Kurt R. Sacksteder

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.

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.

Buoyant downward diffusion flame spread and extinction in partial-gravity accelerations

This paper describes experimental observations of downward, opposed-flow flame spreading made underpartial-gravity conditions aboard NASA research aircraft. Special apparatus and techniques for these tests are described, including schlieren imaging of dim near-limit flames. Flame-spreading and flammability limit behaviors of a thin cellulosic fuel, 1×10 −3 g/cm 2 tested at 1 atm of pressure in oxygen/nitrogen mixtures of 13–21% oxygen by volume, are described for effective acceleration levels ranging from 0.05 to 0.6 times normal earth gravity (1 g ). Downward-burning flammability increases in partial gravity, with the limiting oxygen fraction falling from 15.6% oxygen in 1 g to 13–14% oxygen in 0.05–0.1 g . Flame-spread rates are shown to peak in partial gravity, increasing by 20% over the 1- g value in air (21% oxygen). Partial-gravity flame-spreading results, corrected for fuel density and thickness, are consistent with results obtained at acceleration levels above 1 g in a centrifuge. The results compare qualitatively with predictions of flame spreading in buoyant flow by models that include finite-rate chemical kinetics and surface and gas-phase radiative loss mechanisms. A correlation of experimental buoyant downward flame-spread results is introduced that accounts for radiative heat losses using a dimensionless spread rate, V f o , a radiation/conduction number. S R , and the Damkohler number, Da , as parameters. The correlation includes data from 0.05 g to 4.25 g and oxygen/nitrogen mixtures from 14% to 50% oxygen.

Upward Flame Spread Over Thin Solids in Partial Gravity

Experiments to observe upward and downward flame spread and extinction over a thin solid fuel in partial-gravity environments were conducted in an aircraft flying parabolic trajectories. In the upward spreading case, flames with constant lengths and steady spread rates were observed using narrow fuel samples in reduced pressures. The upward flame spread rates and the flame and pyrolysis lengths increased linearly with the gravity level. The proportionality constants, however, increased with pressure and sample width. For comparison, downward spreading tests were also conducted using the same reduced-pressure atmospheres needed to obtain steady flames in the upward spreading case. In downward spreading, the steady spread rates and the flammability boundary exhibited a non-monotonic dependence on gravity. This behavior is attributed to competition between finite-gas-phase residence times in the flame stabilization zone and radiative heat losses from the flame. Throughout the accessible range of partial gravity, the upward spreading flames propagated at higher speeds than the downward spreading flames and the fuel is more flammable in the upward spread direction. A three-dimensional concurrent-flow flame-spreading model, originally developed for forced flows in a duct at microgravity, was reformulated and numerically solved for buoyant flow. The numerical flame spread simulation provides detailed flame structure including gas flow and temperature fields, oxygen and fuel transport, and solid temperature and thickness distributions and predicts the essential three-dimensional features observed for the narrow, reduced pressure flames

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.

Development of a Reactor for the Extraction of Oxygen and Volatiles From Lunar Regolith

The RESOLVE (Regolith and Environment Science, Oxygen and Lunar Volatiles Extraction) Project, aims to extract and quantify useful resources from lunar soil. The reactor developed for RESOLVE is a dual purpose system, designed to evolve both water, at 150 C and up to 80 psig, and oxygen, using hydrogen reduction at 900 C. A variety of laboratory tests were performed to verify its operation and to explore the properties of the analog site soil. The results were also applied to modeling efforts which are being used to estimate the apparent thermal properties of the soil. The experimental and numerical results, along with the analog site tests, will be used to evolve and optimize future reactor designs.

Solid fuel combustion experiments in microgravity using a continuous fuel dispenser and related numerical simulations

The conventional way of determining the flammability characteristics of a material involves a number of tedious single-sample tests to distinguish flammable from non-flammable conditions. A novel test device and fuel configuration has been developed that permits multiple successive tests for indefinite lengths of thin solid materials. In this device, a spreading flame can be established and held at a fixed location in front of optimized diagnostics while continuous variations of test parameters are made. This device is especially well-suited to conducting experiments in space (e.g. aboard the International Space Station) where the limited resources of stowage, volume, and crew time pose major constraints. A prototype version of this device was tested successfully in both a normal gravity laboratory and during low-gravity aircraft trials. As part of this ongoing study of material flammability behavior, a numerical model of concurrent-flow flame spread is used to simulate the flame. Two and three-dimensional steady-state forms of the compressible Navier-Stokes equations with chemical reactions and gas and solid radiation are solved. The model is used to assist in the design of the test apparatus and to interpret the results of microgravity experiments. This paper describes details of the fuel testing device and planned experiment diagnostics. A special fuel, developed to optimize use of the special testing device, is described. Some results of the numerical flame spread model are presented to explain the three-dimensional nature of flames spreading in concurrent flow and to show how the model is used as an experiment design tool.

Solid Inflammability Boundary at Low Speed (SIBAL)

This research program is concerned with the effect of low-speed, concurrent flow on the spreading and extinction processes of flames over solid fuels. The primary objective is to verify the theoretically predicted extinction boundary, using oxygen percentage and flow velocity as coordinates. Of particular interest are the low-speed quenching limits and the existence of the critical oxygen flammability limit. Detailed flame spread characteristics, including flame spread rate, flame size, and flame structure are sought. Since the predicted flame behavior depends on the inclusion of flame and surface radiation, the measured results will also be used to assess the importance of radiative heat transfer by direct comparison to a comprehensive numerical model. The solid fuel used in this experiment is a custom-made fabric consisting of a 1:1 blend of cotton and fiberglass. This choice was made following an extensive search to yield a material with favorable properties, namely, rollability, non-cracking behavior during combustion, strength after combustion, and flammability in a range of oxygen limits permissible within the Combustion Integrated Rack (CIR) on the International Space Station. At the present time, an effort is being made to characterize both the radiative properties of the fuel and the flame spreading behavior in normal gravity at reduced pressure. These will provide a basis for comparison with the microgravity results as well as aid in bracketing the anticipated flammability boundary for the flight experiment. An overview of recent work, with emphasis on theoretical results, is presented.

Observations of methane and ethylene diffusion flames stabilized around a blowing porous sphere under microgravity conditions

This paper presents the experimental and theoretical results for expanding methane and ethylene diffusion flames in microgravity. A small porous sphere made from a low-density and low-heat-capacity insulating material was used to uniformly supply fuel at a constant rate to the expanding diffusion flame. A theoretical model which includes soot and gas radiation is formulated but only the problem pertaining to the transient expansion of the flame is solved by assuming constant pressure infinitely fast one-step ideal gas reaction and unity Lewis number. This is a first step toward quantifying the effect of soot and gas radiation on these flames. The theoretically calculated expansion rate is in good agreement with the experimental results. Both experimental and theoretical results show that as the flame radius increases, the flame expansion process becomes diffusion controlled and the flame radius grows as gamma t. Theoretical calculations also show that for a constant fuel mass injection rate a quasi-steady state is developed in the region surrounded by the flame and the mass flow rate at any location inside this region equals the mass injection rate.