Research On A Carbon Dioxide Automobile Air Conditioning System Using Microchannel Heat Exchangers

Environmental protection has been a crucial challenge for the gloable community. In automobile industry, the manufacturers and the researchers are facing high pressure from the R134a replacement issue because it has a high gloable warming potential. Carbon dioxide (CO₂) has been regarded as the potential alternative refrigerant due to its high efficiency and environmental safety. Investigating the CO₂ refrigeration technology will avoid the situation that the R134a technology is entirely imported from overseas, promote the technology in the car air conditioning application and form the competitive power in China.In this thesis, based on the previous CO₂ refregerating system development and fundation from Shanghai Technology Development Fundation, a new CO₂ automobile air conditioning system using microchannel heat exchanger has been developed. The thesis includes the following topics:1. Considering the paticular characteristics of carbon dioxide as a refrigerant, the steady models of evaporator and gas cooler using microchannel tube and louvered fin have been developed and validated. Using the newly developed refrigerant-side two phase heat transfer coefficient and pressure drop correlations, the evaporator model can predict the performance of evaporator with a reasonable accuracy compared with the experimental data, especialy in the high vapor quality region. Moreover, the pressure losses both in the headers and at the port inlet of the microchannel tube are also considered. The errors of RMS for cooling capacities, refrigerant-side pressure drops and condensation rate are±1.9%,±12.3% and±8.6% , respectively. The program can be used to analyze and design a CO₂ microchannel evaporator.2. The new generation CO₂ transcritical automobile air conditioning system has been established which consists of microchannel heat exchangers, variable displacement compressor, co-axial internal heat exchanger, oil separator and accumulator. By using the high performance microchannel heat exchangers, the general problem of relatively low performance for the previous CO₂ automobile air conditioning system can be improved. The real on-road test with the designed CO₂ air conditioning system shows a comparable performance with the original R134a system.3. A transcritical CO₂ automobile air conditioning system test bench has been built for this research. The influnces of different valve openings, compressor speeds, refrigerant charge amount, with/without accumulator and face velocity of gas cooler on the sysem performance have been experimentally studied. The test results reveal that the phenomenon of optimum charge amount also exists for the system using accumulator. However, the effect can be neglected after the system have been charged by more than the standard refrigerant charge amount. The maximum COP is 2.83 when the system is operated under the compressor speed of 900 rpm at ambient temperature 25℃,which is 34.8% higher compared with the previous CO₂ system using the tube and fin heat exchangers. The major factor affecting system performance is attributed to the refrigerant pressure drop in gas cooler.4. Steady and dynamic simulation models for the automobile CO₂ refrigeration sytem have been proposed. The steady model employs a tube-by-tube finite volume method instead of port-by-port method. Moreover, the factors such as internal heat exchanger and connecting pipes also been considered. The differences between the calculated and tested results are within±5%. The dynamic model is based on lumped parameter method and boundary-moving method for the heat exchanger. The compressor efficiencies are curve-fitted based on the experimetal data and accumulator is added to the evaporator outlet in order to simulate the real system.The results show that the dynamic characteristics of the system with accumulator is better than the sytem without accumulator.