Axisymmetric Flame Spread Across Propanol Pools in Normal and Zero Gravities

Abstract

Axisymmetric ignition and flame propagation across propanol pools is investigated numerically with finite-rate one-step chemical kinetics, variable properties, and an adaptive finite-difference gridding scheme without forced gas-phase flow. Propanol fuel is allowed to evaporate into the air after filling the fuel tray and before the igniter is activated. The propanol vapor profile in the gas phase is examined numerically as a function of time. At 1 -g0, a steady fuel vapor-concentration boundary layer is established in the gas phase 5 s after filling a fuel tray. Conversely, at 0-g0, the profile of fuel vapor concentration is always unsteady and keeps growing in the gas phase as time elapses. Axisymmetric flame spread over a shallow circular pool is examined as a function of oxygen concentration. The flame spread rate at \\-g0 is comparable to but lower than that at 0-g 0 for χO2,∞ ≥ 21% and T0 = 22.1°C with ΔtPI = 5s, where ΔtPI denotes the time lapse between filling the fuel tray and activating the igniter. As the oxygen concentration is reduced to 17.5% with the same ΔtPI the flame at 1-g0 pulsates whereas the flame at 0-g0 extinguishes. At 1-g0, the flame spread rate does not change for ΔtPI ≥ 5s; on the other hand, at 0-g0, the flame spread rate increases as ΔtPI is increased. Flame propagation is also examined as a function of initial pool temperature T 0 both for axisymmetric and two-dimensional configurations with pool depth 2 mm in normal gravity. For the uniform spread regime, flame spread rate in the axisymmetric configuration is nearly identical to that in the two-dimensional configuration except for a short initial time. For the nonuniform (or pulsating) spread regime, flame spread in the axisymmetric configuration is quite different from flame spread in the two-dimensional configuration. The axisymmetric flame spreads very slowly without pulsation for some initial time depending on T 0 and then either spreads with pulsations or remains stationary as long as the igniter is activated.