Numerical Study on the Design of Microchannel Evaporators for Ejector Refrigeration Cycles

Two-phase ejectors are devices capable of improving the performance of refrigeration and air conditioning cycles by means of expansion work recovery. Ejector studies often focus on the design and performance of the two-phase ejector and the effect it can have on the performance of the ejector cycle. However, the ejector is not the only component of the system that can have a significant influence on the performance of the ejector cycle. Recent experimental work has shown that the effect of evaporator design on ejector cycle performance can be quite significant, though there is very little research available on the relation between evaporator design and ejector cycle performance. In this paper, a numerical model of a microchannel air-to-refrigerant evaporator, capable of accounting for heat transfer and pressure drop effects, is developed and used to investigate the effect that different evaporator dimensions have on the performance of ejector cycles. The model is validated with experimental data from previous ejector cycle studies. There are two ejector cycles of interest: The standard ejector cycle, in which the ejector is used to directly lift the compressor suction pressure, and the ejector recirculation cycle, in which the ejector recirculates excess liquid through the evaporator but does not directly lift compressor suction pressure. The effects of microchannel port hydraulic diameter, number of evaporator passes, refrigerant outlet state, and air-side resistance are investigated. The analysis is performed with refrigerants R134a and CO2 (R744) in order to demonstrate how proper evaporator and ejector cycle design is different for different refrigerants