Experimental comparison of the performance of a CO2 ejector cycle for transcritical and subcritical operation.

The use of a two-phase ejector to improve the COP and capacity of CO2 vapor compression systems has been a topic of much research in recent years. Studies often investigate the performance of the CO2 ejector cycle when operated in transcritical mode, focusing on controlling and optimizing high-side pressure. However, less focus is given to subcritical operation of CO2 ejector cycles as well as how well the same ejector and system can operate in both modes. This paper presents the results of an experimental investigation of a CO2 ejector cycle over a range of ambient temperatures, allowing the system to operate in both transcritical and subcritical modes. An adjustable ejector using a needle to control nozzle throat size, often used in CO2 ejector studies, is used to control high-side pressure and improve COP for transcritical mode. It is observed that the adjustable ejector can be used to control and optimize the performance of the system for subcritical operation as well with a similar control concept as used for transcritical operation.

近年来，采用两相喷射器（two-phase ejector）提升二氧化碳蒸气压缩系统的性能系数（Coefficient of Performance，COP）与制冷容量，已成为近年的研究热点。现有研究多围绕跨临界模式（transcritical mode）下的二氧化碳喷射器循环性能展开，核心聚焦于高压侧压力（high-side pressure）的调控与优化。然而，针对二氧化碳喷射器循环的亚临界运行（subcritical operation）模式，以及同一喷射器与系统在两种模式下的运行适配性，相关研究的关注度相对不足。本文针对不同环境温度下的二氧化碳喷射器循环开展实验研究，实现了系统在跨临界与亚临界两种模式下的稳定运行，并给出相关实验结果。本研究采用二氧化碳喷射器研究中常用的针阀式可调喷射器——通过针阀调节喷嘴喉部尺寸（nozzle throat size），用于跨临界模式下的高压侧压力控制并提升COP。实验结果表明，采用与跨临界模式一致的控制思路，该可调喷射器同样可实现亚临界运行模式下的系统性能调控与优化。