A novel relaxation drift model for simulating liquid-vapor momentum non-equilibrium in two-phase ejectors

Abstract

A novel relaxation drift model (RDM) was proposed to simulate the interphase momentum non-equilibrium (slip) inside two-phase work-recovery CO2 ejectors. The RDM was validated using the experimental data of 5 different ejectors under 15 trans-critical conditions in total. The proposed model was compared with the existing homogeneous model (HM) and algebraic slip model (ASM) through case studies. The effects of ejector mixer design and nozzle expansion state were also disclosed. The results showed that the prediction errors of ejector performance by the RDM were within 17 %, which was smaller than the maximum error of 23 % predicted by the HM and the ASM. The numerical results demonstrated that the liquid droplets had lower velocities than the vapor when the liquid-vapor mixture accelerated, but the droplets travelled faster than the vapor during deceleration of the mixture. It was also revealed that the velocity slip would affect the shock pattern at the diffuser inlet if the mixer was choked. The effects of slip, however, became insignificant when the mixer was designed too large. In addition, the RDM might overestimate the ejector performance if the motive nozzle was strongly under-expanded. This study facilitates understanding of interphase velocity slip in two-phase CO2 ejector flow.

Keywords

• Two-phase ejector
• Momentum non-equilibrium
• Slip velocity
• Relaxation
• Particle diameter