Effects of ambient conditions on concurrent-flow flame spread over a wide thin solid in microgravity

Abstract

Two-dimensional numerical simulations were performed to study concurrent-flow flame spread over a thin solid in microgravity. The main variable is the ambient flow velocity. Results were validated against recent microgravity experiments (Saffire) where samples 41 cm wide were burned in two different flow velocities. The numerical results showed that, when flow velocity increases, the average radiative heat flux out of the solid surface in the pyrolysis region remains constant. The average convective heat flux decreases and the average radiative heat flux into the solid surface increases. The average net heat flux remains approximately constant. Additional simulations using different ambient pressures and oxygen percentages further showed that, while away from extinction conditions, the average net heat flux in the sample pyrolysis region and hence the average solid mass burning rate are insensitive to the ambient conditions (i.e., oxygen percentage, flow speed, pressure). Furthermore, the steady-state flame length is mainly controlled by the rate of oxygen entrainment into the gas-phase reaction zone and the average solid mass burning rate. Based on these observations, analytical model was developed to correlate the flame spread rate and flame length to ambient conditions. It predicts that, for concurrent-flow flame spread at steady state, the flame length and flame spread rate have linear dependency on the ambient pressure and the forced flow velocity, and a second order dependency on the ambient oxygen percentage. The proposed correlations were tested against numerical simulations as well as a collection of data from previous microgravity experiments in a wide range of ambient conditions.