Investigation on key geometries optimization and effect of variable operating conditions of a transcritical R744 two-phase ejector

Abstract

There was no literature that involved the combined investigation on the key geometries optimization and the effect of variable operating conditions (temperature, pressure and dryness fraction of primary and secondary flows) for a R744 two-phase ejector. In this paper, CFD simulation on geometries optimization and effect of variable operating conditions of a transcritical R744 two-phase ejector was carried out for the first time. Firstly, each of three geometries of area ratio (AR), nozzle exit position (NXP) and constant-area mixing chamber length (L2) was optimized to determine their relative sensitivity on ejector performance. Then, two or three geometries under different optimization orders were optimized. Finally, the effects of temperature, pressure and dryness fraction of primary and secondary flows as well as back pressure on ejector performance were analyzed based on the optimal ejector geometries. The results showed that: (1) the relative sensitivity of entrainment ratio (ER) to AR is the highest with value of 44.56%, followed by NXP and L2; (2) optimizing two or three of these parameters under different optimization sequences can achieve certain optimization effect with ER improvement within 5.96%–33.91%, but the result depends on the optimization order; (3) the effect of dryness fraction of the primary flow on the ejector performance is greater than that of temperature; (4) the change of secondary flow pressure has a greater impact on the ejector ER as compared to the dryness fraction of the secondary flow; (5) not only increasing the primary and secondary flow pressure but also reducing the back pressure is conducive to improving the ejector performance.