Experimental study on control methods for transcritical CO2 two-phase ejector systems at offdesign conditions.

Recent studies have shown that expansion work recovery by use of a two-phase ejector offers significant opportunity for improving the COP of refrigeration systems, especially transcritical CO2 systems. However, performance improvement is known to be less significant at off-design conditions that are not as favorable to the ejector cycle. This paper investigates methods for controlling the high-side pressure of a transcritical CO2 ejector cycle. The use of an ejector with an adjustable motive nozzle throat area as well as a simple expansion valve upstream of the ejector to actively control high-side pressure are investigated and compared experimentally. Proper control of evaporator flow rate for different conditions is also investigated, and the penalty for not actively controlling the evaporator metering valve is also investigated. The control methods are investigated over a range of capacities and ambient temperatures in order to see the performance penalties to a transcritical CO2 ejector cycle when it is not controlled properly.

近期研究表明，采用两相喷射器（two-phase ejector）实现膨胀功回收，可显著提升制冷系统的性能系数（COP），跨临界二氧化碳（transcritical CO₂）系统的提升效果尤为突出。然而，在对喷射器循环不利的非设计工况下，其性能提升幅度会显著降低。本文针对跨临界二氧化碳喷射器循环的高压侧压力控制方法展开研究。本文通过实验研究并对比了两种高压侧主动控制方案：一是采用带可调驱动喷嘴喉部面积的喷射器，二是在喷射器上游增设简易膨胀阀以主动调控高压侧压力。本文同时研究了不同工况下蒸发器流量的精准控制策略，并探讨了未主动调控蒸发器计量阀所带来的性能损耗。为探明未精准控制时跨临界二氧化碳喷射器循环的性能损耗情况，本文在覆盖不同制冷量与环境温度的工况范围内对上述控制方法开展了实验测试与分析。