Performance Analysis And Experimental Study Of Co₂ Trans-critical Cycle Heat Pump Of Different Evaporator

As awareness raising of environmental protection, refrigerants alternative becomes a research focus in the refrigeration and air conditioning industry for the conventional refrigerants with ozone depletion potential（ODP） and global warming potential（GWP） having damaging effects on the environment. As natural working substance, CO₂ has good environmental protection performance and thermodynamic properties. It is a good choice to replace conventional refrigerants. With the further application of CO₂ refrigeration system and the expansion of application scope, the system capacity will increase. Therefore, in this paper CO₂ trans-critical cycle water-water heat pump system of different evaporators was experimented and analyzed.Combined with the concept of sustainable development and environmental protection back to nature, this paper put forward refrigeration and air conditioning system in the following quantification of three aspects: The first, using natural perfectibility to evaluate refrigerant influence on the nature; The second, using thermodynamics perfectibility evaluate system efficiency close to the extent of the Carnot cycle; The third, it has put forward ideal perfectibility of the system for refrigeration and heat pumps system, including heat source and terminal. And quantitative calculation was carried out respectively, the results could provide reference for the system of evaluation and improvement.First of all, the double-pipe evaporator of CO₂ water- water heat pump has been studied in this paper, as a test benchmark of CO₂ water- water heat pump in this paper. The effect on the system performance was discussed about the internal and external parameters of water temperature and the water flow. The results were as follows: Improving the suction gas superheat temperature has little significant influence on the efficiency of system, but it can increase the discharge temperature of the compressor. Reducing the CO₂ temperature of gas cooler outlet can significantly improve the efficiency. Reducing cooling water flow or increasing cooling water inlet temperature, hot water temperature could increase, but the COP decreased. Increasing cooled water flow or inlet temperature, the hot water temperature and COP both increased.Falling film evaporator has been widely adopted in conventional refrigerants system for its good heat transfer performance, so in this paper CO₂ falling film evaporator was studied. In the process of falling film evaporation, dry spots will easily appear at someplace where the liquid film is thin. So in this paper, CFD numerical simulation was carried out to analysis the CO₂ falling film process and the variation of the liquid film thickness and the influence of tube diameter, tube spacing, hole diameter and height of liquid feeder was also ananlyzed.The CO₂ falling film evaporator was designed based on the CFD results, and the heat transfer performance of the evaporator was carried out through experiments. The results show that heat transfer coefficient of the refrigerant side and the total heat transfer coefficient both increased with the increase of the evaporation temperature, heat flux and refrigerant flow. The heat transfer coefficient of the upper tubes was greater than that of the bottom tubes, and the heat transfer coefficient of the middle tubes was greater than that of the two side tubes. The flow modes were jet mode between the tubes when changing evaporation temperature and the heat flux. There appeared droplet mode transforming to jet mode when changing the refrigerant flow. When the flow rate was 212kg/h, droplet mode and jet mode appeared simultaneously.Based on the analysis of structural strength and the experiment of the heat transfer and system performance, flooded evaporator and falling film evaporator in the application of CO₂ trans-critical system could run safely and efficiently. At the same external heat source parameters, system performance of falling film evaporator is better than that of the flooded evaporator and double-pipe system.