Combustion Science and Technology

This article was downloaded by:[Nasa Glenn Research Centre] On: 21 August 2007 Access Details: [subscription number 770433347] Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Combustion Science and Technology Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713456315

Mechanisms of Microgravity Flame Spread Over a Thin Solid Fuel: Oxygen and Opposed Flow Effects S. L. Olson a a NASA Lewis Research Center, Cleveland, OH

Online Publication Date: 01 April 1991 To cite this Article: Olson, S. L. (1991) 'Mechanisms of Microgravity Flame Spread Over a Thin Solid Fuel: Oxygen and Opposed Flow Effects', Combustion Science and Technology, 76:4, 233 - 249 To link to this article: DOI: 10.1080/00102209108951711 URL: http://dx.doi.org/10.1080/00102209108951711

PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. © Taylor and Francis 2007

Downloaded By: [Nasa Glenn Research Centre] At: 20:28 21 August 2007

(9 Gordon and Breach Science Publishers S.A.

Combust, Sci. (lml T(·(-II.. 1991. Vol. 76. pp. 233-249 Photocopying permitted by license only

Printed in United Kingdom

Mechanisms of IVlicrogravity Flame Spread Over a Thin Solid Fuel: Oxygen and Opposed Flow Effects S. L. 0 LSON mail stop 500-217. NASA Lewis Research Center. Cleveland. OH 44135, (216) 433-2859 (Received July 19. 1990; in final form November 5, 1990) Abstract-e-Microgravity tests varying oxygen concentration and forced flow velocity have examined the importance of transport processes on flame spread over very thin solid fuels. Flame spread rates, solid phase temperature profiles and flame appearance for these tests are measured. A flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process. In the near-quenching region (very low characteristic relative velocities) a new controlling mechanism for flame spread-oxidizer transport-limited chemical reaction-is proposed. In the near-limit, blowoff region. high opposed flow velocities impose residence time limitations on the name spread process, A critical characteristic relative velocity line between the two near-limit regions defines conditions which result in maximum flammability both in terms of a peak flame spread rate and minimum oxygen concentration for steady burning. In the third region, away from both near-limit regions, the flame spread behavior, which can accurately be descibed by a thermal theory, is controlled by gas-phase conduction. Key words: microgravity, flame spread. diffusion flame. thermally-thin solid, opposed-flow, mechanisms

INTRODUCTION The effects of oxygen concentration and opposed flow velocity on flame spread over solid fuels has been extensively studied in normal gravity. A consensus has developed among researchers as to the controlling mechanisms of flame spread in normal gravity. A Damkohler number/thermal theory correlation, summarized in FernandezPello and Hirano (1983), successfully predicts flame spread rates for either thermallythick or thermally-thin fuels. The Damkohler number describes the transition from gas-phase conduction-limited flame spread, which is accurately described by the de Ris (1969) thermal theory, to residence-time-limited flame spread. which has been numerically modeled by a number of investigators, including Frey and T'ien (1979), Chen (1986a, b), di Blasi et al, (1989), and Bhattacharjee et al. (1990a). A number of investigators, but most notably Fernandez-Pella et al. (1981), have experimentally shown that for low opposed flows ii.e. large Damkohler numbers), flame spread rates for both thin and thick fuels become independent of opposed flow velocity. For thin fuels this is in agreement with the de Ris thermal theory, but for thick fuels it is not because the thermal theory predicts a linear dependence on opposed flow. Explanations of experimental data have pointed out that buoyant flows induced by the flame at low opposed flow velocities distort the flow field such that the velocity gradient at the leading edge near the fuel surface remains fairly constant (Hirano and Sato, 1975) despite variations in free stream velocity. Wichman (1983) used a linear velocity gradient to describe the flow field at the flame leading edge. West et at. (1990) have demonstrated through numerical modeling that the effect of increasing gravity is the same as if the opposed forced flow is increased for a spreading flame because the boundary layer flow near the fuel surface is similar in the two cases despite large differences in the ambient flow. Low gravity permits the study of quiescent and low-speed opposed flow without the complications of induced buoyancy. Experimental studies of flame spread over a 233


234

S.L. OLSON

thermally-thin fuel have been conducted for quiescent and very low speed opposed flow in microgravity (Olson et af. 1987, 1989; Ferkul, 1989). These studies have revealed a low-velocity quenching limit which, combined with existing high-velocity blowoff limit data, define a flammability map for the material in opposed flow. Whereas the blowoff extinction is due to reaction time limitations compared with residence times in the reaction zone, the quenching limit is concluded to be due to excessive heat losses. Radiative loss, either solid surface radiation or gas-phase radiations, has been used to predict the experimental results (Chen, 1986a; Bhattacharjee et al.. 1990a, b). Opposed flow testing in microgravity has been limited to very low velocities, thus data have been extrapolated over the range of velocities not experimentally obtainable in order to relate trends in microgravity flame spread with trends in normal and elevated gravity. Although a mechanism of radiative heat loss (either gas or solid) can predict the flame spread trends observed experimentally, other mechanisms of heat and mass transfer which increase the ratio of heat loss to heat generated by the flame may also be adequate to predict the observed trends. The purpose of this work is therfore to extend opposed flow testing in microgravity to higher velocities in order to bridge the gap in flame spread data and provide additional insight into the mechanisms of flame spread over the entire range of opposed flow velocities and oxygen concentrations. EXPERIMENTAL The flame spread experiments have been conducted with a thermally-thin cellulosic fuel in a 20 em diameter combustion tunnel rig, shown in Figure 1, which provides low-speed uniform flows from 0-35 cm/s through the test section. In order to reduce buoyant convection, free-fall experiments are conducted in the NASA Lewis Research Center's 2.2 second Drop Tower (Lekan, 1989). In this facility the experiment package is encased within a drag shield and suspended at the top of the 30.5 m high drop area by a single wire. Pre-drop experiment operations such as activating the flow and cameras, are controlled through remote switching. The drop is initiated by notching a support wire, which causes it to fail and release the experiment package and drag shield into free-fall. Although the drag shield is experiencing air drag, the experiment package is freely falling within the drag shield and falls approximately 20 cm relative to the drag shield during the test. The combined package is decelerated in an aerated sand pit at the end of the 2.2 second free fall. Although gravity levels during free fall have never been measured in this facility, estimates indicate that the gravity level is on the order of IO-'/g. As shown in Figure I, the flow through the combustion tunnel is controlled during the test from a high pressure supply bottle containing a selected O,-N, mixture by regulating the pressure upstream of a critical flow nozzle. The metered flow enters the plenum section of the tunnel and passes through four sintered flow-straightening plates before entering the test section. The flow then passes through the test section at atmospheric pressure and exits through a large tee vent at the top of the apparatus. The sample holder, shown in Figure 2, is made of very thin steel sheets between which the fuel sample is clamped. This holder is suspended in the flow in the test section of the combustion tunnel. The velocity profiles for a range of flows from 5-30cm/s through the central 10cm diameter region of the test section have been measured in microgravity using a hot-wire, smoke-pulse, flow visualization technique, and are uniform to within 10%


235


236

.-


MICROGRAVITY FLAME SPREAD

237

of the theoretical plug flow velocity through the test section. In this flow visualization technique, thin lines of smoke are produced perpendicular to the direction of flow, and the displacement of the smoke line as a function of time is measured at different radial positions to determine the magnitude and profile of the velocity through the test section. Velocities less than 5 cm/s could not be measured with the smoke-pulse technique due to field-of-view and flow-time constraints, so velocities in this range have been estimated based upon plug flow calculations from bulk flow measurements made using a calibrated air rotameter. Since the velocities higher than 5 cm/s agree well with the plug flow estimates from rotameter measurements, it has been assumed that the flow at the low velocities is also well behaved and can be estimated in this manner. The lowest velocity reported here is 1.5 ern/sec. Boundary layer velocities across the sample holder were also measured using the hot wire smoke-pulse flow visualization technique and compared within 3% of Blasius theory. The velocity profiles were checked periodically between drop tests using a 3-30 cm/s TSr hot-wire velocity meter to verify flow conditions. The fuel used in the forced flow experiments is a 5 em wide by 15em long by 0.0076 em thick (full thickness) laboratory wipe whose composition is 99% cellulose, I % polyamide resin. The sample width has been increased from the 3 em used previously (Olson, 1987) to 5 em to ensure that the flame is nearly two-dimensional even near extinction. Under all test conditions reported here, the flame pyrolysis front is observed to be two-dimensional except near the edges of the sample holder. The material is dried in a vacuum for more than 4 hours before being exposed to the test atmosphere. Because of its very low area density (I x 10- 3 g/cm', based on its half-thickness), this material has a very rapid burning rate. This enables flame spread and extinction data to be obtained in the short test times. A high-resistivity wire (typically 3-4 ohms cold resistance) is energized at approximately 28 Volts during the first 0.4 sec of the test to ignite the sample. After that time the ignitor is deactivated. At ignition a bright yellow flame appears, but as the flame develops it typically becomes less intense and frequently changes colors as it progresses toward steady state. The flame shapes for many of the tests reported here do not reach a complete steady state in the 2.2 sec of low gravity test time. Flame widths and flame standoffs stabilize rapidly, but the overall flame length needs more time to develop fully. Opposed flow flame spread is controlled by processes at the leading edge of the flame, however, and the leading-edge transition to a steady state configuration occurs within approximately I sec even for the weakest flames. Flame spread rates are measured after steady state is achieved by recording the leading-edge position of the flame as a function of time from the 16mm motion picture color films of the flame spread process using a digitized projection screen. Reproducibility of the flame spread rate measurement is estimated to be 6% for the weakest flames. The largest error associated with this measurement is the uncertainty in the location of the dim blue leading edge of the flame. Surface temperature measurements have been made with type K 0.0 I em diameter exposed-bead thermocouples which are positioned along the surface of the sample so that the thermocouple leads were perpendicular to the direction of flame spread. This orientation reduced conductive losses by minimizing the temperature gradient along the thermocouple leads through the two-dimensional flame. The thermocouples are positioned axially along the fuel sample such that the leading edge of the flame passes the thermocouple after the flame has had sufficient time to reach steady state at the leading edge. The thermocouple signals are cold-junction compensated, signal conditioned and


238

S.L OLSON

a) 0.4 sec

b) 0.6 sec

c) 0.8 sec

d) 1.0 sec

g) 1.6 sec

h) 1.8

I-----l 1 em

e) 1. 2 sec

f) 1.4 sec

sec

FIGURE 3 Photographs of a name in air with an opposed now of approximately Semis as a function of time after ignition. See color plate at end of issue.

stored on an on-board data acquisition and control system. The complete system is calibrated using an Omega Type K thermocouple calibrator. No radiative or conductive corrections to the thermocouple data are made because the corrections are estimated to be small (~ 10 K) at these temperatures (surface temperatures 750 K) and in this configuration. Response times are estimated to be 0.0002 seconds for this size thermocouple in contact with a solid using a conduction heat transfer central temperature-history analysis of the thermocoupole bead. The steepest gradients recorded in these tests are - 1000 Kjsec, so time resolution of the solid phase temperatures is adequate. Flame Appearance Figure 3 shows the transition of a flame in air at 5 cmjs opposed flow velocity from ignition at 0.4 sec to steady state flame spread, which occurs a t approximately 1.4 sec, after which no further changes are noted in the leading edge region of the flame. For the photographs at 1.4, 1.6 and 1.8 sec in time, the flame standoff distance remains constant, the overall flame color remains constant, and the dark gap between the two flame halves (just behind the leading edge) remains constant in width. The overall flame is growing in length, but the leading-edge position versus time slope is linear. Therefore it is concluded that this flame has reached its steady-state flame spread condition. Notice the 2.2 sec flame shape for this test in Figure 4 has the same leading-edge appearance but increased overall length. Figure 4 shows photographs of six flames in air and Figure 5 shows photographs


239

MICROGRAVITY FLAME SPREAD

t....."

-

a) 5 cm/s

b) 10 cmys

e) 15 eml'

1--1 1 em

d) 20 Cr.1/s

FIGURE 4 of issue.

e) 30 eml'

f) IG

Photographs of flames in air with a range of opposed flow velocities. See color plate at end

of three flames at 30% oxygen at different opposed flow velocities. The overall flame width increases with decreasing flow. A linear relationship between flame width and velocity boundary layer thickness is observed. Both the thermal and oxidizer boundary layers become thicker as flow is reduced so this trend is plausible. Additionally, the magnitude of the fuel blowing velocity at the surface (estimated to be as high as 0.5 cm/s) becomes more significant as the ambient flow is reduced. The flame is positioned where the fuel and oxidizer mix, so as the boundary layer for oxygen thickens and fuel blowing velocity becomes relatively more important, the flame moves away from the surface. As has been noted previously in low gravity tests (Olson, 1987; Ferkul, 1989), the standoff distance at and near the leading edge of the flame also increases with decreasing characteristic relative velocity. The leading edge of the flame also changes from yellow (mostly sooting) to blue as the characteristic relative velocity decreases. Lastly, a dark region develops within the flame as characteristic relative velocities are reduced below approximately IOcm/s. Figure Sa is a photograph of a 30% oxygen flame with a characteristic velocity of 0.1 ctn]«, This low velocity is obtained by directing the forced flow in the same direction, but at a slightly slower rate, as the flame spread. Since the definition of the characteristic velocity for forced flow (Olson et al., 1989) is the vector sum of flame


S.L. OLSON

240

a) 0.1 cm/s

b) 5 cm/s

1---4 1 em

c) 10 cm/s

FIGURE 5 Photographs of flames in 30% oxygen with a range of characteristic relative velocities. See color plate at end of issue.

spread rate and opposed flow velocity (in this case the opposed flow velocity is negative), the characteristic relative velocity between the ambient flow and the flame becomes extremely slow, less than purely diffusive transport, which has been estimated to be I em Is (Olson et al., 1989). In this case, the dominant velocity is the blowing velocity (estimated to be 0.3 cmls for this case) from the fuel surface due to fuel vaporization, and a totally symmetric flame shape results (the very bright objects behind the flame are parts of the ignitor wire). While the outline of the Figure 5a flame shape is symmetric, the flame colors are not. It is apparent that the leading edge is dim blue and encloses a large dark zone. The rest of the flame is very sooty. FLAME SPREAD RATES Figure 6 displays flame spread behavior at three oxygen concentrations over the entire flammable range of characteristic relative velocities: i.e. from the quenching extinction limit in microgravity to blowoff extinction limit at high characteristic relative velocities. Figure 6 includes data gathered in this study, normal gravity forced convective experimental data (Fernandez-Pelle et al., 1981), elevated gravity natural convective data (Altenkirch et al., 1980), and previously reported microgravity quiescent and forced convective data (Olson et al., 1989). The spread rates for both the elevated gravity tests and the normal gravity forced flow tests were corrected for fuel area density using the thermal theory inverse relationship (de Ris, 1969) between spread rate and area density for thin fuels to compensate for the different material thicknesses used in the different experiments. To construct this figure, which compares data from different experiments, a characteristic relative velocity for each type of flow environment is defined. In flame-fixed coordinates, a flame spreading in microgravity encounters an opposed flow which equals the sum of the flame spread velocity and the opposed forced convective flow. For quiescent flame spread the characteristic relative velocity is thus simply the flame


MICROGRA VITY FLAME SPREAD

241

2.5 (f)