Research On Compact And Saving Energy Heat Exchanger For Trans-critical Carbon Dioxide Air Conditioning

Trans-critical CO2 circulatory system has excellent environmental characteristics, good heat transfer properties, low flow resistance and a large cooling capacity per unit volume so that it has broad application prospects, while there are some problems to be solved, such as low efficiency of system and security problems of equipments and pipes caused by high working pressure of system. In this thesis, supercritical CO2 as the research object, numerical simulation as the research method, heat exchangers in trans-critical CO2 air conditioning system are researched. The main contents and results are as follows:1 The conventional heat exchangers in trans-critical CO2 refrigeration system are analyzed comprehensively, which concludes that the efficiency of trans-critical CO2 refrigeration system depends on performance enhancement of heat exchangers, and development of micro-channel compact heat exchangers has a crucial role in solving low efficiency and secure issues of system equipments.2 The physical properties of supercritical CO2 are analyzed. The characteristics of heat transfer and pressure drop of supercritical CO2 in the micro-channel are simulated by the low Reynolds L-B model under different mass flow rate and pressure, and the data of simulation is in good agreement with the experimental data in the literature. At the same time, it is concluded that heat flux has a little effect on the characteristics of heat transfer and pressure drop of supercritical CO2 in the micro-channel.3 Louvered fin is used to enhance heat transfer of the air side. The simplified fluid-structure interaction physical models of three-dimensional louvered fin are simulated by Laminar model, and it is found that the heat transfer and friction performance are in good agreement with the experiment and the correlations in the literature. A research method is provided for further optimizing structural parameters of louvered fin and developing new louvered fin.4 To further clarify the performance of louvered fin in the air side, the fin height, the louver height, fin thickness and fin pitch are changed to obtain the characteristics of heat transfer and pressure drop of louvered fin; then the heat transfer-pump power consumption comparison which considers both heat transfer and pressure drop is used to optimize the structure parameters of louvered fin; last it is obtained that a group of structural parameters of louvered fin, the comprehensive performance of which is better.5 New arched louvered fin is developed, and it is researched at different radians of fin and angles of louvered fin by numerical simulation. It is concluded that the characteristics of heat transfer and pressure drop are increased, and the most rate increment of the average heat transfer coefficient and pressure drop is up to 9.47% and 3.2% separately; the overall rate of increase of heat transfer coefficient of arched louvered fin is faster than it of pressure drop, so the new arched louvered fin can enhance heat transfer.Supercritical CO2 in micro-channel and air flow in louvered fin of conventional micro channel parallel flow heat exchanger are simulated. Under different mass flow rate, pressure and heat flux, the performance of heat transfer and pressure drop of supercritical CO2 in tube and air flow in louvered fins are researched, and the arched louvered fin is developed, so this thesis has a certain engineering value and academic significance.