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Abstract
Pulsating and uniform spread across n-propanot and ethanol liquid fuel pools is simulated via a two-dimensional, transient, numerical model which incorporates finite-rate chemical kinetics, variable properties, and a partially adaptive finite-difference gridding scheme. The model is compared to detailed, independent experimental data. The explanation and characterization of the pulsating and uniform flame spread phenomena are developed
Pulsating flame spread requires a gas-phase recirculation cell just forward of the flame. δ flow This cell entrains evaporating fuel vapor. The size and existence of the recirculation cell is determined by the extent of liquid motion ahead of the flame (δ flow) and by opposed flow in the gas phase, naturally induced by buoyancy. The amplitude and period of the pulsations each increase with δ flow Over the range of pool depths that were investigated (2 to 10 mm) the liquid-phase flow is primarily affected by large surface-tension variations along the liquid surface and not by buoyancy. The critical role of gas expansion near the reaction zone in the flame pulsation cycle is identified. Contrary to previous conjectures, mass diffusion at the flame leading edge is vital for both uniform and pulsating spread.
Although there is little liquid motion more than 2 mm ahead of the flame in the uniform spread regime, liquid-phase convection driven by thermocapillarity is the dominant mechanism for transporting heat ahead of the flame for both the pulsating and uniform regimes in normal gravity. As the gravity level decreases, the fuel consumption rate and maximum flame temperature decrease, the flame standoff height increases, and the gas-phase flow field and flame spread rate are governed more by hot gas expansion and very lean, premixed chemical reactions than by the coupling with the liquid phase. Pulsating flame spread does not occur in microgravity without forced, opposed gas-phase flow because a gas-phase recirculation cell does not form ahead of the flame leading edge. Unlike normal gravity flame spread, the microgravity flame spread rate is very sensitive to the thickness y, of the initially uniform layer of fuel vapor above the liquid surface. For relatively small y,, the model predicts that conditions yielding pulsations in normal gravity correspond to extinction in microgravity. For relatively large y 1 the flame spread rate increases with decreasing gravity level. Parametric studies show that the flame spread character and rates are strongly affected by fuel volatility and chemical kinetic constants and less strongly by the liquid viscosity and the surface- tension temperature coefficient. These parameters affect the initial pool temperature at transition to flame pulsations but do not change the dependence of flame pulsations on the existence of the gas-phase recirculation cell. Pulsating flame spread occurs at conditions that rates (e.g., lower initial liquid temperature, higher latent heat of vaporization, lower height of the initially uniform fuel vapor layer in the gas phase, or higher activation energy). An increase in surface-tension coefficient or a decrease in the liquid viscosity results in a faster spread rate in the uniform regime but does not appreciably change the mean flame spread rate in the pulsating regime.
