Numerical analysis of two-stage vacuum ejector performance considering the influence of phase transition and non-condensable gases

Abstract

Multi-effect distillation with thermal vapor compression system has not been able to penetrate the market because of the low coefficient of performance. The two-stage vacuum ejector is an effective vehicle to maintain system condensing temperature, discharge non-condensable gases and improve the system performance. Improving performance of two-stage vacuum ejector relies on the understanding of the fluid dynamic phenomena. Unfortunately, few studies paid attention to the effect of inner fluid phase transition and non-condensable gas mass fraction on the performance of vacuum ejector. Therefore, this paper contributes to proposing a Species transport and Phase transition model to investigate the two-stage vacuum ejector numerically considering the physical properties of non-condensable gases, thermodynamic transformation caused by condensation and evaporation with experimental validation. The results indicate that the entrainment performance of the ejector increases by about 14.95% when the non-condensable gases mass fraction increases from 0% to 100%. Meanwhile, with the consideration of heat and mass transfer in the phase change process, the mean error is 4.9%, which is much lower than single-phase model. Meanwhile, the internal fluid pressure, Mach number, shock waves and expansion degree can be largely influenced by the consideration of Species transport and Phase transition. The results of this paper proved that influence of non-condensable gases and phase transition cannot be ignored in the design and prediction of vacuum ejector and it can contribute to ejector optimization and enhancing the stability and efficiency of multi-effect distillation with thermal vapor compression system.