Numerical Analysis for Parametric Study and Shape Optimization of Two-Phase Ejector with R134a Refrigerant

Abstract

A two-phase ejector is a passive pumping device to increase the flow rate and to enhance compression of the fluid flow by geometrically induced secondary flows. In particular, a high-speed two-phase ejector has attracted attention as an alternative to the throttling valve, because it has the potential to improve significantly the performance of refrigeration systems by compensating the throttling loss that appears in expansion devices. However, flows inside the ejector are so complex that it is not easy to characterize the relevant flow and thermodynamic behaviors through the theoretical and experimental methods. In contrast, the numerical approach is relatively favorable to elucidate the relevant flow physics inside the ejector, and is considered useful to improve the performance of the ejector. Unfortunately, there have been few relevant numerical studies because it is challenging to resolve high-speed flows accompanied with phase transitions. In this thesis, we present numerical methods for representing the high-speed flows inside a two-phase ejector accurately. An evaporation-condensation model is implemented and the real-fluid properties of refrigerant R134a are input in our RANS simulations to resolve phase transitions. Prior to applying numerical methods to the parametric study, we validate the computational apparatus by comparing the numerical results for the baseline ejector model with experimental data. Then, we perform the parametric study to identify the effects of design variables on the entrainment performance. Finally, the ejector shape is optimized to maximize the entrainment performance using both an evolutionary and a micro-genetic algorithms. An optimal ejector design shows a 55% improved entrainment performance as compared to the baseline ejector model. Therefore, it is expected that our study can provide specific guidelines to be considered when designing supersonic two-phase ejectors as well as contribute to studies associated with supersonic two-phase ejector-equipped refrigeration systems.