Optimization of microchannel evaporator design in HVAC&R applications using two-phase ejectors.

Expansion work recovery through the use of two-phase ejectors is receiving increased attention as a method to improve the performance of refrigeration and air conditioning systems. However, the majority of the published ejector studies focus on the design and performance of the two-phase ejector itself and the effects certain design variations have on the overall performance of the ejector cycle. Most of the studies overlook that there are other components that can greatly affect the performance of an ejector system and which should therefore be optimized. Recent experimental work has demonstrated that the effect of evaporator design on ejector cycle performance can be significant, although very little research has been published on the impact of evaporator design on the overall performance of the ejector cycle. In this study, a numerical simulation model of a microchannel air-to-refrigerant evaporator, capable of accounting for heat transfer and pressure drop effects, has been developed and is used to investigate the effects of key evaporator dimensions on the performance of ejector cycles. Experimental data available from earlier studies have been used to validate the evaporator model. This model has been linked with two ejector cycles of interest: the standard ejector cycle, in which the expansion work recovered by the ejector is used to directly increase the compressor suction pressure, and the ejector liquid recirculation cycle, in which the recovered expansion work is used to provide evaporator liquid overfeed. In the latter cycle, the recovered work is utilized to increase the effectiveness of the evaporator by reducing the dry-out zone rather than to use the recovered work to reduce the compressor pressure ratio. The effects of microchannel port hydraulic diameter, number of evaporator passes, and refrigerant outlet state are investigated. R410A is used as the refrigerant.