Flame Acceleration in Narrow Channels: Applications for Micropropulsion in Low-Gravity Environments

A laminar flame propagating towards 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 the velocities close to the sound speed without creating strong shocks. This combustion regime is of particular interest for micropropulsion because it allows an efficient use of fuel and a gradual development of the trust. I. Introduction This work is based on a micropropulsion idea generated from a relatively recent discovery concerning the behavior of laminar flames propagating in very narrow tubes. When a slow laminar flame is ignited near the closed end of a tube, the material ahead of the flame is accelerated and a boundary layer is formed along the walls in the unreacted material. Due to the presence of the boundary layer, the flow velocity changes across the channel, increasing from zero at the wall to a maximum in the middle of the tube. Previous simulations [1-3] have shown that the nonuniform flow stretches the flame, so that the shape of the flame becomes similar to the velocity profile. This increases the flame surface area, thus accelerating the energy generation and the flow. There is little change in the laminar burning rate, but the speed of the flame that propagates with the flow grows very rapidly in the laboratory frame of reference, from centimeters per second to hundreds of meters per second for adiabatic walls. This phenomenon, explained by Ott et al. [1-3], is different from the well known case of flame propagation in a channel with cold walls. When the walls are cold (the case originally considered in the classical work of Mallard and Le Chatelier [4]), the flame quenches near the walls, and the flame velocity oscillates. We consider regimes in which the flame remains laminar until the end of the tube and does not produce strong shocks or detonations. The high-speed flow created by the accelerating flame has the potential to provide a significant thrust that can be further enhanced by attaching an appropriately shaped exist nozzle. The thrust will gradually increase as the flame and flow accelerate, and reach a maximum when the flame approaches the open end of the tube or the nozzle. In contrast to the majority of other combustion-based propulsion concepts, most of the thrust here can be provided by the material accelerated ahead of the flame. Therefore, a propulsion device could be only partially filled with a combustible mixture. The rest of the material can be an inert gas or combustion products from a previous pulse. This micropropulsion concept is of particular interest for low-gravity environments, where relatively small, controlled thrusts could be used for navigational corrections. In such cases, there might not be a need for the high repetition rates required for pulse combustion and pulse detonation engines. The purpose of this paper is to confirm original results [1-3] and to carry the study further to examine the possibility of using flame acceleration in narrow tubes for micropropulsion. As a model energetic system, we consider the same stoichiometric acetylene-oxygen mixture used in [3], perform numerical simulations for two-dimensional (2D) and three-dimensional (3D) channels of different widths and lengths, and analyze effects of channel sizes on integral propulsion characteristics.