Difficulties in Defining the Degree of Superheat in Flash Boiling Liquid Nitrogen Sprays

Abstract

Technology development for propulsion systems of upper stages and reaction control thrusters is driven by green propellants to substitute hydrazine. At high-altitude conditions prior to ignition the liquid propellants are injected into the combustor at near-vacuum. Due to the sudden pressure drop the liquid is in a superheated thermodynamic state resulting in an eruptive evaporation and fast expansion, a process called flash boiling. To know the composition related to phase and atomization is important for both to determine the probability of a successful ignition and to avoid destructive pressure peaks. Hence, we visualized flash boiling liquid nitrogen (LN2) sprays by means of shadowgraphy at the cryogenic test bench M3.3 at DLR. The data analysis of these sprays revealed that the commonly used degree of superheat Rp on its own is not sufficient to describe the flash boiling process. This paper focuses on the description of this issue by comparing shadowgraph images of different LN2 sprays to each other and by the discussion of the kinetics of the sprays in terms of velocity and pressure distributions obtained by numerical simulation. Despite similar degrees of superheat Rp, the spray morphology as well as the spray kinetics show serious deviations from each other.