Simulation Of CO₂ Ejector Based On HEM And Its Application In Supermarket Refrigeration System

The equipment that consumes the most significant proportion of electricity in the supermarket is the refrigeration system.Affected by environmental factors,the refrigeration system of the supermarket that operates inefficiently for a long time will waste many power resources,and the use of high GWP and ODP refrigerants will insensibly harm the surrounding environment.Ozone layer destruction and global warming are the most obvious phenomena.China abides by the provisions of the Kigali Amendment of《The Montreal Protocol》and will complete the 80%reduction of HFCs before 2045.Therefore,the promotion of natural refrigerants and the optimization of their supporting units are essential.In order to improve the refrigeration system’s operation efficiency,this paper completed the optimization of the CO2 refrigeration system of the traditional supermarket.The CO2 two-phase ejector was used to replace the original throttling part,and its structure was optimized.Combined with numerical calculation and physical model,the key parameters affecting the efficiency of the ejector were explored.The main research contents are as follows:(1)According to the revised empirical formula,Matlab software was used to conduct mathematical modeling for the CO2 two-phase ejector,and the basic structural parameters were obtained.The two-dimensional and three-dimensional unstructured mesh models of the ejector were established by ICEM,which were imported into Fluent for CFD simulation.Then the HEM model was constructed,and the static simulation of two-phase flow was realized through the UDF function of the software.Combined with the experimental data,the model is concluded to meet the application requirements.(2)The dimensions of NXP,area ratio,diffuser length,and ejector structure were optimized,and the optimal dimensions of each parameter were obtained under the boundary condition of supermarket refrigeration working conditions.The results show that the mixing efficiency is low when NXP is in the pre-mixing section;the value out of range will lead to the reduction of ejection ratio,area ratio,and reverse flow in the diffuser can be eliminated by adjusting the channel diameter.Under the condition of supermarket refrigeration with CO2 as refrigerant,the throat type of convergent-constant area nozzle is the best mixing structure.(3)Using Matlab software,the thermodynamic models of BPR system and EPR system with ejector in traditional supermarkets are built,the relationship between outlet pressure and temperature of gas cooler and the relationship between ejector entrainment ratio and the ratio of pressure lift are optimized,and the performance of the two systems under different environmental conditions is compared.The results show that the higher the ambient temperature,the higher COP of the EPR system,the smaller the compressor power,the smaller the volume flow of refrigerant needed in the system.In cities with higher annual ambient temperature,the increase of SEER of the EPR system is more obvious.(4)The exergy models for BPR and EPR systems were built,and the exergy loss and exergy loss rate of each part of the two systems changed with ambient temperature were analyzed,as well as the proportion of exergy loss of each part of the EPR system.The results show that the ejector’s exergy efficiency in the EPR system is higher than that of the combination of high-pressure control valve and auxiliary compressor in the BPR system,and the priority of optimized components in the EPR system should be:high-pressure compressor>gas cooler>medium-temperature evaporator.