Modeling and Simulation of Fuel-Oxidizer Mixing in Micropower Systems

This paper estimates the range of Reynolds numbers and diffusive mixing lengths associated with fuel-oxidizer mixing in micropower systems and then develops analytical and numerical models to explore how mixing performance varies with device size. Both axial and transverse diffusion of species are considered. The models show that Reynolds numbers associated with mixing in micropower systems fall in the laminar-transitional flow regime, where relatively little experimental data exists. They also indicate that fuel-oxidizer mixing lengths decrease with decreasing device size and that the relative importance of axial diffusion to fuel-oxidizer mixing on the microscale depends on the ratio of the diffusive to the convective velocity. At high flow velocities, the mixing length is proportional to the convective velocity and the physical dimensions of the device. At low flow velocities, diffusion dominates, and the mixing length is only proportional to the physical dimensions of the device. This transition in behavior is the result of axial diffusion, which becomes important at Re < 20. Overall, these results suggest that axial diffusion may impose additional limits on the degree to which a combustion-based micropower system can be miniaturized.