Research Of Surface Drainage Characteristics And Heat Transfer Performance Of Micro-channel Heat Exchanger

The micro-channel heat exchanger has a dominant position for its high heat transfer efficiency,compact size and light weight relative to other types of heat exchangers.Its high surface area-volume ratio and scale effect lead to high heat transfer efficiency.It has been widely applied to automotive air conditioning field in recent years,but it is mainly used as condenser of the air conditioning system.When it is used as the evaporator of a single cold air conditioner or the heat exchanger of the cold and hot air conditioner,it will inevitably encounter air condensation on the surface of the heat exchanger,and the condensation will further affect the heat exchange performance.In this paper,the air side heat transfer characteristics and surface drainage characteristics of micro channel heat exchangers under condensation conditions are researched experimentally and numerically.It will try to determine the optimum wind speed and optimum inclination angle for strengthening the heat transfer effect of the micro-channel heat exchanger.The conclusions by experiments of this paper are as follows:(1)The outlet air temperature,the formation rate of condensate,heat exchange and the air side heat transfer coefficient increase with the relative humidity when inlet air temperature is 30?.The relative humidity raises 10%,the outlet air temperature increases averagely by 1.1?.The formation rate of condensate increases averagely by 0.4g/s,the heat exchange rate increases by 8.5%and the heat transfer coefficient increases by 2.3%.The enhanced condensation degree is beneficial to the increases of heat exchange and the heat transfer coefficient of the air side.While the inlet air temperature is 33?,the relative humidity raises from 60%to 70%,the heat exchange and air side heat transfer coefficient decrease,because a large amount of condensate is gathered on the surface of the heat exchanger,which greatly increases the flow resistance and thermal resistance on the air side,then the increase of the condensation degree begins to be unfavorable to the increase of heat exchange and heat transfer coefficient.(2)The outlet air temperature,air side pressure drop,condensate formation rate,condensate removal rate,heat exchange and air side heat transfer coefficient increase with wind speed,the wind speed raises 0.5m/s,the outlet air temperature increases by 1.2?averagely,the pressure drop increases by 32Pa,and the rate of formation of condensate increases by 0.28m/s averagely,removal rate of condensed water increased by 0.49g/s averagely,heat transfer increases by 1.4kW,and the heat transfer coefficient increases by9.4W/(m2?K).When the wind velocity increases from 2.5m/s to 3.0m/s,the heat exchange and the heat transfer coefficient began to decrease.The heat transfer effect of micro-channel exchanger is best at wind speed 2.5m/s(3)The outlet air temperature and air pressure drop will be reduced.The main reason is that the air velocity is constant,when inclination increases,the equivalent air volume is reduced,and the unit mass air exchange heat more fully.When the inclination angle increases from 10 degrees to 20 degrees,the condensate generation rate decreases by 0.1g/s,and the removal rate of condensed water increases by 0.36g/s.The heat transfer coefficient and heat exchange raise about 4.9%and 1%respectively.The increase of inclination angle reduces the surface aggregation of condensing water and strengthens the effect of air side heat transfer to a certain extent,When the inclination angle increases from 10°to 20°,the heat transfer rate increases by 1%,but the air volume that can be treated for the test system is reduced by 5%.Therefore,the comparison of the three sets of experimental conditions shows that the heat transfer effect of the air side at the inclination angle of 10degrees is the best.The conclusions by simulator investigation of this paper are as follows:(1)When the air runs through the fin angle,the air disturbance is strongest.The pressure drop increases gradually along the flow direction,the air temperature near the flat tube is lower,the temperature decreases step by step along air flow direction,and the change gradient becomes smaller and smaller;the volume fraction of liquid raises step by step along the flow direction,and the condensation intensity along the flow direction is enhancing.The air in the first half of the model go through sensible heat transfer mainly and the latter half go through mainly latent heat transfer.(2)The outlet air temperature and pressure drop raises with the increase of the air velocity,the air velocity increases by 0.5 m/s.Outlet air temperature increases averagely by0.95?.the air pressure drop increases by 27.8Pa.When the air velocity raises,the location of the condensation will take ahead.The formation rate of condensate and the coefficient of air side heat transfer increase with the increase of air velocity.Air velocity increases by0.5m/s,the air side heat transfer coefficient increases by 12W/(m~2?K),the air velocity increases,and the surface temperature distribution uniformity of the heat exchanger is improved.(3)The outlet air temperature and pressure drop decrease with the increase of the obliquity.The wind speed is certain,when the heat exchanger obliquity is increased,the equivalent air volume decreases,which makes the effective heat transfer area of the unit mass air increases,the heat exchanges more fully,and the outlet air temperature is reduced.When the obliquity increases 10 degrees,the air side pressure drop decreases by 6.4Pa on average,and the outlet air temperature decreases by 0.3 degrees.Air side heat transfer coefficient raises by 4.512W/(m~2?K)averagely.In this paper,the heat transfer characteristics and drainage characteristics of the air side surface of micro-channel heat exchanger under different wind speed and dip angle are experimented and simulated.Contrasting the simulation datas with the experimental ones,it is found that the trend of outlet air temperature,pressure drop,formation rate of condensate,heat transfer coefficient and the other parameters changing with the air velocity and the inclination are consistent in experimental research and simulation research,and the error is within 15%.It has a certain scientific significance and hopes to provide a reference for the use of micro channel heat exchangers as evaporators.