| Air source heat pump has been widely used in building energy-saving projects as a mature,efficient and environmentally friendly cooling and heating air conditioning equipment.However,the frosting phenomenon of the air source heat pump evaporator during heating operation in cold areas in winter will reduce the heating capacity of the system and affect the comfort experience.When the frosting is severe,it may even cause the system to shut down or irreversibly damage the heat pump system.Therefore,in-depth study of the frost/defrost characteristics of the air source heat pump evaporator has great theoretical and engineering practical significance.The whole process and all-round visual research on the overall frosting and defrosting process of the air source heat pump evaporator under different environmental conditions and different defrosting methods were carried out by designing and building a visual experiment platform for frost/defrost characteristics of the air source heat pump evaporator.Taking the whole evaporator as the research object,the frost characteristic experiment was carried out.The frost layer morphological characteristics,formation and growth rules were analyzed during the frost process of the evaporator,and the influence of the evaporator structure and air parameters on the frost characteristics was studied.Taking the whole evaporator as the research object,the air defrosting characteristic experiment and the electric heating assisted air defrosting characteristic experiment were carried out,and the defrosting characteristic of the air source heat pump evaporator and the influencing factors of the defrosting process were analyzed.Ansys numerical simulation method and air source heat pump continuous frost/defrost cycle experiment were combined to analyze the formation and growth mechanism of retained droplets.The effects of retained droplets on the frosting characteristics of the system and the effects of large retained droplets on the defrosting characteristics of the system are analyzed.Under low temperature environment(<0 ?),the lower the air temperature and the lower the moisture content of the air,the smaller the frost thickness on the surface of the evaporator.The greater the relative humidity of the air,the more frost the heat exchanger,and the relative humidity has a greater influence on the formation and development of the frost layer than temperature.When using low-temperature weathering frost,the higher the air temperature and the lower the relative humidity,the faster the frost layer on the surface of the evaporator will melt.The temperature has a more significant effect on the melting stage of the frost layer.When the relative humidity of the air is 45% and 60%,the defrosting temperature is increased from 3?to 5?,and the defrosting time is shortened by 29.3% and 39%,respectively.The removal process of retained droplets in the defrosting process accounts for about20%-40% of the entire defrosting time.The effect of the electric heating method on the melting process of the frost layer is more significant.When the electric heatingwith 1000 W and 1600 W power is used to assist the air defrosting at 10°C,the melting rate of the evaporator frost layer is increased by 33% and 41.7% respectively,and the flow of defrosting water The time was shortened by 20% and 30%,the elimination time of retained droplets was increased by 24% and 48%,and the total rate was increased by 22.9% and 37.1%,respectively.Due to the effects of gravity,adhesion,and surface tension at the gas-liquid interface,liquid bridges will form during the flow of defrosting water,and eventually develop into large droplets.The retention of large droplets has an adverse effect on the system's frosting and defrosting processes.Through the continuous frost/defrost cycle experiment of the air source heat pump,the optimal defrosting time under the control method of freezing point suppression time and the method of defrosting time control without freezing point was obtained. |
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