Flow Characteristics of Micro-Scale Planar Nozzles

Abstract

Flow in micro chemical propulsion systems (µCPS) based on etched silicon deviates strongly from its conventional, macroscopic counterparts. This paper reports on peculiarities of small scale planar nozzles with a high aspect ratio, rectangular cross section. Design and analysis paradigms based on the assumption of rationally symmetric flow with a dominant isentropic core are shown to be no longer valid. We will point out insufficiencies of treating planar nozzles as two dimensional, inviscid, or assessing their performance with classical analytical isentropic 1D analysis. Instead, the resulting low Reynolds number flow is boundary layer dominated. Boundary layer build-up from the top and bottom walls threaten to choke the expansion. The geometrical expansion ratio is found to be essentially irrelevant, the length from throat to exit plane is found to be a much more important design parameter. The work has been carried out within the European PRECISE project which is focused on designing and testing a prototype using catalytically decomposed hydrazine as propellant.