| Heat exchanger is one of the core components that affect the energy efficiency of heat pump system.Because the existing hot water-condensing heat exchanger is mainly connected in series with the compressor outlet and between the condenser and the evaporator,it is difficult to guarantee and solve the heat transfer performance of the heat exchanger and the imbalance of refrigerant during operation,resulting in the low heat transfer efficiency of the heat exchanger.Therefore,the heat transfer characteristics of heat exchanger in heat pump hot water system are studied to strengthen its heat transfer performance,and the influence of heat transfer of heat exchanger on system performance is studied to solve the instability of heat transfer of heat exchanger.It will be of great significance to improve the heat transfer capacity of the heat exchanger and the overall performance of the system.In this paper,the heat exchanger of multi-functional heat pump system is mainly composed of finned tube heat exchanger and plate heat exchanger.As evaporator and condenser,the main function of finned tube heat exchanger is to cooling and heating of air conditioner,while the function of plate heat exchanger as hot water-condenser is to make hot water.According to different structures of R410A refrigerant finned tube heat exchanger and heat transfer characteristics of air side and tube inner side are analyzed and studied,the optimal structure type for enhancing heat transfer performance of heat exchanger is obtained.Through the establishment of three-dimensional model of plate heat exchanger and numerical simulation with FLUENT software,the heat transfer characteristics of R410A-water in herringbone plate of plate heat exchanger are studied and the optimal structural parameters of heat transfer performance of herringbone plate are obtained.The influence mechanism of heat exchanger heat transfer on system performance of 6HP multi-functional heat pump system under different operating modes is studied,the relationship between heat transfer performance of finned tube heat exchanger and plate heat exchanger and system energy efficiency is revealed.By optimizing system structure and control strategy,the overall performance of the system is further improved.The main research contents and conclusions of this paper are as follows:(1)The effects of flow arrangement,flow direction,split uniformity,subcooling section and fin type on heat transfer characteristics of finned tube heat exchanger were studied and analyzed.The results show that under standard heating conditions,the heat transfer capacity of type 140 heat exchanger can be increased by nearly 23%and that of type 160 heat exchanger can be increased by 16.5%.Moreover,the heat transfer performance and energy efficiency ratio of the type 160 heat exchanger after adjusting the capillary shunt are improved by 9.15%and 18.7%respectively.The design of subcooling section has little effect on improving the performance of condenser,but the defrosting period without subcooling section is 88min longer and the defrosting time is390s longer than that with subcooling section.By comparing and analyzing the heat exchangers with three different fin types,it can be seen that the heat exchanger with corrugated slotted fin(STEPFIN)type has the best heat transfer performance,the standard cooling and heating capacity of type 140 heat exchanger reaches 17722W and13933W respectively,the energy efficiency ratio reaches 2.85 and 3.19 respectively.The standard cooling and heating capacity of type 160 heat exchanger can reach18956W and 15149W respectively,and the energy efficiency ratio can reach 2.51 and3.06 respectively.(2)The heat transfer characteristics of air side and R410A-lubricating oil mixture in internally threaded tube of corrugated slotted finned tube heat exchanger were studied experimentally.The results show that:(1)When the fin spacing increases from1.2mm to 1.9mm,the heat transfer capacity on the air side also increases,while the energy efficiency ratio of the heat exchanger increases first and then decreases.When the fin spacing is 1.7mm,the maximum cooling and heating energy efficiency ratios reach 2.6 and 2.85 respectively,and the cooling and heating capacities are 13746 W and 15793 W respectively.(2)The lubricating oil has dual effects on R410A refrigerant which can enhance heat transfer at high dryness and weaken heat transfer at low dryness in the threaded pipe.With the increase of refrigerant mass flow density,the effect of lubricating oil on heat transfer of refrigerant is smaller.(3)The three-dimensional model of R410A-water plate heat exchanger was established and the numerical simulation was carried out by using FLUENT software.The heat transfer characteristics of R410A refrigerant and water in the plate heat exchanger were studied by changing the structural parameters of the herringbone plate of the plate heat exchanger.The results show that the corrugation angle?is 60o,the corrugation pitch?is 14mm,the plate with 2mm corrugation height h,the goodness factor j/f on the refrigerant side and water side is the best,reaching 0.44 and 0.06,0.49 and 0.07,0.97 and 0.08 respectively.(4)The multi-function heat pump system in hot water mode:through experimental research and analysis on the problem of low performance of system hot water production under low-temperature working conditions,a new circulation system with refrigerant recovered into the system and then cut off is established,which solves the problem of low heat transfer capacity caused by lack of refrigerant in the plate heat exchanger due to unbalanced refrigerant.The heating capacity of the plate heat exchanger after optimizing the system structure is increased by 150%as compared with that before,and the system energy efficiency ratio is increased by 133.3%.(5)The multi-function heat pump system in the mode of cooling+hot water:(1)When the inlet water flow rate of the water module is 0.55m~3/h?2.0m~3/h,the heating capacity of the plate heat exchanger and the cooling capacity of the fin tube heat exchanger both increase,and the total energy efficiency ratio of the system is in direct proportion to them.At this time,the optimal inlet water flow rate is 2.0m3/h,the heating capacity and waste heat recovery ratio can reach 10650W and 71%respectively,and the total energy efficiency ratio of the system can reach 5.37.(2)When the inlet water temperature increases from 10?to 53?,the cooling capacity of finned tube heat exchanger and the heating capacity of plate heat exchanger decrease by 23.7%and99%respectively,and the total energy efficiency ratio of the system decreases by as much as 80%,indicating that the total energy efficiency ratio of the system has a linear relationship with the heat transfer performance of the heat exchanger.At this time,adjusting the EVO opening of the outdoor unit electronic expansion valve to 10%?40%,the heat transfer between the finned tube heat exchanger and the plate heat exchanger can be balanced,the cooling capacity of the fin tube heat exchanger can be increased by 20%and the heating capacity of the plate heat exchanger can be increased by 16%,and the total energy efficiency ratio of the system can be increased by 22%at the most.(3)When the outdoor temperature rises from 10?to 40?,the cooling capacity of the finned tube heat exchanger only increases by 12.1%,but the heating capacity and waste heat recovery ratio of the plate heat exchanger increase significantly,with the heating capacity increasing by 670%and the waste heat recovery ratio increasing by 589%.The total energy efficiency ratio of the system increases almost linearly after 15?,with an increase of 84.8%.(4)When the operating capacity of the indoor unit changes,adjusting the EVO opening of the electronic expansion valve of the outdoor unit to 10%?20%,the cooling capacity of the fin tube heat exchanger and the heating capacity of the plate heat exchanger can be increased by 10%and 8%respectively,and the total energy efficiency ratio of the system can be increased by 15%;It shows that adjusting the opening of EVO can balance and distribute the refrigerant flow between finned tube heat exchanger and plate heat exchanger to a certain extent,so as to ensure the stability of heat transfer performance between heat exchangers,and improve the total energy efficiency ratio of the system.(6)The multi-functional heat pump system in the mode of heating+hot water:(1)When the inlet water temperature of the water module increases from 20?to 40?,the heating capacity of the finned tube heat exchanger increases by 15.5%,while the heating capacity of the plate heat exchanger decreases by 13.1%,indicating that the heat transfer capacity of the finned tube heat exchanger and the plate heat exchanger is inversely proportional,and the total energy efficiency ratio of the system presents a nonlinear relationship with the change of inlet water temperature;By adjusting the opening of EVM to 30%?60%,the heating capacity of finned tube heat exchanger and plate heat exchanger can be balanced and stabilized,and the total heating capacity can be increased by 20%and the total energy efficiency ratio of the system can be increased by 16.3%.(2)When the outdoor temperature is-15?,the heating capacity of finned tube heat exchanger and plate heat exchanger are very low,being 9833W and139W respectively;When the outdoor temperature increases from-5?to 20?,the heating capacity of finned tube heat exchanger increases by 31%,while that of plate heat exchanger increases by 130.8%,and the total energy efficiency ratio of the system increases from 1.7 to 3.0,with an increase of 76.5%,indicating that the total energy efficiency ratio of the system is in direct proportion to the heat transfer capacity of the heat exchanger.(3)The greater the operating capacity of the indoor unit,the greater the heating capacity of the finned tube heat exchanger,while the heating capacity of the plate heat exchanger decreases continuously.It can be seen that the heat transfer capacity between the finned tube heat exchanger and the plate heat exchanger is inversely proportional;At this time,the inlet water temperature is controlled within the range of 35±5?for adjustment,which can balance and stabilize the heat transfer between the fin tube heat exchanger and the plate heat exchanger,and can affect up to80%of the total heat generation amount,and the total energy efficiency ratio of the system can be increased up to 15.7%. |
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