Influence of Wall Heat Transfer on Supersonic Micronozzle Performance

摘要

A numerical model to characterize the influence of wall heat transfer on performance of a microelectromechanical systems (MEMS)-based supersonic nozzle is reported. Owing to the large surface-area-to-volume ratio and inherently low Reynolds numbers of a MEMS device, wall phenomena, such as viscous forces and heat transfer, play critical roles in shaping performance characteristics of the micronozzle. Viscous subsonic layers inhibit flow and can grow sufficiently large on the nozzle expander walls, potentially merging to cause the flow to be subsonic at the nozzle exit, and result in reduced efficiency and performance. Heat flux from the flow into the surrounding substrate can mitigate subsonic layer growth and improve overall thrust production. In this study, subsonic layer growth is quantified to characterize the impact on performance of micronozzles with a flowfield that is subject to wall heat transfer. Both two- and three-dimensional (3-D) simulations are performed for varying expander half-angles (15 deg, 30 deg, and 45 deg) and varying throat Reynolds numbers (30-800), whereas the depth of the 3-D nozzle is varied (25-300 μm). Simulation results and nozzle efficiencies are compared with inviscid theory, previous adiabatic results, and existing numerical and experimental data. It is found that heat loss to the substrate will further accelerate the supersonic core flow via Rayleigh flow theory and can reduce subsonic layer growth. These effects can combine to alter the micronozzle expansion angle, which maximizes thrust production and specific impulse efficiency.