Under NASA-Lewis Grant NAG3-1611, starting date 6/1/94, a three-year experimental and theoretical study of the effects of ambient atmosphere on the properties of flame spread over thin and thick solid fuel beds has been initiated. In particular the effect of the type of inert gas, which affects the Lewis numbers of fuel and oxidant, and the effect of the addition of sub-flammability-limit concentrations of gaseous fuels to the oxidizing atmosphere will be studied. The effect of convection will be studied through one-g and mu g experiments with and without a forced flow. Moreover, the influence of thermal radiation, whose effect is known to be markedly different depending on the convection level, will be addressed

Ronney, Paul D.

English

NASA Technical Reports Server

N96- 15585
Premixed Atmosphere and Convection Influences
on Flame Inhibition and Combustion (PACIFIC)
Paul D. Ronney
Department of Mechanical Engineering,
University of Southern California, Los Angeles, CA 90089-1453
Objective
Under NASA-Lewis Grant NAG3-1611, starting date 6/1/94, a three-year
experimental and theoretical study of the effects of ambient atmosphere on the
properties of flame spread over thin and thick solid fuel beds has been initiated. In
particular the effect of the type of inert gas, which affects the Lewis numbers of fuel
and oxidant, and the effect of the addition of sub-flammability-limit concentrations
of gaseous fuels to the oxidizing atmosphere will be studied. The effect of
convection will be studied through one-g and lig experiments with and without a
forced flow. Moreover, the influence of thermal radiation, whose effect is known to
be markedly different depending on the convection level, will be addressed.
Approach
The emphasis of this study is on thermally thin fuels because of the limited
lig test time available in ground-based facilities, but preliminary scaling analyses
suggest that it may be possible to study thermally thick fuels as well when gaseous
fuel is added to the oxidizing atmosphere.
The experiments will be conducted in a combustion chamber in which a
convective flow of a few cm/sec can be imposed in the direction opposite the flame
spread. The oxidizing atmosphere will be mixed by the partialpressure method.
For tests of Lewis number effects, the inert gases He, Ne, N2, CO2 and SF6 will be
used since they provide Lewis numbers from about 0.3 to 1.4. CO and CH4 will be
used for the gaseous fuels. Thin fuel samples will be ashless filter paper and thick
fuel samples will be PMMA. Fuel samples of varying thickness will be ignited by
the heat generated by a current passed through a coiled nichrome wire coated with
nitrocellulose.
The primary diagnostics are video and an array of fine-wire thermocouples to
measure the temperature simultaneously at several locations. The video records
provide information on the spread rate and flame shape. The thermocouples give
an independent check of the spread rates and the existence (or lack thereof) of a
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separate flame front in the case of added gaseous fuel. The temperature data may
also be used to determine the heat flux from the gas phase to the fuel bed, which can
be related to the spread rate.
Significance
The understanding and control of accidental fires is a critical safety issue in
both terrestrial and space-borne environments. The proposed work would provide
insight that could be used to assess the fire hazards associated with non-standard
atmospheres that might be employed in future manned spacecraft. Also, fires in
enclosures produce a considerable amount of unburned vaporized fuel and partially
combusted gases such as CO. One-g experiments have shown that the addition of
combustible gases such as CO to the oxidizing atmosphere may increase the flame
spread rate substantially. This study could provide information to improve models
of fire development and spread in enclosures at one-g and _tg.
The influence of weak forced convection is particularly important for studies
of flame spread at _tg because there is very little buoyancy-induced flow- at _tg.
Experiments by Olson and collaborators shows that the presence of forced
convection currents (for example due to ventilation systems in manned spacecraft)
can have a profound effect on the spread rate and extinction conditions.
Consequently, the understanding of these effects is critical to understanding how
fires might start, spread, and be extinguished at lxg conditions.
ProKrress
v
To improve the sensitivity of the video imaging system for the very weak
near-extinction flames, a shearing interferometer (Fig. 1) has been designed and is
being constructed for evaluation. The shearing interferometer has no parts that
have critical alignment requirements, and thus may be especially suitable for drop
tests. The interferometric measurements may also be useful to supplement the
thermocouple temperature measurements. A system has been constructed and
bench-tested and has been found to provide excellent resolution and sensitivity for
flame spread experiments (Fig. 2). Both finite fringe spacing and infinite fringe
spacing configurations have been tested. The infinite fringe spacing configuration
has been selected for the drop apparatus because it appears to be easier to quantify
the recorded images from the infinite fringe system to obtain temperature
information. The system is sensitive enough to detect temperature differences of
only a few degrees Kelvin in air, though sensitivity of this level is not required to
image flames. This system is now being integrated into our drop-tower flame
spread experiment. All optical components and cameras have been obtained, and
the mounting system is being fabricated.
A graduate student has built a new test chamber and flame spread apparatus
to supplement the PI's existing apparatus. The test combustion chamber is based on
Lewis's now-standard layout for 2.2 second drop tower experiments. The new
chamber and apparatus provides improved fuel sample mounting, optical access for
direct photography and interferometry, a rapid ignition system and an updated
214
Tattletale-based data acquisition system. This equipment is now being integrated
into a standard 2.2 Second drop tower frame. Preliminary one-g tests have been
conducted to verify previous results in the Prs laboratory and elsewhere. Initial _tg
experiments are planned for the summer of 1995.
The PI's existing computer-controlled partial pressure gas mixing system has
reconfigured for the planned experiments. An undergraduate student has re-
written the partial pressure gas mixing software using a nonlinear least-square fit
algorithm to obtain successively improved estimates of the final pressure after gas is
added to the chamber, to minimize the total time required to mix the gases and
maximize the accuracy of the final mixture. This gas mixing system is now
operational.
While the focus of this study is on opposed-flow flame spread, corresponding
to downward flame spread at one-g, it is useful to compare these results to
concurrent-flow flame spread, corresponding to upward flame spread at one-g. It is
found (Fig. 3), as expected, that the spread rate is higher for upward flame spread
because heat transport is in the same direction as the spread. However, what has
not been shown previously is that upward flame spread over thin fuels can be
steady, but only for sufficiently narrow samples and/or sufficiently tall samples.
This is sensible considering that the early stages of upward flame spread, the spread
rate is accelerating due to the fact that the flame length is increasing and thereby the
rate of heat transfer to the fuel surface is increasing. When the flame length has
grown to the point where transverse losses match the rate of heat generation by the
flame, a steady spread rate is observed. Since the transverse losses will be greater for
smaller sample widths, the spread rate will be smaller for narrow samples.
Morevoer, because these considerations are absent for downward flame spread, the
downward spread rate is nearly independent of the sample width.
An analytical study has been initiated in conjunction with Dr. Mike
Delichatsios of Factory Mutual Research Corporation in Norwood, MA. The goal of
the work is to extend his exact solution of flame spread over a pyrolyzing fuel bed to
consider the effects Lewis number, finite-rate chemistry, and gaseous fuel addition
on flame spread rate. A preliminary theory of Lewis number effects on flame spread
over thin fuel beds, including finite-rate chemistry, has been obtained and compared
with prior one-g data from Zhang et al (1992). The results (Fig. 4) are in reasonable
agreement except for SF6-diluted mixtures, which have the lowest oxygen Lewis
number (= 0.30); current theoretical efforts are aimed at trying to understand the
unusual behavior of SF6.
Publications / Presentations:
Delichatsios, M. A., Ronney, P. D., "Horizontal and Lateral Flame Spread on Solids:
Closure and Diffusional Lewis Number Effects," Fall Technical Meeting,
Combustion Institute, Eastern States Section, Dec. 5-7, 1994, Clearwater Beach, FL.
Liu, J. B., Ronney, P. D., "Robust Interferometer System for Drop Tower
Experiments," SPIE International Symposium on Optical Science, Engineering, and
Instrumentation, July 9-14, 1995, San Diego, CA (to be presented).
215
Test Chamber 
Side View 
Figure 1. Schematic diagram of laser shearing interferometer system 
for 2.2 second drop tower. 
Figure 2. Digitized image of the plume above a burning paper match 
obtained from the shearing interferometer system. 
I 216 
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c_
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1 01
i I i 4 |
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• • 00
I • Upward spreadDo nward spread
0.1 , , , , I , , ,
5 6 7 8 910 20 30 40
Sample width, cm
Figure 3. Observed spread rates for upward and downward spread
r-......,.;__ ^¢ plover _;,thin solid .t-,old,,..as a ,-,,,.L,u,, u, sam e width
• 4
• • SF 6
1 0 "" • 002
Q- 5 m Ar
, , , , ! , , , , I , , , • ! , , ,
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(AH - L)/[Cp(Tp- To)keol;2S] "
Figure 4. Comparison of model of spread rate over thin solid fuels with Lewis
number effects to experimental results.
217


Premixed Atmosphere and Convection Influences on Flame Inhibition and Combustion (PACIFIC)

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