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PDR says, " Flame spread over thin solid fuels is a useful paradigm for studying the behavior of more complex flame spread problems, e.g., in building fires. Our recent experiments on flame spread in non-standard atmospheres have shown that the unequal rates of diffusion of thermal energy and oxidizer (the "Lewis number" effect) are important influences which have not received much attention in prior studies. We have also discovered a new type of flame spread process in which the flame front is cellular and explained this in terms of Lewis number effects in conjunction with reactant interdiffusion near extinction. Our theoretical modeling has confirmed the importance of Lewis number effects and shown that such effects can be described via extensions of classical theory. We have also assessed the effect of "partial premixing" of gaseous fuel in the oxidizing atmosphere, which can occur in building fires due to the incomplete combustion of the solid fuel vapors. We found that the presence of gaseous fuel in the ambient atmosphere can accelerate flame spread in ways which previous flame spread models could not predict. Consequently, we extended these models to include the effects of finite-rate chemical kinetics of the gas-phase fuel, which experiments show is the cause of the discrepancies. Good quantitative agreement between model and experiment was found. The influence of gravity-induced buoyant convection in flame spread over both thin and thick fuel beds is also being tested. Results on upward flame spread over thin fuel beds has shown a limit on the growth of the flame length, and thus a limit on the spread rate, occurs because of heat losses to the sides of the fuel sample and/or by radiation. For small fuel bed widths, these losses are predicted to cause the steady upward flame spread rate to vary with the cube of the sample width, and experiments verify this prediction. For wider samples and thick fuel beds, other predictions have been obtained and experiments to test these predictions are in progress. Additionally, µg experiments have shown that radiative transport and partial premixing effects influences are extremely important at reduced gravity because the thermal transport zone thickness increases remarkably, leading to drastically different heat fluxes to the fuel bed and an increased impact of heat losses (primarily through radiation). Remarkably, for some cases where the diluent gas is radiatively active (e.g. CO2) the µg spread rate is actually larger than the one-g value, whereas the opposite is true for radiatively-inert diluents (e.g. N2). This difference is extremely important because the fire suppression system on the International Space Station will use CO2 as the extinguishing agent." |
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