跨临界CO2引射制冷循环临界背压分析

为了优化设计或控制引射系统的性能,建立了跨临界CO2引射制冷循环中引射器性能的设定背压模型,定义引射器临界背压为忽略引射器出口动能时的引射器迭代背压,分析了引射器背压和出口速度对系统性能的影响.结果表明:当引射器背压低于临界值时,引射系数保持不变,且在不同系统高压侧压力下均成立；系统性能系数(COP)随着背压升高而增大,在系统状态可控的情况下引射器背压应尽量靠近临界值.同时,不同背压下COP随高压侧压力变化的趋势相同,背压作为设计或控制参数不会影响高压侧压力的优化.引射器出口动能会因流动摩擦和冲击而转化成热量并造成系统的热力学损失,背压选取不当会造成较大的引射器出口速度,且随着背压降低,出口速度增大,随着系统高压侧压力升高,出口速度先急剧降低,再缓慢变化.

Critical Back Pressure of Transcritical Carbon Dioxide Refrigeration Cycle with Ejector

The mathematical model of the ejector with contrivable back pressure in a transcritical carbon dioxide ejector-expansion refrigeration cycle was established.The critical back pressure of the ejector was defined as an iterative value by ignoring its outlet kinetic energy.The effects of the ejector back pressure and outlet velocity on the system performance were analyzed to optimize design or control the ejector system performance.The results show that the entrainment ratio can maintain constant when the ejector back pressure is lower than critical value at different high-side pressures.The coefficient of performance(COP) increases with the ejector back pressure.It seems that the ejector back pressure under controlled system status should be close to the critical value as far as possible.Moreover,the variation tendency of COP changes similarly with the high-side pressure at different back pressures.Back pressure as the design parameter or control parameter does not affect the optimization of high-side pressure.Due to flow friction and impact,the outlet kinetic energy of the ejector will transform into heat and eventually lead to thermodynamic loss.In addition,the improper back pressure will raise the outlet velocity,and the outlet velocity increases with the decrease in the back pressure.The outlet velocity decreases dramatically first and then varies slowly with an increase in the high-side pressure of the system.