Flash-Evaporation of Oxidizer Spray During Start-Up of an Upper-Stage Rocket Engine

Abstract

The low-pressure oxidizer preflow during start-up of an upper-stage rocket engine is analyzed computationally. An iterative Euler-Lagrange approach is used to describe the three-dimensional two-phase flow of oxidizer vapour and spray which is characterized by intense phase transfer. Multipoint injection and flash-atomization of liquid oxidizer is modelled by stochastic droplet injection according to injector locations and spray characteristics. Based on the framework of classical D2-theory, a flash-evaporation model is developed to describe heat and mass transfer between superheated liquid droplets and vapour flow. The computed transsonic flow field agrees well with experimental characteristics such as pressure level and temperature decrease and further provides detailed local information on vapour flow homogeneity and liquid wall deposit. Introduction During start-up of an storable propellant upper-stage rocket engine, a steady flow of vaporizing liquid oxidizer is maintained in the combustion chamber to ensure well-defined conditions prior to injection of liquid fuel and hypergolic ignition. Due to the absence of combustion, this flow regime is characterized by thermodynamic conditions far from nominal operation of the engine. The pressure and temperature levels in the chamber are predominantly controlled by the evaporation rate of the oxidizer. Due to the extreme pressure drop from oxidizer dome to combustion chamber (see figure 1) the injected liquid is superheated. Rapid vapour formation and expansion within the liquid leads to flash-atomization, a mechanism which is characterized by extended spray cones and significantly reduced droplet sizes. Accordingly, heat and mass transfer between spray and vapour flow is governed by flash-evaporation. In contrast to conventional droplet evaporation, this mechanism is controlled by heat transfer within the droplet. However, only a fraction of the injected oxidizer is evaporated in the combustion chamber during preflow. Aside of a finite-rate phase-change and the short residence time in the chamber flow, this is mainly due to the limited superheat energy content and the absence of other energy Research Engineer, Aerothermodynamics Section Research Engineer, Aerothermodynamics Section Copyright c © 2003 by European Space Agency. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. 200 250 300 350 400 10 10 10 10 10