Performance Research On Transcritical CO₂ Ejector Refrigeration Cycle

As the ozone layer is destroyed increasingly aggravation by traditional refrigerants, the environment-friendly natural refrigerant CO2 comes back to people’s horizon in refrigeration fields. As an effective energy-saving technology, trans-critical CO2 injection refrigeration cycle is attracted national scholars’ attention and research.Currently, although researches on CO2 refrigeration are more concentrated in the field of heat pump water heaters, it is still rare for comprehensive energy exergy analysis about trans-critical CO2 injection refrigeration cycle. Besides, contracting-expanding nozzle is the critical section, which can plays an important role for cycle performance through influence the efficiency of injector. However, study on the CO2 two-phase flow within injector nozzle is little. For this reason, on the basis of the analysis of the mixed flow inside the injector, a new definition of the pressure coefficient is established to evaluate the pressure boost performance of ejector’s expansion segments, and thus improved the p-h diagram of trans-critical CO2 injection refrigeration cycle and establish the mathematical model to do thermodynamics theories analysis for trans-critical CO2 injection refrigeration cycle. Study the effect of variable condition parameters on system performance, and the exergy loss of various components of system as well. Therefore, this paper established the mathematical model of trans-critical CO2 two-phase flow in injector’s converging-diverging nozzle based on the delay balance theory, then analyze the pressure distribution within the nozzle and the pressure changes with the increasing of the diverging angle. The main conclusions are as follows:1.The effect of variable condition parameters on cycle performance:as pressure coefficient increases, the COP, injector’s pressure ratio and efficiency increases whereas the compression ratio of the compressor is reduced. The injection coefficient, however, is almost the same. When the cooler’s working pressure increases from 8MPa to 9.5MPa in the low pressure section, the entrainment ratio, the ejection efficiency and COP increase rapidly, and the COP will reach a maximum as well. In high-pressure section where pressure is greater than 9.5MPa, the injection coefficient and efficiency are basically keep unchanging, yet COP is gradually reduced. With the increase of the cooler exit temperature, entrainment ratio, COP and the ejector efficiency were decreased, but the corresponding pressure of the maximum COP is raised. With the evaporation temperature increases, the entrainment ratio and COP increases, the ejector efficiency is somewhat reduced, whereas the corresponding pressure of the maximum COP is substantially constant.2. Exergy analysis of the system:the changes of pressure coefficient and cooler outlet temperature primarily have greater impact on ejector’s exergy loss coefficient, but the impact on other parameters is not obvious; With the increase of evaporation temperature, exergy efficiency of the cooler and evaporator increase significantly, and the system’s exergy efficiency is also improved; With the increase of cooler working pressure, exergy efficiency of the cooler reduces significantly, and the exergy loss coefficient of system gets the minimum value at about 8.4MPa.3. By comparison, discover the calculations of converging-diverging nozzle’s pressure distributions agree well with the related experimental data, and the established mathematical model was verified having a certain feasibility, then get that the nozzle exit pressure decreases with the divergent angle increases, and the pressure of divergent section is essentially the same when the divergent angle reaches a certain value. Through the analysis of Equilibrium shock waves, the high pressure side of the shock increases with the length of the divergent section reduces, which makes the shock becomes strong. Therefore, it needs to avoid shock wave generated inside the nozzle in order to design a high performance CO2 injector, so it’s important to design the length and angle of the converging-diverging nozzle’s divergent section rationally.This paper enriches the theory research of trans-critical CO2 injection refrigeration cycle, and is instructive to the cycle’s application to some extent.