/ ) (j? r Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

NATIONAL BUREAU OF STANDARDS REPORT

6830

EXPERIMENTS ON THE BURNING OF CROSS -PILES OF WOOD

by

D o Gross *

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT

1002-12-10120 April 27, I960

NBS REPORT

6830

EXPERIMENTS ON THE BURNING OF CROSS-PILES OF WOOD

by

D. Gross

IMPORTANT NOTICE

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U. S. DEPARTMENT OF COMMERCE
'

.
EXPERIMENTS ON THE BURNING OF CROSS-PILES OF WOOD

by

D. Gross

ABSTRACT

Experiments have been performed in which

geometrically-scaled, unenclosed cross-piles of

wood have been burned under controlled conditions.

For a range of stick sizes from 1/16 to 3°6 in.

in cross section, the typical weight-time curve

illustrated the three characteristic stages of

ignition, active combustion, and glowing embers.

For the active combustion stage, the maximum rate

of burning (rate of weight loss) has been determined and all the test data correlated in terms

of a porosity factor involving the vent area of • the pile and the total exposed surface area of

the sticks. The correlation between the scaled

rate of burning and the porosity factor may be

simply considered in terms of three regions:

(a) diffusion-limited , in which the scaled rate

of burning is very nearly proportional to

the porosity factor,

(b) n on diffusion-limited , in which the scaled

rate of burning is independent of the

porosity factor, and

(c) nonsustained combustion, in which the openness of the pile prevents the establishment

of sustained combustion.

1. Introduction

The lack of basic knowledge on the growth and propagation of fires in building structures has hampered efforts

to evaluate quantitatively the importance of the interior

finish or lining material. The costs involved in performing

an extensive number of full-scale tests of room and building

size are considerable, and, in addition, are usually performed

in the open where they are subject to uncontrollable weather

conditions. Although full-scale tests are indeed necessary,

2

it appears likely that a basic understanding of the mechanism

involved in fire spread can be achieved through experimentation on a reduced geometric scale.*

In using the model-study approach to research on fire

growth, it is possible to achieve a systematic compilation

of results based on varying the physical, chemical and

geometrical properties of combustible and surroundings under

more readily reproducible conditions. Of even more significance is the opportunity for analyzing and understanding

those processes which control the burning of combustible

material and their relation to the modelling parameters.

Although the ultimate goal is the use of models for analysis

in enclosed rooms and building structures, the initial tests

were limited to simple, symmetrical, unenclosed piles of

sticks o

The purpose of this paper is twofold: first, to present

some experimental results on the fundamental burning characteristics of geometrically-scaled cross-piles of wood, and

secondly, to attempt to analyze and correlate these results

in terms of the important parameters which govern its behavior.

2. Experimental Details

The wood used for the majority of these experiments was

Douglas Fir, D grade, clear, kiln-dried lumber. The mean

density was approximately 30 lt> per cu ft (0.48 gm/cc) although

considerable density variation was observed. The wood was

cut to size and conditioned to equilibrium in an atmosphere

maintained at 73°F and 50% rh yielding an equilibrium moisture

content of 9-2 ± 1.5 % based on the oven-dry weight. Several

experiments were also performed using mahogany, ash and balsa

woods to explore the effects of density and thermal properties

on the maximum rates of burning.

The sticks were of square cross section and had a length L equal to ten times the width b. The construction of

the pile was identical to that employed by Folk [l] and consisted of N layers (usually 10) each containing n sticks, with

alternate layers laid crosswise to the adjacent layers. A

pile configuration was designated by the thickness of the

stick in inches, the number of sticks per layer and the number of layers, or b-n-N. The range explored is listed in

Table 1.

*The importance of this problem was emphasized at the Fourth Session

(Fire Research and Fire Models) of the First Fire Research Correlation

Conference sponsored by the National Academy of Sciences-National

Research Council, November 1956.

USCOMM-NBS-DC

- 3

The pile was ignited by burning normal heptane in a square fuel pan centered a distance equal to b beneath the

pileo See Figure 1. In several experiments, alcohol was

used In place of normal heptane. Wo appreciable difference

in the results was noted for the two fuel types nor of the

quantity of fuel supplied as long as there was sufficient

fuel to Initiate burning of the pile. For almost all tests,

the size of the pan was scaled according to the pile size

and contained a quantity of fuel corresponding to 8 per cent

or less of the Initial pile weight. With this percentage,

the fuel was completely exhausted before any appreciable

weight loss of the pile occurred.

For sizes up to ^/b in. , a dynamometer of the strain

gage type was used for obtaining a continuous weight record.

The dynamometer was arranged to operate in either of two

ways: (a) directly supporting a suspended wire gage basket

containing the pile, or (b) mounted within a ring supporting

the pile on a platform, the deflection of the ring being

transmitted to the internally mounted dynamometer. The type

of mounting had no effect upon the results except that

shielding the dynamometer unit from radiation was much simpler

with the platform support. For sizes 3 A- in. and larger, conventional balance scales were used. In some tests, measurements were made of the time for balance after removing one

of several small weight increments while in other tests, the

pile weight was read directly from a calibrated dial.

All tests were conducted within essentially closed

rooms to minimize effects of wind and draft. Figure 2 shows

the active combustion stage of a pile of 3 ° 6 in. thick wood

arranged seven pieces per layer. The similarity to an actual

building fire Is evident.

3 . Results

Typical weight-time curves are shown In Figure 3? fnom

which three characteristic stages may be noted: (a) the

ignition stage corresponding to a gradually decreasing weight

of pile, (b) the active combustion stage corresponding to a maximum and relatively uniform rate of weight loss, and (c)

the glowing stage corresponding to the deceleration of the

burning and ultimate extinction. The maximum rate of burning

is taken as the slope of the weight-time curve at its maximum

value

.

- 4 -

Investigation of the effects of some conditions external

to the pile was limited to one pile configuration, 1/2-7-10.

It was found that:

(a) A horizontal floor shield, up to 5 times the pile

size on a side, placed at the base level or 1/2 in. below

the base of the pile had no appreciable effect upon the mode

or maximum rate of burning compared with a completely op,en

pile. For uniformity, however, a square floor shield was

used in most tests, its size being 2 to 5 times the pile size

on a side.

(b) A vertical convection shield placed so as to restrict

air from the sides of the pile reduced the maximum burning

rate over 50 per cent. The shield consisted of a squarecornered tube made from iron sheet two times the height of the

pile and providing 3

_ i n » clearance on all sides of the pile.

It was placed in position after Ignition of the pile was

established

.

(c) There was no appreciable difference in the maximum

rate of burning when a highly reflective aluminum foil sheet

or a highly absorptive (carbon-blacked) floor shield was placed

at the base of the pile.

Flame Height : Visual measurements were made of the maximum height of the flames measured above the base of the pile.

The data are given in Table 1. In Figure 4 the ratio of this

flame height 6 to the height of the pile has been plotted

against the maximum rate of burning per unit of projected pile

area. The direct proportionality is similar to results found

by Thomas [2] from model experiments with cross-piles of sticks

within an enclosure, only one side of which was open. From

the data shown in Figure 4, it appears that the proportionality

is not entirely independent of scale size.

Temperature : Measurements of the temperatures in and

around the pile were made with chromel-alumel thermocouples

(0.020 in. dia. wire) for a number of tests. The maximum temperatures along the central axis within the pile were of the

order of 800, 1000 and 1200°C for piles composed of 1/2, 1 and

3=6 in. sticks respectively, although the maximum temperatures

for a given size stick appeared to be somewhat dependent upon

the structure of the pile.
'
- 5 - Radiant Energy : Measurements were also made of the

radiant flux from the pile incident on a single receiver

.

The radiometer consisted of a multiple- junction total radiation thermopile with a thin mica window and a wide-angle field

of view. Taken from a commercial radiation pyrometer, it was

of moderate response speed ( 98% within 2 seconds) and was temperature compensated by means of a nickel coil for ambient temperatures up to 250°F. It was horizontally mounted and arranged

to view the pile plus the entire area of flaming according to

the scheme in, Figure 1. The results are summarized in Table 1.

Figure 5 is a plot on logarithmic coordinates of the maximum

radiant intensity as a function of the maximum rate of burning

.

The ordinate was calculated on the basis of the inverse square

law (considering the flame as a point source) and refers to a unit solid angle for the orientation of the radiometer shown

in Figure 1. The error introduced by the inverse square law

assumption was considered negligible when the radiometer to

source distance was three or more times the maximum flame

dimension. A straight line of unit slope (direct proportionality) yields a good fit to the data. It appears, however,

that the maximum energy radiated may be slightly affected by

the porosity of the pile.

Air Velocity : These measurements were limited to exploratory tests using both titanium tetrachloride smoke (for visual

demonstration) and heated thermocouple anemometers. More work

in this area is planned.

b. Discussion

Experiments conducted by Bryan [ 3 ] led him to the conclusion that the fundamental law governing the combustions of his

wood cribs was the dependence of mass and heat emission upon

the 3/2 power of the scale size. Measurements of heat conduction in bodies subjected to standard fire exposure tests [t-]

have shown that the time for a certain temperature to be reached

is approximately dependent upon the 1.6 power of the thickness.

This has also been verified by means of measurements on electrical models arranged to represent the analogous thermal situation

[5? 6]. Whereas the rate of burning of a stick of square cross

section should properly be considered a two-dimensional system,

this 1.6 power relation is based upon one-dimensional heat flow.

However, in the actual burning, all sides are not equally affected by the exposing fire and a nearly one-dimensional heat flow

predominates. Since the rate of burning depends upon the absorption of heat within a body with resultant release of combustible

decomposition products, it is not unreasonable to expect the

rate of burning to approximate this 1.6 power relationship.

6

This may be shown as follows:

If t - b^"^ then db/dt - k2 b

When sufficient air is present, the analysis for one

stick is identical to that for the entire pile. We define R

as the maximum per cent combustion per unit time, or

R = dM/dt

x 100

Here, dM/dt equals the maximum rate of weight change and MQ

is the initial weight. If it is assumed that during the

burning, the density p remains constant and the volume V

varies, then

Since M0 = pv

dM/dt = p dV/dt = 30pb 2 db/dt

= ,10pb3

then R _ dM/dt _ 30pb2 db/dt _ 3db/dt

100 M0 10pb3 b

3 k2 b ~°-6

b

= k

3

b

- 1.6

where kq , k2 , k^ are constants of proportionality

.

In Figure 6 the "scaled" rate of burning R b^^, is

plotted for three configurations against the pile size, 10 b.

For those pile sizes and stick spacings which do not limit

the air available for maximum burning, it is seen that the

scaled rate of burning has the same value of 8A per cent per

minute

.

For other pile sizes and stick spacings, sufficient air

is not available for burning at this maximum rate. Under

these conditions, the rate of burning is determined by the

rate at which the air can flow into (or gases flow out of)

the pile. We may define a porosity factor, cp, in terms of

the ratio of the actual to the theoretical air flow rates.

The actual air flow rate will be proportional to the product

of the mean air (or gas) velocity and the vent or open area

of the pile. The theoretical air flow rate will be proportional to the rate of volume (or weight) change and therefore

- 7 -

to A s b *6. If it is further assumed that the mean air or

gas velocity is proportional to the square root of the height

of the pile, we may write

Vh A

V

b

-0 * 6

A,

or tp = N° ' ^ b

1 ' 1 Av/As

Here, is the total exposed surface area of the sticks, or

A

s = 2nt> [N(21-n) + n]

The vent area of the pile, A , may be considered the area

of the vertical shafts only or ofv

some unknown fraction of the

.total vent area comprising the top, four sides and bottom of

the pi^e. For simplicity, the open area of the vertical shafts,

A

v

= b ilO-n) , has been taken.

It was evident from visual observations that significant

flaming issued from many of the side openings as well as from

the top. Several experiments may be suggested to investigate

this point, e.g., using a solid slab roof as the top layer,

offsetting sticks in alternate layers to obstruct the fuel

effect, setting the pile directly on the floor after ignition,

closing off all or part of the side openings, etc. In preliminary investigation of the solid slab roof, a 20% reduction,

approximately, in the maximum rate of burning was obtained and

this suggests that the area of the vertical shafts was only

partially limiting.

All the data, including that of Folk [l] , have been plotted in Figure 7 as a function of porosity factor cp. To take

into account the effect of thermal properties in the tests

with mahogany, ash and balsa woods, the scaled rate of burning

ordinate, R bl*°, has been multiplied by a factor F which is

the ratio of the thermal diffusivity of Douglas fir to the

wood under test. For these data points, the greater scatter

probably results from lack of appropriate information on the

thermal properties of the different woods.

The weight-time curves of Figure 3 may be considered in

idealized form as starting at the initial pile weight and

decreasing linearly at the maximum rate of burning. This

approximation permits the following generalizations to be

made with respect to t x ? the time at which the pile weight

has dropped to x% of its initial value:
.

I
maximum rate

- 8 -

(a) For pile configurations for which the

hi • 6

of burning is diffusion-limited, t^cc ——

(b) For pile configurations for which the maximum rate

of burning is not diffusion-limited, t ocb^* 0

Because of the complexity of the problem, no attempt has

been made here to reduce to dimensionless coordinates. It is

realized that other variations or interpretations of the

abscissa grouping might yield equally satisfactory correlation

However, the abscissa grouping chosen serves as a useful means

fo gaging the effect of the porosity or openness of the pile

and, on this basis, the plot is considered to consist of essen

tially three regions:

(a) diffusion-limited, in which the scaled rate of burning is very nearly proportional to the porosity factor cp,

(b) riondif fusion-limited , in which the scaled rate of

burning is independent of the porosity factor, and

(c) nonsustained combustion, in which the openness of

the pile prevents the establishment of sustained combustion.

Mention might be made here of a few supplementary tests

in which the specimens consisted of five vertical slabs each

of dimensions b by 10 b by 10 b. The thickness range investigated was b = 1/b, 1/2 and 1 inch. It was observed that only

the inner three slabs were involved in active flaming during

the period when the maximum rate of burning was recorded. By

computing the per cent rate of burning on the basis of three

slabs only, it was found that these data also plotted along

with the general population of Figure 7»

5- Summary

This report describes experiments performed over a period

of several years to obtain fundamental information on the burn

ing characteristics of cross-piles of wood. This is one of

the initial steps in an overall investigation of the applicability of model techniques to the study of the development and

growth of fires in buildings.
.

\
9

From the results of the experiments to date, it has been

found that weight-time records can be considered in terms of

three characteristic stages*. ignition, active combustion and

glowing. For the active combustion stage, the maximum rate

of burning data have been found to correlate in terms of a po

rosity factor involving the vent area of the pile and the

total exposed area of the sticks. This correlation may be

simply considered in terms of three regions on the porosity

scale: diffusion-limited, nondiffusion-limited and nonsustained combustion.

Future work is planned in which attention will be directed toward the study of the growth of fires in model structures*, in particular, the effect of the interior lining

material on the rapidity of fire spread and measurement of

the velocity and distribution of air flow in openings will be

studied

.

6. Acknowledgements

The experimental work was performed through the cooperative efforts of many members of the Fire Protection Section.

Credit for the bulk of the work is due to the following parttime student aids, K. N. Berk, T. Burns, P. F. Eastman and

R. H. Speier.

10

7. References

[1] F. Folk, "Experiments in Fire Extinguishment" National

Fire Protection Association Quar. 11 , No. 2, pp 115-126,

(Oct 1937).

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

Models, Part II" Research, 13 pp. 87-93 5 (March I960).

[3] JT. Bryan, "Scale Effects in the Burning of Timber"

U. K. Ministry of Home Security, RC (F) 64, (July 1943)*

[4] American Society for Testing Materials, "Standard Methods

of Fire Tests of Building Construction and Materials,"

ASTM Designation E110-55.

[5] A. F. Robertson and D. Gross, "An Electrical-Analog

Method for Transient Heat-Flow Analysis," J. Research

NBS 61, 2, pp 105-115, (1958)

[6 a] D. I. Lawson, and J. H. McGuire, "The Solution of

Transient Heat Flow Problems by Analogous Electrical

Networks" Inst. Mech. Eng. Proc. [A] 167

,

2 75 (195a)

b] J. H. McGuire, "The Scaling of Dimensions in B. S. 476

Fire Resistance Tests" Joint Fire Research Organization

F. R. Note 95/1954.

[7] J. D. MacLean, "Thermal Conductivity of Wood" Heating,

Piping and Air Conditioning, 13( 6): pp. 380-391 (1941)

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FIG. ISCHEMATIC

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EXPERIMENTAL

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SPECIMEN

SUPPORTED

ON

PLATFORM

FIG. 2 -ACTIVE COMBUSTION STAGE IN BURNING

OF CROSS-PILE OF WOOD. 3.6 IN. THICK WOOD,

7 STICKS PER LAYER, 10 LAYERS HIGH.

700

SlAIVH0‘3Hd JO J.H9I3M -

FIG.

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SYMBOL A n N




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.
ILS. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary NATIONAL BUREAU OF STANDARDS

A. V, Aalin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

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