Simulation And Experimental Study Of The CO₂ Air-cooler

CO₂, as natural refrigerant, has good environmental characteristics and thermal performance. It has many advantages, such as eco-friendly（ODP=0, GWP=1）, low motion viscosity and compression ratio, high refrigerating capacity, extensive sources, low maintenance charge, no toxic, odourless, no pollution and so on. CO₂ has been paid more and more attention of the scholars and researchers with the serious environmental problems.Air cooler plays an important role in the refrigerating system and its performance would directly influences the quality of food and energy consumption in the system.The simulation calculation and experimental research of CO₂ air cooler in refrigerating equipment has been conducted. And the effects of different systems（direct expansion system and pump hydraulic system） and different operation parameters on air cooler have been researched. Therefore, the main conclusion could be obtained list as following:（1） The mathematical models for specific air cooler in direct expansion system and pump hydraulic system has been established by using steady state parameter method and MATLAB software.（2） The simulation for the performance of air cooler has been calculated through the models and the influences of structure parameters, such as tube pitch, fin pitch and fin thickness on the performance has been discussed. The heat transfer coefficient would increase and the refrigerating capacity would decrease with the rising of the fin pitch. In addition, refrigerating capacity would increase and the heat transfer coefficient would decrease with the rising of tube pitch. The increasing trend of heat transfer coefficient and refrigerating capacity would occur as the rising of fin thickness. What’s more, the effects of tube pitch and fin pitch on performance of air cooler would be larger than fin thickness.Therefore, the higher heat transfer would be obtained by increasing the fin pitch or decreasing the tube pitch. And the higher refrigerating capacity would be obtained by decreasing the fin pitch or increasing the tube pitch.（3） The regulations of local heat transfer coefficient, the pressure drop gradient, and comparison of two systems are also obtained through the simulation. The local heat transfer coefficient would steady decrease after increasing rapidly as the rising of length of tube.And the pressure drop gradient exists a peak with the increasing of quality.（4） The feasibility and accuracy of simulation has been verified through the experimental research. The maximum relative error of heat transfer coefficient in simulation and experiment in direct expansion system is 20.2%, and the maximum relativeerror of heat transfer coefficient in pump hydraulic system is 12.5%. The data in the condition of variable-frequency fan is 12%.（5） In the direct expansion system, the heat transfer coefficient and refrigerating capacity would show the decreasing trend as the temperature decreasing of evaporating temperature through the experiment. And a peak of heat transfer coefficient exists for different circulation factor in pump hydraulic system（the best circulation factor is 3 in this paper）. The changing trend of heat transfer coefficient in pump hydraulic system is same with the direct expansion system, while the value of heat transfer coefficient is bigger than direct expansion system in the same operation condition.（6） Heat transfer coefficient would increase with the increasing of driven temperature difference. However, the application in actual engineering would consider the energy consumption. The reason is the increasing of driven temperature difference means the lower evaporation temperature which would cause more energy consumption. Therefore, the optimal driven temperature difference should be selected for maximum heat transfer coefficient.The simulation and experiment has systematically researched the performance of air cooler in different systems and operating parameters, which could provide some reference for optimal operating conditions and structure parameters.