Investigation On Effect Of Different Structure Forms Of Wrap-around Condenser Coil On The Performance Of Heat Pump Water Heater

The low-temperature heat in the air can be efficiently absorbed by the domestic air source heat pump water heater to produce hot water,and the continuous research and optimization of its performance can make an important contribution to the national energy conservation and emission reduction cause.In recent years,researchers have made some achievements in the aspects of heat pump system circulation,control operation optimization,etc.,however,in terms of component structure design,the coordination mode of condenser coil and water tank needs to be improved.In order to further optimize the wrap-around forms of the condenser coil,this paper proposes the wrap-around variable-pitch condenser coil structure.The effects of different wrap-around coil structural forms on the energy storage/consumption performance of the heat pump water heater were studied through simulation and experiment,which could provide ideas and methods for the optimal design of the coil structure.Firstly,the heat pump mathematical model and the two-dimensional flow and heat transfer simulation model of water tank were established,and through the heat transfer process of the refrigerant in coil and the hot water in water tank,the two were combined to form a coupled model.The heat pump model including compressor,condenser coil,throttling device and evaporator was compiled by MATLAB,and the water tank simulation model was established by CFD.In order to optimize the boundary conditions and improve the accuracy of the model,the multi-heat flux boundary conditions including the heat flux density in the superheated steam,two-phase flow,and subcooled liquid regions were proposed.Secondly,the water tank with wrap-around variable-pitch coil was designed and manufactured,and a certain brand model of heat pump main engine was used to build the domestic air source heat pump water heater experimental device in Nanjing,which could realize the production and use of 150L/50℃domestic hot water.The hot water temperature distribution and system power consumption were measured experimentally,and the average temperature of the hot water,the amount of heat in the water tank,the COP,and the energy release efficiency were calculated and compared with the simulation values,which verified the accuracy of the coupled model.Then,the performance of the water tank with wrap-around constant/variable-pitch coils was simulated and analyzed.Taking the heat flux value,temperature field distribution and Nusselt number of natural convection as performance indicators,the influence of two different coil structures and the structure parameters such as pipe pitch and pipe diameter under the variable-pitch coil structure on the heat transfer and flow performance of water tank was analyzed.The results show that the heat flux values of the three phase regions of the variable-pitch coil is higher than those of the constant-pitch coil,the temperature difference among the temperature layers in the water tank is reduced,and the average Nu number is increased by3.48%.It can be known that the heating performance of the water tank can be improved by using the wrap-around variable-pitch coil structure,and the heat transfer and flow performance of the water tank can be further improved by reasonably reducing the wrap-around spiral pipe pitch and diameter.Finally,the processes of energy storage and utilization of heat pump water heater were studied by simulation and experiment.With the heat transfer coefficient,hot water temperature distribution,system COP and hot water extraction efficiency as performance indexes,the performance of heat pump water heater with two different coil structure was simulated and analyzed,the operation results of the system under different operating conditions were analyzed,and the operation performance of the heat pump water heater under various random live conditions was studied experimentally.The results show that the average heat transfer coefficient of the wrap-around variable-pitch coil during the energy storage process is 319.31W/(m~2·K),21.91%higher than that of constant-pitch coil,the average COP is 4.54,10.75%higher,and more uniform distribution of hot water temperature.The average heat transfer coefficient and COP during the water discharging process are slightly higher than the constant-pitch coil,while the hot water extraction efficiency,heat discharge efficiency and fraction of heat recoverable are increased by 7.69%,8.53%,and 5.84%respectively,and the quality of the hot water temperature output from the water tank is improved.It can be concluded that the use of a variable-pitch coil structure can increase the heat exchange area at the bottom of the water tank and enhance convective heat transfer in the spiral tube,which can improve the temperature distribution in the water tank,heat transfer performance between the refrigerant side and the water side,and improve system performance and hot water output performance.When the experimental device built in this article is used in Nanjing where the summer is hot and the winter is cold,except for severe rain and snow weather,the operation performance of the system is good.