Experimental Characterization of Nozzle Performance at Low Reynolds Numbers for Water Microthrusters

This study directly measures the performance of a thruster nozzle using water vapor as propellant in the Reynolds number below 1000. Because of its properties, water is suitable as a propellant for small satellites, often with a low-Reynolds-number flow using a micronozzle. The low Reynolds number of the flow and the phase changes in supersonic vapor flow should affect nozzle performance. It is essential to measure thrust performance and consider the flow dynamics inside the nozzle, but to date there have been few direct thrust measurements at low Reynolds numbers from 100 to 600 and different temperatures of the vapor from 290 to 400 K. In this study, nozzle performance is evaluated using two dimensionless numbers: the discharge coefficient and the specific impulse efficiency. The results presented here are specific to water vapor. The discharge coefficient remains almost the same under these conditions and higher than those of argon and nitrogen. The specific impulse efficiency depends on the degree of superheating above Reynolds numbers of about 250, but shows little dependence below Reynolds numbers of about 250 owing to the combined effects of viscosity and phase change.