Overcoming Low Nozzle Efficiency: A Test-Correlated Numerical Investigation of Low Reynolds Number Micro-Nozzle Flow

The U.S. Naval Research Laboratory (NRL) has been investigating viscous flow through micronozzles applicable to advanced spacecraft propulsion systems under a four-year 6.2 Discovery and Invention research effort. The objective of the program is to create thruster nozzle design tools that perform numerical optimization of the nozzle geometry to minimize viscous losses in the nozzle for challenging viscous flow regimes. This paper summarizes interim results for the program, and provides contextual detail for topics covered more extensively in three related papers from the program by coinvestigators Holman, Williams and Rosenberg. The micro-nozzles under investigation produce thrust levels on the order of 1 mN and have throat diameters of less than 0.010 inches. These nozzles are applicable to many micro-propulsion applications for cubesatellites or other small spacecraft. Flow conditions were selected to be representative of resistojet propulsion, but are applicable to many other electro-thermal devices. The nozzles were operated under a variety of pressure, mass flow and temperature conditions, while exhausting to vacuum, over a range of Reynold’s Numbers below 2000. Thrust was measured using a precision torsional thrust balance, and other diagnostic measurements were collected for correlation to numerical fluid dynamic simulations. Computational Fluid Dynamics (CFD) was used in the continuum regions of the nozzle flow field and transition to Direct Simulation Monte Carlo (DSMC) in the rarified region of the nozzle exit and near field plume. Advanced DSMC techniques were used to efficiently model the complete flow field. The work demonstrates a new capability to numerically optimize the thruster nozzle geometry allowing the user to tailor thruster geometry to maximize the nozzle thrust or specific impulse performance for challenging viscous flow environments.