Transient numerical study of CO2 two-phase flow in a needle adjustable ejector

Steady-state numerical modeling is mostly used for predicting the flow and transport characteristics in an ejector, but it may not be able to reflect the actual performances of an ejector when operating condition varies. Moreover effects of needle movement on the performances of an ejector is not very clear. In this study, a transient heterogeneous mixture model was developed for a two-phase CO2 needle adjustable ejector with motive nozzle inlet in supercritical, critical or sub-critical regions. The dynamic mesh method was used to control the needle movement, and the unsteady coupling solution between the needle movement and the flow field is solved for the adjustable two-phase CO2 ejector. Modeling results show that the transient modeling can predict dynamic performances of CO2 ejector more accurately than the steady-state modeling. Based on the transient modeling, it is found that as the needle moves further into the motive nozzle throat, effective nozzle throat area decreases, expansion angle of motive flow increases then decreases, the velocity at the motive nozzle outlet decreases then increases, and entrainment ratio decreases, then increases sharply, then decreases, finally become negative. High pressure at the motive nozzle inlet leads to high entrainment ratio owning to high velocity at the motive nozzle throat outlet. A sudden increase of pressure at the motive nozzle inlet in different thermodynamic state regions (supercritical, sub-critical) leads to different dynamic performance responses and oscillation times of ejector. The mechanisms of boundary layer separation and backflow vortices as well as shock waves were explored to explain the suction flow chocking phenomena. This study is helpful to understand flow mechanisms inside the CO2 ejector, and improve optimal design and control of an adjustable two-phase CO2 ejector in variable operating conditions in practice.