| Air source heat pump water heaters occupies an important part of the domestic water heater market due to their characteristics of energy-saving,environmentally friendly,stable and efficient.Continuously output high-quality hot water and improve system performance of heat pump water heaters has become urgent problems to be solved.In order to improve product performance and system reliability,the heat pump water heater with an externally wound micro-channel coil water tank is widely used.Since the water tank is still heated during the process of discharging energy,the heat source is transferred to the outer wall of the water tank and new requirements are put forward for the optimization of the inlet structure.A curved baffle-shaped inlet structure is proposed for the micro-channel coil heat pump water heater,which is compared with the traditional direct and side-entry inlet structures through simulation and experimental research methods.The theoretical basis and research methods are provided for the optimization of the inlet structure.Firstly,a mathematical model of the heat pump system was established including compressors,condensers,throttling devices and evaporators.At the same time,the three-dimensional flow and heat transfer simulation model of the water tank was established.The boundary conditions were set as multiple heat flux with the superheated zone,two-phase zone and subcooled zone that changed proportionally with time.A coupled model was formed by the mathematical model of the heat pump system and the simulation model of the water tank which coupled by the heat flux of the condensing coil and the outer wall of the water tank.Secondly,the curved baffle-shaped was designed and manufactured.The experimental system of an air source heat pump water heater was built according to the whole machine transformation,which can provide 200 L/55 ℃ domestic hot water.The temperature distribution in the water tank and the power consumption of system during discharging energy was measured and collected by instruments.Besides,the extraction efficiency and system COP was calculated.By comparing the experimental and simulated values,it is found that the relative errors of the water tank outlet water temperature,hot water output rate,and system COP are within 2.5%,1.9%,and 4.8%,respectively.The experimental and simulation results are in good agreement and the accuracy of the coupling model are verified.Then,the performance of the heat pump water heater with direct type,side-entry type and curved baffle-type inlet under nominal working conditions was simulated.The operation performance of the heat pump water heater in the energy release process under nominal operating conditions is analyzed by the extraction efficiency and system COP.The results show that: heat pump water heaters with curved baffle-type inlet have the best thermal stratification.Compared with direct and side-entry heat pump water heaters,the hot water output rate of curved baffle-type is increased by 47.6% and 14.8%,respectively,and the system COP is increased by 18.6% and 6.03%,respectively.The curved baffle-type inlet can control the mixing of cold and hot water at the lower part of the baffle,forming a stable piston flow on the upper part of the baffle.This phenomenon can greatly improve the thermal stratification in the water tank,delay the drop of outlet water temperature and reduce the condensation temperature.Thus,the water output rate and the system COP are both highly efficient operation effects are achieved.Finally,the variable-condition operation effect of the heat pump water heater during energy discharging process is simulated.The operation performance of the heat pump water heater during energy discharging process under actual working conditions is tested experimentally.The results show that the inlet flow has a greater impact on the thermal stratification in the water tank.The larger the flow rate,the worse the thermal stratification in the tank and the inlet water temperature has a significant impact on the COP of the heat pump system.The lower the inlet water temperature,the higher the COP of the heat pump system.The heat pump water heater with curved baffle-type inlet maintains stable and efficient operating performance under various working conditions.Compared with the side-entry type imported heat pump water heater,the hot water output rate and system COP of curved baffle-type can be increased by up to 18.5% and 6.6%.In addition,the inlet water temperature of the water tank and the ambient temperature increase or decrease at the same time as the season changes under actual working conditions.The air source heat pump water heater can operate normally in Nanjing area throughout the year and can maintain good operating performance,except for a few severe weather. |
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