Research On Performance Of CO₂ Refrigeration System With New Ejection Cycle

The environmental problems brought about by economic development have gradually attracted people’s attention.Among them,the destruction of the ozone layer and the greenhouse effect are particularly prominent.The leakage of refrigerant is closely related to these two phenomena.To study the working fluid system,in order to improve the performance of the system,a new type of ejection cycle is proposed and applied to subcritical and transcritical systems respectively.In the subcritical CO₂ refrigeration system,the use of underground buried pipes as the condenser can use the characteristics of the underground soil temperature lower than the critical temperature of CO₂ to maintain the system pressure in the subcritical range,but the condenser outlet fluid is still in a two-phase state,in order to avoid Among them,the gas without refrigeration enters the compressor again to consume power.In this paper,a schematic diagram of a subcritical CO₂ refrigeration system with a new ejection cycle is designed and the corresponding mathematical model is established.It is calculated and solved by simulation software to study its performance Then,the dimensions of each component of the ejector were calculated and a gas ejector was processed,and a test bench for a subcritical CO₂ refrigeration system with a new ejection cycle was designed and built.In the transcritical CO₂ refrigeration system,in order to prevent the gas at the outlet of the expansion tank from entering the compressor and consuming power,this ejection cycle method is applied to the transcritical CO₂ refrigeration system,taking into account the performance of the ejector at different positions on the system The thermodynamic models of two transcritical CO₂ ejection cycle refrigeration systems and a transcritical CO₂ parallel compression refrigeration system were established respectively,and the two types of transcritical CO₂ introduction were analyzed:the outlet pressure of the air cooler,the pressure in the expansion tank and the evaporation temperature.The impact of the performance of the jet cycle refrigeration system was compared with that of the traditional transcritical CO₂refrigeration system with bypass valve and transcritical CO₂ parallel compression refrigeration system.By simulating the power consumption and COP of subcritical and transcritical CO₂refrigeration systems with a new ejection cycle,and comparing them with systems without an ejection cycle,the conclusions are as follows:（1）In the subcritical CO₂ refrigeration system with ejection cycle,when the condensing pressure and the ejected fluid pressure are constant,the greater the ejection ratio,the greater the ejector inlet and outlet pressure ratio;when the ejection ratio is constant,The greater the pressure of the ejected fluid,the smaller the inlet and outlet pressure ratio of the ejector;therefore,the inlet and outlet pressure ratio of the ejector can be reduced by appropriately increasing the pressure of the ejected fluid,that is,the compressor discharge pressure is reduced,and thus the System performance.（2）In the subcritical CO₂ refrigeration system,as the evaporation temperature increases,the condensation temperature decreases,and the supercooling degree a generated by the plate heat exchanger increases,whether or not an ejector is added,the subcritical CO₂ refrigeration system COP all showed an increasing trend,and after adding the ejector,the COP has increased compared with the original,in which the supercooling degree a has the greatest impact on the COP improvement rate,the COP can be increased by a maximum of 16.19%;with the gas-liquid separator The increase in the supercooling degree b caused by the COP changes before and after the ejector is added.However,after the ejector is added,the COP is significantly improved compared with the original,and the COP increase rate changes from6.72%to 10.77%.Change the system’s ejection ratio（reservoir gas-liquid ratio）,evaporation temperature,condensation temperature,ejector inlet and outlet pressure ratio,the system COP can be increased by 4.59%¹4.04%,1%⁸.55%after adding ejector,2.54%¹1.8%and 3.71%⁴0.31%.（3）In the subcritical CO₂ refrigeration system with ejector cycle,compared with the subcritical CO₂ refrigeration system without ejector,the lower the evaporation temperature of the system,the higher the condensing temperature and the greater the degree of supercooling a 3.The smaller the supercooling degree b,the greater the gas-to-liquid ratio of the accumulator,and the smaller the inlet and outlet pressure ratio of the ejector,the greater the COP increase rate of the system after the ejector is added,and the more obvious the COP improvement effect.The jet-cycle subcritical CO₂ refrigeration system is more suitable for use in environments with lower evaporation temperatures and higher outside condensation temperatures.（4）In the subcritical CO₂ refrigeration system with ejector cycle,the ejection ratio（reservoir gas-liquid ratio）and the ejector inlet and outlet pressure ratio are the two key factors that affect the system performance after the ejector is added factor.Under the working conditions given in this article,when the pressure ratio at the inlet and outlet of the ejector is≤1.5,the performance of the subcritical CO₂ system added to the ejector is significantly improved.（5）In the transcritical CO₂ refrigeration system with ejector cycle,the ejector cycle system（position 1）with the ejector placed behind the compressor,the smaller the outlet pressure of the gas cooler,the higher the pressure in the expansion tank When the evaporation temperature is lower,the COP of the system is significantly larger than that of the traditional transcritical CO₂ refrigeration system with bypass valve,but smaller than that of the parallel compression system.Compared with the traditional transcritical CO₂ refrigeration system with bypass valve,the internal pressure and evaporation temperature can be increased by10.05%,6.09%and 5.71%respectively.（6）Place the ejector behind the gas cooler（position 2）in the transcritical CO₂ ejection circulation system,the COP of which is higher than that of the parallel compression system and the ejector after the compressor（position 1）.When the outlet pressure of the gas cooler is lower,the pressure in the expansion tank is lower,and the evaporating temperature is higher,place the ejector behind the gas cooler（position 2）instead of parallel compression.Large,which changes the pressure in the expansion tank.COP increases by a large amount,which can be increased by 1.08%⁸.81%under the working conditions given in this article.