Effect of Source Geometry on Fully Expanding Free Jets

Abstract

Underexpanded free jets issuing from sonic orifices are a fundamental flow in rarefied gas dynamics and as such have been studied extensively both in theory and experiment. An analysis of jet plumes originating from rather high pressure reservoirs and expanding freely into high vacuum pose difficulties to both approaches. Especially most test facilities cannot provide and sustain a background pressure low enough to permit expansion of the jet into the free molecule flow regime. The High-Vacuum Plume Test Facility for Chemical Thrusters (STG-CT) at the DLR site in Göttingen is designed for the particular purpose of studying free plume expansion, employing a liquid-helium driven cryo pump. In this work we compare mass flux angular profiles of two different sonic flow sources, recorded with a free molecule impact pressure probe ("Patterson probe"). Both sources have a reservoir-to-exit diameter ratio of D/d∗=27 at a common exit diameter d∗ = 0.6 mm, but one is a flat-faced cylinder into whose face a square edged orifice is centrally machined, the other source features a sharp-edged orifice whose outer walls are inclined backwards 135 degrees from the axis. Nitrogen gas is used throughout this study. The free expansion is found to be hindered at angles well smaller than 90 degrees, and comparison with the conical flow source indicates that the disturbance likely arises from interaction of plume gas with the outer surface of the flat-faced source. This effect has to our knowledge previously not been been described before and will be studied here in more detail.