NUMERICAL INVESTIGATION OF THE INFLUENCE OF HIGH AMBIENT PRESSURES ON SUPERCRITICAL JETS AND MIXING PROCESS

摘要

Understanding and predicting the fuel spray characteristics under trans/supercritical conditions is crucial to the design of rocket and diesel engines. In this paper, the effects of different supercritical environmental pressures on the thermodynamics and flow characteristics of a cryogenic liquid jet are investigated numerically. In particular, we discuss the evolution processes and characteristics of the mixing layer on the liquid jet surface. To facilitate the analysis, we use nitrogen as a material to simulate the supercritical jet under the Mayer's experimental conditions by a self-built CFD model. Results demonstrate that, with increasing the supercritical ambient pressures, the pseudoboiling temperatures are increased, but the pseudoboiling behavior is significantly weakened, while the intensity of thermal diffusion increases and also the length of the cold core becomes shorter. However, in the downstream of the cold core, the thickness of the mixing layer is increased due to the reduction of the damping introduced by the density gradient. Also, with increasing the supercritical ambient pressure and weakening of the pseudoboiling effect, the isothermal expansion is more and more replaced by a continuously rising temperature process. Consequently, the jet is more like a gaseous jet, and enters the self-similar state fast. Furthermore, an entropy analysis demonstrates that a high ambient pressure promotes the entropy yield and accelerates the mixing between the injected fluids and surrounding gas, so that the mixing layer gradually retracts toward the nozzle.