Subrata Bhattacharjee

The behavior of flames spreading over thin solids in microgravity

Abstract Experiments were conducted aboard Space Shuttle Orbiters during five different flights to study flame spread over a thin cellulosic fuel in a quiescent, microgravity environment. Data, which include spread rate and temperature measurements in the gas and solid phases, and also recordings of the flame from ignition to extinction using two 16-mm cameras, were gathered for two different oxygen levels and three different pressures. Detailed observation of the flame evolution is described along with theoretical support from steady and unsteady models that include radiation from CO 2 and H 2 O. Experimental results indicate that the spread rate increases with ambient oxygen level and pressure. The brightness of the flame and the visible soot radiation increases monotonically from the slowest to the fastest spreading flame. Steady-state theory compares well with experiments in the vicinity of the flame leading edge. Trends in temperature, spread rate, and structure of the flame are qualitatively reproduced in this region, but the feature of a flame trailing edge curving back to the fuel surface and flame evolution over time is only captured through an unsteady model.

The Effect of Ambient Pressure on Flame Spread Over Thin Cellulosic Fuel in a Quiescent, Microgravity Environment

Results from recently conducted experiments on flame spread over a thin cellulosic fuel in a quiescent, microgravity environment of a 50/50 volumetric mixture of oxygen and nitrogen (oxygen mass fraction 0.53) at three different pressures-101, 152, and 203 kPa (1, 1.5, and 2.0 atm)-are analyzed. The results are compared with established theoretical results and two different computational flame spread models : one that includes gas-phase radiation, and one that does not. The spread rate behavior from experiment, i.e., an increase of spread rate with pressure, is consistent with the theoretical model that includes gas-phase radiation, and side-view photographs of the flames compare favorably with two-dimensional temperature contours produced computationally from the same model. In contrast, neither the dependence of spread rate on pressure nor the flame shape can be predicted with favorable comparison to experiment if radiation is neglected.

Quiescent flame spread over thick fuels in microgravity

Experimental results for flame spread over thick PMMA in microgravity are reviewed. The results were obtained abouard three different space shuttle missions, STS-54, STS-63, and STS-64. For the three conditions, 50% O 2 in N 2 at 1 atm, 50% O 2 at 2 atm, and 70% O 2 at 1 atm, the flame-spread rate slowly decreases with time, which varied from about 50 s to over 300 s. Computational modeling that includes the effects of radiation captures the essential features of the flame position versus time trajectory. When computations are carried out past the experimental time, the flames eventually retreat and then extinguish after spread times of about 450–600 s. With respects to the flame, the flow velocity into the flame is the spread rate. Absent any additional flow to press the flame close to the surface to provide a heat flux that allows the heated layer in the solid to develop., the process remains unsteady. The thermal and mass diffusion scales each are approximately the thermal diffusivity of the gas divided by the spread rate. The computed temperature and oxygen fields show that the distances over which temperature changes take place are small compared to those over which oxygen diffuses. This effect is due to the radiation causing a reduction in the length scale characteristic of the temperature field compared to the mass diffusion scale. The mismatch in the actual thermal scale and the mass diffusion scale grows with time until the oxygen diffusion rate to the flame is unable to sustaint it. For fuels with thickness below some critical value, the fuel thickness is heated fast enough and the spread rate is high enough that the mismatch in the thermal and the mass diffusion scales is unimportant, and the spread rate is steady.

Inherently unsteady flame spread to extinction over thick fuels in microgravity

Results of an experiment for flame spread over thick PMMA in a quiescent, 50% O2 in N2, 1 atm, microgravity environment recently obtained aboard space shuttle mission STS 85 are described. Previous experimental results indicate that the spread process is unsteady with the spread rate decreasing with time. Although experiment time in the earlier experiments was insufficient to determine if steady spread is established or extinction occurs, computational modeling predicts extinction. The sample length was extended over that of the earlier experiments to determine the ultimate fate of the flame. Flame imaging shows that following ignition, the flame leading edge spreads at a continually decreasing rate for approximately 180 s, ceases to progress forward, and then retreats in the opposite direction for approximately an additional 360 s, at which time flame extinction occurs. Computational modeling, including gas and fuel surface radiation, captures the observed behavior, which is predicted for all oxygen concentrations up to pure oxygen at 1 atm. In the presence of a flow, a thin heated layer in the solid develops quickly with the heat transfer driving vaporization and steady spread, while in the quiescent environment, a heated layer of substantial thickness develops over time while the flame spreads, unsteadily, more slowly. As a result, radiation is important, and the length-scale characteristic of the temperature field in the gas is decreased in comparison to the mass diffusion scale, which grows with time. Ultimately, the mismatch in scales results in the flame being in a region to which oxygen is unable to diffuse at a sufficient rate, and the flame extinguishes. Such self-extinction at microgravity has implications for fire safety considerations in spacecraft.

A comparison of the roles played by natural and forced convection in opposed-flow flame spreading

Abstract A computational model of flame spread down a thermally thin solid in a gravitational environment, in which the acceleration of gravity can be varied from zero to some finite value of the order of the Earths acceleration, is presented. Information obtained from the model bridges the gap between experimental data that is most generally obtained in normal gravity and in microgravity environments, an understanding of flame spreading in a microgravity environment being essential for understanding the fire safety aspects of space travel Results of the modeling effort show that gravity levels that might be thought of as low enough such that the effects of gravity are negligible, i.e., 10−2 or 10−3 times that of the Earth, have a significant effect on the flame spread process. A striking similarity between flame spreading in a naturally induced flow and a forced flow, each opposing the spreading flame, is found. The reason for the similarity is the similarity in the velocity profiles near the surface an...

Structure of downward spreading flames: a comparison of numerical simulation, experimental results and a simplified parabolic theory

Temperature and velocity fields in a downward flame spread over flat, solid fuels in a gravitational field are numerically simulated and compared with available experimental measurements and a simplified theory. The two-dimensional steady numerical model solves the mass, energy, species-mass, and momentum equations in the gaseous phase and the energy equation in the solid phase and includes gas-phase and pyrolysis kinetics, gas and surface radiation with radiation feedback. The published experimental results include measured temperature and velocity profiles, surface regression data, visible images of flames, and images taken with interferometers. A simplified parabolic solution for the temperature field in an opposed-flow configuration is extended to the downward configuration and compared with the simulation and experimental results. Flames over thin cellulosic fuels and PMMA, both in the thick and thin limits are considered. With one exception, the numerical model is found to reproduce the observed flame structure for a diverse range of fuel and ambient conditions. The simplified theory, based on a parabolic solution of the coupling functions, is found to reproduce the temperature fields in the gas and the solid reasonably well for flame spreads over thick fuels.

Heat Transfer to a Thin Solid Combustible in Flame Spreading at Microgravity

The heat transfer rate to a thin solid combustible from an attached diffusion flame, spreading across the surface of the combustible in a quiescent, microgravity environment, was determined from measurements made in the drop tower facility at NASA-Lewis Research Center. With first-order Arrhenium pyrolysis kinetics, the solid-phase mass and energy equations along with the measured spread rate and surface temperature profiles were used to calculate the net heat flux to surface. Results of the measurements are compared to numerical solution of the complete set of coupled differential equations that describe the temperature, species, and velocity fields in the gas and solid phases. The theory and experiment agree on the major qualitative features of the heat transfer. Some fundamental differences are attributed to the neglect of radiation in the theoretical model. A scale analysis is developed that makes use of the experimental data at different ambient conditions to support the notion that radiation is important and to investigate the effect of pressure on the spread rate.

SOUNDING ROCKET MICROGRAVITY EXPERIMENTS ELUCIDATING DIFFUSIVE AND RADIATIVE TRANSPORT EFFECTS ON FLAME SPREAD OVER THERMALLY THICK SOLIDS

A series of 6-min microgravity combustion experiments of opposed-flow flame spread over thermally thick PMMA has been conducted to extend data previously reported at high opposed flows to almost two decades lower in flow. The effect of flow velocity on flame spread shows a square-root power-law dependence rather than the linear dependence predicted by thermal theory. The experiments demonstrate that opposed-flow flame spread is viable to very low velocities and is more robust than expected from the two-dimensional numerical model, which predicts that, at very low velocities (<5 cm/s), flame spread rates fall off more rapidly as flow is reduced. It is hypothesized that the enhanced flame spread observed in the experiments may be due to three-dimensional hydrodynamic effects, which are not included in the zero-gravity, two-dimensional hydrodynamic model. The effect of external irradiation was also studied and its effects were found to be more complex than the model predicted over the 0–2 W/cm2 range. In the experiments, the flame compensated for the increased irradiation by stabilizing farther from the surface. A surface energy balance reveals that the imposed flux was at least partially offset by a reduced conductive flux from the increased standoff distance so that the effect on flame spread was weaker than anticipated.

Creeping flame spread along fuel cylinders in forced and natural flows and microgravity

in which the terms containing the constant C account for the enhanced gas-to-surface heat transfer because of the cylindrical curvature, and those containing Lsy, the heated layer depth in the solid, account for a reduction in the solid volume preheated in the cylindrical compared to the flat geometry. The expression is tuned by comparison with complete numerical solutions to the flame spread problem from which the flame energy EFL is determined from the flat surface geometry and the constant C chosen from heat transfer correlations. Results compare favorably with numerical solutions for cylindrical spread in forced and natural flows and microgravity and with experiments on downward flame spread on cylindrical rods in normal gravity and microgravity.

Effect of radiation loss on flame spread over a thin PMMA sheet in microgravity

Flame spread over a thin polymethylmethacrylate sheet in microgravity is investigated experimentally and analytically. A scale analysis yields a simple equation, η+ R /ζ=1, which states the radiation loss is a function of relative flow velocity and the fuel thickness. The prediction with the scale analysis states that the reduction in the relative flow velocity enlarges the size of the preheat zone, which increases radiation loss, and that the presence of radiation loss reduces the spread rate and also may cause extinction. To confirm the prediction, drop experiments are carried out with a 4.5 s drop tower in MGLAB, in Gifu, Japan. Flame-spread rates are measured with varying ambient flow velocity, fuel thickness, and oxygen level. Additionally, the temperature fields near the flame front are measured in quiescent conditions with a Michelson interferometer system. The experimental result shows that the spread rate is actually a function of sample thickness and ambient flow velocity, and that the spread rate becomes the minimum when the relative flow velocity is close to zero. It is also found that steady flame spread is established even in quiescent conditions if R is sufficiently low. The interferometer measurement shows the enlargement of the preheat zone in quiescent microgravity conditions as predicted by the scale analysis. It also shows that the steady heat balance is established when R is small, whereas it is not established in near extinction conditions. These results support the scale analysis and clarify the extinction mechanism via radiation loss.