| The micro-channel heat exchanger was widely used in automobile air conditioning because of its high thermal efficiency and small refrigerant charge, but it would lead to condensation when it was used as an air conditioning system evaporator. Simulation and experimental research on the flow and heat transfer performance of micro-channel heat exchanger under the condensate condition was done in this paper, and the comparative analysis of the simulation results and experiments was carried out.The 3D physical and mathematical model of the fin on the micro-channel heat exchanger was established in the research, and the physical model was simplified according to its structure and heat transfer characteristics. The grid division was carried out by using the structured and unstructured meshes, the influence of inlet air state parameters, head wind speed and freezing water inlet temperature on the flow and heat transfer characteristics of the micro-channel heat exchanger was simulating by using Fluent software, and the following conclusions were got:1) Most air flows through the louvered, the remaining fluid went through the gap between the fins. The air temperature gradually decreases along the flow direction. When flow depth was same, the air temperature near the flat tube(Z=1mm) was relatively lower, and the air temperature of the symmetry plane(Z=5mm) is the highest. The air temperature droped quickly in the first half of the louvered fin, so the heat transfer process mainly occured in the first half of the louvered fin.2) The outlet air temperature of the micro-channel heat exchanger increased with the increasing inlet temperature, relative humidity, head wind speed and inlet temperature of frozen water, and the air pressure drop increased with the increasing head wind speed. The air pressure drop depended on Re value under the conditions of the same size and structure parameters of the louvered fin. The bigger the wind speed was, the bigger the Re number was, so the air pressure drop increased with the increasing wind speed. The air side heat transfer coefficient increased with the increasing inlet temperature, relative humidity and head wind speed, but decreased with inlet temperature of frozen water. Under the condition of constant inlet air temperature, relative humidity and the inlet temperature of frozen water, the growth rate of pressure drop was from the beginning of the 25.3% down to 5.4% with the increase of the head wind speed, and the growth rate of air side heat transfer coefficient was from 12.7% down to 6%. When the air pressure drop was increased to about 85 Pa, the increment began to decrease quickly, which was determined by the shape size and the structure parameters of the fin. When the inlet air temperature, relative humidity and the head wind speed were certain, the average decline of the air side heat transfer coefficient was about 5.4% with the inlet temperature of frozen water increased 1℃.In this research, the performance test bench of micro-channel heat exchanger was established, chilled water was provided by mobile cold source. Through the simulation of surfacial condensation condition of the heat exchanger, the experimental research on the condensate heat transfer surface of micro-channel heat exchanger was carried out by changing inlet air state parameters, head wind speed and inlet temperature of frozen water, and the following conclusions were got:1) The outlet air temperature, pressure drop, air side heat transfer coefficient and refrigeration capacity increased with the increasing inlet air temperature, which was also the same with the increasing inlet air relative humidity. When the air inlet temperature increased 1℃, the pressure drop, air side heat transfer coefficient and refrigeration capacity increased by 5%, 4% and 2% respectively under the condition of constant head wind speed and the inlet temperature of frozen water; When the air relative humidity increased10%, the pressure drop, air side heat transfer coefficient and refrigeration capacity increased by 8%, 12% and 3% respectively; When the influence factors were certain, the pressure drop of late dew condensation increased about 150% than early, while the air side heat transfer coefficient and refrigeration capacity of late dew condensation decreased about 23% and 18% respectively than early.2) The outlet air temperature, pressure drop, air side heat transfer coefficient and refrigeration capacity increased in different degrees with the increasing head wind speed. When the head wind speed increased 0.25m/s, the outlet air temperature, pressure drop, air side heat transfer coefficient and refrigeration capacity increased by 5%, 20%, 13% and 9% under the condition of constant inlet air temperature, relative humidity and the inlet temperature of frozen water. When the wind speed varied in a small range, the pressure drop of late dew condensation at least twice as larger than the initial condensation, and the air side heat transfer coefficient and refrigeration capacity of late dew condensation decreased about 24% and 7% respectively than the early.3) The outlet air temperature, pressure drop, air side heat transfer coefficient and refrigeration capacity decreased in different degrees with the increasing inlet temperature of frozen water. Under the condition of constant inlet air temperature, relative humidity and the head wind speed, the growth rate of outlet air temperature was about 5% when the inlet temperature of frozen water increased 1℃, and the decline rate of the pressure drop, air side heat transfer coefficient and refrigeration capacity decreased about 8.3%, 7.6% and 3% respectively.According to the comparison of results of the simulation and experiment, the pressure drop, air side heat transfer coefficient trend of experimental test and simulation were same with changing head wind speed, error was also within the small range.In summary, the influence of dew condensation on the pressure drop was the biggest. From the early period of dew to the late period, the increment of the pressure drop could reach more than two times; The influence of it on the air side heat transfer coefficient was also large, and the drop of air side heat transfer coefficient was about 20%~30% from the early period of dew to the late period; The refrigerating capacity of heat exchanger was the least affected by dew condensation, and the refrigeration capacity before and after the condensation was reduced about 5%~9%, but from the heat transfer capability of the heat exchanger itself, it was not to be ignored. With the increase of the air inlet temperature, relative humidity and the head wind speed, the air outlet temperature, pressure drop, air side heat transfer coefficient and refrigeration capacity all had different degrees of increase under surface condensate conditions. Therefore, there was the best combination of the inlet air temperature, relative humidity and the head wind speed to keep the outlet air temperature and pressure drop in a small range, and the heat transfer effect of the heat exchanger was considerable. With the increase of inlet temperature of frozen water, air outlet temperature was gradually increased, and the pressure drop, air side heat transfer coefficient and the refrigeration capacity were all decreased gradually. In the premise of ensuring the outlet air temperature and pressure drop to meet the requirements, we should try to reduce the inlet temperature of frozen water to reach the target of increasing the heat transfer performance of the heat exchanger.The performance of the micro-channel heat exchanger under dew condensation conditions was studied, and the effect of air inlet parameters, head wind speed, inlet temperature of frozen water and condensation process on its properties was analyzed, which had certain scientific significance and practical value. It could provide reference for the micro-channel heat exchanger which used in the household and commercial air conditioning system. |
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