CFD study on the optimal nozzle exit position in a CO2 two-phase ejector.

The nozzle exit position (NXP) is a significant parameter for an ejector. In this work, a two dimensional (2D) homogeneous CFD model is developed to simulate the two-phase ejector used in transcritical CO2 refrigeration cycle and NXP is focused on. After model validation, the flow fields in ejectors with different NXPs are simulated and an optimal NXP is obtained. According to the numerical results, with the increase of the NXP, a higher velocity and lower pressure are achieved for the primary fluid in the suction chamber which enhances the entrainment of the secondary fluid. Oppositely, the flow area of the primary fluid at the mixer inlet increases with the NXP which reduces the secondary flow rate. A compromise between these two factors leads to the optimal NXP with the highest entrainment ratio under the same operating condition. The results are helpful to the optimal design and control of the ejector.