Research On Performance Of CO₂ Ejector Refrigeration System For Vehicles

Due to global warming,ozone layer destruction and other environmental problems,R134a,a high GWP（global warming potential）refrigerant widely used in automotive air conditioning,is gradually being replaced.The greenhouse effect of natural working fluid CO₂（R744）is very low and it is a very good alternative refrigerant.However,it has some disadvantages,such as high temperature performance attenuation and large throttling loss.The ejector can recover the expansion work and improve the energy efficiency of the system.It is very important to study the two-phase flow mechanism in the ejector and the characteristics of transcritical carbon dioxide air conditioning system with ejector for the application of carbon dioxide refrigerant.In this paper,simulation and experimental methods are used to study the carbon dioxide air conditioning system with ejector.The main research contents and conclusions are as follows:（1）The ejector is simulated.Based on the mixed multiphase flow,compressibility and phase transformation model,the computational fluid dynamics model of ejector is developed.The changes of the internal pressure and Mach number,cavitation phase transition,evaporation phase transition and supersonic shock wave are analyzed by using the model.The key structural parameters affecting the performance of the ejector are studied,which lays a theoretical foundation for the structural design of the ejector in the field of vehicle.（2）The performance of ejector is analyzed.The influence of the state of the inlet refrigerant on the performance of the ejector is analyzed.The differences among Nakagawa,Butrymowicz and Elbel efficiency models were analyzed.According to Bukingham’sπtheory,the ejector ratio prediction model is established,which can effectively predict the ejector ratio according to the inlet state.（3）The performance of carbon dioxide injection refrigeration system is compared with that of conventional refrigeration system.The effects of charge,the opening of valve,outdoor temperature,indoor air volume and compressor speed on the system performance were studied.The cop and cooling capacity of the injection cycle are 8.6%and 9.4%higher than that of the conventional cycle at 1.5kg charge.With the increase of the opening of the electronic expansion valve in front of the evaporator,the performance of the injection system increases first and then decreases slowly.With the increase of outdoor temperature,the cooling capacity of conventional system is reduced by 31.2%,while that of ejector system is only 15.6%.The conclusion can provide guidance for the application of carbon dioxide ejector in automobile.