Numerical Studies of Supersonic Flow in Bell-Shaped Micronozzles

In this study, numerical computations are performed that examine the thrust production and efficiency of supersonic micronozzles with bell-shaped expanders. The bell geometry is favored on the macroscale for its flow alignment. To date, concerns over microfabrication challenges of the contoured geometry have limited its consideration for microscale applications. Three different bell expander configurations are examined (100% full bell, 80%, and 60%) for two-dimensional and three-dimensional duct configurations of varying depths (25-200 mu m), and a decomposed H2O2 monopropellant is used as the working fluid, and the associated throat Reynolds numbers range from 15 to 800. Owing to the inherently low Reynolds numbers on the microscale, substantial viscous subsonic layers develop on the walls of the nozzle expander, retard the bulk flow, and reduce the nozzle performance. The thrust production and specific impulse efficiency are computed for the various flow scenarios and nozzle geometries to delineate the impact of viscous forces on the nozzle performance. Results are also compared to the inviscid theory and to two-dimensional and three-dimensional results for 30deg linear nozzle configurations. It is found that the flow alignment of the bell nozzle comes at the expense of increased viscous losses, and, on the microscale, a 30deg linear nozzle offers a higher efficiency for Re<320 in two-dimensional micronozzles and over the majority of Reynolds numbers in three-dimensional simulations. The simulation results indicate that a short micronozzle outperforms a longer nozzle at a given Reynolds number, and this result is supported by existing micronozzle studies in the literature.