Solid–gas surface effect on the performance of a MEMS-class nozzle for micropropulsion

Steady-state performance of a three-dimensional flat-shaped micronozzle in underexpanded cold- and hot-gas operation has been analysed with numerical simulation. The flow regime is supersonic and characterized by a Knudsen number varying from slightly slip-flow by the nozzle throat, to moderate transitional regime near the nozzle lip. A second-order in Knudsen model has been implemented to incorporate the effects of the slip in velocity and temperature jump into a Navier–Stokes finite-volume commercial code, so fully thermal coupling between the gas and solid portions of the computational domain is imposed and the heat balance at the solid surface is enforced from the temperature jump at the interface. The results stress the strong dependence between solid–gas surface temperature and gas expansion across the nozzle, favoured by the high-speed of the flow that enhances the heat-transfer by the solid surface. The results also show that thermal coupling at the solid–gas surface becomes an essential part of the modelling for reliable prediction of the nozzle wall temperature and therefore of the performance delivered. As viscous dissipation leads to a performance drop that may be serious in too lengthy nozzles, the maximization of performance demands to consider the three-dimensional nature of the high-speed flow in simulations with microgeometries.