Study On Heat And Mass Transfer Characteristics Of Evaporator Air Side In Refrigerated Dehumidification System In Solar Greenhouse

With the development of the national economy and the annual growth of people’s demand for a better life,facility agriculture has become an important part of modern agriculture in China.Due to the limitation of land resources and natural conditions,facility agriculture in the arid region of the northwest temperate zone of China is densely distributed,among which the area of solar greenhouse is increasing year by year.In order to create a microclimate environment suitable for crop growth in solar greenhouse,it is very important to solve the problem of high humidity.Too much humidity in solar greenhouse greenhouse crops will produce a lot of diseases,resulting in crop yield reduction or even failure.The refrigeration dehumidification system is an effective method to solve the problem of high humidity in solar greenhouse.However,in the process of greenhouse dehumidification,the condensation of wet air in the evaporator is relatively large,which will have a more complex impact on the heat and mass transfer and flow resistance of the evaporator.Current against the sunlight greenhouse with the study of the characteristics of heat and mass transfer in the evaporator is relatively weak,therefore,in this paper,the different structure size of finned tube evaporator shutter in the process of solar greenhouse dehumidification air side performance of thermal mass transfer and fluid flow characteristics on numerical analysis of key to exploring the requirements under the condition of the sunlight greenhouse shutters finned tube evaporator air side heat mass transfer law and the analysis of performance,and the comprehensive heat transfer performance of the advantages and disadvantages are analyzed.The main contents are as follows:Firstly,the numerical heat transfer model of the louver finned tube evaporator under different louver angles and different fin spacing is established,and the independent verification of the mesh system is carried out.Experiments were used to verify the correctness of the numerical simulation.The results show that under the same Re number,the average error of the resistance coefficient and the average Nusselt number is 10.38%and 16.36%,respectively,and under the same relative humidity,the average error of the resistance coefficient and the average Nusselt number is 10.71%and 14.33%,respectively.Secondly,under the dehumidification condition,Fluent was used to numerically analyze the heat and mass transfer and resistance characteristics of the louver finned tube evaporator with louver Angle of 22°and fin spacing of 2mm.The results show that,compared with the dry condition,the contours of temperature field on the center surface of the evaporator heat exchange channel are more uniform and the contours of velocity field are denser under the moisture dissection condition,and the decreasing interval is smaller,and the decreasing velocity is faster.The average Nusselt number and resistance coefficient under wet condition are 1.70 times and 2.15 times of those under dry condition,respectively,which indicates that the wet condition is beneficial to heat transfer performance,but has a negative effect on the resistance.Compared with the flat-finned tube evaporator,the louved-finned tube evaporator can effectively improve the air retention on the leeward side of the round tube and improve the heat transfer efficiency.The maximum increase of average Nusselt number and the average resistance coefficient of the louved-finned tube evaporator are 1.67 times and 1.61times of that of the flat-finned tube evaporator,respectively.This indicates that the louver fin can not only improve the heat transfer performance of the flat-finned tube evaporator,but also have a negative effect on the resistance.However,by analyzing the heat transfer enhancement factor JF,it is found that the comprehensive heat transfer performance of the louver fin tube evaporator is generally better than that of the flat-finned tube evaporator.Thirdly,the shutter Angle for 22°,fin spacing for 2 mm blinds finned tube evaporator wet performance analysis of the research,the results show that the fin surface and the amount of condensation heat transfer channel center face are increased with the increase of relative humidity,and with the increase of flow velocity,finned surface and condensation water of the channel center size and quantity of condensation were decreased;Compared with the flat-finned tube evaporator,the condensate in the heat exchange channel of the louver finned tube evaporator is more distributed,while the condensate on the fin surface is less.The average increase of moisture dehydrating volume of louver fin and finned tube evaporator is 1.02×10^-6kg/s and 6.69×10^-8kg/s,respectively,and the former is about 15 times that of the latter.Fourthly,under the conditions of relative humidity RH_in=50～80%and flow rate u_in=2m/s,numerical simulation analysis is carried out on several different structure louver finned tube evaporators with louver Angle（15°,22°,28°）and fin spacing（1.5mm,2mm,2.5mm）.The results show that when the fin spacing is 2mm,the average growth rate of Nusselt number and drag coefficient is 20.03%and 20.77%,respectively,with the increase of the shutter Angle.When the shutter Angle is 22°,the drag coefficient decreases with the increase of fin spacing,and the average decrease rate is 57%.The average Nusselt number first increases and then decreases.According to the analysis of enhanced heat transfer factor JF,when the distance between fins is 2mm,the increase of the louver Angle can improve the comprehensive heat transfer performance of the louver fin tube evaporator.When the Angle of the louver is 22°,the comprehensive heat transfer performance of the louver fin tube evaporator increases with the increase of the distance between fins.