| Direct expansion solar heat pump system absorbs solar energy and air energy at the same time,which not only overcomes the defect of unstable solar energy flow density,but also solves the problem of low efficiency and frosting of air source heat pump in cold season.It has been paid more and more attention because of its high efficiency and environmental protection.As a key component to absorb solar energy and air energy,the performance of collector/evaporator directly affects the performance of heat pump system.In order to optimize the structure of collector/evaporator in the heat pump system,the work done in this paper are as below:The one-dimensional heat conduction model of a finned tube in collector/evaporator was established under the condition that the base tube temperature is known.The two-dimensional numerical simulation based on C programming language was used to verify the correctness of the theoretical solution.The effects of solar radiation intensity,solar radiation area,fin height and excess temperature on heat transfer characteristics of collector/evaporator were analyzed.The results showed that after the fin absorbs solar radiation,the temperature difference between the top and the root of fin is increased,the average temperature of fin is increased,and the heat transfer temperature difference between the fin and the environment is decreased,which is not conducive to the absorption of air energy.With the increase of fin height,the convection heat transfer capacity of fin is firstly increased and then decreased,which has a peak value.Therefore,the fin height is not the bigger the better.At the condition of large fin height and solar radiation intensity,the fin will release heat to the environment,and the smaller convection heat transfer coefficient is beneficial to increase the heat transfer capacity.The another theoretical model of a finned tube in collector/evaporator was established under the condition that the fluid temperature and heat transfer coefficient in base tube are known.The two-dimensional numerical simulation based on C programming language was used to verify the correctness of the theoretical solution.The effects of fin height,solar radiation area,solar radiation intensity,convection heat transfer coefficient and inner diameter of base tube on heat transfer characteristics of collector/evaporator were analyzed.The results showed that,although increasing the effective solar radiation intensity on the fin surface and increasing the solar radiation area on the fin could reduce the convention heat transfer capacity of fin which unexposed to sunlight,it could still effectively increase the total heat transfer of finned tube.When the inner diameter and the convection heat transfer coefficient in the base tube are small,the fin which unexposed to sunlight temperature will be increased by solar radiation,and even release heat to the environment,which is harmful for the utilization of solar energy.Considering that the theoretical solution is too complex to use in practice,the theoretical solution was simplified,and the influence of the copper tube thickness and the convection heat transfer coefficient on the calculation accuracy of the theoretical solution was discussed.The results showed that when the copper tube thickness is much larger than fin thickness,the temperature field and heat transfer capacity of finned tube calculated by the simplified solution have high accuracy.Based on the analysis of the collector/evaporator heat transfer characteristics,this paper adopted T.L.Jordon clear-day solar radiation model to calculate direct solar radiation intensity,combined with theoretical solution under the condition of fluid temperature and the heat transfer coefficient in base tube,the total heat transfer capacity calculation formula of collector/evaporator considered time-varying characteristics was given.In order to optimize its structure,the three structural parameters of collector/evaporator inclined angle,the angle between fin and collector tube row and fin height were taken as optimization parameters,MATLAB was used to write the objective function and constraints,and carry out optimization analysis.The results showed that for the working conditions studied in this paper,the optimized structure is as follows:the fin height is 0.0470 m,the collector/evaporator inclined angle is 42.038°,and the angle between fin and collector tube row is 28.132°.Compared with the control group,the optimized total heat transfer capacity increased by 7.3%.The research in this paper has certain theoretical guiding significance for the design and optimization of collector/evaporator. |
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