gr

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) [ j on October 9, 2015.

TWv-..- — NATIONAL BUREAU OF STANDARDS REPORT

7306

IGNITION OF SOME WOODS EXPOSED

TO LOW LEVEL THERMAL RADIATION

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1002-11-104-27 July 26 , 1961 7306

IGNITION OF SOME WOODS EXPOSED

TO LOW LEVEL THERMAL RADIATION

by

A. J. Buschman

IMPORTANT. NOTICE

NATIONAL BUREAU OF STANDARDS REPORTS are usually preliminary or progress accounting documents

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to additional evaluation and review. For this reason, the publication, reprinting, reproduction, or opcn-lltcraturc

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however, by the Government agency for which the Report has been specifically prepared If that agency wishes

to reproduce additional copies for its own use,

II S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

IGNITION OF SOME WOODS

EXPOSED TO LOW LEVEL THERMAL RADIATION

BY

A. Jo Buschman

ABSTRACT

Critical intensities for the pilot ignition of some common materials have been determined. An effort is made to correlate data

from a flame-spread test in which the specimen

is subjected to a non-uniform radiant exposure

with data from the pilot ignition of uniformly

irradiated specimens in order to understand the

effect of the presence of a flame.

Intensity of irradiation and time of ignition have been employed in computation of

surface temperatures at the time of ignition.

Equations are presented which show the dependence of ignition time and surface temperature

upon the physical and thermal properties of

density, conductivity, and specific heat.

I. INTRODUCTION

The radiant ignition of wood has received considerable attention during the past few years. This attention has been focused

primarily upon short pulses of high intensity thermal radiation

although some work has been done in the range where intensities

are less than one calorie per square centimeter per second and

the time of exposure relatively long. It is the purpose of the

present report to present some work in this latter range.

The flame-spread test, described in detail in reference 1,

had been developed to study the rate of travel of a flame along

a surface which was receiving a non-uniform, low level, thermal

radiant input. The introduction of the flame-spread test has

increased interest in the behavior of materials exposed to thermal

radiation at levels effective during fires since these tests

showed that materials would burn at incident intensities which

were below those considered as critical in reference 2. Critical

intensity is defined as the level of irradiation below which

ignition does not occur. While it is reasonable to expect some

increased heat transfer due to the presence of the flame, this
- 2 -

effect was not expected to be of the magnitude found in the

flame-spread data when compared with the results of reference 2.

It was therefore necessary to determine the critical intensities

and the intensity-time relationships for typical materials and

to make a direct comparison with the flame-spread test results

on the same materials.

2. EXPERIMENTAL DETAILS

A twelve by eighteen inch vertical gas-fired refractory

panel, described in detail in reference 1, was used as the source

of the radiant energy. The panel was maintained at a radiant output corresponding to that of a blackbody at a temperature of 670°C

for the flame- spread test and for this reason was operated at this

level for both the uniform flux and delayed ignition tests.

All specimens were conditioned at 75°E and 50 per cent relative humidity for at least forty eight hours prior to any test.

The density of all materials was determined by weighing and measuring the volume. The conductivity was obtained from equation

6-29 of reference 3 relating the thermal conductivity to the density of the material. The specific heat of all materials tested

was taken to be .34 cal/g°C.

3. UNIFORM FLUX TESTS

This series of tests deals solely with pilot ignition, that

is a small pilot flame was placed one-half inch above and one-half

inch in front of the specimen in order to ignite the evolved gases.

Direct contact between the pilot and the specimen was avoided in

order to reduce to a minimum the heat added by the pilot. Reference 1 presents some work on the effect of pilot position upon

ignition time.

Intensities were varied by positioning the specimen at various

distances normal to the panel along its centerline. . This yielded

a range of intensities from .890 cal/cm^ sec at a distance of ^+.25

inches to 0 31+2 cal/cm^1 sec at a distance of twelve inches from the

panel. Visual observations show that even at the closest position

convection currents from the panel do not reach the specimen. Therefore, the predominant mode of heat transfer was by radiation with

natural convection effects at the specimen surface qLtfe only to the

temperature of the specimen.
- 3 -

The intensity was measured at selected distances normal to

the radiant panel and along its centerline using a copper sheet

calorimeter. The calorimeter consisted of a blackened 1/16-inch

thick copper sheet which was two and a quarter inches square with

a 2b gauge chromel-alumel thermocouple swaged into the rear surface.

From the rate of temperature rise measured on a high speed recorder

and the weight and heat capacity of the copper sheet, the intensity

was calculated assuming blackbody absorption of radiation.

Specimens were two and one-quarter inches square and onequarter inch thick. Surface area was made as small as possible

in order to have a more uniform input yet large enough to avoid

errors due to insufficient area. Reference b presents some work

on the effect of specimen area upon ignition time. The specimen

thickness of one-quarter of an inch was the same as that of the

standard sample used in the flame-spread test reference This

thickness was sufficient so that no appreciable temperature rise

occurred on the rear surface prior to ignition and the specimen

could therefore be considered semi-inf inite prior to this time.

The specimen was held in position in front of the panel by

a holder fabricated of cement-asbestos board. The holder provides

support as well as freedom from drafts therby minimizing forced and

natural convective losses from the rear and along the edges. Aluminum foil was placed around the opening on the front of the holder

and along the edges in order to reduce the heat input to the holder

as much as possible. The holder was then placed on a pivoted arm

so that it could be rapidly placed in position in front of the panel.

In performing a test, the lighted pilot was placed in position

and then the specimen was pivoted into, position at time zero. The

time for ignition to occur was recorded from visual observations.

The observation of sustained flaming on the surface of the specimen

was taken as evidence of ignition. Flashing in the hot gasses above

the specimen was not considered as evidence of ignition. This

flashing effect did not occur in the higher intensity exposures

but was quite common in the range of the critical intensity of the

material.

The materials tested in this manner were tempered hardboard,

and maple, oak, poplar, spruce? and balsa woods. These materials

were used to give a wide range in density and thermal conductivity.
During a test, the intensity of radiation reaching the specimen tends to increase due to reflective effects. Since the

specimen is not a blackbody, it reflects some of the indicdent

radiation and returns a portion of it back to the panel. The

panel, also not a blackbody, absorbs some of this reflected radiation and reflects a portion of it back to the specimen. This

process continues indefinitely until the equilibrium is maintained.

This new equilibrium will result in a higher panel temperature (the

panel being originally set for a blackbody temperature of 670°C)

and therefore in a higher incident intensity at the surface of the

specimen. The effective blackbody temperature of the panel after

a ten minute exposure was found to increase from 670°C to 686°C

and the intensity at the surface of the specimen was increased from

I89Q to .94-7 cal. /cm2 sec at the 4.25 inch position.

In order to reduce this effect, a servo-control system was

employed to maintain the panel temperature reasonably constant.

The panel was originally set for a blackbody temperature of 670°C

using the radiation pyrometer employed for this purpose in the

flame-spread test. A second radiation pyrometer placed adjacent

to the specimen and facing the panel was used to control the supply

of gas to the panel. Its output in series with a pre-set potentiometer whose output in opposition cancelled out the pyrometer when

the panel was at the proper temperature was fed to the servocontrol amplifier which controlled the supply of gas to the panel.

In this manner small changes in panel temperature were sensed and

corrected by changes in the flow of gas to the panel. Since the

refractory panel has a large thermal inertia it did not respond

rapidly to the changes in flow ratt=5. However, the output of the

control pyrometer was kept within - .02 millivolts corresponding

to - 2.3 degrees Centigrade of the desired temperature.

4. DELAYED IGNITION TESTS

This series of tests was designed to provide ignition data

to supplement the uniform flux tests and the flame-spread tests.

The specimen exposure was the same as in the flame-spread test

except that the pilot was not applied at the start of the test

but was delayed for selected time periods.

The test specimen was six. by eighteen inches and was placed

in position facing the panel at an angle of inclination of thirty

degrees. The intensity along the specimen has been measured and

is reported in reference 1. The ignition times measured .for these

tests corresponds to the delay period plus ten seconds. The additional ten seconds were allowed to permit the flame to become
- 5 -

established thereby leaving a mark on the surface which clearly

indicated the position of the flame front. After ten seconds of

pilot ignition, the flames were immediately extinguished and the

maximum distance reached by the flames was measured. The intensity

at this position was determined from reference 1, figure 8.

5. RESULTS AND DISCUSSION

The data obtained from the present series of tests are presented in Table I. The times in the data for the delayed ignition

tests include the ten seconds allowed for the flame to become established. The critical intensities are also presented in Table I.

The critical intensities were obtained from the uniform flux

data by plotting the intensity against the reciprocal time of ignition and extrapolating to zero on the reciprocal time scale.

This corresponds to the intensity required for ignition at infinite

time. A typical plot of this type is shown in Figure 1. It was

then possible to determine the relationship between excess intensity, (intensity above critical), and time. Figure 1 is a logarithmic plot of intensity above critical versus time for tempered

hardboard. Similar plots for the other materials tested have been

prepared and it has been found that the slope, which appears constant for each material, is not the same for all materials as had

been suggested in reference 2.

It is possible to represent the data using an empirical formula of the type

(I-I

p

) t

n = A

where Ip is the critical intensity for pilot ignition, n is a constant determined from the slope of the line as in Figure 2, and

A is a constant.

Attempts have been made to express the values of the parameters

Ip, n, and A in terms of the physical and thermal properties of

the materials. The solution of the heat flow equation for a semiinfinite slab initially at zero temperature heated on the surface

by radiation from a medium at a constant temperature, reference

6, page 72, equation 5, shows the surface temperature rise to be

a function of the Kp Cp product, where K is the thermal conductivity

p

is the density, and Cp is the specific heat.
- 6 -

Figure 3, therefore, shows the dependence of Ip, n, and

A upon the product KpCp. Both Ip and n can be represented' by

linear functions of 1 KpCp while A can be expressed' as a simple

quadratic in (KpCp)2". The relations are as follows:

Ip = 4.24 x 10" 1 - 8.75 x 10~4 (KpCp x 106

) n = 9.75 x 10

_1

- 1.20 x 10“3 (KpCp x 10 6

) A = Kp'Cp [ 6.90 x 10 2

- 3.15 x 104

(KpCp)

2

] 2

Intensities at the time of ignition when calculated using the

above equations to determine the constants are within six percent

of the intensities obtained when using the experimentally determined values.

Figure 4 compares all the data for tempered hard board for

the three conditions of testing. The solid line represents the

empirical equation with the parameters evaluated experimentally.

The dashed line represents the flame-spread data while the delayed ignition and uniform flux data are points as indicated bn

the figure.

The delayed ignition data and the uniform flux data agree

very well over the entire range shown in Figure 4. At high intensities and short times, it appears as if the flame-spread data

would closely agree with the uniform flux data as represented in

this range by the solid line. At the lower intensities and longer

times, the flame-spread data falls far below the Critical intensity.

This is undoubtedly due to the presence of the flame since the delayed ignition data are only slightly, below the solid line and

agree very well with the uniform flux data. The predominant mode

of heat transfer producing the increased heat flow is not known.

Certainly some increased radiant energy results from the presence

of the flame both directly to the specimen as well as by reflections from the panel and the surroundings.

Figure 5 presents a sketch of the basic geometry of the flamespread test showing the position of the flame as it travels down

the specimen. It Is td be noted that the flame does not extend

out from the surface appreciably at its leading edge. It is

therefore assumed that the flame contributes a negligible quantity

of direct radiant energy to the specimen in front of the flame

front
- 7

-

The radiant energy from the flame is directed towards the

panel as well as towards the specimen behind the flame' front.

That portion which reaches the panel is partially absorbed

thereby increasing the panel temperature. The increased panel

temperature along with the partially reflected energy from the

flame cause increased radiation to reach the specimen in front

of the flame front. The section of the specimen behind the

flame front also receives additional heat from the flame due

to convection currents. The flame temperature is considerably

higher than the specimen temperature and could add large amounts

of heat to the system.

Surface temperatures in front of the flame front can be

calculated, as will be shown later, but temperatures behind the

flame front have not been determined. However, it is expected

that large gradients exist in the neighborhood of the flame front

causing a higher flow of heat down the specimen than that obtained

in the absence of a flame. All of the above mentioned modes of

heat transfer contribute towards producing ignition below the

critical intensity of incident radiation.

Another possible source of energy is self -heating. At the

ignition temperature considerable heat is generated per unit

volume for materials such as wood fiberboard. Surface temperatures at the time of ignition can be estimated* However, these

temperatures exist only in a region close to the surface and it

is difficult to estimate the contributions of self heating.

The results are similar for maple, poplar, spruce, and

balsa woods as shown in Figures 6, 7, 8, 9, and 10 respectively.

It can be seen that the deviation between the uniform flux data

and the flame-spread data varies from species to species. This

deviation is a measure of the heat added to the system by the

flaming specimen and by associated heat transfer processes and

should therefore be related to the physical and thermal properties of the materials. A correlation in terms of the thermal

properties is currently being sought utilizing the data reported

herein and additional flame-spread data on other wood species.

6. SURFACE TEMPERATURES

The data from the uniform flux tests has been used to calculate the surface temperatures at the time of ignition. These

temperatures were obtained through the use of. an equation presented by Lawson, Fox, and Webster-, reference 7, by modification

of previously referenced work, reference 6c e

( 0 ,t) = Wax [ l-espp^rfc P ] where e (Q,t) is the surface temperature rise at time t,
- 8 -

T is the temperature on the Kelvin scale, T0 is the initial

temperature on the Kelvin scale, © max is the maximum temperature ri^e on the surface, p is a non dimensional parameter,

f r t | is a heat transfer factor which permits radiative

iKpCpJ

and convective heat transfer to be lumped and considered as a

linear function of temperature difference, t is time in seconds,

K is the thermal conductivity, p

is the density and C is the

specific heat. p

The surface temperature at the time of ignition can be obtained from the above equation if the value of Tm o X can de determined. Knowing T

max , ^ can be obtained from the following:

6

max

“

so that = I . T - T

“•max x o A heat balance at the surface of the specimen can be arranged

to provide the radiant interchange configuration factor in terms

of the temperatures of the souhce, the specimen, and the room as

follows:

ct(T 2 ^ - T:A) + H(T 0 - T-,)

1 * 2 ^

T * V- 4,

3

a(TiM - T

3

m

) where cp is the radiant interchange configuration factor from the

specimen to the panel, cn is the Stefan-Boltzmann constant, 1.37

x lCr 1 ^ cal/crrr sec (°K) , T is the absolute temperature, H is

the free convection heat transfer coefficient for a vertical

plane. The subscripts 1, 2, and 3? refer to the source, the specimen, and room respectively. Since both the source and room

temperatures can be measured, the configuration factor can be

plotted as a function of Tp max. The configuration factor can

also be obtained from the following:

cp = _I

crT“*F

1

thereby making it possible to obtain T from the plot of - versus Tp max.

max

Surface temperatures at the time of ignition, calculated from

the above equa-tdons, are shown in Table 2 and presented graphically

in Figure 11. The data can be well represented by the linear expression / T ign = 667-. 527 (KpC

p

x 10°).
- 9

-

7. SUMMARY

Critical intensities have been determined for the following

materials: tempered hardboard, maple, oak, poplar, spruce and

balsa woods. Intensity above critical has been related to the

time of ignition in the following manner:

(I - I ) t

n - A.

P

A general equation expressing the constants in terms of the physical and thermal properties has been obtained. They are:

I = 4.24 x 10" 1

- 8.75 x 10~h

(KpC

p

x 10^)

n = 9-75 x 1CT 1

- 1.20 x 10"3 (KpC x 106

) A = K^ CP [6.90 x 10 2

- 3.15 x 10** (KpC )^

2

. P

Surface temperatures at the time of ignition have been calculated from a solution, reference 6, for a semi-infinite slab

initially at temperature zero and heated on the surface by radiation from a medium at a constant temperature. The radiation

and free convection boundary conditions have been lumped and are

represented by a heat transfer factor which is linearly dependent

upon temperature difference. The surface temperatures calculated

in this manner are reasonably constant over a wide range of incident radiation. The calculated surface temperatures at the

time of ignition can be well represented by the linear relationship

Tign = 667 - .527 (KpC

p

x 10 b

) The deviation at the lesser intensities between the uniform

flux and the flame-spread data is a measure of the heat added to

the system by the flame. This too should show a dependence upon

the physical and thermal properties of the material. A correlation in terms of the thermal properties is currently being sought.
8. SYMBOLS

Constant depending upon species.

Specific heat, cal/g°C.

Convective heat transfer coefficient, cal/cm^ sec °C

2 Intensity of irradiance, cal/cm sec.

2 Critical intensity of irradiance, cal/cm sec.

2

Thermal conductivity, cal/cm sec (°C/cm).

Constant exponent depending upon species.

Time, seconds.

Temperature, degress K.

Initial temperature, degrees K.

Maximum temperature, degrees K.

t Non-dimensional parameter, )

Maximum temperature rise above T

q

degrees K.

Density, g/cm

Stefan-Boltzmann constant, 1.37 x 10 cal/cm2 sec

(

Radiant interchange configuration factor from the

specimen to the panel.

Heat transfer factor which permits radiative and con

vective heat transfer to be lumped and considered

as a linear i function of temperature difference,

cal/cm2 sec (°C)
REFERENCES

CJJ Robertson, A. F. , Gross, D. , and Loftus, J.J. : A Method

for Measuring Surface Flammability of Materials Using

A Radiant Energy Source. Proc. ASTM Vol. 56? pp. 288-292,

1956.

/ 2 7 Lawson, D. I. and Simms, D. L. : The Ignition of Wood

by Radiation. British Journal of Applied Physics, Vol. 3?

pp. 288-292? September, 1952.

Z3_7 Jakob, M. : Heat Transfer. John Wiley and Sons, Inc.,

Vol. 1, New York, 1949.

/~^+ 7 Simms, D. L. : Ignition of Cellulosic Materials by

Radiation. Combustion and Flame, Vol. 4, No. 4, Dec. I960.

/~~7 7 NBS Technical News Bulletin, Vol. 45, No. 5? PP* 82,

May, 196l.

ZJ>_7 Carslaw, H. S. , and Jaeger, J. C. : Conduction of Heat

in Solids. Second Edition, Clarenton Press, Oxford, 1959*

/ 7 7 Lawson, D. I., Fox, L. L. and Webster, C. T. : The

Heating of Panels by Flue Pipes. Fire Research Special

Report No. 1. Dept, of Scientific and Industrial Research

and Fire Offices' 1 Committee, London, 1952.

U S-C OMM-NB S- DC

Table 1. - Results of Uniform Flux and Delayed Ignition Tests

UNIFORM FLUX DATA

Tempered

Hardboard Maple Red Oak Poplar Spruce Balsa

X• oo ON o 52.8 39.9 42.5 32.4 22.9 7.5

.780 66.9 * 45.0 38.3

.695 88.2 66.8 45.3 12*8

.445 281 327 347 280 392 155

• 392 422 64l 575

•

• -r rv> 600

h .265 .355 .363 .385 .395 .417

DELAYED IGNITION DATA

K

20 .425

40 .729 .301

70 • 739 .734 0 569 .490 .230

100 0 688

130 0650 • 586 0609 .468 .385 .200

145 .588

190 0 545 .430 .531 .435 .375

310 .V73 • 375 A13 .239 .270

490 .378

730 .289

[Intensities are given in cal/crX sec.

Times are given in seconds.
Table 2. - Surface Temperatures at the time of Ignition from

the Uniform Flux Data.

r\ Intensity (Cal/cm^ sec)

Material

.890 .780 .695 .445 .392 .342 Ave T

Temperature U K

Tempered

Hardboard 591 577 577 567 559 556 571

Maple 620 612 616 616

Red Oak 638 613 615 618 621

Poplar 6 78 657 647 666 662

Spruce 652 645 649

Balsa 664 663 664
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400

6fc0

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3-

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CONSTANTS

AND

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FIG. ARESULTS

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UNIFORM

FLUX,

DELAYED

IGNITION

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FLAME

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U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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front cover.

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Navigation Systems.

Upper Atmosphere and Space Physics. Upper Atmosphere and Plasma Physics. Ionosphere and Exosphere

Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.