Robert A. Altenkirch

Buoyancy effects on flames spreading down thermally thin fuels

Experiments show that buoyancy influences the downward spread rate of flames consuming thermally thin fuel beds. For index cards (9.8 × 10−3 cm half-thickness) and adding-machine tape (4.3 × 10−3 cm half-thickness), an increase in the buoyancy level causes the spread rate to drop until no flame propagation is possible. A dimensionless spread rate is found to correlate with a Damkohler number. As the Damkohler number increases with decreasing buoyancy level brought about by an increase in pressure or a decrease in gravity, the dimensionless spread rate approaches unity. It is also found that a small change in orientation with respect to the vertical is equivalent to a change in the magnitude of gravity in the direction of spread, and power-law relations between the dimensional spread rate and pressure are only valid over a small pressure range.

A Study of Boilover in Liquid Pool Fires Supported on Water Part I: Effects of a Water Sublayer on Pool Fires

Abstract Under certain circumstances, the water on which a burning pool of liquid fuel is supported may begin to boil. The water vapor that is released and escapes through the fuel surface tends to atomize the oil, which results in an emulsive-droplet flame above the fuel surface. This phenomenon, called boilover, has been observed for large scale pool fires, but the mechanism causing it to occur has not been fully investigated yet. This paper describes fundamental aspects of the effect of a boiling water sublayer on the behavior of pool fires. A burner system in which the burning surface of the fuel can be fed into the flame so that the fuel/water interface with respect to the edge of the container remains fixed was used. Ten different single-component and six different multicomponent fuels were tested. Conventional flow visualization techniques were applied to study the liquid motion, and results for an ethylbcnzene pool in a 4.8cm diameter pan are presented. Data obtained include temperatures and mass ...

Radiation-controlled, opposed-flow flame spread in a microgravity environment

The effects of surface and gas-phase radiation on laminar flame spread over thin solid combustibles are explored computationally. The opposed-flow configuration considered covers the entire possible range of opposing velocity: from the quiescent environment obtainable at microgravity to extinction (blowoff) at high velocity (up to 60 cm/s). Radiation is coupled to the hydrodynamics using the emission approximation through three simple parameters: a Planck mean absorption coefficient, the fraction of total emission directed towards the surface, and a shape function that represents the distribution of gas-to-surface radiation. These parameters are evaluated separately from the hydrolynamic solution, providing a degree of decoupling, by using a global radiation energy blanace that employs sophisticated models for accurate determination of radiative heat flow. Only CO2 and H2O radiation is considered, and the surface emittance is treated as a parameter to account for the lack of experimental values for it. Numerical calculations, complemented by scaling arguments, show that radiation effects are unimportant at high velocities of the oxidizer flow; the spread rate decreases with increasing opposing velocity due to finite-rate, gas-phase kinetics. However, if the opposing velcity is below a certain value, radiation becomes progressively important: the flame cools, shrinks in size, and its spread rate sharply falls as the opposing velocity decreases. Competition between finite-rate chemistry and radiation produces the peak in spread rate as a function of opposing velocity. This predicted peak has been observed in drop tower experiments providing support for the existence of a new class of flames that are radiatively controlled at low flow velocity. Such flames cannot be observed experimentally in normal gravity because the buoyancy generated flow is strong enough to mask the radiative effects.

The effect of surface radiation on flame spread in a quiescent, microgravity environment

Abstract In most theories of laminar flame spread over solid combustibles, radiation is neglected. However, when gas motion is completely absent, which may occur in the quiescent, microgravity environment of a spacecraft, the importance of radiation compared to convection is enhanced. We develop a theoretical model of flame spread over a thin solid fuel into an opposing flow of oxidizer, the opposing flow being present in the quiescent environment in flame-fixed coordinates, including the effects of radiative heat transfer from the fuel surface. Numerical solutions to the conservation equations in the gas and the solid phase that describe the spreading flame are presented for a variety of ambient conditions and surface emittances. When surface radiation is significant the solid surface acts as a heat sink, causing the flame to cool, shrink in size, and spread more slowly. Better agreement with experiment for net heat transfer to the surface is obtained when surface radiation is included. For low ambient oxygen levels, radiative effects lead to flame extinction. A radiation/conduction parameter is identified that adequately describes the importance of surface radiation.

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.

A Theoretical Description of Flame Spreading over Solid Combustibles in a Quiescent Environment at Zero Gravity

Abstract A theoretical model is presented that can be used to predict the structure and rate of spread of an attached diffusion flame moving over a thermally thin, pyrolyzing combustible placed in a gravity-free, quiescent, oxidizing environment. The gas-phase model includes steady-state, two-dimensional momentum, energy, and species equations while the solid-phase model consists of continuity and energy equations, the solution to which provide boundary conditions for the gas-phase problem. The spread rate appears as an eigenvalue in both the gas- and solid-phase equations. The numerical procedure developed to solve the system of equations is stable even for spread rates comparable to normal velocities present at the fuel-gas interface. Solid fuel pyrolysis is modelled using a first-order Arrhenius decomposition while both finite-rate and infinite rate chemistry in the gas phase are considered. Computed spread rates increase with increasing oxygen concentration in the ambient and are generally a factor of...

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.

A Method for Particle and Gas Temperature Measurement in Laboratory-Scale, Pulverized-Coal Flames

Abstract A multiple-wavelength, infrared pyrometer suitable for making line-of-sight particle and gas temperature measurements in pulverized-coal flames is described. The pyrometer uses lead selenide detectors covered with narrow-band filters to measure emitted and transmitted radiation. Scattering effects may be and are here incorporated into data reduction schemes for calculating temperatures. Measurements were made on one-dimensional, coal-dust/oxygen/argon flames stabilized on a flat-flame burner. Profiles of particle and gas temperatures and optical depths as a function of distance from the burner are presented. In general, particle and gas temperatures do not differ much close to the burner, but farther downstream the gas temperature exceeds the particle temperature.

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...