NATIONAL BUREAU OF STANDARDS REPORT

/O.

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) ^

on October 9, 2015.

IGNITION OF SOME WOODS EXPOSED

TO LOW LEVEL THERMAL RADIATION

SECOND PROGRESS REPORT

By

A, J. Buschman, Jr.

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 KBS REPORT

1002-11-10427 April 5 g 1963 7848

IGNITION OF SOME WOODS EXPOSED

TO LOW LEWL THERMAL RADIATION

SECOND PROGRESS REPOST

BY

A0 Jo Buscbjnaii.9 Jr 0

FOR

Office of Civil Defense

PCD Project Order QCjEM)S -62-89

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

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the Report has been specifically prepared if that agency wishes to reproduce additional copies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

IGNITION OF SOME WOODS EXPOSED

TO LOW LEVEL THERMAL RADIATION

SECOND PROGRESS REPORT

By

A. J. Buschman, Jr.

ABSTRACT

Delayed ignition flame spread data by the radiant

panel test are compared with standard flame spread and

uniform flux data on a typical compressed cellulosic

board. The importance of thermal conduction within a thin surface layer in the spread of flames is shown.

Results of tests performed to evaluate the effects

of longitudinal thermal conduction are presented.

Surface temperatures were measured and the effect

of the presence of the flame upon them determined. An

attempt is made to show the possible compatibility of two

ignition criteria, a minimum surface temperature and a minimum rate of volatile generation.

1. INTRODUCTION

The spread of flame over the surface of materials has been of interest in

the field of experimental fire research for several years. Empirical tests

are generally used to classify .materials according to their flame spread

behavior^ two of the most common being a "tunnel" test [1], and a "radiantpanel" test [ 2] . It is the purpose of the present report to present the results of some experiments using the radiant-panel flame spread test in an effort to explain the mechanism by which a flame travels along a surface.

In the tunnel test a gas burner forces ignition at one end of a 25-ft

long horizontal specimen which forms the "ceiling" of a tunnel furnace en

closure. The flame then travels along the specimen aided by the preheating

effect of the burner flame itself in contact with the unignited portions.

The heat input to the material Drior to ignition is not known due to the

complex radiative and convective transfer from the specimen and burner

flames. The role of thermal conduction within the material is masked by

the obscure flame front, complexities of the experimental arrangement,

and the unknown heat input at tne surface.

A variation of the tunnel-type test employs an enclosed 8-ft long specimen ignited by a small burner at its lower end and receiving supporting
thermal radiation from a steel plate separating the specimen from a gasfired combustion chamber [ 3 J , The specimen is inclined both lengthwise

and across its short axis to simulate natural draft conditions.

The radiant panel test permits close observation of flame front progression on a much smaller specimen and with a minimum of convective and

radiative feedback from the flames of the burning specimen. The geometry

of the test apparatus provides a steady, low level, thermal irradiance to

the 18-in, long specimen with maximum exposure at the top, A small pilot

burner forces ignition at the top of the specimen and the flame front

travels down the specimen succeeding portions of which are exposed to a continuously decreasing irradiation from the radiant panel. The flames

from the burning portions do not brush over the unignited portions, and

generally lie fairly close to the surface so that the thermal input to

the unignited portions is primarily that received from the radiant panel.

Since flaming occurred on the surface at positions which were receiving less than the critical flux (irradiance) under uniform flux conditions [4] s it appeared as though conduction both within the material

and in the gas phase might be responsible for the more rapid flame travel.

If tests were performed under conditions which completely removed the

radiative and convective transfer by direct contact with the flames on

the specimen surface, the role of thermal conduction within the specimen

could be determined.

Prior to ignition, the problem is that of two-dimensional, unsteadystate heat transfer with non-linear boundary conditions. In the presence

of a flame the problem becomes more complex due to heat transfer from a decelerating flame front, A complete analysis should also include the

effects of self-heating due to the elevated temperatures in the vicinity

of the flame front but this Is probably a second order effect and will be

neglected for the present.

The tests described in this report were made fe© gain a better understanding of particular aspects of the problem. They were not expected

to present a complete solution but only to serve as a foundation upon

which a better understanding of the entire problem could be formed,

2 . EXPER'XMENTAL DETAILS

A twelve by eighteen inch, vertical, gas-fired, refractory panel,

described in detail in reference 2, was used as the source of radiant

energy. During most of the tests, the radiant flux output of the panel

corresponded to that of a blackbody at a temperature of 670 S G„ A few

tests were performed with a panel blackbody temperature of 482°C„ The

gas supply to the panel was controlled manually and the temperature was

monitored before each test with a calibrated radiometer.
3

Specimen size was six by eighteen inches unless otherwise stated. The

material selected was a commercial wood-base hardboard* of fairly uniform

surface flammability; asbestos millboard of comparable density** served as an inert material. All specimens were conditioned at 24°C and 50 percent

relative humidity for at least 48 hours prior to any test.

Specimen thickness, unless otherwise specified, was one-quarter of an

inch. Some tests to determine the effect of specimen thickness employed a range of thickness from 0.070 to 1.0 in.

3 . RESULTS

3.1 Delayed Ignition Tests

The results of a series of delayed ignition tests reported earlier [4]

will be summarized here for convenience.

Delayed ignition was obtained by standard exposure of the specimen as in the flame spread test except that the pilot flame was withheld for

selected time periods. After forcing ignition by application of the pilot

flame, flames travelled down the specimen rapidly to a position determined

by the selected delay. The flame was allowed ten seconds to become established and was then extinguished. This permitted determination of the posi

tion of the flame front for each selected exposure period. The equivalent

blackbody temperature of the panel was pre-set at 670°C as it was during

the regular flame spread test. From the calibration of irradiance at each

position on the specimen it is possible to obtain the level of irradiance

at the leading edge of the char produced by the delayed ignition test.

This irradiance-time data was compared with uniform flux data, the latter

having been obtained from irradiance-time measurements with pilot ignition

of two inch square specimens that were uniformly irradiated.

The delayed ignition data is compared with the uniform flux data for

hardboard and balsa wood in figure 1. The denser -hardboard material shows

good agreement between the uniform flux and delayed ignition data. The

less dense balsa wood shows that under delayed ignition, the balsa wood

ignites below the critical uniform flux as shown on the right edge of

the ordinate scale. The critical flux was obtained from the uniform flux

data by plotting irradiance versus the reciprocal ignition time and extrapo la ting to zero of the reciprocal time scale. The critical flux is the

highest irradiation at which ignition would not occur during very long

exposures of uniform irradiation in the presence of a small pilot flame.

Considering the delayed ignition of balsa wood, figure 1 shows the time

of ignition was twenty seconds for an incident flux only slightly greater

than the critical. This corresponds to the energy flux at the nine inch

position on the specimen. Early ignition of this point near the critical

irradiance is undoubtedly affected by the flow of heat within the material

* K = 4.65 x 10

4 cal/cm^ sec ~ P= .91 o C = .34 cal/g°C cm ? cmJ i ** K = 2.9 x 10”^ cal/cm“ sec — P= .86 —

C = *20 cal/g°C

cm ^ cm-5 ?
- 4 -

from regions which are far above the critical value. The data presented

for hardboard do not show this effect since the two types of test give very

nearly the same results.

A comparison of the same tests on maple, red oak, poplar, and spruce

woods show that the observed separation increases with decreasing density.

The above order of woods is with decreasing KPC but with increasing diffusivity. It appears as if the higher irradiance and heat absorption at

the top of the specimen accelerated the release of volatiles appearing

at the surface. It is to be expected that this effect would be more pronounced in the less dense materials since lighter materials attain a higher

surface temperature for a given irradiance and time of exposure than denser

materials. This, coupled with the increased diffusivity of the lighter

materials, suggests that conduction of heat along the surface plays an

important role in the spread of flame along a surface.

If the application of the pilot flame is withheld beyond the minimum

time for the appearance of a flame, it is possible to have a net flow of

heat into a given region which would exceed that received directly from

the panel. This increased heat will prepare the specimen so that a flame

would travel over its surface at a greater rate. In order to show this

effect an additional series of delayed ignition flame spread tests were

performed

.

In this series, the pilot was not applied at the start of the exposure

but was again withheld for selected time periods. After a selected delay,

ignition was forced at the top. The flame front travelled down the surface very rapidly and after establishing itself, was allowed to continue

along the specimen. The time of arrival of the flame front at each three

inch position was recorded from visual observations. Position along the

specimen was converted into irradiance and the irradiance-time data compared with the uniform flux and regular flame spread data in figure 2.

Figure 2 presents the results of the uniform flux, the delayed ignition

flame spread, and regular flame spread tests for the hardboard specimens.

The upper curve is the pilot ignition data using uniformly irradiated 2 in.

square specimens. This curve approaches the critical value asymptotically

with increasing time. The lowest curve is the flame spread data. The delayed ignition flame spread data for delays of 180, 240 and 300 seconds

are represented by the curves which separate from the upper curve and

approach the lower curve at longer times.

The. uniform flux and the regular flame spread curves appear to converge

at the higher irradiance levels. At the point corresponding to the appearance

of flame during the flame spread test, the curve for the irradiance-time data

begins to fall below the data for uniform flux. The separation continues

so that after 300 seconds the flame was travelling along surfaces which

were receiving less than the critical value of irradiation. The flame continued to the lower end of the specimen where the irradiance was .08 cal/cm^

sec. From these results, it may be concluded that specimen flaming contributes additional heat input to the unignited portions and results in ignition

at sub-critical values of irradiance.
5

The three delayed-ignition flame spread curves appear to converge with the

regular flame spread curve at about 600 seconds. From a position-time plot

(not shown) it is evident that the velocity of the flame at each position

on the specimen increased with increased delay, emphasizing the importance

of the additional heating time. This additional time allowed the material

to be heated to a greater depth as well as allowing the surface temperature

to more nearly approach the ignition temperature. Both effects speed up

the thermal decomposition process and result in increased flame velocity

at each position.

The empirical formula developed in reference 4 for the pilot ignition

of uniformily irradiated material is

(I-I )tn

=A. (X)

The experimentally determined time of flame arrival at a given position on

the regular flame spread specimen corresponds to an irradiance from this

formula greater than that which the specimen receives directly from the

radiant panel. This difference continues to increase as the flame progresses

down the specimen. It can be well represented at any time by

AI - Bt

m

. (2)

where AI is the irradiance excess in the presence of a flame, i.e., the

difference between that required for ignition under uniform flux conditions

and that incident at a point on a flame-spread specimen at the corresponding

time. The: -value of: B for each material is related to thermal diffusivity

as follows,

B = 24. 4<* -3 ,-56xl0”

2

(3)

when 0/ is in cgs units. No simple expression for m as a function of the

physical and thermal properties was found, and it was considered constant

with a value of 0.71. This is the average value of m for hardboard, maple,

red oak, spruce, and balsa woods where the individual values ranged from 0.59

for balsa to 0.97 for hardboard.

Using the above expression for AI it is possible to represent the flame

spread data by

(I+AI-I ) tn = A,

P

and the constants 1^, n, and A by the following:

I = 4.24 x 10" 1 - 8.75 x lO

-4

(KpC x 10

6

) m = 9.75 x 10" 1 - 1.20 x 10' J (KpC x 10°)

2

A = KPC[6.90 x 102 - 3.15 x 10

4 (KpC) .

-3

(4)

(5)

It was found that at any time after delayed ignition, except for very long

times, AI at a given position along the specimen varied inversely with the delay. If we consider that the temperature of the flame is constant this again

indicates that the increased exposure has prepared the surface so that the

flame itself need supply less energy and can therefore proceed at a greater rate.
6

3.2 Reduced Irradiance Tests

The uniform flux data asymptotically approach - - the critical value while

the flame spread data continue to much lower irradiance levels at a decreasing

rate. Attempts were made to determine if a minimum critical flux exists for

surface ignition during the non-uniform irradiation of the flame-spread test.

To do this, radiant panel blackbody temperature specimens were tested as in

the regular flame spread test except for a reduced radiant input (by operating

the radiant panel at a blackbody temperature of 487 °C) and it was found that

the flame continued to travel in regions where the flux was as low as .04

cal/cm^sec. However, it was also observed that a flame will not spread beyond the area of the igniting pilot in the complete absence of additional

irradiation. Other tests have shown that flames will not continue to travel

along the surface if the specimen is shielded from the radiating panel during

the course of a regular flame spread test. In the latter case the flame

travel stopped and all flaming ceased within a few minutes.

It is not likely that a critical irradiance level less than .04 cal/cnrsec

is realistic. This value was the measured flux input at the lower end of the

specimen during a reduced irradiance flame spread test and hence was a measure

of the incident radiant flux at the surface at the start of a test only. It

does not consider the effect of increased panel temperature due to the presence of a flaming specimen.

In order to determine the effect that re-radiation from a flaming specimen could have upon the radiant panel temperature, a few tests were performed

while the center portion of the panel was monitored by an auxiliary radiometer. (The radiometer used to measure and control the panel temperature

prior to the start of a test does not permit viewing the panel during the

test.) In this series of tests the control radiometer was used to set the

panel at an effective blackbody temperature of 487 °C. Two tests were performed while monitoring the panel temperature: a reduced intensity flamespread test and a thirty-seven minute delayed-ignition, reduced intensity

test. The results are shown in figure 3 in terms of the uncalibrated

millivolt output versus time. The broken line shows flame position versus

time during the reduced intensity flame spread test. It can be seen that

re-radiation from the flaming specimen increases the panel temperature considerably more than the presence of a non-flaming body (whose surface temperature is increasing). It should be noted that both specimens caused a slight

increase in the radiometer output as soon as they were placed in test position. This is attributed to the reflection of the incident radiation from

the specimen surface and not to an increased panel temperature since the

thermal inertia of the radiant panel is appreciable. It is also likely

that some portion of the output of the radiometer during the flaming tests

was due to reflection from the specimen and not all due to increasing panel

temperature.

Panel temperature was not monitored during the regular flame spread

test. The control radiometer at the start of each test had an output of

1.95 millivolts corresponding to a blackbody temperature of 670°C. After

a ten minute flame spread test, the output was found to be 2.05 millivolts

corresponding to 680°C. At this time the flaming specimen had been removed

so that ref lections from the specimen were absent. Additional measurements

using calibrated radiometers during flame-spread tests should enable more

quantitative estimates of this effect to be made.
- 7 -

3„3 Variable Thickness Tests

To determine the effects of thickness upon flame spread, specimens were

prepared from 1/4-inch thick tempered hardboard in the following uniform

thicknesses; .070, <,145, .25, .50 and 1,00 inches. The thin specimens were

prepared by sanding the 1/4-inch material while the thick specimens were

made up of layers of the 1/4-inch stock and held together with a wood glue.

All except the one-inch thick specimens were backed with half-inch thick

asbestos millboard.

The specimens were all exposed to the regular flame spread test and

the results are shown in figure 4 in terms of the time of flame arrival

and the corresponding irradiance at the position of the flame. The thinnest

specimens (.070 in.) ignited earliest and the flame travelled at a greater

rate over the top twelve inches than it did for all other thicknesses. For

a specimen twice as thick (.145 in.) the primary ignition took longer and

the flame did not spread as fast. In both tests the flame appeared earlier

and spread faster than during the standard test with the .25-ineh thick

specimen.

Flame spread tests performed with .50 and 1.00 inch thick specimens

showed the same trend but were only slightly different from the standard

thickness specimen. It appears as if specimens of .25 in. thickness or

greater all give very nearly the same flame spread factor when tested

in the standard manner. This is not the case when thicknesses below the

standard thickness are tested.

A series of regular flame spread tests were performed on hardboard

specimens which had horizontal grooves (i.e., parallel to the specimen

width) on either the exposed or unexposed surfaces. The grooves were saw

cuts 1/16 of an inch wide and 3/16 of an inch deep in 1/4 inch thick specimens. When the cuts were on the unexposed surface, no measurable differences from the regular flame spread test were observed. This suggests

that conduction effects are unimportant beyond 1/16 of an inch from the

surface.

Specimens tested with the saw cuts on the exposed surface gave temporary differences. The flame arrived at the edge of the saw cut at the

usual time, but was then delayed for a longer time, than it would normally

take for the flame to travel 1/16 inch. After jumping the gap, the velocity

of the flame increased slightly so that it arrived at the next saw cut

again at the usual time. The same effect to a lesser degree was noticed

in the delayed ignition flame spread test when specimens were tested with

horizontal grooves on the exposed surface.

3,4 Surface Temperatures

Surface temperatures have been measured during the standard flame spread

test and compared with those measured on a specimen where pilot ignition

was delayed. Approximate surface temperatures were measured using a series
- 8 -

of stripes of temperature-indicating paints placed at each three-inch position along the specimen. Upon reaching the specified temperature the paint

underwent a prescribed color change and the time of this change was recorded

from visual observations. The manufacturer claims the paints to be accurate to within 1% and when applied in thin coats the time lag is reported

to be a small fraction of a second.

Figure 5 presents the positions of the 300°F, 400°F, 500°F, and 600°F

temperature fronts as they traveled down the specimen. It also shows the

flame front as it traveled along the specimen in a standard flame spread

test. These measurements indicate that a temperature of 600°F was reached

at the three-inch position prior to the arrival of the flame. The 600°F

front also reached the six-inch position just shortly before the flame and

it appears as if the flame front starts to lead the measurable 600°F front

soon afterwards. After this time, the 600°F paint did not change color until it was actually touched by the flame. At the twelve-inch position the

flame and the 500°F front approached at about the same time and by the

time the flame reached the fifteen inch position it was very close to

overtaking the 400°F front. After this time the 400°F, 500°F, and 600°F

paints were unchanged until touched by the flame front.

A comparison of temperature fronts during a 600-sec delayed ignition

test shows, figure 5, that these were very similar except when the flame

approached a given temperature front. In this case the apparent velocity

of the temperature front increased when the flame approached indicating

that the effect of the flame was restricted to fhe^mmediat6 n~eigh,bofho6d of

the flame front.

Approximate surface temperatures were also measured on some specimens

in the complete absence of flaming to investigate the effects of longitudinal conduction. Specimens included those with horizontal grooves

previously described and short-length specimens centered at the nine-inch

position with the remainder of the test area consisting of asbestos millboard. In all, five types of specimens were tested using temperature,

indicating paints without flames. They were:

a - 6 by 1 inch specimens

b - 6 by 3 inch specimens

c - 6 by 18 inch specimens

d - 6 by 18 inch specimens with 1/8

inch deep saw cuts each inch

along the unexposed surface

e - 6 by 18 inch specimens with 3/16 inch

deep saw cuts each inch along

unexposed surface

For all specimens, the time for attainment of each of several surface

temperatures at each position was noted. Temperature indications were

obtained from 300°F to 600°F in 50°F and 100°F steps and are shown in
- 9 -

figure 6 for the 9-in. positon. The greatest difference noted, 30 sec, was

for the 600°F surface temperature reached on the one inch long specimen

in 210 seconds and on the three inch long specimen in 240 seconds. Although

measured times from repeat tests were generally within 15 seconds, there

was no apparent correlation with longitudinal conduction. These results

indicate that conduction along the surface without a flame is not significant for this material. Due to the increased flame spread of balsa wood

and the resulting short ignition times, temperature measurement was not

attempted until reduced irradiance tests could be performed satisfactorily.

It is thought however that conduction in less dense materials will be more

pronounced and surface temperatures will be greater for a given exposure

on the longer specimens.

It was reported earlier [6] that surface temperatures could be calculated at the time of ignition. From all the irradiance-time data obtained

experimentally for each material the method gave reasonably constant surface temperatures at the time of ignition. Correlation on this basis has

also been shown by Simms [ 5 ]. Since the hardboard in the present report

is slightly less dense than that reported previously [4], use was made of

the relation between surface temperature at the time of ignition and thermal inertia, KPC, namely

T ign(°K) = 667 -.527 (KP C x 10

6

) . (6)

This equation was based upon an assumed natural convection heat transfer

coefficient of 4.7x10"^ cal/cm^sec °C. For a

KpC of 144x10”^ cal^/cm^ sec (°C)^,

T. = 591°K = 604°F. ign

This compares favorably with the surface temperatures measured with the

temperature-indicating paints.

Previous work (4) showed that pilot ignition occurs when the surface

reaches a given temperature. Actually it was shown that the calculated

temperature for ignition decreased slightly as the irradiance was decreased.

However, the time of ignition increases as the irradiance is decreased and

this permits the temperature front to penetrate farther into the material

so that the surface temperature requirement may be relaxed slightly.

It can also be shown that surface temperatures in excess of 610°F

may not result in ignition if the time is excessively long. The uniform

flux test gave the critical irradiance for pilot ignition of hardboard

as .30 cal/cm^sec. Although this irradiance may give a maximum surface

temperature of 710°F, ignitiondid not occur at this irradiance level.

Since the supply of volatiles is exhaustable, time becomes an important

factor.
- 10 -

In such cases, it appears as if this effect might be explained byconsidering the rate of volatile generation as suggested by Bamford, Crank,

and Malan (4). From this work, it was considered that a minimum rate of

volatile generation was necessary in order for a sustained flame to exist

on the surface of combustible materials,,

In order to check the compatibility of a minimum surface temperature

and a minimum rate of volatile generation for the spread of flames, it

was necessary to determine the effect of the flame itself* So far, it

has been established that conduction in the solid is not significant in

the absence of a flame and that, in the presence of a flame, conduction

is restricted to the immediate neighborhood of the flame front and to

a thin surface layer not greater than 1/16 inch thick* Furthermore, an

ignition oriterion based on the attainment of a critical surface temperature is in good agreement with surface temperature measurements* However,

even if the critical surface temperature is exceeded, lengthy irradiation

at flux levels just above critical may not result in ignition because of

excessive volatile depletion*

3*5 Numerical Solution: Sub-surface Conduction

To determine to what extent sub-surface conduction in the solid could

contribute to flame propagation, numerical solutions were sought for the

temperature distribution in the immediate vicinity of the flame front.

The analytical solution to the problem of a surface of a semi-infinite

combustible material originally at room temperature and receiving radiant

energy as a function of position along the surface y=Q and which loses

heat through convection and radiation is further complicated by the

presence of the flame which moves at a nonuniform rate along the surface.

The model chosen as a first approximation in the numerical solution

was a semi-infinite slab initially at uniform temperature throughout with

a step temperature input applied over a portion of its surface at t=0

while the remainder of the surface was cons idered impervious to the flow

of heat, i.e* perfectly insulated* To represent conditions at the moment

of flame arrival, the measured surface temperature at a given position

along the specimen was taken as the initial temperature and the surface

temperature of the portion covered by the flame was assumed to be 1400°F.

A two dimensional numerical solution was obtained for the above

model (see appendix) with initial temperatures of 590°F, 540°F, 480°F,

405 °F, and 345 °F, representing the 6, 9, 12, 15, and 16*5 inch positions

respectively on the flame spread specimen at the time the flame reached

each position* Since earlier work with uniform flux input had shown the

surface temperature to be 610°F at the time of pilot ignition of hardboard.
- 11 -

this temperature was considered to be critical in the spread of the flame.

The velocity of the 610°F front along the surface was determined for a given initial temperature and the time at which this velocity equaled

the measured flame velocity was determined,. The position of the 610°F

front both along the surface and in depth was obtained at the time of

equal velocities.

The results of the numerical solution presented in the appendix are

given in Table 1.

Table 1„ Results of the numerical solution obtained by equating the

velocities of the 610 °F front and the flame.

is it ion. I _ vf

t T

o xa ya R

(in.) cal/cm^sec cm/sec (sec) (°F) (cm) (cm) g/cm^sec

9 ,420 .115 175 540 .020 .025 ,1330

12 .300 .049 300 480 .028 .033 .0530

15 „ 195 , 026 500 405 .033 .041 .0300

16.5 ,130 .018 675 345 .037 .047 . 0220'

where

I is the irradiance on the specimen,

v^ is the velocity of the flame front,

t is the time of flame arrival,

T

0 is the initial (measured) temperature at the time of flame arrival,

xa

is the position along the surface of the 610°F front at the time of

equal velocity,

ya is the depth of the 610°F front at the time of equal velocity,

R is the rate of volatile generation given by

R = Op ya

7 f (7) xa with 9 the fraction of mass converted into volatiles at the time of the

appearance of the flame. 9 was considered constant since the position of

a constant front was investigated.

The work of reference 6 concluded that in order for a sustained flame

to exist on the surface of a material a minimum rate of volatile generation

was necessary. This minimum value was stated to be 2.5x10*” g/cm sec. It

follows that a sustained flame on the surface will travel as soon as this

minimum rate of volatile generation is established in the area ahead of

the flame front. The results of Table 1 showed that 9 was not constant

but that it varied with irradiance and ignition time. It also showed

that the minimum rate of volatile generation was not the criterion where

the irradiance was greater than the critical value.
12

The attainment of a minimum surface temperature appears to be the

criterion where the irradiance is above the critical value. The time

required for the velocity of the temperature front to equal the flame

velocity must be the controlling factor. This criterion could also

be applied to irradiance levels below critical when modified by the

time required to attain equal velocity.

In order to investigate both criteria in the range of the critical

value of irradiance, a more refined numerical solution than that attempted here is required. Additional flame-spread tests and surface

temperature measurements at reduced incident flux would also be useful.

4. SUMMARY

Uniform flux and delayed ignition tests gave very nearly the same irradiance-time results for cellulosic materials which have a high thermal

inertia, C « As the thermal inertia was reduced, with a corresponding

increase in thermal diffusivity, more pronounced separation of the data

was observed. In the delayed ignition tests on materials of low thermal

inertia, pre-flaming thermal conduction in the solid allows heat to flow

from positions of high irradiance to those of low irradiance so that the

total heat flow into a given volume at the surface is larger than the

direct incident irradiation. It is suggested that this effect is the

reason that uniform flux data and standard flame spread data separate.

This effect would be more pronounced in the neighborhood of the flame

front due to the increased temperature of the flame. This accounts for

the spread of flames at incident flux levels which are below the critical value.

The delayed Ignition flame-spread tests have shown that flame velocity

is increased at each position according to the delay chosen so that the

flame arrives at the 15-inch position at the same time for delays of 180,

240, and 300 seconds. In the case of hardboard, Figure 1 showed the effect of pre-flaming conduction to be negligible. The increased flame

velocity during a delayed ignition flame spread test compared to a standard

flame spread test shows the significance of the pre-heating period. This

allows the surface to attain a slightly higher temperature prior to the

application of the pilot as well as allowing the material to be heated to

a greater depth. More volatiles are generated and the time required for

the flame to accomplish its mission is reduced thereby increasing flame

velocity.

Surface temperatures were measured during the standard flame spread

and delayed ignition flame spread tests. Results showed that the effect

of the flame i& limited to a small area in the vicinity of the flame front.

Measured surface temperatures compared well with those calculated for the

case of the uniform flux tests. For hardboard, ignition occurred when the

surface temperature was approximately 610°F. Measured temperatures were

found to be slightly greater than 600°F when the flame passed a test point..
- 13 -

An attempt was made to show the possible compatibility of two criteria

for ignition, i.e., attainment of a minimum surface temperature and the

generation of a minimum rate of volatilization,, In determining the minimum rate of volatilization, it was assumed that, at the time of ignition,

all specimens were converting the same fraction of their mass into volatiles o The results show that if the flame spreads as soon as a minimum

rate of volatiles is present, then the assumed constant is a function of

intensity and total time of exposure . However, if the assumed constant

is a constant, then the minimum rate of volatile generation criterion is

not valid o This could be checked by a series of tests similar to the uniform flux tests with the addition of thermogravimetric analysis.

Weight loss measurements on specimens which were receiving a uniform

flux could take into account both criteria „ The method outlined using

the numerical solution could then determine which criterion was governing the spread of flames.

If the attainment of a certain temperature on the surface is the governing criterion, then the time required for the velocity of the temperature

front to equal the flame velocity would be important in determining when a flame would cease to travel. This could be accomplished with a more refined

numerical analysis and with additional surface temperature measurements

at irradiance levels below the critical value.

2
14

5 . REFERENCES

1) "Method of Test for Surface Burning

Characteristics of Building Materials"

ASTM E 84-61 , 1961 Book of ASTM Standards.

2) Robertson, A. F., Gross, D., Loftus, J. J.,

A Method for Measuring Surface Flammability

of Materials Using a Radiant Energy Source.

PROC. ASTM Vo 1. 56, pp 1437-53, 1956.

3) Proposed Method of Test for Surface Flammability

of Building Materials Using an 8-Foot Tunnel Furnace,

PROC, ASTM Vo 1. 60, pp 462-7 (1960).

4) Buschman, A. J., Ignition of Some Woods Exposed

to Low Level Thermal Radiation. NBS Report

7306, 1961.

5) Simms, D. L. "Ignition of Cellulosic Materials by

Radiation", Combustion and Flame, 4, pp 293-300,

Dec. 1960

6) Bamford, C. H. , Crank, J., and Malan, D. H.

,

The Combustion of Wood, Part I, PROC, Cambridge

Phil Soc., Vol . 42, pp 166-182, Oct. 1945.
15

6 .. SYMBOLS

A, B

C

I

X

P

At

K

M

m, n

N|

R

T

tv

W

Ax, Ay

At

CD

P

eSubscripts

ave

ign

f

Constants depending upon material.

Specific heat, cal/g°C.

2 Irradiance (or flux), cal/cm sec.

2 Critical irradiance (or flux), cal/cm sec.

2 Irradiance excess in the presence of a flame, cal/cm sec.

2 °C

Thermal conductivity, cal/cm sec

^ • Weighting modulus in numerical computation.

Constants depending upon material

Ratio of flame velocity and the temperature front velocity.

r Rate of volatile generation per unit surface area, g/sec cm*

Temperature.

Time, seconds.

Velocity, cm/sec.

Width for numerical computation, cm.

Thermal diffusivity, cm /sec.

Space increments in numerical computation, cm.

Time increment in numerical computation, cm. Fraction of mass volatilized at the time of flame arrival.

Density, g/cm\

Temperature rise.

At the time when the flame velocity equals the velocity of the

temperature front.

Average value.

At the time of ignition.

Flame.

o Initial.
15

7 . APPENDIX

Several attempts were made to study the spread of flames along a surface analytically. In each case the most realistic models were too complex and simplifying assumptions had to be employed. The model chosen

was a semi-infinite slab with a step temperature input over a portion of

its surface while the remainder of the surface was considered to be impervious to the flow of heat. Numerical solutions were obtained to determine the temperature distribution in the vicinity of the leading edge

of the step input. The numerical solution employed a square-grid network,

Figure 7a. The initial temperature of the slab was constant but was changed

according to the specimen position, each of which was analyzed separately.

For this reason temperature rise is considered instead of temperature.

The initial temperature at each position was considered to be equal

to that determined experimentally through the use of temperature indicating

paints. At time t=0 a portion of the model surface is raised to 1400°F

and held at this temperature for the duration of the calculations. A

general solution has been obtained in terms of Tq for ten time intervals.

By using the square network, the temperature rise during a time interval is given by

6l + fl2 + 0

3 + 04 + (M-4) ec (At)

°’ At ZM

2 where M = (Ax) /oAt

I is the space interval

is the time interval

is the thermal diffusivity

is the temperature of the center point (9o) after the time

interval At

is the temperature above the initial at the start of a time

interval

.

The temperature distribution throughout the specimen was determined at

each time interval for a total of ten time intervals. Figure 7b presents

the temperature distribution along the surface, y=0, in ,1 /St intervals for

Tq= 345°F corresponding to the 16.5 inch position. In this way the position

of the 610°F front c&n :be',d£t ermined at leach time interval. The position

of the 610°F front at each time interval presented in figure 7b can be

plotted and the velocity of the front determined at each time interval.

The results of .this intermediate plot are shown in figure 7c.. The velocity of the flame front had been determined for the standard

flame spread test at each position. It is now possible to match the flame

velocity at the 16.5 inch position with the 610°F front and determine the

time at which they are equal.

Ax

At

<y e o,At

6m
17

Arbitrarily assign a value to the time interval and determine the

corresponding value of the space interval for M=4. By trial and error,

adjust the time interval to a value which will allow N from figure 7e to

be in the range of 6 to 10 At, Then from figure 7c determine the time

at which the velocities are equal* Figure 7c will give this time in

terms of the time interval but this is known from the trial and error

procedure*

Divide the time found above into ten time intervals and determine

the corresponding space interval* With these time and space intervals

it is possible with figure 7d to determine the depth of the 610°F temperature froht at the time of equal velocities* Figure 7d is a graph of the

position below the surface of the 610°F front as a function of the time

interval* The same type of graph was prepared in order to obtain figure

7c from figure 7b. The position of the 610°F front at the time of equal

velocities along the surface and in depth will be designated xa and ya respectively.

At the time of equal velocities the volumetric rate at which material

is heated above 610*^ is taken as ”yav

f (A2>

The mass above 610°F per unit time is pWyav^

while the surface above 610^ at this time is Wx^*

Therefore, the mass per unit time per unit surface area is

°yav

f (as)

Xa

The rate of volatile generation per unit surface area will be

R

pyavf xa. (A4)

where ep is an assumed constant representing the fraction of mass converted

into volatiles at the time of flame arrival* The results of the above procedure are given in table 1*
i

;;
Q

cr

o oo & * cu - d d o o

oas

2ujo/|do ‘3DNVI0VUUI

w2

5

10

20

50

100

200

500

1000

TIME

.SECONDS

FIG. I

-

COMPARISON

OF

UNIFORM

FLUX

AND

DELAYED

IGNITION

TESTS

ON

HARDBOARD

AND

BALSA

WOOD
6 O

(oas

2ujo/|do) 3DNVIQVaai

TIME,

SECONDS

FIG.

2-

RESULTS

OF

UNIFORM

FLUX,

STANDARD

FLAME

SPREAD,

AND

DELAYED

IGNITION

FLAME

SPREAD

TESTS
<_>

S3H0NI ' NOIllSOd 3WVld

TIME,

MINUTES

FIG.

3-OUTPUT

OF

PANEL-MONITORING

RADIOMETER

DURING

REDUCED

INTENSITY

TESTS

(

INITIAL

PANEL

TEMPERATURE,

487
o

<

LU

DC

a.

CO

LU

coQ

z

o

c_> UJ

CO

Ll)

to

to

LU

Z

O

LU

O

LU

CL

CO

O

I

<_>

LU

Ll

Ll

LU

l d

Ll
I

0

100

200

300

400

500

600

700

800

900

TIME,

SECONDS

FIG.5-

POSITIONS

OF

SURFACE

TEMPERATURE

FRONTS

ON

HARDBOARD

DURING

REGULAR

AND

DELAYED

-

IGN

ITION

FLAME

SPREAD

TESTS
600 0

100

200

300

400

TIME,

SECONDS

FIG.6-

SURFACE

TEMPERATURE

MEASUREMENTS

ON

VARIOUS

SIZE

HARDBOARD

SPECIMENS

SHOWING

EFFECT

OF

LONGITUDINAL

CONDUCTION
o O o o o

o o o o o

CD o

CD ° o
10

At

^-J 1 lo

o o o

o o o Is- iO rO

do ‘3anivH3dW3i 3DVdans

oII 0

1 ^

x m

i

L0 rO

a?

0- o

o

LU

H

LlI

CJ —

i2 in

or co

co

i n

ro

Ll.

5

o

o

cr>
0.200

0.150

0.100

0.050

610° F FRONT

T0

= 345° F

2 4 6 8 10

NUMBER OFt TIME INTERVALS( At)

v

f

FIG. 7c- VELOCITY RATIO( -^r ) AS A

At
2.50

2.00

1.50

Ay

1.00 —

0.50

610° F FRONT

T

0 =345’F

/

/

0 2 4 6 8 10

NUMBER OF TIME INTERVALS(At ) FIG.7d- POSITION 0F6I0°F FRONT IN

DEPTH
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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Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

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