

No observable difference from 20 to 110 kW/m^ (± 3%)

Vertical Orientation

o Measured fluxes (kW/m^)

Total Radiant Convective

(By Different

10.0 8.1 1.9

20.0 17.6 2.4

29.9 26.7 3.2

40.0 34.7 5.3

49.7 43.8 5.9

59.8 54.9 4.9

70.2 64.7 5.5

80.0 72.2 7.8

90.0 82.7 7.3

100.2 91.8 8.4

110.5 102.6 7.9

37
Table 6

Flux Deviation Summary

Nine-point grid, spacing at 32 mm

Ratio

Orientation Irradiance center Peak deviations from

(kW/m^)

average

horizontal 25 1.01 -2%, +1%

50 1.03 -2%, +1%

75 1.00 -3%, +2%

vertical 25 1.02 -11%, +8%

50 1.02 -8%, +7%

75 1.03 -6%, +4%

avg.
Table 7

Linearity of Response

CH4 supply Measured

rate^^^ response^^^ Fraction Noise

(kW) (kW)

0.98 0.94 0.96 ±2.4%

2.01 1.82 0.95 ±1.7%

2.99 2.94 0.98 ±1.9%

3.95 3.77 0.95 ±2 . 6 %

4.99 5.01 1.00 ±0.5%

5.97 5.88 0.98 ±1 . 1 %

6.94 6.89 0.99 ±1 . 1 %

8.04 7.89 0.98 ±1.7%

8.96 8.76 0.98 ±0.7%

9.93 9.93 1.00 ±2 . 0%

10.93 10.68 0.98 ±1.4%

11.94 11.66 0.98 ±1 . 0%

Exhaust flow » 0.022 m^/s

^^^Calculated using net heat of combustion of 50.0 MJ/kg

^^^Wlth apparatus calibrated at 10 kW

39
Table 8

Burner Height Effect for 10 kW Methane Flow

Burner head

distance below

Calculated Heat Release Rate (kW)

100 X 100 mm

lip of hood

(mm)

Standard burner burner Ratio

12 9.75 10.18 1.012

38 9.87 10.29 1.043

64 10.05 10.30 1.025

89 10.08 10.40 1.032

114 10.12 10.39 1.027

140 10.05 10.41 1.036

165 10.06 10.36 1.030

191 10.13 10.42 1.029

216 10.09 10.34 1.025

40
Table 9

Effect of Air Flow Rate on Noise

Methane Fan

•V Measured Q

Supply Setting

(kW) (Arbitrary) (m^/s) (kW)

2 50 0.015 2.00 ± 0.04

2 60 0.019 1.98 ± 0.02

2 70 0.024 1.98 ± 0.008

2 80 0.028 2.00 ± 0.03

2 90 0.032 1.99 ± 0.02

2 100 0.033 1.97 ± 0.01

10 50 0.014. 9.86 ± 0.14

10 60 0.018 9.82 ± 0.08

10 70 0.022 10.00 ± 0.17

10 80 0.027 10.14 ± 0.14

10 90 0.030 10.13 ± 0.15

10 100 0.032 10.14 ± 0.19

41
Oxygen/

Carbon

Dioxide

Balance

o m

I •—I I—

(

I • • CM CM CS CM

CM OS 00 CO •o Os 0 • 0) • • • CO *j CM CM cd 0

at at X a.XM

CM O

<

O ^ CM CM o LO O O o O o o

•K CM OO w

O 0> CM CO 00 CM Q CO•••••• NO <r CM O O -H CM CO >»• |8

CM m m CM CO CO

ON 0> ON O' o o • ••••• O O O O ^ -H

.o^

Cu

a.d

(0 • O'

I o

I •

CM ^ -H On CO O O O ON o • • * • • —I i—t »—( o <—

t

(0«

at 3

jKi NOstONOvO.4-

00 NO ^ CO -H uo O • • • • • • —I CO vO 00 O CM

•O

<U

i-i 3

00

cd

CO

'

oo

00 CM ON CM Sf ON ON NO CM

•-H CM nO OO

• • • • • O O o o o

CMo

•o

<u NO o CM m CO

00 no ^ CM CM o • • • • • • <—t CO NO 00 O CM

03

(VP

3

CO

CO

ON ON o •—t t—

(

ON 00 o CM r«^ ON O '—I CO I/O NO

• ••••• o o o o o o

I

CO CO NO >-H 00 ON O ON r>^ ^ ON I/O • •••••• O O ON 00 NO no CO

CM CM •—I •—t I I

•s 00

CM ON o 00 CM CM I-H

I CM i-H n-H t—( I—I ^

I O• o••••• O o O o

o o o o o p

(U .U CM U 0

V <

s* •e

'3

at 4J CM'

CJ 0 . V

s* •s ,

00

00

00 00 00 NO o CM 00 00 cco CM

<—( CM NO 00 On

• ••••• o o o o o o

CM 00 NO n-h 00 O On CO n^ vo NO

•—I •—( CO '3‘ no NO

• ••••• o o o o o o

cd < eu

I O vj- ^ CO •-(

I CM 00 'O’ CM <—• O I CM f-H 1-^ r-H

H u: no NO 00 i-H CO CM 00 no CO no no no nO

>N iHa

o.3

CO

CM CM 'd- no CO CO C' CM NO o <t O O ^ I-M CM CM

• ••••• 000000

CO0> H

O n-h CM CO >d' no NO

u

CO

(V H

•—t CM CO 'd- no NO

*

42

Baseline

values

subtracted

out
Table 11

Rate of Heat Release for Additional Gases

Propane (Ah^/r^ 12.78 x 10^ kJ/kg)

Supplied

(kW)

Measured

(kW)

Meas

Suppl

3.13 3.20 1.02

5.43 5.50 1.01

6.96 6.95 1.00

8.7 8.8 1.01

10.2 10.1 0.99

Ethylene (Ah /r - 13.78 x

c o 10^ kJ/kg)

Supplied

(kW)

Measured

(kW)

Meas

Suppl

3.38 3.30 0.98

5.97 5.75 0.96

7.83 7.55 0.96

9.4 9.2 0.98

11.8 11.5 0.97

43
fIII

44

Figure

1.

Over-all

view

of

apparatus
VERTKM.

POSITION

LKK

BRACKET

45

Figure

2.

Exploded

view,

horizontal

orientation
VERTICAL

rosmoN

46

Exploded

view,

vertical

orientation
I -I

i

47

Figure

4.

Elevation

and

side

views,

horizontal

orientation
48

Figure

5.

Elevation

and

side

views,

vertical

orientation
fI sE

49

Figure

6.

Gas

sampling

ring

probe
WASTE

FLOW

CONTROLLER

PARAMAGNETIC OXYGEN ANALYZER

7/um RLTER-f -aP

ROTAMrER

OUTFLOW

ABSOLUTE

BACK PRESSURE REGULATOR

Figure 7. Gas analyzer Instrumentation

50
\

FROM COID TRIP

All dimensions in mm

(except where noted]

SEPARATION CHAMBER

Figure 8. Thermoelectric cold trap

51
</,

20

TAPERED

PIPE

TAP

52 '

MILIPORE

STAINLESS

STEEL

FILTER

SUPPORT

DISK

90

mm

SUE.

No.

YY2009064
53
I

54

WELD

BRACKETS

TO

PLATE

IN

POSITIONS

INDICATED
It

HOLE

55

Figure

12.

Mandrel

for

winding

cone

heater
1/8”

STAINLESS

PLATE
10-24

TAPPED

HOLE.

2

PLACES

57

Figure

14.

Base

plate

and

support

pieces
a>

b*o>

O)oX

09

4J

CO

CO

4J

CO3

CO 'SWm

cu3

bO

•H

Pl4

58
MATERIAl:

STAINLESS

STEEL,

24ga. S

59

Figure

16.

Exhaust

duct
60

Figure

17.

Horizontal

specimen

holder
4

111.3

I

/

9 0

'

n

L j

ALL DIMENSIONS IN mm

54

i U L

I

I 111.3 — H

INSIDE DIMENSION

|

( Stainless Steel, 14ga.]

Figure 18. Retainer frame for horizontal ignitability testing

61
SECTION

B-B

62

Figure

19.

Vertical

specimen

holder
5/32”

HOLE, 4

PLACES,

COUNTERSUNK

FOR

#10

FLAT

HEAD

SCREW

a>

a»

CO(U

CJ

(U •H

iH

60G 3

•H (1) ^ CO O CO

CO cO

4-1 4J >-l CO O

<4-1 I—I

tH CM

CUO • 60 T3 *H

cO LU o

h-3

O

oeo

4-1 CO • >-l O 0) a CM «4-4 < CU LiJ CU >-4 M — 3

60 < •H 1 Ll.

[*4 4

o>

o> CO

CO

CO

a>

ra CO

X CM

CO

63
1^-14

TAPERED

PIPE

TAP

[TAP

FROM

THIS

SIDE)

64

Figure

21.

Spark

plug

carrier
|S^

Figure

22.

Power

supply

for

spark

plug
f

13.46

t

17.73

I

MATERIAL: BRASS

4 PLACES

ALL DIMENSIONS IN mm (EXCEPT WHERE NOTED)

Figure 23. Holder for Gardon gage

66
28.83

<146

HOLE,

COUNTERSUNK

FORI/

#0

FLAT

HEAD

SCREW, 2

PLACES &Es

67

Figure

24.

Calibration

Burner
ALL CAPACITORS POLYPROPYLfNE FILM

Figure 25. Analog signal module

68
TIME (s)

60

50

40

30

20

10

0

Figure 26. PMMA (horizontal) rate of heat release and

heat of comubstion at 50 kW/m^ irradiance

69

EFFECTIVE

HEAT

OF

COMBUSTION

(MJ/kg)
±

200 400 600 800 1000 1200 1400 1600 1800

TIME (si

Figure 27. Red oak (horizontal) rate of beat release

heat of combustion at 50 kW/m irradiance

— 60

- 50

- 40

- 30

- 20

- 10

0

2000

and

70

EFFECTIVE

HEAT

OF

COMBUSTION

(MJ/kg)
Figure 28. PMMA (horizontal) rate of heat release at

several irradiances

71

IPMMA-H-F-25]
TIME (s)

Figure 29. PMMA (vertical) rate of heat release at

several irradiances

72

IPMMA-V-F-251
TIME (sj

Figure 30. Red oak (horizontal rate of heat release

at several irradiances

73

(RO-H-F-201
Figure 31. Red oak (vertical) rate of heat release

at several irradiances

74

(RO-V-F-20)
TIME |s)

Figure 32. PMMA (horizontal) variations during three runs

at 25 kW/m^ irradiance

75

(PMMA-H-F-251
NBS-H4A (REV. 2><0) U.S. OEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2611

2. P«rfonnlnt Organ. Report No 3. Publication Date November 1982

4. TITLE AND SUBTITLE

Development of the Cone Calorimeter — A Bench-Scale Heat Release Rate Apparatus Based on Oxygen Consiimption

5. AUTHOR(S)

Vytenis Babrauskas

6. PERFORMING ORGANIZATION (If joint or other than NBS, see Instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

I. Type of Report & Period Covereo

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. Clty.Jiate. ZIP)

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, Is attached.

f

calorimeter utilizing the oxygen consumption principle has been developed for use in fire testing and research. Specimens may be of uniform or composite construction and may be tested in a horizontal, face-up . orientatxon, or, for ones which do not melt, also vertically oriented. An external rradxance of zero to over 100 kW/m*^ may be imposed by means of a temperature- controlled radiant heater. The rate of heat release is determined by measuring combustxon product gas glow and oxygen depletion, while the mass loss is simultaneously recorded directly. The instrument has been designed to be capable of higher accuracy than existing instruments and yet to be simple to operate and moderate in construction cost. The instrument is termed a "cone calorimeter" because of the geometric arrangement of the electric heater.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

calorimeters, combustion, fire tests, heat of combustion, heat release rate^ ignition, oxygen consumption, plastics ’

13. AVAILABILITY

[It] Unlimited

I I For Official Distribution. Do Not Release to NTIS

I I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS). Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

84

15. Price

$10.50

uscomm-dc e043-pao

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