Gravity modulation study on opposed flame spread over thin solid fuels

In this work a numerical investigation has been carried out to study the effect of g-jitter on zero-gravity (0g(e)) opposed flow spreading flame over thin solid fuels. For comparison simulations have also been carried out for normal gravity (1g(e)) downward spreading flames. G-jitter is emulated by gravity modulation of sinusoidal (A(ge) sin(2 pi t/T-ge)) gravity perturbation (g-perturbation) of a particular time-period (T-ge) and amplitude (A(ge)) over a selected base gravity level (0g(e) or 1g(e)). The response of flames at 0g(e) base gravity and at 1g(e) base gravity was different to the imposed g-perturbation. While at 0g(e) the mean and the amplitude of the oscillatory flame spread rate (FSR) magnified with increase in the time period of g-perturbation, interestingly for the 1g(e) flame a maximum mean FSR and oscillation amplitude occurs at certain perturbation time period. Further, at very small perturbation time-periods (T-ge) the FSR at 1g(e) was lower than the steady state FSR. The amplitude of oscillatory FSR increased with increase in perturbation amplitude (A(ge)). However, the 0g(e) flame which is little affected (compared to 1g(e) flame) at small perturbation amplitude (A(ge)) is affected severely at large perturbation amplitude (A(ge)). Both the gas phase and fuel pyrolysis (or fuel response) follow perturbation signal with a lag but fuel pyrolysis is more sluggish and lags behind gas phase. The phase lag between fuel pyrolysis and gas increases at smaller time-periods (T-ge) and tends to enhance the effect of external perturbation whereas at larger time-periods (T-ge) this lag inhibits the effect of external perturbation. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.