Opposed flame spread over a thermally-thin charring material under varied flow velocities and ambient pressures in a narrow channel apparatus

The opposed flame spread over thermally-thin filter paper under constrained buoyancy at varied flow velocities and ambient pressures was investigated, which will be conducive to develop ground-based techniques for simulating microgravity. With increasing flow velocity, three regimes can be roughly identified: 1) near quenching regime, the periodic fingering flamelets appear for oxygen starvation, expanding the flammability range; 2) steady-state spreading regime, symmetric flames occur among the sample surface due to buoyancy suppression, and the flame spread rate reaches its peak; 3) near blow-off regime, the flame is affected by finite chemical kinetics. The whole pyrolysis region is identified into the primary/secondary pyrolysis regions characterized by blue/orange attached flames, and the frequent transformation between them intensifies the flame instability. With increasing flow velocity, the global equivalence ratio increases firstly and decreases finally under higher pressures, while decreases monotonously under lower pressures. The optimal utilization of the narrow channel apparatus for suppressing the buoyancy effect should be located in the range of moderate flow velocities far away from the extinction limits, where the heat loss effect of the parallel plates can be neglected. By tracking the extinction limit, the flammability boundary under different pressures was obtained. It is found that the quenching boundary is dependent on the diffusion rate of fuel vapor, while the blow-off boundary is determined by the critical global equivalence ratio. The flammability range narrows with the decreasing pressure, and relatively higher ambient pressure is required to achieve better effectiveness of simulating microgravity in future space missions.