U.S. FOREST SERVICE 

RESEARCH PAPER INT-30 

1966 

FIRE SPREAD CHARACTERISTICS 

DETERMINED IN THE LABORATORY 

RICHARD C. ROTHERMEL and HAL E. ANDERSON 

This file was created by scanning the printed publication.

Errors identified by the software have been corrected;

however, some errors may remain.
THE AUTHORS 

RICHARD C. ROTHERMEL received his bachelor's degree in 

aeronautical engineering from the University of Washington 

in 1953. He served for 2 years as project officer at the Air 

Force Special Weapons Center after which he worked in the 

armament group at Douglas Aircraft Company. From 1957 

to 1961 he worked on General Electric's Aircraft Nuclear 

Propulsion project in Idaho Falls, Idaho. He joined the 

staff of the Northern Forest Fire Laboratory as Aeronautical Research Engineer in 1961. He is responsible for the 

study of wind effects on fire in the Fire Physics project. 

HAL E. ANDERSON received his bachelor's degree in physics 

from Central Washington State College in 1952. He was 

employed by General Electric Company from 1952 to 1961 

except for 2 years of military duty. He worked in gas turbine instrumentation, nuclear reactor instrumentation and 

operation, and data system operation. In 1961 he came to 

the Northern Forest Fire Laboratory as a member of the 

Fire Physics project. He is now project leader for that 

project and has responsibility for thermophysics research. 
U.S. Forest Service 

Research Paper INT -30 

1966 

FIRE SPREAD CHARACTERISTICS 

DETERMINED IN THE LABORATORY 

by 

Richard C. Rothermel and Hal E. Anderson 

INTERMOUNTAIN FOREST AND RANGE EXPERIMENT STAT~ON 

Forest Service · 

U.S. Department of Agriculture 

Ogden, Utah 

Joseph F. Pechanec, Director 

·.· .. 

,_.-_· 
INTRODUCTION 

PURPOSE •••. 

METHOD o 

CONTENTS 

FACILITIES . • • • • • • 

EXPERIMENTAL TECHNIQUE • • • • • 

INSTRUMENTATION 

RESULTS • • • o • o • • 

INFLUENCE OF MOISTURE IN ABSENCE OF WIND 

EFFECT OF WIND UPON RATE OF SPREAD. 

DISCUSSION OF RATE OF SPREAD 

SUMMARY OF DISCUSSION • • • • 

CHARACTERISTICS OF RATE-OF-SPREAD CURVES • 

RELATION BETWEEN FIRE BEHAVIOR AND ENERGY RELEASE RATE 

FIRE CHARACTERISTIC CURVE • • • 

DIFFUSION FLAME ANALYSIS. 

SUMMARY AND CONCLUSIONS 

LITERATURE CITED • 

APPENDIX A. 

APPENDIX B • o • 

INFLUENCE OF SURFACE FLOW OF WIND UPON RATE OF SPREAD 

Page 

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24 

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i,' ' 
Fire Spread Characteristics Determined in the Laboratory 

INTRODUCTION 

A study of the mechanisms of fire spread was conducted from January 1961 to January 1964 

by the Fire Physics Research project located at the Northern Forest Fire Laboratory. The research program was supported by the U.S. Forest Service and the National Science .Foundation. 

This report presents the major findings of that study and is part of a continuing program 

conducted by the Forest Service to obtain a better understanding of forest fires. 

Light forest material such as pine needles, leaves, and rotten wood are recognized as the 

fuels in which fires start and spread in early stages (7, 9, 15). 1. Such fires often burn on the 

ground in a thin layer of fuel without generating enough heat to carry them into the tree crowns. 

Running crown fires are dangerous to approach and are therefore difficult to control or instrument for research purposes. The small initial fire burning in a flat bed, however, can be 

studied successfully in the laboratory. 

Moisture content of the fuel and velocity of the air over it are two primary factors that 

determine the rate of spread and other burning characteristics of a fire. Laboratory research 

on the mechanisms of fire spread was directed at understanding the influence of these two 

factors on the behavior of fire in a mat-type fuel bed of randomly placed pine needles. 

Past research in forest fuels (~, 10, JJ) was hampered by the variability of outdoor 

weather conditions. Consequently, most of the work in the last decade (5, 13, 14, 17) has been 

directed toward controlled environmental conditions and controlled fuel bed characteristics. 

In most of these investigations, fuel particles larger than 0. 25 inch thick have been arranged 

in a geometric pattern. This provides an idealized model which is easily duplicated, but does 

not represent the randomized arrangement nor the particle size of forest fuels that contribute 

to the start and early spread of a forest fire. This study of the mechanisms of fire spread is 

aimed at providing this information. 

PURPOSE 

This study was designed to find the relative importance of fuel moisture and wind upon the 

rate of fire spread in light forest fuels. Systematic investigations of a physical phenomenon 

often produce an insight into its cause and effect. Through this technique, the authors hoped to 

gain a better understanding of the basic mechanisms of fire spread, which would enable them to 

develop hypotheses to be analyzed in future experiments. 

Several questions of technique and instrumentation also needed answers. Could a reproducible fuel bed be constructed with pine needles? Could the environmental system of the 

laboratory maintain stable conditions long enough to complete a test fire? How could flame 

dimensions be measured consistently? How could the energy released by the fire be measured? 

What is the best technique for measuring low velocities of air movement? 

1 Italic numbers in parentheses refer to Literature Cited, page 24. 
RETURN AIR PLENUM 

METHOD 

FACILITIES 

Figure 1 • - - Profile 

section of the 

combustion research facilities 

at the Northern 

Forest Fire 

Lab oratory, 

Missoula, Mont. 

These experiments were conducted at the Northern Forest Fire Laboratory in Missoula, 

Montana. The combustion facilities there consist of two wind tunnels, a still-air combustion 

laboratory, and an environmental conditioning section (fig. 1). The large wind tunnel has a 10-

by 10-foot cross section and is capable of velocities up to 8 m.p.h. The combustion laboratory in which the no-wind experiments were conducted is 66 feet high and 44 feet square. A 

flue- exhaust hood, located in the center of the room, may be raised or lowered to accommodate 

the size of the fire. The exhaust velocity may be controlled automatically. The air circulated 

through the combustion laboratory and wind tunnels is conditioned to the desired temperature 

and humidity in a series of heaters, chillers, and water spray nozzles. The tabulation below 

describes the range of environmental conditions and limits of control used. 

Relative humidity (percent) 

Air temperature (° F.) 

Air velocity (ft. /min.) 

Range 

6.4 ---7 75 

89° --7 97 ° 

0 --7 704 

EXPERIMENTAL TECHNIQUE 

Control 

±1.5 

±1 oF • 

±2 . 0 percent 

The experiments were conducted in two phases. In the first phase we investigated the 

effect of fuel moisture on rate of fire spread in still air. In the second, we studied the effect 

ot wind on fire spread with fuels at three distinct moisture content levels. 

All primary experiments were conducted on a fuel b~d 8 feet 1long and 1. 5 feet wide. The 

fuel was needles of ponderosa pine and white pine. The fuel was precp~ditioned for several 

d£:)-3 at the same air temperature and relative humidity conditi~ns that would prevail during the 

test burnings. This determined the fuel moisture content. The ponderosa pine needle beds 

2 
Figure 2 • - - Instrumentation u s e d 

in the combustion 

laboratory. A 

similar arrangement was used in 

the wind tunnel. 

RADIOMETERS 

were 3 inches thick and loaded at 0. 5 lb./ft. 2 • The white pine needle beds were 2 inches thick 

and loaded at 0. 33 lb./ft. 2 • Uniform loading was achieved by spreading 1 pound of needles 1 

inch deep over 4 feet of the 8-foot bed. Successive layers were added until loading was 

completed by techniques for collecting and processing already described by Schuette (~). Additional information concerning initial experiments and determination of fuel bed size was 

published by Anderson (__!_). 

Fuel beds were ignited at one end across their entire width by means 10f an alcohol trough. 

In the absence of wind the fuel bed was moved on a track at the same rate as the fire progressed (fig. 2). This kept the flame centered under the exhaust hood and overhead instrumentation. In the wind tunnel, the fuel bed was stationary 2 feet above the floor. A ground plane 

surrounded the bed and was held flush with the top surface of the fuel. 

In the wind tunnel a boundary layer trip fence or air spoiler was placed on the ground 

plane 1 foot ahead of the fuel bed. The spoiler started a wake that developed quickly into a turbulent boundary layer over the entire fuel bed. Without the spoiler, the free-stream air passed 

directly over the surface of the pine needles in the first portion of the fuel bed. As a consequence the rate of spread was four to seven times faster over the first half than over the last 

half of the fuel bed. The spoiler prevented this acceleration and permitted the fire to stabilize 

within the first 2 feet. (For further explanation of this phenomenon and results of boundary 

layer investigation with a hot wire anemometer, see Appendix B.) 

INSTRUMENTATION 

The instrumentation was designed to eliminate need for visual observations. However, 

visual observations of rate of spread and flame dimensions were used until the instrument 

system was proved reliable. Similar instrumentation (fig. 2) was used in the combustion laboratory and the wind tunnel; provisions were made for the fact that the flame front was 

stationary in the combustion laboratory but mobile in the wind tunnel. 

Weight-loss system. --A system for weighing the fuel bed continuously during the fire was 

developed to permit the energy release rate of the fire to be determined. The original system 

3 
used four cantilever beams with a strain gage on the top and bottom of each beam. The gages 

were arranged in a Wheatstone bridge circuit. The output of the bridge was amplified and recorded on a strip chart recorder. An improved version of the system incorporates force rings 

in place of the cantilever beams (~). 

The recorded trace of weight loss was linear with time during the steady state of the fire. 

The slope of the trace began decreasing when the fire reached the end of the bed, and was essentially flat when the flames ceased. This indicated that most of the fuel is consumed in the fire 

front, and not in the after burning and glowing combustion. 

Fuel bed thermocouples.--Thermocouples of 30- gage chromel-alumel wire· were placed 

in the fuel bed one-fourth inch beneath the upper surface of the pine needles <±> • The thermocouples were spaced 9 inches apart along the longitudinal centerline of the fuel bed and were 

used primarily to locate the leading and trailing edges of the fire. The thermocouple signal 

was recorded continuously on strip chart oscillograph recorders. When the fire reached the 

thermocouple, the recorder trace rose sharply. When the rear of the fire passed over the 

thermocouple, the trace became erratic and finally dropped. The drop indicated that the major 

flame front had pas sed. 

From these traces, the rate of spread of the fire was determined by the time required for 

the flame front to travel the 9 inches between thermocouples. Residence time of the fire, i.e. , 

the time flaming exists at one point, was taken as the difference between rise time and drop 

time of the thermocouple. The flame depth of the fire, i.e. , the distance from the leading edge 

of the flame to the trailing edge, was computed by multiplying rate of spread by residence time. 

Radiometers.- -Gier & Dunkle 2 directional radiometers were used to measure irradiance 

from the fires. In the wind tunnel, only overhead measurements were made. In the combustion 

laboratory, measurements were taken with an instrument 18 feet in front of the fire. 

Photographs.-Several photographic techniques were tried for recording flame dimensions. 

The best results were obtained with Plus-X film with 1-second exposure at f:11. Flame height, 

flame length, and flame angle were measured from the photographs. Flame depth measured 

from photographs was shorter than flame depth obtained by thermocouple techniques when the 

fire was being driven by wind. 

Environment monitors. --Air temperature was measured with a platinum resistance bulb. 

Dewpoint temperature was measured with a salt-saturated heater bobbin and a resistance bulb. 

Air velocity was measured with pressure probes connected to a microdifferential transducer 

and amplifier. 

RESULTS 

The results were analyzed with the object of exploring phenomena and relative influences 

of various parameters. No hypotheses were tested, but empirical formulas were developed to 

demonstrate the effects of wind and fuel moisture on the fire. The results of these tests, together with the studies by others, are beginning to reveal the characteristics of fires and 

should eventually lead to an understanding of the basic laws governing fires. 

A complete list of the results for each fire is presented in Appendix B. 

2 Trade names are used for identification only and do not imply endorsement or recommendation by the U.S. Forest Service. 

4 
Q 

~ 0:: 

a. 

en 

1.1 

~ .3 

w 

~ 0:: 

0 e PONDEROSA PINE0 WHITE PINE ----

0 

• ......... 

.......... ......... 

o' 

5 6 7 8 9 10 II 12 13 14 15 16 

MOISTURE CONTENT 11 Mt' % 

INFLUENCE OF MOISTURE IN ABSENCE OF WIND 

Figure 3.--Corre~ 

lation of experimental values of 

rate of spread 

and prediction 

equation. 

We noted early in the program that the rates of spread in both kinds of fuel were strikingly 

similar (!). A regression analysis over the range of fuel moistures used showed that a linear 

relation gave the best fit with the data (fig. 3). The equations for rate of spread are: 

ponderosa pine 

white pine 

where: 

1.04 - 0.044 Mf 

1.12- 0.051 Mf 

R0 rate of spread in ft. /min. , and 

Mf = fuel n1oisture content, fraction of ovendry weight. 

(1) 

(2) 

Equations (1) and (2) may be made dimensionless by dividing both sides of the equation by 

the rate of spread at Mf = 0. 

R 

Thus for ponderosa pine 0 1 - 0.0423 Mf (3) 

RMf=O 

R 

and for white pine 0 1 - 0.0455 Mf (4) 

RMf=o 

Tht~;.e equations show that the rate of spread in our ponderosa pine needle fuel beds decreased 

by 4 0 23 percent for each 1-percent increase in fuel moisture 0 Rate of spread in white pine 

needle fuel beds decreased by 4 0 55 percent for each 1-percent increase in fuel moisture. If the 

effect of moisture remains linear as moisture content increases, the ponderosa pine needles 

would not sustain a rate of spread at a fuel moisture of 24 percent in the still air environment. 

Similarly, the limit for white pine needles would be 22 percent. 

5 

., .. 
The similarity of the equations for these two fuels is surprising because of the contrast 

in flame size. The ponderosa pine beds burned with flames 3 to 4 feet high, and the fuel was 

well burned out. The white pine beds burned with flame heights closer to 1 foot high, and considerable unburned fuel was left beneath the ashes on the surface. Even though the equations 

are similar, we should not expect that they will predict rate of spread over the wide variety of 

compactness and fuel geometry that can be found in the field. 

EFFECT OF WIND UPON RATE OF SPREAD 

Ponderosa pine.- -A plot of the logarithm of rate of spread in ponderosa pine needle fires 

against air velocity on a linear scale produced a straight line graph for each fuel moisture 

level. The equation for rate of spread in ponderosa pine needles assumes the form: 

where: 

Rpp rate of spread in ponderosa pine needles, ft. /min. 

R0 rate of spread at U = 0, ft. /min. 

U air velocity, ft. /min. 

r constant, min. /ft. 

e base of natural logarithms. 

A regression analysis for each moisture condition gave r values of: 

0. 00389 at Mf 4. 5 percent 

0.00376 at Mf 7.3 percent 

0. 00362 at Mf = 9. 6 percent 

A weighted average r value = 0. 003 8. 

Substituting the average value of r and the equation for R0 gives 

RPP = (1. 04- 0. 044 Mf) exp 0. 0038U. 

(5) 

(6) 

Equation (6) is plotted using the experimental points in figure 4. The equation predicts 

rate of spread from U = 0 to at least U = 700ft. /min. where U is the free-stream air velocity 

above the boundary layer of the fuel. A linear relation with moisture is predicted by equation (6). 

White pine. --The rate of spread of fire in white pine needles also increases as winds peed 

increases, but at a faster rate than in ponderosa pine needles. An exponential equation can be 

fitted to the data (fig. 5), but it is good for free-stream air velocities only up to 440 ft./min. 

The equation is 

(7) 

substituting for R0 gives 

Rwp = (1.12- 0.051 Mf) exp 0.006U. (8) 

6 
Figure 4. -.-Influence of z air velocity upon rate ~ ....... 

of spread at three 1-

LL 

fuel moisture levels = ~ 

in ponderosa pine ~ 

a: 

.. needles. c 

C( 

1U 

a: 

ll. 

U) 

LL 

0 

1U 

~ 0::: 

26 

24 

22 

20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

Oo 

0 R.H.= 15 "'o M1= 4.51% 

0 R.H.= 30% Mt= 7.33"/o 

OR.H.=50% Mf= 9.62% 

Rpp=(I.04-Q.044Mtl e 0.0038 U 

100 200 300 400 500 

AIR VELOCITY "U" ( FT I MIN ) 

0 

600 700 

Amore elaborate equation fits the data up to 704 ft./min.; however, itiseasiertocompare 

coefficients if the equations are of the same form. 

If we assume the exponential curve is the correct form for describing rate of spread with 

wind, then the deviation of the data from the curve at 704ft. /min. is probably due to a cooling 

effect of the increased windspeed. Possibly there is an upper limit to air velocity in which fire 

can burn without being blown out. Such a limit would be at a lower velocity for small flames. 

The white pine fire with its small flame may show the effects of this limit even though rate of 

spread is still increasing. Comparative flame sizes of ponderosa pine and white pine may be 

seen in figures 6 and 7 at 704 ft. /min. and 15 percent relative humidity. 

26 I I I 0 

24 I I I z I I I i 0 R.H.= 15% Mf= 5.5% ....... 22 I I I 1- 0 R.H. = 30,.o M f = 8.5% I I I LL 0 20 0 R.H. = 50"/o Mf = 11.6% 

I I = I 18 I I 1 Q. 

~ 

a:: 16 I I 1 

c 14 

I I I <( I I I ILl o.oosu a:: 12 Rwp .. ( 1.12- o.051Mt> e II I ll. 

(/) 

5. --Influence of LL 10 I Figure 0 I 

air velocity upon ~ 8 

rate of spread at <( 

0::: 6 

three moisture levels 4 

in white pine needles 

2 

0 

0 100 200 300 400 500 600 700 

AIR VELOCITY "u" CFT/MIN) 

7 
Figure 6. -- Flame size in pondcroba pine needles aL 704 ft. /min. airspeed. 

Figure 7.--Flame size in whi te pine nccdh:s at 704 ft. /min. airspeed. 

8 
DISCUSSION OF RATE OF SPREAD 

The experiments demonstrated that fuel moisture and wind do affect rate of spread and 

showed the relative magnitude of that effect in a specific fuel bed. We have not shown how these 

parameters exert their influences on the fire, however. Some insight into the processes 

involved rna y be obtained from the following observations. 

It is not difficult to picture moisture as a dampening agent which must be driven from a 

fuel before its temperature can be raised to ignition point. Just how the heat gets to the fuel to 

drive out the moisture by radiation, convection, or mass transport is not clear and is a matter 

of controversy among some fire researchers. We must answer this question before we can 

successfully develop mathematical models for rate of spread. 

A fuel bed with a propagating fire bums much differently in the absence of wind than in 

the presence of wind. In the absence of external air movement, the fire creates an indraft and 

moves against this airflow into the unburned fuel. The leading flame surface is a thin, welldefined sheet (fig. 8). The base of this sheet forms in the gases issuing from the newly ignited 

fuel. If there is any preheating of the fuel ahead of the fire, it must be due to radiation because 

there is no mass transport (firebrands) and the convective flow is toward the fire, not away 

from it. The radiation source may be from either the overhead flames or from within the 

fuel bed. 

Wind creates a much more complex situation. Wind tips the flame forward, and sporadic 

burning occurs at the base of the flame where it contacts the unburned fuel (fig. 9). This burning appears to play a key role in propagation of the fire. Observation of this phenomenon with 

high-speed motion pictures and visual observation of many laboratory fires indicate that the 

sporadic flames are caused by an accumulation of ignitible gases which have collected at the 

surface of the fuel bed just ahead of the fire. The fire itself provides the pilot flame that ignites 

the gases--which burn in a direction away from the fire-, and appear as a jet of hot gas issuing 

from the fire. 

The source of the unburned gases is not clear, but rna y be very important in forming a 

mathematical model of rate of spread. Gases may be formed in the actively burning portion of 

the fuel bed and rna y be blown 

forward through the fuel to rise 

in front of the fire; they rna y 

not ignite before reaching the 

surface because of an improper 

fuel/air ratio. They may be a 

product of pyrolysis of the unburned fuel ahead of the fire • 

Figure 8. --Flaming zone 

combustion characteristics in the absence 

of wind. 

IGNITI0'4 OF GASES 

ABOVE COOL ZONE 

WHERE PROPER 

FUEL-AIR RATIO 

EXISTS. 

GLOWING 

ca.1BUSTION 

CHARRING 

FUEL 

UNBURNED 

FUEL 

9 
MIXED GLOWING MIXED CHARRING UNBURNED PYROLVZING FUEL 

AND BURNING FUEL AND BURNING FUEL 

Figure 9. --Flaming zone 

combustion characteristics in the presence 

of wind. 

If they are combustible gases from unburned fuel ahead of the fire, then this fuel must be 

receiving sufficient heat to drive the gases off before it is engulfed by the fire. Again the heat 

may come from several sources- -radiant heat from the fuel bed, radiant heat from the overhead 

flame which is now tipped forward very close to the fuel, or it may be convective heat carried 

by the wind blowing through the fire. Mass transport of firebrands was not observed in laboratory fires but may be very important in natural uncontrolled fires. Some simple experiments 

were conducted to answer some of these questions. 

Heat shield. --The most straightforward method of observing the effect of radiation is to 

shield the unburned fuel so that radiation from the flame cannot heat it. A horizontal sheet of 

asbestos was placed above the fuel bed in the combustion laboratory just high enough to permit 

the fuel bed to be moved under it without scraping the needles out of position. The fuel bed was 

ponderosa pine needles, 3. 5 feet wide, 8 feet long, and 3 inches deep. Burned in still air, the 

rate of spread in the first half of the bed (which was not shielded) was 1.18 ft. /min. When the 

fire reached the 4-foot mark, the bed was positioned so the shield stayed just ahead of the flame 

but did not touch it. The rate of spread decreased to 0. 72 ft. /min. Blocking the radiation from 

the flame thus reduced the rate of spread by 39 percent. Interestingly, the fuel burnout was 

much more nearly complete when the fire was slowed by the shield than when it was unimpeded. 

This was very evident to the eye because ash content changed from black residue to grey ash 

where the fire had been slowed. 

A similar experiment was performed in the wind tunnel with the air velocity at 5 m.p.h. , 

air temperature at 90° F., and relative humidity at 14.5 percent. The ponderosa pine needle 

fuel bed was standard size- -1. 5 feet wide, 8 feet long, and 3 inches deep. The shield was 

moved so that it stayed 1 to 1. 5 inches ahead of the fire. The rate of spread was 1.42 ft. /min., 

whereas without the shield it would have been 4. 5 ft. /min. The shield therefore reduced rate 

of spread by 68. 5 percent. This reduction cannot be attributed solely to radiation, however, 

because convective heat would also be blocked from the fuel downwind from the fire. 

Needle temperature. --In a second experiment, a 5-mil (.005-inch) chromel-alumel 

thermocouple was placed inside the fascicles of a hollowed-out ponderosa pine needle; the 

needles we:re held tightly around the thermocouple with fine thread wrapped around the fascicle 

needle. The thermocouple inside the needle provided a temperature history of whatever heat 

impulse wa$ received by the needle. Because of the low mass of the needle and the small size 

of the thermocouple, the temperature response was reasonably fast. 

10 
The needle was placed on the surface of the fuel bed. In still air, any temperature rise 

in the needle must be attributed to radiation because the induced airflow caused by the fire is 

toward the fire; hence no convective heat could be carried from the fire to the needle. 

Figure 10 shows the temperature rise of a needle which was placed 7 feet in front of the 

fire. The ponderosa pine needle fuel bed was of standard size. The average flame height was 

3. 7 4 feet. Even though the instrumented needle was 7 feet from the end of the fuel bed where 

the fire was ignited, the temperature began to rise immediately when the fire had burned 6 

inches into the bed and the flames were 1 to 2 feet high. The temperature continued to rise as 

the fire burned closer and the temperature measured inside the needle passed the boiling ternperature of water when the fire was still 1 foot away. This test clearly shows tl}at radiation 

from the flame can preheat the needle while it is two flame lengths away. When it is close to 

the fire, radiation from the fuel within the bed may also be contributing to the temperature rise. 

The combined effect is sufficient to drive the moisture out of the fuel and raise the temperature 

toward ignition point before the fire engulfs the fuel. The surface temperature of the needle 

would be hotter than the temperature measured inside the needle. It is reasonable to assume 

that products of pyrolysis would also be coming from the needle during the preheating. 

A fire being driven by wind is more difficult to analyze becfluse the airflow is from the 

fire toward the unburned fuel. The needle temperature indicates 'nly the total heating and cannot separate the methods of heat transport. To make this sep ration, an aspirated thermocouple was placed just beneath the surface of the fuel next to he instrumented needle. To 

assure rapid response, the aspirated thermocouple was made 1-mil wire; the housing was 

3-mil brass shim stock covered with insulation. The tube wit the thermocouple inside was 

oriented vertically so that the thermocouple 

would respond only to a small collimated 

portion of the flame when it was directly 

overhead. Air was drawn off the surface 

of the fuel bed and pulled past the thermocouple with a vacuum pump and hoses. 

Figure 10.--Temperature 

rise of fuel in front of 

still-air fire. 

440--

420 

400 

380 

360 

340 

320 

300 

280 

260 

-

~ 240 

220 

~ 200 

~ Q: 180 LIJ 

a. 

::!! 160 LIJ 

1- 140 

120 

60 

40 

20 

I 

TEMPE~ATURE RISE OF FUEL 

IN FRONT OF STILL AIR FIRE 

0 

UJ 

!z~ 

~~ 

(/) 

~~ 

~ 

BOILING 

BASE OF FIRE 

(FEET) 

5 MIL THERMOCOUPLE 

oL---~--~2----L3--~4L---~5----6~--~7----~e 

TIME (MINUTES) 

11 
If the temperature rise of the needle is to be attributed to convective heating from the air 

passing over it, then the air temperature must be hotter than the needle. For this test a 1-mil 

thermocouple was placed inside the needle- -instead of the 5-mil used in the "no wind" test. The 

needle was hollowed sufficiently to allow the thermocouple to be positioned just beneath the surface. The needle and inlet to the aspirated thermocouple were placed 7 feet from the forward 

edge of an 8-foot fuel bed. The bed was 3 o 5 feet wide and 3 inches deep with ponderosa pine 

needles. The free-stream air velocity was 5 m.p.h., air temperature was 90° F., and relative humidity was 12.5 percent. Fuel moisture content was about 4 percent. Even though 1-mil 

thermocouples were used in both the aspirated thermocouple and the needle, the response of the 

needle thermocouple to heat impinging on the surface would be slower than that of the aspirated 

thermocouple because of the time necessary for the heat to be conducted from the surface of the 

needle to the thermocouple. 

The temperature trace from the two thermocouples (fig. 11} shows that the air temperature began to rise almost immediately. The needle temperature began to rise when the fire was 

still more than 6 feet away. Except for a dip in the curve when the fire was 1. 5 feet from the 

instruments, the air temperature remained higher than the needle temperature. If it is assumed 

that the lag of the needle temperature was not enough to account for the difference in temperatures, then the air temperature was indeed hotter than the needle and capable of transferring 

heat to it. This test does not rule out radiant heat transfer because the needle temperature is 

responsive to both types of heating. It would have eliminated convective preheating if the air 

temperature had been cooler than the needle temperature. 

IL 

. 

w 

0:: 

::::> 

~ 0:: 

w 

Q. 

:z 

U.l 

1-

12 

480 

460 

440 

420 

400 

380 

360 

:340 

320 

300 

280 

260 

240 

220 

200 

180 

160 

140 

120 

100 

eo 

60 

40 

20 

t ASPIRATED THERMOCOUPLE 

AND 

INSTRUMENTED NEEDLE 

BOILING TEMPERATURE 

2' 

3' 

4' ..._{!STANCE FROM 

I t 

1 o.o 

I 

I 

I 

I 

BASE OF FIRE (FEET) 7' 

I MIL THERMOCOUPLES 

o~~~~~--~~~~~--~~~~-

o .05 .10 .IS .20 .25 .30 .35 .40 .45 .50 .55 .60 .65 

TIME (MINUTES) 

Note also the sharp increase in both 

temperatures as the fire comes close and 

the flame comes overhead. Boiling temperature of the needle was passed when the 

fire was about 8 inches from the needle 0 

The chart did not extend to ignition temperature of 608° F., but both temperatures 

were heading for it simultaneously. During 

this portion of the fire, the sporadic burning at the surface was close upon the needle 

and possibly touching it. The high temperature of the needle before the fire reached it 

presents the possibility that preheating releases combustible gases ahead of the fire. 

Figure 11. --Temperature rise 

of air and fuel in front of 

wind-driven fire. 

: ~ ;· 
SUMMARY OF DISCUSSION 

The thermocouple inside the needle shows that preheating of the fuel begins at a considerable distance (two or more flame lengths) ahead of the fire. The temperature of the needle 

passes boiling temperature of water before the fire reaches it. Heating in the absence of wind 

must result from radiation because the direction of airflow is from the needle toward the fire. 

In the presence of wind, preheating may be caused by both convecti.on and radiation. Fires 

burning in wind exhibit burning on the surface of the fuel ahead of the main flame front. This 

burning is in gases which have accumulated there from an unknown source. Possibly these 

gases may result from pyrolysis in the fuel ahead of the fire or from the combustion zone within 

the flame. 

The mechanism of fire spread in fine fuels may be described as a series of ignitions in 

which the temperature of fuel ahead of the fire rises, slowly at first as the water is expelled 

and then rapidly as the fire draws close. In a steady state fire, the fuel reaches pilot ignition 

temperature simultaneously with the arrival of the fire. 

CHARACTERISTICS OF RATE-OF-SPREAD CURVES 

There has been considerable conjecture about the shape of the rate-of-spread curve in a 

wind -influenced fire. Thomas and Pickard (_!i) have shown in crib fires that as windspeed increases, its effect on rate of spread progressively decreases. They show that this effect of 

wind speed appears to approach a limit as wind velocity increases. Byram et al. (~) at the 

Southern Forest Fire Laboratory in Macon, Georgia, have shown a linear increase in rate of 

spread as air velocity increases in crib fires. Velocities thus far tested have not been high 

enough to indicate whether a limiting value will be approached. Our tests with two similar mattype fuel beds have produced curves that increase as win(~ velocity increases. Perhaps they too 

will approach a maximum rate of spread at a higher velocity. It seems reasonable to assume 

that an upper limit exists. For large outdoor fires, however, the upper limit rna y fall outside 

the atmospheric range of wind velocities. Similarly, increasing rate-of-spread curves were 

found in the lodgepole pine and Douglas-fir logging slash fires ,

3 and by Pons (!J) in outdoor 

ponderosa pine fires. These curve shapes are shown in figure 12. 

This diverse behavior may be explained by the surface area-to-volume ratio of the fuel 

particles, cr, and the porosity, A, of the fuel bed. Fine fuels, such as pine needles, have a 

large surface -to -volume ratio. They have a short ignition delay time, and individual needles, 

once ignited, sustain combustion. Emmons(_~) shows how the surface-to-volume ratio is important in preheating fuel elements by radiation. A large value of cr also facilitates better convective heat transfer. Table 1 shows the rate of spread of several fuels having different values 

of cr and A at the same air velocity and having nearly the same fuel moisture content. Recent 

work at the laboratory indicates the burning characteristics may be related to the product of 

the fuel particle surface area-to-volume ratio and the fuel bed porosity (erA). This is a nondimensional parameter. A plot of the data in table 1 vs. (erA) is shown in figure 13. 

3 

Anderson, Hal E. , Arthur P. Brackebusch, Robert W. Mutch, and R. C. Rothermel. 

Mechanisms of fire spread research progress report no. 2 (field study). U.S. Forest Serv. 

Res. Pap. INT-28, 1966. (In press.) 

13 

-. ·_- .. 
a 

<( 

LLJ 

a:: 

a.. 

en 

IJ... 

0 

LLJ 

~ a:: 

PINE NEEDLES 

AIR VELOCITY "U" 

114"STICKS 

Figure 12.--Possible 

curve shapes for 

rate of spread in 

wind as influenced 

b y s i z e o f fuel 

particles. 

Table 1.--Fuel particle size and fuel bed porosity used in these and 

other fire experiments 

Fuel 

White pine needles 

Ponderosa pine needles 

!-inch sticks 

!-inch sticks 

1 

Fuel surfaceto-volume 

ratio 

2 3 (5 ft. /ft. 

2,790 

1,741 

192 

96 

Meanings of the terms "compactness, " "porosity, " and the symbol A are 

sometimes confused. The porosity of a 

fuel bed is defined as the void volume of 

the bed divided by the surface area of the 

fuel in the bed and is represented by the 

symbol A with the units ft. 3 /ft. 2 • Compactness is the reciprocal of porosity. 

The void volume of the fuel bed is the 

total geometric volume minus the volume occupied by the fuel. The surface 

area of the fuel is that of all the fuel 

within the volume. 

Figure 13 

14 

Fuel bed void 

volume-to- Rat U = 

surface area (erA) 600 ft. /min. 

ratio 1 

3 2 'Aft. /ft. Dimensionless Ft./min. 

6 

-3 

X 10 16.7 12.0 

-3 

8.46 X 10 14.7 6.5 

-2 

2.47 X 10 4.74 0.775 

3.86 

-2 

X 10 3.70 0.456 

16 

z 

i 14 

' 1- 12 ~ 

0 

<( 

10 

w 

a::: 

a.. 8 (/) 

u.. 6 

0 

4 

~ a::: 2 

0 

0 2 4 6 8 10 12 14 16 18 20 22 2· 

PRODUCT OF r:TJ\ (DIMENSIONLESS) 

~ ' . . . 
RELATION BETWEEN FIRE BEHAVIOR AND ENERGY RELEASE RATE 

Fire increases the temperature of the air associated with it and thereby lowers its density. 

Buoyancy causes the less dense air to rise and thus form the familiar convection column. If 

wind is present, the vertical buoyancy forces compete with the horizontal wind force, and the 

flame tilts away from the vertical. The amount of tilt depends on the relative magnitude of the 

fire and wind forces. We are very much interested in the amount of tilt and the amount of heat 

that is carried horizontally, because of the resultant preheating in the unburned fuel and the in--

crease in rate of spread (fig. 14). Unfortunately the vertical forces that a fire generates over 

its entire area are difficult, if not impossible, to measure or compute. However, the energy 

rates of the fire and wind can be measured and these energy values may help to explain fire 

behavior. 

The conservation of energy relationship for the fire may be written: 

E potential = E combustion + E residue + E loss (9) 

where: 

E potential = heat content of the fuel 

E combustion = heat released by the fire 

E residue = heat content of the unburned fuel and ashes 

E loss = heat content of unburned fuel in the convection column. 

Since the potential energy of the fuel is equal to the weight of the fuel multiplied by the heat 

content per unit weight, the change in weight is proportional to the energy released by the fire. 

6 wt. ~ E potential - E residue = E combustion + E loss (10) 

Figure 14.--Relation of flame 

angle and rate of spread 

u n d e r i n f l u en c e of air 

velocity. 

z 

:i 

...... 

..-= 

LL. 

0 

~ 0::: 

a.. 

C/) 

LL. 

0 

1.1.1 

~ 0::: 5 

WHITE PI~OSA PINE 

.j]" 

~ 

-----.5 1.0 

TAN t 

15 
Fans (_!_~) showed that under no-wind conditions E loss is approximately 5 percent of the 

potential energy of the fuel. Assuming that E loss remains small with wind, E combustion = 

6. wt. x heat content of fuel. Since we are interested in rate changes rather than total changes, 

the rate of weight loss is used and gives the rate of combustion(B.t. u. per minute). The energy 

release rate per unit area of burning fuel may now be defined as the equivalent unit energy release rate, ER, and is obtained by dividing the combustion rate by the combustion area. 

E _ (Weight loss rate) (Energy equivalent of fuel) B /f 2/ . R - (C b . ) . t. u . t. m1n. om ust1on area (11) 

The energy rate per unit area of the wind tunnel airstream equals the dynamic pressure times 

the air velocity. The tangent of the flame angle should then be proportional to these two 

quantities: 

(12) 

where: 

cp = flame angle 

q dynamic pressure of airstream lb. /ft. 2 

U air velocity ft. /min. 

ER = equivalent unit energy release rate B. t. u. /ft. 2 /min. 

J mechanical equivalent of heat ft. lb./B. t. u. 

Tan cp plotted against qU /ERJ (fig. 15) is a straight line on log log paper, with no evidence 

of dependence upon moisture or species for all conditions under which fires were burned. This 

relation also shows a correlation to rate of spread when a sufficiently wide fuel bed is used. 

10-7 o-6 lo-s 

UNIT ENERGY RATE OF AIR STREAM 

EQUIVALENT UNIT ENERGY RELEASE RATE OF FIRE 

16 

The relation qU /ERJ may 

also prove to be a valuable dimensionless parameter for scaling indoor 

fires to outdoor fires. 

Figure 15.--Dependence 

of Tan cp upon unit 

energy rate of fire 

and airstream. 
The flame angle and the rate of spread have been shown to depend upon the combustion 

rate per unit area of the fire. Similarly, ER can be shown to be dependent upon wind. Figure 

16 shows a reduction of ER as air velocity increases. An analysis of variance did not indicate 

a significant difference in ER for the fuel moistures used in these experiments. The independence of ER to fuel moisture is less obvious in the absence of wind. Without wind, the flame 

depth is shallow, and edge effects are stronger. Further testing with a wider range of fuel 

moisture and a better determination of combustion area rna y show a dependence of ER upon fuel 

moisture. 

A simple regression analysis of ER on U produces the equations: 

ponderosa pine 

white pine 

where: 

3240 

ER = 1+0.0069U 

2740 

1+0.0088U 

ER equivalent unit energy release rate B. t. u. /ft. 2 /min. 

U air velocity ft. /min. 

(13) 

(14) 

Further study is needed to develop a hypothesis that will relate ER to fuel bed loading, fuel 

particle size, fuel bed compactness, and air velocity. 

Figure 16.--Influence of air 

velocity upon equivalent 

unit energy release rate. 

tH 2000 

LLI 

~ 0: 

100 200 300 400 500 

AIR VELOCITY •u• FT./ MIN. 

17 
A cursory look at fires would not indicate that ER should decrease as air velocity increases. In mat-type fuel beds, this decrease is explained by the fact that as air velocity increases, combustion area increases faster than total heat release from the fire. It is true that 

total heat output of the fire, after first decreasing slightly, increases with additional air velocity; but at the same time the fire is burning over a much larger area and not burning as deep--

thereby lowering the heat output per unit area. In fuel beds such as ours, that are restricted 

in width, the combustion area is directly proportional to flame depth, where flame depth is the 

distance from the leading edge of the fire to the rear of the fire front. The fire front does not 

include residual burning behind the main fire. The relation between flame depth and rate of 

spread is shown in figure 17. In this figure, moisture and wind influences have already shown 

their effect on rate of spread, and they are not apparently separable. 

While flame depth is increasing, the vertical depth of burn into the fuel bed is decreasing. 

At 704 ft./min. (8 m.p.h.)unburned needles could be found in the lower layers of the ponderosa 

pine fuel beds. Unburned needles could not be found when air was moving slower. Data on 

burning depth are presented only in the data summation tables of this paper, pages 31-34. 

If the fuel bed porosity, A , were high enough to permit burnout of fuel to the full depth 

even in very high air velocities, combustion rate might increase faster than flame depth and ER 

would not decrease as air velocity increases. Again, the need for more research on fuel bed 

parameters 'A and a becomes apparent. 

FIRE CHARACTERISTIC CURVE 

The preceding discussion has shown a relation between the rate of unit energy release of 

the fire and the resulting flame angle in the presence of wind. This relation indicates that if 

ER is large the flame will not be tipped and rate of spread will be sma 11. As wind increases, 

the flame tips, and rate of spread increases as ER decreases. A plot of rate of spread versus 

28 • PONDEROSA WHITE 

26 PINE PINE 

e4.6% M.C. 0 5.5%M.C 

24 • 7.3% M.C. a 8B%M.C 

.9.6% M.C. <)I 1.6% M.C 

22 

20 

...,: 

IL. 18 

"'o 16 

14 :I: 

..... 12 0.. 

LIJ 

0 10 

LIJ 

::!: 8 <( 

.....J 

u.. 

18 

6 

4 

2 

0 

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 

RATE OF SPREAD "R" FT./MIN. 

Figure 17.--Influence of 

m o i s t u r e and air 

velocity on rate of 

s pre ad and flame 

depth. 
ER confirms this and produces the fire characteristic curve shown in figure 18. Fires that 

have a very low and nearly constant rate of spread, a large ratio of flame length to flame depth, 

and a high unit energy release rate are at one end of this curve. These characteristics are 

similar to those of a fire storm, which is a high -intensity stationary fire. At the other end of 

the curve are fires that show a very rapid rate of spread, a low ratio of flame length to depth, 

and a low but nearly constant rate of unit energy release. These are the characteristics of 

runaway fires that burn in fine light fuels. This curve illustrates the rapid increase in rate of 

spread that results when the convection column weakens and tips because the unit energy 

release rate is low. 

The knee of the curve (transition zone) marks the area of minimum values for several 

important fire characteristics- -including flame length, total energy release rate' and the peak 

irradiance (measured by an overhead radiometer). Presence of the transition zone may reflect 

a change in the mechanism of rate of spread as was discussed earlier (see figures 8 and 9). 

This is the only curve that could be found that relates rate of spread of both fuels to a 

common parameter along the same line. Each point on the curve represents a single velocity 

at which all values of fuel moisture for that velocity are averaged. The effects of varying size 

of the fuel particles and configuration of the fuel bed upon this curve will be very interesting 

to see. 

DIFFUSION FLAME ANALYSIS 

The general principles of diffusion flame theory presented by Thomas (~, 20) applied to 

fires in mat -type fuel beds (_~, p. 18) if the characteristic length, D, in equation (15) is revised 

to equal the distance from the front of the fire to the rear: 

Figure 18. --Fire characteristic curve relating rate of spread 

and equivalent unit 

energy release rate. 

:a:: 

c 

~ 

g: 10 (/) 

LL. 

0 

LLJ 

~ 5 

'-' ~RUNAWAY FIRES 

~ 

~ 

\ /TRANSITION 

AIR VELOCITY 

FT./ MIN. 

PONDEROSA e O.O 0 WHITE • 132 0 

PINE • 264A PINE • 44()[) 

ZONE 

$ 7040 ER EXTRAPOLATED 

FROM EQUATIONS 13 a 14 

~ FIRE STORM\, 

~-----·-------------

EQUIVALENT UNIT ENERGY RELEASE RATE •ER• BTU/MIN./FT.2 

19 
L 

D 

where: 

L 

D 

m" 

1-<m") · ..]n K~ (gD)2 

flame length in feet (distance from base to tip of fire) 

flame depth in feet 

2 unit mass flow rate in lbs. /min. /ft. 

-2 3 

p air density (8. 1 X 10 lbs. /ft. ) 

5 ::::> 

g = gravitational constant (1. 152 X 10 ft. /min.-). 

(15) 

To represent the orifice diameter, we used flame depth rather thun some equivalent 

dimension based on the square root of flame depth multipUed by fuel bed width. The value of 

m" (mass flow rate of combustion gases per unit area) was determined from the weight loss 

data in much the same manner as the equivalent unit energy release rate ER was computed. 

When results of this analysis were compared to the relation developed by Thomas' equation 

( 16), they showed good correlation (fig. 19). 

_!::_ = 42 (m ") _ • 

[ ] 

0 .. 61. 

D p(gD)2 

( 16) 

The dimension of flame depth appears to be the correct measure to use, and radial symmetry 

is unnecessary. For our fuel beds the numerical values on both sides of the equation increased 

as moisture content increased. 

L 

0 

20 

5 

0 WHITE PINE 

e PONDEROSA PINE 

Figure 19. --Diffusion 

flame char u c t e ri stic s of fl res under 

no-wind conditions 

compared to values 

in Thomas' equation. 
Thomas extended his work to include the influence of air velocity; we checked our data 

against his analysis. The values for each moisture level were averaged at each air velocity. 

The relations tested were of the form: 

(17) 

These were compared to Thomas' results (fig. 20). A slight difference is evident between white 

pine and ponderosa pine fires, but a single line can approximate the relation for both fuels. The 

equation of this line, plotted from our data, is: 

L (U2_}0-ll 

D gD 

164 (m") 

p(gD)2 

-0.97 

whereas Thomas obtained this equation: 

- -- = 7 0 ----'----'-.....- L (U2) o. ll [ (m ") 1o .43 

D gD p(gD)2 

( 18) 

(19) 

The difference apparent between the two sets of data may be attributed to differences in me asurement techniques, or possibly to variations in size of fue 1 particles. 

A series of outdoor fires was studied in prepared fuel beds of logging slash during the 

summer of 1962 (_~, p. 18). The fuel was lodgepole pine and Douglas-fir needles, and branch 

wood up to 2 inches in diameter. The fuel beds, 6 feet wide by 60 feet long, were ignited at one 

end. We recorded data similar to those recorded for the laboratory fires. 

Figure 20.--Diffusion 

flame characteristics for labor atory fires under 

the in f 1 u en c e of 

wind. 

~ c 

0 

I~ r-----------~----------~----------,-----------, 

~ ~~----------~------~~~r-----------;-----------~ 0 

~ 

,.O(gD)r/2 

WHITE PINE 

0 WIND 

C NO WIND 

PONDEROSA PINE 

e WIND 

• NO WIND 

21 
The outdoor fires were checked by the diffusion flame analysis for wind to determine their 

correlation with laboratory fires. Since the actual rate of weight loss could not be measured, 

we used maximum burning rate to compute mass flow rate of the combustion gases. The mass 

flow rate was determined from the equation: 

mil 

W0 R 

- -D- lbs./ft. 2/min. (20) 

where: 

W 0 = loading in lbs • /ft . 

2 

R rate of spread in ft. /min. 

D = flame depth in feet I 

The relation given tn equation) 1 7) was applied and the data were plotted (fig. 21). 

The field d~ta (fig. 21f fell to the right of the pine needle laboratory data and Thomas' 

!-inch stick data ·I This was expected because all the fuel was assumed to be contributing to the 

mass flow rate m 11 in the fire front. In a heterogeneous fuel bed the fine fuels ignite first and 

therefore contribute most heavily to the fire front. If the fine fuels up to one-fourth inch in 

diameter are assumed to be supporting the initial flame size, then rn 11 is reduced to a value 

that alines the field data with the laboratory pine needle data. If all the fuel up to one- half 

inch in diameter is assumed to be contributing, then the field data aline with Thomas' !-inch 

crib data. 

This demonstrates a possible use for the diffusion flame characteristic equation provided 

the size classification of the fuel is known. When this is known, the burning rate per unit area, 

or mass flow rate, can be determined. If forest fuel can be classified so that loading and size 

percentages are known, estimates of mass flow rate (rn 11

) and equivalent unit energy release 

rate (ER) can be made. If ER can be so estimated for outdoor fires, it may be possible to 

relate fires of different sizes with the fire characteristic curve (fig. 18). 

~·r-----------~----------~------------~----------~ 

<> LODGEPOLE 

PINE 

• DOUGLAS FIR 

J 

0 

N~ 10'~-----------+--------~~~~--------~----------~ ::J 

0 

....... 

_J 

22 

~~~~----~--~~--------~------------._----------~ 10• 4 I0-3 10-2 10-1 100 

m• 

p (gO) 112 

Figure 21 . - -Diffusion 

f 1 a rn e characteri s t i c s f o r field 

fires under the influence of wind. 

\- . ~ . '... 
SUMMARY AND CONCLUSIONS 

Our studies of the effects of fuel moisture and wind upon the rate of fire spread in ponderosa pine and white pine needle fuel beds have shown the following: 

1 • As wind increased, the fire spread at an increasing rate. 

2. The fire was carried in the surface fuel particles. 

3. Flame depth increased and vertical depth of burn decreased as windspeed increased. 

4. In the absence of wind, rate of spread decreased linearly as fuel moisture increased, 

Additional experiments were conducted to aid in explaining how fire is propagated. For 

these, a thermocouple was placed inside a needle to register changes in temperature. These 

experiments showed the following: 

1 . The thermocouple began to show rise in temperature when the fire was still two flame 

lengths away. 

2. Temperature inside the needle became higher than that for the boiling point of water 

before the fire reached it. 

3. In the absence of wind, the initial rise in temperature must be due to radiation from 

the overhead flame because of the large distance from the fire and because air is flowing 

toward the ~ire, not away from it. 

4. When the fire draws close to the needle, radiation from within the fuel bed may also 

contribute to the temperature rise. 

5. In the presence of wind, the air temperature at the surface of the fuel bed was found 

to be hotter than the fuel; therefore, both convective heating and radiant heating can contribute 

to the rise in temperature of the fuel particles, 

6. In the presence of wind, combustible gases may form on the surface of the fuel bed 

ahead of the main flame front. The exact source of these gases is not known. These gases 

seem to be ignited periodically by the fire and to sweep ahead of the main fire front along the 

surface of the fuel. They appear to play a major role in the mechanism of fire spread in the 

presence of wind. 

Results of this study of the effect of wind upon rate of spread in fine fuels do not corre~ 

spond to re~ults of other studies that used cribs made of dimensioned sticks, Possibly this may 

be explained by differences in the fuel particle size, o, and fuel bed porosity, 'A , There was a 

correlation between rate of spread and the product of cr and 'A. 

These studies revealed the importance of the equivalent unit energy release rate. Rate 

of spread plotted against equivalent unit energy release rate produce$ a curve on which the 

average values of both fuel types fall. This curve clearly demonstrates the characteristics of 

a fire; therefore it is named the fire characteristic curve. Fires having low rates of ~pread 

and high rates of energy release are at one end of the curve and have many characteristic~ of a 

fire storm. Fires with rapid rates of spread and low, nearly constant, energy r~lease values 

23 
are at the other end of the curve and have the characteristics of runaway fires. The knee of the 

curve represents a transition zone in which the mechanism of spread is changing from the nowind to the wind model. Several important parameters reach minimum values at the transition 

zone. 

Diffusion flame analysis that Thomas used on crib fires was applicable to mat-type fuel 

bed fires if flame depth was used in place of equivalent orifice diameter. The use of this 

analysis may enable us to predict burning rates of outdoor fires from their flame dimensions. 

LITERATURE CITED 

( 1) Anderson, Hal E. 

1964. Mechanisms of fire spread research progress report no. 1. U.S. Forest Serv. 

Res. Pap. INT-8, 20 pp., illus. 

(2) , and R. C . Rothermel. ---

1964. Influence of moisture and wind on the characteristics of free-burning fires. 

Tenth Int. Symp. on Combust. Proc., Cambridge, England. 10 pp., illus. 

( 3) Breuer, Erwin H. 

(4) 

1965. A strain gage differential weighing system. U.S. Forest Serv. Res. Note INT40, 8pp., illus. 

1965. Thermocouples for forest fire research. U.S. Forest Serv. Res. Note INT -31, 

7 pp., illus. 

(5) Bruce, H. D., W. Y. Pong, andW. L. Fons. 

1961. The effect of density and thermal diffusivity of wood on the rate of burning of 

wood cribs. U.S. Forest Serv. , Pacific Southwest Forest and Range Exp. 

Sta. Tech. Pap. 63, 14 pp., illus. 

(6) Byram, G. M., H. B. Clements, E. R. Elliott, and P. M. George. 

(7) 

(8) 

(9) 

24 

1964. An experimental study of model fires. U.S. Forest Serv. , Southeastern Forest 

Exp. Sta. Tech. Rep. 3, 36 pp., illus. 

Curry, John R., and W. L. Fons. 

1938. Rate of spread of surface fires in the ponderosa pine type of California. J. Agr. 

Res. 57:239-267, illus. 

___ ,and W. L. Fons. 

1940. Forest fire behavior studies. Mech. Eng. 62: 219-225, illus. 

Emmons, Howard. 

1963. Fire in the forest. Fire Res. Abstr. and Rev. 5: 163-17 8. 
(10) Fahnestock, George R. 

1960. Logging slash flammability. U.S. Forest Serv., Intermountain Forest and 

Range Exp. Sta. Res. Pap. 58, 67 pp., ill us. 

(11) Fons, Wallace L. 

1946. Analysis of fire spread in light forest fuels. J. Agr. Res. 72: 93-121, illus. 

(12) H. D. Bruce, W. Y. Pong, and S. S. Richards. 

1960. Project fire model summary progress report. U.S. Forest Serv. , Pacific 

Southwest Forest and Range Exp. Sta. , 56 pp. , ill us. 

(13) H. B. Clements, E. R. Elliott, and P. M. George. 

1962. Project fire model summary progress report II, U.S. Forest Serv ., Southeastern Forest Exp. Sta. , 55 pp. , ill us. 

(14) Gross,D.J. 

1962. Experiments on the burning of cross piles of wood. J. Res. Nat. Bur. Stand. 

(C. Eng. and Instrum.) 66 C. 

(15) Hawley, L. F. 

1926. Theoretical considerations regarding factors which influence forest fires. J. 

Forest. 24: 756-763. 

(16) Schuette, Robert D. 

1965. Preparing reproducible pine needle fuel beds. U.S. Forest Serv. Res. Note 

INT-36, 7 pp., illus. 

(17) Thomas, P. H. 

(18) 

1958. On the rate of burning of cribs of wood. Joint Fire Res. Organ., Boreham 

Wood, Herts, England. Fire Res. Note 370. 

1963. The size of flames from natural fires. Ninth Int. Symp. on Combust. , pp. 844-

859. New York: Academic Press, Inc. 

( 19) , and R. W. Pickard. 

1961. Fire spread in forest and heathland materials. Report on Forest Research, p. 

105. Stockport, England: The North Western Printers, Ltd. 

(20) , C. T. Webster, and M. M. Raftery. 

1961. Some experiments on buoyant diffusion flames. Combust. and Flame 5: 359-367. 

25 
APPENDIX A 

Symbol Definition Measure 

Ac area of combustion at surface of fuel ft.2 

B = rate of weight loss (burning rate) lbs. /min. 

D = depth of flame zone from leading edge of fire to 

trailing edge of fire front ft. 

ER = equivalent unit area energy release rate B.t.u./ft. 2 /min. 

H = low heat of combustion value of fuel B.t. u. /lb. 

10H = peak overhead irradiance B. t. u·. /ft. 2 /HR 

IT = total radiant heat received during passage of the flame B. t. u. /ft. 2 

IJ.e = irradiance to a point 18 feet forward of the fire B. t. u . /ft. 2 /HR 

J = mechanical equivalent of heat ft. lb./B.t.u. 

L = flame length from midpoint of flame depth to tip of flame ft. 

Lv = vertical height of flame tip above fuel surface ft. 

Ma = unit mass of air lb. /ft. 3 

Mf = fuel moisture content percent 

Mg = unit mass of convection column gases lb. /ft. 3 

Q = total heat release rate B. t. u. /lb. 

R = rate of spread at any condition ft./min. 

RH = relative humidity percent 

Ro = rate of spread in absence of wind ft. /min. 

Rpp = rate of spread in ponderosa pine needles ft. /min. 

Rwp = rate of spread in white pine needles ft. /min. 

To = ambient temperature op. 

tb burn thickness ft. 

u = velocity of air ft. /min. 

v = velocity of convection column ft. /min. 

Wo = unit area loading of ovendry fuel lb. /ft. 2 

g = acceleration due to gravity ft./min.2 

m" = mass flow rate of combustible gases per unit area lb. /ft.2 /min. 

q = dynamic pressure of air stream lb. /ft. 2 .·· 

-e ;:: residence time of flaming zone at any point in fuel bed min. 

f... ;::: void volume per unit of fuel surface area ft._s;~-

air density 3 p ~ lb. /ft. 

a :;:: fuel particle surface area per unit volume ft. 2 /ft. 3 

<p deflection angle of flame from vertical degrees '. = 

26 ... 
APPENDIX B 

INFLUENCE OF SURFACE FLOW OF WIND UPON RATE OF SPREAD 

A series of preliminary fires in the wind tunnel was used to check -instrumentation and 

experimental techniques. During these tests the fire spread four to seven times faster in the 

first half of the fuel bed. The flames were tipped considerably farther in the first 4 feet; this 

indicated a stronger influence of the wind upon this portion of the fire. 

In a smoke test, a tight rolling vortex was observed over the first 2 feet of the fuel bed; 

its diameter gradually increased, and the vortex dissipated over the last 4 feet of the bed. The 

boundary layer was investigated with a constant-current hot wire anemometer. The hot wire 

confirmed the visual observations of the smoke and showed the vortex formation, breakup, and 

dissipation (fig. 22). The center of the vortex gradually rose from 1 inch at fuel bed station, 

0. 5 foot to 3 inches at station 1 . 0 foot, and to 5. 0 inches at station 2. 0 feet. During this time 

If) 

w G I z 

0 

w 

Ill 

6 

4 

2 

FUEL BED STA. 0.0 FT. 

FUEL BED STA. 2.0 FT. 

• 

FUEL BED STA. 5.0 FT. 

FUEL BED STA. 0.5 FT. 

FUEL BED STA. 3.0 FT. 

~ ~ 

\ 

....... ·-· ·-·--

FUEL BED STA. 60FT. 

'\.'------. 

) 0 5 1015 2025 30 354045 50 

TURBULENCE LEVEL, PER CENT 

FUEL BED STA. 1.0 FT. 

·-· ·~ -· 

FUEL BED STA. 4.0 FT. 

\ FUEL BED STA. 7.0 FT. 

.'----

.\ 

., 

I ...... /. 0 5 10 15 20 25 30 30 40 46 50 

Figure 22.- -Surface layer turbulence over ponderosa pine needle fuel bed at freestream air velocity of 3 m.p.h. 

27 

.. -.·. 
the -------- diameter of the vortex grew larger until at 4. 0 feet it no longer held any well-defined shape 

and broke up. At 5, 6, and 7 feet the air motion appeared to be random; and the turbulence 

level, which had been as high as 50 percent, decreased to 25 to 30 percent at 7 feet. 

These measurements were made over the fuel bed in the absence of any fire and, of 

course, the same turbulence pattern would not exist if the fuel bed were burning. The me asurements do show, however, that when the fuel bed is ignited at station zero, the free-stream 

air velocity extends down to the surface of the fuel bed and can tip the weak flame almost 90° 

over the unburned fuel. The free-stream air velocity is then dominant over the flame until the 

energy output of the fire builds up and straightens the flame. This did not occur until the fire 

was well into the fuel. 

The solution to the problem was to lift the free-stream air 'velocity above the surface of 

the fuel at station 0. 0 by placing a trip fence or spoiler across the fuel bed 1 foot ahead of the 

fuel bed. The trip fence was 1-5/8 inches high and induced a turbulent boundary layer which 

was surprisingly uniform over the entire length of the fuel bed. Results of a hot wire turbulence survey with the spoiler in place (fig. 23) showed that beyond 2 feet the vortex appears 

to have dissipated and the maximum turbulence levels are about 30 percent. 

28 

.J 

w 

:::> 

LL 

w 

~ ID 

<f 

FUEL BED STA. QO FT. 

\ ---~- --...... _ 

.... ~ 

·-. FUEL BED STA. 2.0 FT. 

10 \ 

8 

---· --..... 6 .. , 

........... 4 

0 

2 

~ 

FUEL BED STA. 5.0 FT. 

2 

~~~----._._~~~~_.~~~_.50 

Fl.eL BED STA. 05 FT. 

FUEL BED STA. 3.0 FT. 

• \...,. __ __ 

\._ 

FUEL BED STA. 6.0 FT. 

~ I • \ 

' .... . , 

Z-... 

\. 

•• 0 5 10 15 20 25 ~ 35 ~ 4~ 50 

TURBULENCE LEVEL - PER CENT 

I 

I 

• 

FLU BED STA I 0 FT. 

FUEL BED STA. 4.0 FT . 

I FUEL BED STA. 7.0 FT. I • I 

I 

.,,_ ....... 

l,. 

~-- 0 510 15aJ2530M404550 

Figure 23.- -Surface layer turbulence over ponderosa pine needle fuel bed with spoiler 

at free-stream air velocity of 3 m.p.h. 
Fuel beds burned behind the spoiler produced a uniform rate of spread from 2 feet to 7. 5 

feet. Typical rate of spread data for fires burned with and without the spoiler are shown in 

figure 24. All wind tunnel data presented in this report were taken with the spoiler in place. 

Figure 24. --Effect of 

surface layer turbulence on rate of 

spread with and 

without a spoiler. 

t-= 

1L.. 

LLI 

(,) 

z 

(/) 

~ 

c 

,,. ..... / 

/ 

//~/ WITHOUT SPOILER -----;v _,.- /• . .... . 

,/:"' ./ 

//•""/// A-wiTH SPOILER 

//

/. /' / PONDEROSA PINE 

'/ • 264 FT./MIN. 

/ / 15% R.H. 

T • 1/ 

I 

2.5 3.0 35 

TIME 

29 
w 

0 

Pire no. 

PP-1-C 

:-1'-2-C 

!'P-3-C 

PP-17-C 

<'P-1f'-C 

l -l -: 

PP-26-C 

PP-27-C 

PP-28-C 

PP-4-C 

PP-5-C 

PP-6-c 

PP-7-C 

PP-8-C 

PP-9-C 

?P-32-C 

PP-33-C 

PP-34-C 

PP-10-C 

PP-ll-C 

PP-12-C 

PP-29-c 

PP-30-C 

PP-31-c 

PP-13-c 

PP-14-C 

PP-15-C 

PP-16-c 

PP-20-c 

PP-21-C 

!'P-22-C 

PP-23-C 

Te~!n:~::o:b!rt 

To RH 

•p ! 

93 13 

92.5 13.5 

92 12 

91.8 15.4 

90.7 15.5 

~~-0 15.6 

91 15.6 

90.8 15.7 

90.6 15.7 

92 75 

91 75 

92 75 

91 52 

92 53 

92 53 

90.5 50 

90.7 50.4 

90.5 50.8 

92 33 

92 34 

91 34 

90.5 30.2 

91 30.6 

90.8 30.1 

91 22 

!'11.2 22.5 

92 22.5 

92 21.5 

90.9 9.8 

92.0 8.1 

91.8 9.2 

99 6.6 

l"uel and tuel bed partlllletera 

Mr •o H 

1. 1b/tt2 Rtu/1b 

4.20 0.4? 8869 

4.41 0.48 8869 

4.38 0.48 8869 

4.97 0.48 8869 

4.78 0.48 8869 

5.26 0,47 c'86C 

4.46 0.48 8840 

4.51 0.48 8840 

4.93 0.48 8840 

13.15 0.48 8869 

12.21 0.49 8869 

12.88 0.48 8869 

9.26 0.48 8869 

9.39 0.48 8869 

9.26 0.48 8869 

9.16 0.45 8840 

9.11 0.45 8840 

9.20 0.45 8840 

6.92 0.48 8869 

7.28 0.48 8869 

7.25 0.48 8869 

6.48 0.47 8840 

6.52 0.47 8840 

6.53 0.47 8840 

5.96 0.47 8869 

5.54 0.47 8869 

5.64 0.47 8869 

5.76 0.47 8869 

3.81 0.48 8869 

3.83 0.48 8869 

3.77 0.48 8869 

2.24 0.49 88t\9 

Table 2 

Burnins part~~~~~~iera 

R e B Ac tb 

tt/min lllin 1b/m1n tt2 t't 

0.845 0.50 - 0.63 - 0.860 0.50 - 0.65 - 0.900 0.51 - 0.69 - 0.863 0.59 0.540 1.29 0.13 

0.805 0.66 0.550 1.14 0.17 

0.792 l-.69 0.".60 1.20 l.h 

0.840 1.09 0.601 1.37 0.25 

o.80o 1.27 0.548 1.53 0.24 

0.846 1.07 0.532 1.36 0.22 

0.470 0.31 - 0.23 ~ 

0.495 0.32 - 0•24 - 0.495 0.21 - 0.17 -

0.60 1.10 - 0.99 - 0.63 0.95 - 0.90 - 0.625 0.87 - 0.83 - 0.532 1.33 0.481 1.06 0.33 

0.586 1.57 0.433 1.38 0.27 

0.603 1.39 0.394 1.25 0.25 

0.804 0.73 - 0.98 - 0.735 0.58 - 0.97 - 0.690 0.65 - 1.05 

0.674 1.176 0.« 1.19 0.23 

0.?18 1.318 0.418 1.42 0.20 

0.740 1.313 0.438 1.46 0.21 

0.830 0.720 - 0.90 - 0.719 0.550 - 0.60 - 0.669 o.~ - 0.50 - 0.781 0.730 - 0.86 -

0.88 0.59 0.61 1.25 0.15 

0.91 0.65 0.55 1.22 0.15 

0.82 0.60 0.63 1.19 0.17 

0.92 0.58 - 1.S5 -

!"lame fleai 

d1mena1ona 

D L 'l ~ 

t't t't Btu/lllin ~tt2 aia !

0.42 3.2 - - 

0.43 .:. 2 - - 

0.46 3.8 - - 

0.86 3.5 4789 - 

0.?6 3.5 4€?8 - 

0.80 3.5 4080 - 

0.915 3.55 5315 3880 

1.02 3.51 4844 326~ 

0.91 3.29 4700 M40 

.e-;15 3. 2 - - 

0.16 3.2 - - 

0.11 3.3 - - 

0.66 4.0 - - 

0.60 4.0 - 

0.55 4.0 - - 

0.708 2.85 4250 4000 

0.918 2.91 3830 2780 

0.836 2.75 3330 2660 

0.65 3.4 

0.64 4.0 - - 

0.70 3.8 - - 

0.793 3.16 «20 3715 

0.946 3.28 3980 2800 

0.972 3.04 3862 2650 

0.60 3.58 - - 

0.40 3.17 - - 

0.33 3.16 - - 

0.5'7 3.44 - - 

0.83 2.83 5410 4S28 

0.81 2.79 4878 3998 

0.'79 2.74 5587 4695 

0.90 3.30 - - 
w 

,_. 

Table 3 

Pire no. Ta:!n!:~o:"Jlent Jlllei u4 tiiil bed parametfl"ll Fuming parsmetera on!!:': one - ~ n.r1ab1e• 

To 1111 lit 'lo R I! e B Aa 1:b D L :1 Sa ltl[ 

., ~ ~ ~:!!!n2 l!tu/lb t't/min min 1b/m1n n 2 :rt n n !!!im1n Btu/rto2 lll1n ~n hr 

IP-1..0 113 u ~-1.6 o.u 8476 o.~ o.M - 0.86 - 0.57 0.9 - - - WP-2-C 92 13 5.55 0.47 8476 0.942 0.40 - 0.87 0.58 1.1 - - IP-3-C 93 12 5.37 0.4? 8476 0.865 0.34 - 0.83 - 0.55 1.0 - - 111'-17-C 91.5 115 5.71 o." 8476 0.829 0.44 - 0.60 - 0.40 1.02 - - - 111'-18-C 90.15 115.!1 s.w 0.47 84'n 0.785 0.:58 0.115 0.51 0.14 0.34 1.07 2967 5818 - IP-19-C 91.3 16.3 6.0'1. '0.4'7 84.76 0.811 0.42 0.30 0.36 0.19 1!.24 0.94 2543 7064 - •-26-C 91 115.7 5.23 0.47 8422 0.93 0.67 0.185 0.94 0.07 0.6!4 ~-42 1557 16156 19.0 

IP-27-C 90 15.7 5.52 0.411 8422 0.'.!1 0.63 0.222 0.86 0.09 0.574 1.36 1870 2174 12.68 

IP-28-C 90.'1 u.s 15.42 0.47 M2Z 0,8215 0.46 0.140 0.157 0.06 0.380 1.05 11'79 :2068 u.oe 

'11'-+-C 91 74 U.33 0.'&8 84'16 0.:586 0.36 0.21 - o.u 0.75 - - - .. -5-c 92 74 17.62 0.15 8476 - - - - 0.50 - - - IP-6-c 92 74 lll.!59 0.48 84'16 0.4015 0.!8 - 0.17 - 0.11 0.79 - - - .,-7-c 93 '13 1.5.4'1 0.<&'7 8476 0.344 0.21 - o.u - ·0.0'1 0.88 - -

11141-Cl 92 52 ll.·tl! 0.<&'1 8476 0.515 0.48 - 0.39 - ·0,26 0.86 - - ....... c 92 51 11.28 0.<&'1 8476 0.$4 0.48 - 0.3'.! - 0.26 1.20 - - - IP-10-C 92 52 1.!.32 0.<&7 8<&'16 0.515 0.43 - 0.315 - 0.23 1.1'1 - - - _..::52-C !10.7 50.2 10.30 o . .s 8422 0.638 0.562 0.183 0.15<& o.u O.Sll9 1.1<& 1540 2860 13.<&6 

IP-33-C 90.9 50.1 10.89 0.-&5 8<&22 0.668 0.36'1 0.1'18 0.38 0.10 0.2415 1.02 1495 4068 5.55 

liP-34-C !10.4 150.8 10.89 O;o&S 8<&22 0,663 0.342 0.18 0.34 0.10 0.227 0.96 1515 444'1 4.'15 

IP-1.1-c n 33.15 8.13 0 .• 4'7 8476 0.'158 0.60 - 0.60 - 0.40 1.3 - - WP-12-C 12 32.15 l'.M '0.48 8676 0.5815 0.43 - 0.47 - 0.31 1.1 - - - W'-1$-C 91!.15 33 8.00 0.47 84'16 0.68<& 0.64 - 0.38 - 0,!15 1.1 - - - .,_29-C 90.0 30.1 '7.U 0.46 94ft 0.'71515 0.41.'7 '0.140 0.4'1 0.0'1 O.:llll 1.4 11'19 Mtll ll!.83 

•-a-c 91 19.8 '1.95 0.46 8422 0.804 0.150 0.164 0.60 ·0.07 0•402 1.31 1383 2294 15.83 

•-a-c 111.11 JQ,g '1.!13 0.46 8422 0.878 0.43 0.176 0.5'1 0.0'1 0,3'nl 1.4 1482 2615 14-215 

•-u.c 111.11 11.5 6.46 0.47 8476 o.eo4 O.M - 0.611 - 0.43 0.'7415 - - - IP-l.S-C 111-11 13.0 6.153 Q.4'T 8476 0.?14 0.61 - 0.615 - 0.43 0.980 - - - w-11-C 9&.0- J!4.11 1.11 0.4'1 84'16 0.'1153 o.o&& - 0.153 - O.Sll 0.'143 - - - 'W..al-1: n.s 8.t 4.6'1 0.411 8"6 1.02 0.63 - 0.'12 - 0,4f! 0.88 - - - -..a3.-C -------lld -- 7.8 1.'157 0.48 84'16 0.11:1 0.4'1 0.11 0.415 o.oa 0.30 0.'71 118'1 2638 - IP..a-c 

" 10.8 4.66 o.ts ~ o.~ 0.33 0.31 0.51 0.1? O.M 0.91 l!6l!8 151153 -

•-a-c uo 1.4 !.59 0.49 8476 1.10 0.66 O.!!l! 1.19 0.06 0.'19 1.115 1!107 1603 -

Y~.-.~~x-1 (, ... .,. .. 111tte Jtu • -n•o DoQil.) 

Y ou..r ~er uU ~1'011111""' \II• •tudJ: .... 1'0 .. JiM: 

,. • 31.1 1111n3 

u • lUI nttn3 

A • OAM n'3/,_a 

...,._*IHJlM 

,. • ~-· ~ftS r • u. nt/n3 

)\ • O.OM W/n1 
VJ 

t-.J 

F1re Ng 

"P-1-RL 

PP-2-1.1. 

PP-3-"RL 

PP-4-lrl 

PP-5-lll.. 

Pl'-6- 'o'L 

PP-:57-'IfL 

PP-38-'IL 

PP-39-IfL 

PP-7-WL 

Itt~ PP-45-"RL 

PP-46-'IL 

PP-47-'.'L 

!>P-41-.'.'1. 

PP-42-"',1 

PP-43-.';L 

I'P-44--.\'L 

PP-10-.'IL 

PP...ll-:¥1. 

PP-12-¥1. 

PP-31-','/I. 

'""-32-'IL 

l'P-13-lll. 

l'P-14- 'IL 

''P-15-'o'L 

!'!'-16-I'L 

iP-17-RL 

PP-1R-IL 

PP-34-'fL 

PP-35-'.fL 

PP-2f'-WL 

f P-2"-11. 

-P-3·~-n 

l'P-1~- 11 

PP-2C-WL 

-·P-::'1- :'1 

!"P-2 ... -'fl 

PP-?3-'II 

PP-~4-11 

PP-:>!i-WL 

PP-26-WL 

!'P-27-WL 

Teet erea abient 

oondlttone !/ 

u To RH 

ft/p1n .. ' 132 95.5 16.3 

132 97.0 15.5 

132 91.8 15 

264 96 14.3 264 9'1 13 264 92.3 15.9 264 90.3 115.1 

264 90.6 15.7 

264 90.7 15.5 440 95 12.7 440 94..5 ig:~ 440 90.0 440 91.0 15.6 440 91.2 15.5 

440 91.6 u;.o 

704 90 15 704 90 15 

704 90 15 704 90 15 

132 91 30.8 132 91.3 31.8 132 92.5 30.0 132 89.9 30.6 132 90.6 so.o 264 92.2 31.4. 

?64 91.8 30.7 

l'64 91.6 30.5 440 !'1.5 31.8 

>l40 92.0 30.5 >i40 91.5 31.2 

>lf.O 90 •• 30.0 

440 90.8 so.o 704 90 30 70>i 90 30 704 90 so 

132 90 50.9 132 90.2 50.5 1~::! 90.1 50.2 264 90.!1 50.8 264 90.9 50.9 2~4 90.1 50.5 44C 90.8 48.5 

440 90.5 4.9.6 4tf.O 90.7 50.!1 

:r.l u4 fall bed 

par ... tere 

1ft •o H 

' Jbltt2 Bt;n/Jb 

4.53 0.48 8869 

4.56 0.48 8869 

4.22 0.48 8869 3.'14. 0.48 8869 3.78 0.48 8869 5.10 0.4.7 8877 

4..63 0.4.8 8812 4.63 0.48 8812 

4..52 0.48 8812 

4.14 0.4.8 8869 

~:&i 0.48 88'1'1 0.48 8869 4.96 0.48 8812 4.80 0.48 8812 

4.72 0.48 8812 

4.'18 0.48 8812 4.75 0.48 8812 

4.97 0.48 8812 5.00 0.48 8812 

7.46 0.46 8877 7.60 0.46 8877 7.50 0.46 8877 6.93 0.4.7 8812 7.17 0.46 8812 

7.54 0.46 8877 

7.50 0.46 8877 7.27 0.46 8877 7.87 O.>i6 8812 7.72 0.46 8812 7.77 O.>i6 8812 

8.06 0.46 8812 

8.20 0.46 8812 7.32 O.>lti 881'2 7.32 0.46 8812 6.12 0.47 8812 

9.49 0.45 8812 9.50 0.45 8812 9.43 0.45 8812 9.82 0.4.5 8812 9.94 0.45 8812 10.03 0.45 8El12 9.51 0.45 8812 

9.80 0.45 8812 10.07 0.45 8812 

Table 4 

Barnillc parBMtere 

R e B .lo Tb 

ft/m1p m1p Jb/wtn tt;2 f* 

1.52 0.96 0.65 2.19 0.15 

1.68 1.38 0.562 3.48 0.12 

1.67 1.13 0.605 2.82 0.12 

2.40 1.86 0.791 6.69 0.12 2.&& 1.60 1.03 6.81 0.13 1.89 1.4.7 0.48 4.1'1 0.13 

2.12 1.62 0.58 5.15 0.09 2.01 1.40 0.593 4.22 0.10 2.06 1.70 0.624 5.24 0.06 3.71 1.'12 1.12 9.57 0.11 3.52 1.88 1.02 9.90 0.10 3.95 1.74 0.97 9.24 0.09 4.88 1.64. 1.22 12.0 0.09 5.34 2.03 0.80 16.2 0.05 

5.48 1.42 0.87 11.7 0.05 

16.7 2.04 1.93 51.2 0.04 12.7 1.99 -- 38.0 -- 13.8 2.15 2.24 44.6 0.06 11.6 1.84 2.10 32.0 0.07 

1.17 1.02 0.25 1.79 0.08 1.24 1.12 0.31 2.09 0.09 1.17 0.98 0.33 1.71 0.10 1.11 0.94 0.321 1.56 0.11 1.33 0.76 0.393 1.52 0.11 1.72 1.66 0.50 4.28 0.11 1.96 1.53 0.48 4.49 0.09 1.85 1.56 0.35 4.32 0.07 3.76 1.50 o.8>l 8.45 o.o8 4.10 2.01 0.83 12.33 0.08 4.95 2.25 0.70 16.68 0.05 3.57 1.52 0.85 8.15 0.08 3.60 1.69 0.74 9.12 0.07 9.81 2.02 1.19 29.7 0.04 10.5 1.74 1.26 27.5 0.05 9.3 1.87 1.45 26.1 0.06 

0.794 0.78 0.20 0.93 0.10 0.8'14 0.86 0.206 1.13 0.09 0.921 0.93 0.21 1.29 o.o8 1.543 0.91 0.352 2.10 0.09 1.568 1.03 0.358 2.43 0.09 1.436 0.98 0.366 2.16 0.09 3.54. 1.31 0.638 6.96 0.07 s.e 1.4.3 0.689 7.83 0.07 S.23 1.38 0.700 6.69 o.o8 

J'l.aM 4S.ne1one Heat 

D L H 

• E ~ 

n 1'1; a d .. Btul•ja B:li11ll::li1 •J

1.46 2.39 2.13 28.6 5,760 2,617 

2.32 2.61 2.38 2t!.7 4,4.90 1,423 

1.88 2.13 1.99 20.8 5,360 1,892 

4..4.6 2.12 1.'11 4S '1,020 1,04.:5 

4.54 2.28 1.94. 4S 9,140 1,34.4 

2.78 2.02 1.'12 4.3 4,265 1,01'1 

3.4.3 2.12 1.'14. ss 4.,870 999 

2.81 2.28 1.'12 39 5,000 1,238 

3,4.9 2.32 1.99 30 5,500 1,04.:5 

6.38 2.83 1.'15 53.2 10,630 1,034 

6.60 2.68 1.75 46.6 :·~ 911 

6.16 2.94. 1.92 4'1.8 928 

8.00 -- -- 45 10:'150 &&0 

10.8 -- -- 41 '1,060 436 

7.'18 -- -- 41 7,650 656 

34.1 -- -- 57 1'1,000 332 

25.3 3.66 1.82 56 -- -- 

29.7 3.37 1.53 61 19,'100 4.42 

21.3 3.53 1.63 57 18,500 583 

1.19 1.84 1.73 21 2,219 1,238 

1.39 1.69 1.55 25 2,779 1,326 

1.14 1.67 1.52 23 2,956 1,724 

1.04 2,85 2.49 28 2,830 ~:~~f 

1.01 2.32 1.93 34 3,.SO 

2.85 1.61 1.36 31 4,4.39 1,034. 

2.99 1.48 1.28 30 4,261 946 

2.88 1.55 1.38 32 3,107 716 

5.63 1.72 1.29 4.6 7,4.02 875 

8.22 2.0. 1.5. 40 7,314 592 

11.12 1.90 1.47 44 6,168 371 

5.43 2.71 1.90 44 7,500 919 

6.08 2.84 2.10 n 6,550 716 

19.8 3.32 1.59 56 10,420 354 

18.3 3.68 1.63 61 11,170 4.07 

17.4 •• 19 1.71 61 12,800 486 

0.62 1.73 1.51 26 1,763 1,901 

0.75 1.35 1.24 19 1,818 1,653 

0.86 1.47 1.32 20 1,853 1,441 

1.40 1.85 1.47 36 3,105 1,476 

1.62 1.78 1.4.9 31 3,160 1,308 

1.41 1.82 1.46 35 3,230 1,547 

"·" 2.59 1.'12 4.6 5,620 813 

5.22 2.69 1.'18 48 5,586 778 

4.46 2.31 1.56 46 6,16!1 919 
w 

w 

l"1re No. 

"R"-1-'!IL 

'fP-:>-lf! 

'V"-3-WL 

'IP-45-":IL 

'IP-4-WL 

'IP-5-'NL 

'1"-6-Wl. 

'NP-41-WI. 

lf?-42-WI.. 

wP-~-wc 

IP-e-n 

lrf'-9-WI. 

1"'-43-WL 

'1/P-44-Wl 

IP-46-',\'L 

1"-4?-WL 

i¥1'-48-','(L 

!li'-49-WL 

IP-10-WL 

IP-11-WL 

'IP-12-WL 

IP-13-WL 

IP-14-WL 

lfP-15-WL 

IP-20-WL 

IP-32-WL 

WP-33-WL 

IP-16-WL 

·,fP-17-WL 

IP-18-WL 

IP-19-WL 

ifP-34-WL 

IP-3!5-WL 

WP-37-WL 

r-38-WL 

WP-39-WL 

111'-40-WL 

IP-2!5-WL 

IP-26-WL 

WP-27-WL 

IP-21-WL 

IP-21-WL 

IP-22-liL 

IP-23-WL 

iiP-24.-Wl 

W'-29-n. 

•-so-n 

IP-31-WL 

Teet uee lllllbient 

con41 tionay' 

u To Ill! 

tt/min •r • 132 9<6 1!5.!5 

132 93 16 

32 96 14. 

132 91 14..9 

264. 92.!5 16.1 

264. 94. 13.4. 

264. 94. 13.!5 

264. 89.!5 1!5.4 

264 90 15.2 

440 91 16.1 

440 94..!5 16.4 

440 94. .1!5.1 

440 90.4 1!5.1 

440 90.!5 15.3 

'104 91.2 u.8 

704 91.6 u.8 

704. 90 15 

704 90 1!5 

132 92.2 31.1 

132 92.'1 30.8 

132 91.5 31.!5 

264 92.1 31.2 

264 92.1 30.4. 

264. 91.6 31.0 

264 91.!5 31.3 

264 90.3 30.9 

26<6 90.9 30.!5 

440 91.6 31.4 

440 92.2 30.9 

440 92.!5 30.!5 

440 92.1 30.9 

440 90.6 30.1 

440 90.8 29.9 

704 90 30 

'104 90 30 

'704. 90 30 

704 90 30 

132 90.2 50.!5 

132 90.!5 50.5 

132 91 50.0 

132 90.1 50.0 

2M 90.2 !50.1 

264. 90.1 50.0 

264 90.8 !50.3 

211<6 91 50.0 

440 90.6 50.2 

440 90.8 !50.0 

440 90.3 50.5 

Table 5 

Jllel lllld tiiii bid J!Um1ng p~tere 

P&1'111118ter• 

"r Wo Jl R e B 

• 1b/tt2 Btu/1b tt/min miD 1b/m1n 

!5.2!5 0.<6'1 8<6'16 1.'16 o.u 0.135 

!5.31 0.4.7 8<6'16 2.22 0.401 -- !5.76 0.33 8<676 1.78 0.669 0.162 

!5.!51 0.32 8<622 1.91 0.298 0.14.2 

!5.17 0.32 8<6'16 !5.00 0.36 -- 4..40 0.32 8<676 4..16 0.62 o.u 4..60 0,32 8<6'16 4..81 0.!57 -- 5.40 0.32 8<622 5.3<6 0.35 0.40 

5.19 0.32 8422 5.64. 0.36 0.:'\92 

6.02 0.31 84'16 10.64 0.68 Oo79 

6.p1 0.31 8<676 12.83 0.38 o.8« 

6.30 0.31 8<6'16 10.20 0.4.8 -- 5.60 0.32 8<622 10.65 0.40 0.6'18 

!5.20 0.32 8<622 13.30 0.38 0.673 

5.!5'1 0.32 8422 32 0.40 1.80 

s.u 0.32 8<622 26.9 0.56 1.3'1 

5.70 0.31 84.22 20.9 0.44 1.55 

5.88 0.31 8422 21.2 0.40 1.67 

8.'10 0.30 8<622 1.40 0.45 0.122 

8.61 0.31 8422 1.34. 0.66 0.117 

8.94. 0.30 8<622 1.40 0.54. 0.22 

8.19 0.31 8<622 3.91 0.74 0.35 

8.5'1 0.31 8422 -- 0.61 0.36 

8.31 0.31 84.22 4.72 0.621 0.30 

8.19 0.31 8422 5.00 0.67 0.36 

8.30 0.31 8422 4.14. 0.36 0.325 

8.55 0.31 8422 4.16 0.26 0.356 

8.!51 0.31 8422 10.63 0.51 0.38 

9.31 0.30 8422 9.4.3 0.504. -- 9.4.3 0.30 8422 11.11 0.709 0.60 

9.39 0.30 8<622 11.11 0.4.!5 0.312 

8.26 0.31 8422 8.4.2 0.4.6 0.361 

8.!51 0.31 8<622 7.4'1 0.47 0.326 

9.42 0.30 8<622 22.2 0.478 0.734. 

9.70 0.30 8422 2!5.8 0.4.4. 0.4.5!5 

9.'10 0.30 8<622 17.!5 0.426 0.755 

8.50 0.31 8<622 17.5 0.415 0.926 

11.1!5 0.30 8<622 0.781 0.69 0.124. 

11.50 0.30 8<622 -- 0.4,6 -- 11.09 0.30 8422 1.12 0.61 0.218 

11.23 0.30 8<622 1.01 0.55 0.138 

11.81 0.29 8422 2.17 0.'12 0.20 

12.19 0.29 8422 2.4.4. 0.62 -- 11.'19 0.29 8<622 2.'10 0.50 0.14.4. 

11.68 0.29 8422 2.88 0.52 0.1'11 

11.60 0.30 8<622 6.U 0.4.8 0.24.3 

11.8'1 0.29 8<622 '1.4.8 0.4.6 0.3'1 

11.'16 0.29 8<622 7.04 0.58 0.323 

!'181118 dimensions H

Ac 1'b D L R 4l I 

tt2 tt tt rt rt deg. l'tu/1"1n B

1.11 0.03 0.'14. 0.59 0.!54 25 1 ,14.3 

1.3<6 -- 0.89 0.85 0.71 34..2 -- 

1.79 0.0!5 1.19 -- -- 25 1,372 

1.68 0.02 1.12 1.00 0.82 32 1,230 

2.70 -- 1.80 -- -- 38 -- 

3.81 0.05 2.!58 0.61 0.4.5 37.5 3,475 

4.11 -- 2.74 0.81 0.61 37.5 -- 

2.81 0.04 1.87 1.28 0.87 « 3,370 

3.05 0,04 2.03 1.15 o.ee 39 3,300 

10.86 0.04 7.24 0.95 0.67 « 6,696 

7.31 0.04 4.87 0.94 0.6!5 4.7 7,150 

7.35 -- 4.90 0.81 0.57 4.5 -- 

6.38 0.03 4.25 1.18 0.'12 4.3 5,710 

7.58 0.03 5.05 1.23 0.82 42 5,660 

19.20 0.03 12.80 1.18. 0.67 52 15,200 

22.59 0.03 15.06 1.72 0.8'1 58 11,500 

13.00 0.04. 9.20 1.33 0.64. 58 13,100 

12.72 0.04 8.4.8 1.12 0.61 51 14,050 

0.95 0.05 0.63 0.48 0.45 30 1,027 

1.32 0.05 0.88 0.4.5 0.40 30 985 

1.14. 0.09 0.'16 0.52 0.48 24 1,853 

4..34 0.05 2.89 0.48 0.45 30 1,027 

6.11 0.03 4.07 0.45 0.4.0 30 985 

4.40 0.04 2.93 0.52 0.48 24. 1,853 

5.03 0.04 3.35 0.52 0.40 4.3 3,032 

2.24. 0.04 1.49 1.00 0.75 39 2,740 

1.62 0.05 1.08 1.00 0.75 39 3,000 

8.13 0.02 5.42 0.72 0.46 53 3,200 

7.13 -- 4.75 0.00 0.49 57 -- 

11.82 0.03 7.ae 0,79 0.48 !55 !5,011 

7.50 0.01 5.00 0.72 0.48 51 2,630 

5.82 0.02 3.88 1.62 0.86 4.!5 3,040 

5.28 0.02 3.52 1.29 0.'19 51 2,750 

15.4.5 0.02 10.3 1.55 0.70 60 6,180 

17.03 0.01 11.35 1.49 0.72 57 3,830 

11.18 0.02 7.4.!5 1.54. 0.85 !55 6,360 

10.89 0.03 7.26 1.59 0.85 57 7,800 

0.81 0.09 0.54, 0.68 0.59 25 1,04.!5 

0.50 -- 0.33 0.59 0.!51 2!5 -- 

1.02 0.11 0.68 0.81 0.69 28 1,836 

o.a. 0.0'1 0.56 0.72 0.62 31 1,162 

2;34. 0.06 1.56 0.85 0.62 40 1,68<6 

2.27 -- 1.51 0.72 0.53 4.2 -- 

2.03 0.03 1.35 0.87 0.63 4.2 1,213 

2.25 0.04 1.50 0.82 0.57 43 ~.«0 

4..65 0.02 3.1 1.15 0.78 4.6 2,04'1 

5.16 0.04. 3.4.4. 1.15 0.'11 50 3.117 

6.08 o.os 4,.05 1.19 0.'16 50 2,7,20 
~ 0 

a. 

C1> 

"' 

c:; 

.. ;:r 

w 

~ 

Fire 

No. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

Species Loading 

tonaL acre 

D.!'. 32.5 n.r. 20.0 

D.l'. 7.5 

L.P.P. 32.5 

L.P.P. 20.0 

L.P.P. 7.5 D. F. 32.5 D.P'. 20.0 

D.P'. 7.5 

L.P.P. 32.5 

L.P.P. 20.0 

L.P.P. 7.5 

n.r. - 32.5 

D.F. 20.0 

D.F. 7.5 

r..n.p. 32.5 

L.P.P. 20.0 

L.P.P. 7.5 

'feet area aabint 

cOD4it1ons 

'~o lJ Jill llr 

.,. ttLain ~ ~ 

8'1 •• o 28 8.-1 8'1 -1.5 28 7.9 

90 3.0 24 7.3 

93 5.0 20 8.1 

92 7.0 21 7.7 

92 7.0 22 '1.4 

81 3.0 32 7.0 

85 7.5 27 6.7 

89 4.5 23 6.3 

93 3.5 18 6.3 

93 3.5 18 6.3 

94 5.0 16 6.3 

91 2.0 22 5.8 

92 4.0 21 5.8 

92 2 •. 0 21 5.7 71 2.5 61 8.1 

70 0.5 61 7.7 

70 0.5 61 7.4 

Table 6 

l'uel ad tue1 be4 - BurniDg peraaeters l'l.alle 

paramaters dimensions 

•o H Il e B D L Q 

1bsLtt2 BtJllb DLm !IliA J.lllawa n n mll

1.35 8332 5.U5 1.85 41.64 9.51 5.98 346,9

0.'14 8332 5.05 0.'15 22.32 3.80 4.0'1 185,"

0.50 8332 2.59 0.78 1.01 2.03 2.64 8,4

1.45 8709 9.10 8.46 

0.93 8709 5.90 1.14 6.28 6.'12 6.58 54,6

0.34 8709 -1.70 0.94 1.49 4.43 3.48 12,9

1.35 8332 7.20 0.82 7.99 5.90 6.40 66,5

0.74 8332 4.82 o.8o 2.81 3.84 4.14 23,4

0.50 8332 5.30 0.65 1.72 3.43 2.59 14,3

1.45 8709 3.40 1.94 9.58 6.58 7.17 83,4

0.93 8709 2.70 1.33 3.34 3.58 5.83 29,0

0.34 8709 5.22 0.72 1.27 3.77 3.77 ll,0

1.35 8332 5.40 1.00 7.26 5.38 7.09 60,4

0.74 8332 3.39 0.91 2.28 3.10 3.88 18,9

0.50 8332 -- -- -- -- 2.50 

1.45 8709 4.80 1.21 8.47 5.83 9.52 73,7

0.93 8709 1.79 2.76 4.64 4.94 3.93 40,4

0.34 8709 1.80 -- -- -- 3.82 
Rothermel, Richard C . , and Hal E. Anderson. 

1966. Fire spread characteristics determined in the laboratory. U.S. 

Dep. Agr., Forest Serv., Intermountain Forest and Range Exp. Sta., 

Ogden, Utah. 34 pp., illus. (U.S. Forest Serv. Res. Pap. INT-30) 

Fuel beds of ponderosa pine needles and white pine needles were burned 

under controlled environmental conditions to determine the effects of fuel 

moisture and windspeed upon the rate of fire spread. Empirical formulas 

are presented to show the effect of these parameters. A discussion of rate 

of spread and some simple experiments show how fuel may be preheated 

before the fire reaches the fuel. The interrelationship between unit energy 

release rate and rate of spread produces a fire characteristics curve. 

Diffusion flame analysis shows good agreement when working with !-inch 

stick fires. 

Rothermel, Richard C. , and Hal E. Anderson. 

1966. Fire spread characteristics determined in the laboratory. U.S. 

Dep. Agr. , Forest Serv. , Intermountain Forest and Range Exp. Sta., 

Ogden, Utah. 34 pp., illus. (U.S. Forest Serv. Res. Pap. INT-30) 

Fuel beds of ponderosa pine needles and white pine needles were burned 

under controlled environmental conditions to determine the effects of fuel 

moisture and windspeed upon the rate of fire spread. Empirical formulas 

are p:::-esented to show the effect of these parameters. A discussion of rate 

of spread and some simple experiments show how fuel may be preheated 

before the fire reaches the fuel. The interrelationship between unit energy 

release rate and rate of spread produces a fire characteristics curve. 

Diffusion flame analysis shows good agreement when working with ~-inch 

stick fires • 

Rothermel, Richard C. , and Hal E. Anderson. 

1966 . Fire spread characteristics determin

Dep. Agr. , Forest Serv. , Intermountain F

Ogden, Utah. 34 pp. , ill us. (U.S. Fore

Fuel beds of ponderosa pine needles and white 

under controlled environmental conditions to d

moisture and windspeed upon the rate of fire sp

are presented to show the effect of these param

of spread and some simple experiments show 

before the fire reaches the fuel. The interrelat

release rate and rate of spread produces a 

Diffusion flame analysis shows good agreeme

stick fires. 

Rothermel, Richard C. , and Hal E. Anderson

1966. Fire spread characteristics determin

Dep. Agr. , Forest Serv. , Intermountain 

Ogden, Utah. 34 pp., illus. (U.S. Fore

Fuel beds of ponderosa pine needles and whit

under controlled environmental conditions to 

moisture and windspeed upon the rate of fire s

are presented to show the effect of these param

of spread and some simple experiments show 

before the fire reaches the fuel. The interrela

release rate and rate of spread produces a 

Diffusion flame analysis shows good agreeme

stick fires . 
Headquarters for the Intermountain 

Forest and Range Experiment Station 

are in Ogden, Utah. Project headquarters are also at: 

Boise, Idaho 

Bozeman, Montana (in cooperation 

with Montana State University) 

Logan, Utah (in cooperation with 

Utah State University) 

Missoula, Montana (in cooperation 

with University of Montana) 

Moscow, Idaho (in cooperation with 

the University of Idaho) 

Provo, Utah (in cooperation with 

Brigham Young University) 
"The Forest Service of the U.S. Department 

of Agriculture is dedicated to the principle of 

multiple use management of the Nation's 

forest resources for sustained yields of wood, 

water, forage, wildlife, and recreation. Through 

forestry research cooperation with the States 

and private forest owners, and management 

of the National Forests and National Grasslands, it strives- as directed by Congressto provide increasingly greater service to a 

growing Nation."