A Thermodynamic Model for Performance Prediction of an Ejector with an Adjustable Nozzle Exit Position

Improving the efficiency of ejectors during off-design operations can be effectively achieved through the automatic adjustment of the nozzle exit position (NXP). A thermodynamic model for predicting the performance of an ejector with an adjustable nozzle position is proposed and validated. The key factors influencing the optimal nozzle exit position under variable operating conditions are analyzed using the model. The dimensionless optimal nozzle exit position (DONXP) of the ejector is fitted as a function of these key factors, and a nozzle exit position adjustment scheme for variable operating conditions is further derived. The proposed model has maximum errors in the entrainment ratio, critical back pressure, and nozzle exit position within ±10.70%, ±7%, and ±15.85%, respectively. When the area ratio increases, with the transition point located in the mixing chamber, the increase rates of the DONXP are within 0.068~0.195 for the R245fa, R600a, R141b, and R134a ejectors. However, when the transition point is located before the entrance of the mixing chamber, the increase rates are within 0.0009~0.0034. When the area ratio is fixed, the larger the expansion ratio, the smaller the DONXP. The DONXP can be fitted according to different cases where the turning point is located either before or after the entrance of the mixing chamber, to meet the demand for automatic adjustment of the nozzle exit position.