Research On Flow Distribution And Convective Heat Transfer Characteristics Of Microchannel Heat Exchanger

As a high-efficiency and energy-saving device,the microchannel heat exchanger has the advantages of high integration,high heat exchange efficiency,and low refrigerant charge.In this paper,the flow distribution and convective heat transfer characteristics of microchannel heat exchangers are researched using numerical simulation and experimental methods.The research results have important scientific practical significance for improving the heat exchange efficiency of air conditioners and reducing the refrigerant charge.The influence of flat tube size and inlet form on the flow distribution of microchannel heat exchanger are discussed employing numerical simulation method;and the gas-liquid two phse flow and heat transfer characteristics in microchannel heat exchangers are studied based on user-defined function(UDF).Meanwhile,the microchannel heat exchangers are designed and manufactured and the microchannel heat exchanger test system are set up.The flow and heat transfer characteristics of microchannel heat exchangers with different inlet positions(Z-type,Y-type,U-type)are tested experimentally and compared with finned tube heat exchangers.The main results show as following:(1)By changing the size of the flat tubes and the spacing between the flat tubes,when the inlet is parallel to the microchannel heat exchanger of the header,the countercurrent 1#～4# flat tubes are ignored,and the average change width of the flat tube widths 2mm and 3mm is 19% and 48,respectively %;With the increase of the flow rate,the distance between the flat tubes is 8mm,10 mm and 12 mm,and the unevenness gradually increases,and the flat tubes have reverse flow phenomena.Therefore,the greater the flow rate,the more uneven the flow distribution of each flat tube.The average growth rate of the relative standard deviation of the flow rate of the flat tube spacing 8mm,10 mm and 12 mm is 16.67%,23.64% and 17.02%,respectively.(2)The flow distribution uniformity of the microchannel heat exchanger is improved by changing different inlet angles(0 °,30 °,45 °,60 °,90 °)and inlet positions(Z-type,Y-type,U-type).When the inlet angle is 0 ° and the Z-shaped inlet,the relative standard deviation is 0.1,and the flat tube has no reverse flow phenomenon.(3)Two-phase flow simulation was performed on micro-channel heat exchangers with different inlet positions(Z-type,Y-type,U-type),and it was found that the temperature difference between the inlet and outlet of Z-type,Y-type and U-type is basically the same,and the heat exchange of the three heat exchangers The amount is basically equal.For Y-type and U-type,the temperature uniformity of the surface of the micro-channel heat exchanger is relatively poor,and the phenomenon of "overheating" or "too much liquid supply" is likely to occur,which will affect the uniformity of the air temperature,so the Z-type inlet is better.(4)Compared with the finned tube heat exchanger,the microchannel heat exchanger can effectively reduce the refrigerant charge.When the charge amount is in the range of 800g～1400g,The cooling capacity of the microchannel heat exchanger is588 W higher than that of the finned tube heat exchanger,and the EER has increased by 12.4%.The best charge for original cabinet air conditioning is 1520 g predicted by the fitting curve.The optimal charge quantity of three inlet forms of the microchannel heat exchanger is very close.Compared with the original refrigerant,the optimal refrigerant charge quantity is reduced by about 180 g.The input power of the microchannel heat exchanger is slightly higher than that of the original cabinet type air conditioner.(5)The entrance position of the microchannel heat exchanger has a certain influence on its heat transfer performance.In the range of 800 g ～ 1400 g of refrigerant charge quantity,the average growth rate of the cooling capacity of the three inlet type is about 11% for each additional 200 g of refrigerant quantity.When the refrigerant charge quantity is 1400 g,the cooling capacity of Z type is increased by 5.9% and 8.8%comparing with the Y type and U-type;when the refrigerant charge quantity is less than 1200 g,the average growth rates of cooling capacity of Z-type,Y-type and U-type ware 4.9%,5.2% and 5.3% respectively.Therefore,with the increase of the refrigerant charge quantity,the Z-type is more stable than the Y-type and U-type;under the same charge quantity,the EER of Z-type is the highest.When the refrigerant charge quantity is 1000 g,the EER of Z-type is 6.5% higher than the U-type.In summary,the Z-type microchannel heat exchanger has the best performance.