Simulation of the phase change process of carbon dioxide inside an ejector geometry

Abstract

In the current work, we perform three dimensional simulations of the phase change process of CO2 inside an ejector geometry. We use the droplets based non-equilibrium phase change solver in Ansys CFX for this purpose. The real gas properties of CO2 are based on the Redlich Kwong equation of state already built-in Ansys CFX. We observe an ‘in-phase’ variation of liquid mass fraction and the Mach number both of which vary together but out of phase with the pressure and the temperature. The solver predicts a maximum liquid mass fraction of 0.3 inside the motive nozzle and shows a small amount of liquid fraction (∼ 0.075) leaving from the ejector outlet. The drop nucleation rate is observed to be higher wherever surface tension coefficient is lower, which in turn, depends on the vapor temperature. The drop diameter and number density vary mutually out of phase inside the motive nozzle and in the mixing section. The drop diameter is smaller and the number density is higher wherever the nucleation rate is higher and the liquid mass fraction is lower. These are also the locations where the pressure and temperature are higher as well. Therefore, we conclude that the structure of the oblique shock train outside the motive nozzle has an intimate connection with the condensation behavior inside the ejector.