Flame acceleration in narrow channels: Applications for micropropulsion in low-gravity environments

A laminar flame propagating toward the open end of a narrow channel filled with a gaseous combustible mixture can accelerate or oscillate, depending on the wall temperature and the channel width. The accelerating flame is able to produce a high-speed flow that has the potential to provide significant thrust. We study these phenomena using multidimensional reactive Navier-Stokes numerical simulations, and show that for adiabatic walls, the maximum flame acceleration occurs when the the channel is about five times larger than the reaction zone of a laminar flame. In three-dimensional square channels, the flame speed increases roughly two times faster than in two-dimensional channels. The accelerating flame generates weak compression waves that propagate with a local sound speed and can accelerate the unreacted material ahead of the flame to velocities close to the speed of sound without creating strong shocks. This combustion regime is of particular interest for micropropulsion because it allows efficient use of fuel and gradual development of thrust.