Study On High-Performance Of Air Source Heat Pump Under Frosting Conditions

Air source heat pump has a large number of market sales and amazingdevelopment prospects due to the characteristics of the heating and cooling dual-use,energy-saving and environmental protection, etc. However, the air source heat pumpface a serious problem in winter heating operation is its outdoor airside heat exchangersurface frosting easily which leading to the degradation of the heating capacity. Thisphenomenon is more common in China’s Yangtze River and south of unrealized centralheating areas. One heat pump air conditioners in Enshi is measured in this issue. Then,two air source heat pump outdoor units are processed and use the existing wind tunnelexperiment station completing the compared experiment under the frosting conditions.Finally, the writer analysis the flow and heat transfer in dry conditions for the flat finand the hole fin by the numerical simulation software. The research laid the foundationfor improving the performance of the air source heat pump under winter frostingconditions and has an important guiding significance for the unit structural optimizationand design. The main conclusions are as follows:The research in Enshi for the heat pump in the winter frosting conditions shows:When the unit operates under low ambient temperature of-2℃to5℃and the outdoorrelative humidity higher than65%, the outdoor unit fins surface frost easily. The unitbegins to defrost while frosting to a certain extent. The frosting-defrosting continuouscycle of the unit leads to lower indoor temperature and worse thermal comfort. Thelowest average heating capacity only accounted39.3%in the test period. In the heatingdefrost cycle, the COP less than1accounted41.7%in the total unit operation time.Extending the analysis of the Enshi heating period to the hot summer and cold winterzone, the heat pump can’t normal heating about1/3of the heating period. The air sourceheat pump is not saving.On the basis of actual measurement research, this writer processed the twodifferent fin type air source heat pump outdoor prototypes-flat fin and louver fin.Thecomparative experiment conducted in the existing wind tunnel test under four differentwind speeds. The results indicate:①The evaporator resistance exists one moment thatthe resistance of the flat fin and the louver fin equal to each other, called the momentresistance point of balance. It is significative to discuss the influence of the different fintype on frost before resistance equilibrium point.②Under different wind speeds, the flat fin heat exchanger performance is significantly better than the louver fin which isclosely related to the louver fin’s cumulative amount of frost more than the flatfin’s(9.95%).Among them, the average heat transfer and the average equivalentconvective heat transfer coefficient of the flat fin has increased by15.78%and31.18%than the louver fins.③The COP of two different fin type heat exchanger is lower underfrosting conditions. The COP of the flat fin has increased of11%~17%on the basis ofthe louver fins.④The heat exchanger possessing the excellent properties under thedry(wet) conditions does not necessarily apply to the frosting conditions. Therefore, it isnecessary to develop the new fin type aiming at the fin operating under frostingconditions.Finally, it proposed the idea of the air source heat pump according to preliminaryresearch. Moreover, it has the numerical simulation for the flat fins and the hole finthrough the software FLUENT.The simulation results show: the hole fin has greatlyreduced the average temperature of the entire surface of the fin (336.74K~337.54K)and the lower wind speed (less than1m/s) range has decreased. In addition, the hole inthe fin damage the fin surface air boundary layer development, and strengthen the localheat transfer, so that the average convective heat transfer coefficient of fin surfacecompared to the flat fins increased from16.36%~24.01%. Whereas, the resistance of thehole fin has littler increased relative to the flat in the same simulation conditions. Usinga metal material consumption to evaluate the economy of the hole fin found the DK-10fin saving9%of the metal material in the same heat transfer and temperature differencerelative to the flat fin. The other hoe sheet type can save7%to8%of a metal material.Therefore, the hole fin designing should pay attention to the balance between the heatexchanger and the heat transfer area loss.