The mathematical model of the ejector with contrivable back pressure in a transcritical carbon dioxide ejector-expansion refrigeration cycle was established. The critical back pressure of the ejector was defined as an iterative value by ignoring its outlet kinetic energy. The effects of the ejector back pressure and outlet velocity on the system performance were analyzed to optimize design or control the ejector system performance. The results show that the entrainment ratio can maintain constant when the ejector back pressure is lower than critical value at different high-side pressures. The coefficient of performance(COP)increases with the ejector back pressure. It seems that the ejector back pressure under controlled system status should be close to the critical value as far as possible. Moreover

the variation tendency of COP changes similarly with the high-side pressure at different back pressures. Back pressure as the design parameter or control parameter does not affect the optimization of high-side pressure. Due to flow friction and impact

the outlet kinetic energy of the ejector will transform into heat and eventually lead to thermodynamic loss. In addition

the improper back pressure will raise the outlet velocity

and the outlet velocity increases with the decrease in the back pressure. The outlet velocity decreases dramatically first and then varies slowly with an increase in the high-side pressure of the system.