A novel quasi-one-dimensional model for performance estimation of a Vaporizing Liquid Microthruster

Abstract

The present work aims to propose a novel quasi-one-dimensional model for the performance estimation of a Vaporizing Liquid Microthruster (VLM). The analytical model was applied to the analysis of a MEMS-based VLM composed of a rectangular inlet chamber, a set of parallel microchannels as heating chamber, and a planar convergent–divergent micronozzle. It combines a steady-state boiling model for the analysis of the heater with a real nozzle flow model for the evaluation of actual thrust force and specific impulse, based on iterative procedure aiming at the convergence of the actual mass flow rate and the heat flux. For the purpose, a set of semi-empirical formulas found among both theoretical and experimental scientific works have been introduced for the estimation of the critical heat flux condition and the local heat transfer coefficient. In addition, the real nozzle flow model predicts the performance and the viscous losses due to the boundary layer growth inside the micronozzle. The last ones are estimated by introducing analytical expressions for the discharge coefficient and the Isp-efficiency into the isentropic nozzle flow theory. The resulting performance predictions of the 1D model referred to the on-design operating conditions. They well agreed with the experimental data, with a maximum estimated error of 7.3% on the thrust and the specific impulse. Furthermore, the analytical model of the micronozzle predicted a reduction of the mass flow rate up to about 8%, as well as thrust losses up to 15% due to the contraction of the cross sectional area.In addition, 2D and 3D computational fluid dynamics (CFD) simulations were performed in order to enforce the analysis of the viscous effects. Predictions of 2D computations overestimated the performances of the microthruster with respect to experiments, up to about 19% of the thrust and 20% of the specific impulse. On the other hand, the 3D predicted thrust approached to the experimental one with an error of about 9.2% below. In addition, a severe reduction of jet thrust in favor of the pressure thrust was observed at the nozzle exit. Furthermore, 3D computations pointed out the influence of the micronozzle depth on the boundary layer growth and the viscous losses. In particular, they revealed the establishment of the nozzle blockage and the thermal chocking of the supersonic flow owing to the subsequent viscous heating.