Numerical Investigation of the Effect of Microchannel Evaporator Design on the Performance of Two-Phase Ejector Automotive Air Conditioning Cycles 2015-01-0362

Much attention has been given in recent years to the use of two-phase ejectors and particularly to the performance of the standard ejector cycle with a liquid-vapor separator. However, this cycle may not be the best choice for automotive applications due to the large size required by an efficient separator as well as the cycle's performance at conditions of lower ejector potential. A limited amount of recent research has focused on alternate two-phase ejector cycles that may be better suited for automotive applications. One of these cycles, using the ejector to allow for evaporation at two different temperatures and eliminating the need for a separator, will be the subject of investigation in this paper. Previous investigations of this cycle have been mainly theoretical or experimental; this paper aims to provide a numerical analysis of the effect of evaporator design on the performance of the ejector cycles.

In this paper, a numerical model of a microchannel evaporator is developed and incorporated into thermodynamic models of the standard and alternate two-phase ejector cycle. R134a is used as the working fluid, though the results could be expanded to R1234yf due to the similar properties of the two fluids. Data from the authors' previous experimental investigation of the alternate ejector cycle is used to validate the numerical model. The evaporator model for the alternate cycle is modified to allow for two different evaporation temperatures. The effect of the ratio of low-temperature to high-temperature evaporator area on the performance of the cycle is explored, and comparison is made between the performance of the alternate ejector cycle and the standard ejector cycle. The effect of operation at conditions of lower ejector potential, or reduced ejector efficiency, on the performance of the ejector cycles is also investigated, and both cycles are compared to a baseline cycle without an ejector in order to obtain a realistic idea of the improvement that can be achieved with ejector air conditioning cycles. It is seen that evaporator design, not just ejector design, can have a significant effect on the COP of ejector cycles.