| Compared with traditional finned-tube heat exchanger,micro-channel heat exchanger has the advantages of high heat transfer efficiency,small volume,small copper consumption and small refrigerant charge,it has been widely used as a condenser of refrigeration system.However,when it is used as an evaporator of air source heat pump,there are many problems such as difficulty in discharging condensed water,easy frosting and poor system performance.In this paper,the performance and characteristics of micro-channel heat exchanger as a evaporator of air source heat pump are studied,and some improvement measures are explored.The specific work and research results are as follows:Firstly,the condensate discharge and refrigerant distribution of micro-channel heat exchanger as the evaporator of water-cooled air source heat pump and system performance and characteristics are studied experimentally.It is found that the distribution of refrigerant in the heat exchanger is inhomogeneous,that is to say,liquid refrigerant flows from the lower flat tube of each process,while gas refrigerant flows from the upper flat tube.As the flat tube of micro-channel heat exchanger is horizontal,condensate water accumulates at the root of the fin and is difficult to discharge,which has a great impact on the performance of the unit,and the COP decreases from 3.03 to 2.58,with a reduction of 15%.Then,the optimization measures of rotating 90 degrees installation are put forward,the results show that the uniformity of refrigerant distribution in heat exchanger and the condensate discharge effect are greatly improved,the COP of the unit is maintained at about 3.30,but there are still some problems in the intermediate process,such as gas-liquid reversal and non-uniform refrigerant separation between the microchannels of the flat tube.Secondly,the frosting/defrosting characteristics and performance of air source heat pump system with micro-channel heat exchanger are studied experimentally.It is found that the uneven distribution of refrigerants leads to uneven frosting on the surface of heat exchanger,which will further affect the distribution of refrigerants and eventually lead to rapid deterioration of unit performance.The average EER of the first frosting/defrosting cycle is only 1.8 when the micro-channel heat exchanger adopts traditional arrangement,and the EER decreases by 41.6% during the heating cycle.Through the experimental study on the continuous periodic operation of air source heat pump system with micro-channel heat exchanger,it is found that defrosting water is difficult to discharge as the flat tube of micro-channel heat exchanger is horizontal,which leads to the defrosting is not complete.When the unit is reheated,the ice block is formed between the fins,which not only affects the heating,but also makes the defrosting more difficult.The experimental data show that the performance coefficient of heat pump decreases gradually and the defrosting time increasesgradually when the micro-channel heat exchanger is used for continuous frosting/defrosting operation.The results show that the EER is as low as 1.13 at the end of frosting operation of the third frosting/defrosting cycle,which is 0.29 and 0.15 lower than that of the first and second frosting/defrosting cycle,respectively.And the defrosting time of the third frosting/defrosting cycle reaches 440 s,which is 200 s and 120 s longer than that of the first and second frosting/defrosting cycle,respectively.The energy consumption of defrosting in the third frosting/defrosting cycle reaches 0.062 KW.h,which is 0.028 KW.h and 0.018 KW.h higher than that of the first and second frosting/defrosting cycle,respectively.Thirdly,the preliminary optimization research is carried out on the problems of poor heating performance and defrosting during frosting/defrosting operation of micro-channel heat exchanger air source heat pump system.Through the frosting experiment of air source heat pump system with micro-channel heat exchanger,we can see that the frosting on the surface of the traditional microchannel heat exchanger with 90 degrees rotation is still not uniform,and it is not obvious to improve the performance coefficient of heat pump,the average EER is only increased by 0.04.If a single-flow micro-channel evaporator with vertical flat tube is used in the air source heat pump system,the refrigerant distribution uniformity is improved and the average EER is increased by 0.33.Research on the defrosting characteristics of air source heat pump system with micro-channel heat exchanger,it is found that the method of reversing the air supply by the outdoor unit fan to promote the defrosting water discharge is ineffective,although the frosting on the surface of heat exchanger mainly distributes on the flat tube outside the upwind side of heat exchanger.The effect of defrosting water discharge can be improved by rotating 90 degrees to install micro-channel heat exchanger,however,the drying cycle of surface defrosting water is long and it is difficult to achieve complete defrosting.According to the experimental results,a defrosting method of placing the flat tube vertically and supplementing it with the low-frequency air to dry defrosting water on the outdoor unit fan is proposed,the results of repetitive experiments show that this method can completely defrost the micro-channel heat exchanger,and this method is reliable and effective.The research work in this paper provides theoretical support and technical reserve for promoting the use of micro-channel heat exchanger as evaporator of air source heat pump system,and provides a new idea for developing modern products of air source heat pump with high reliability and energy efficiency. |
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