Experimental Study On The Effect Of PM2.5 On The Frosting Process Of Outdoor Finned-tube Evaporator

Air source heat pumps have been widely used in the world for the superior energy saving performance and environmental friendliness.But when the ASHP system is running in the low-temperature environment,it is easy for the evaporator to frost,which has a great impact on the operation of the heat pump unit.In recent years,haze weather has occurred frequently in northern China,and the measured data show that the outdoor haze weather conditions are beneficial to the formation of frost layer on finned tube evaporator.To study the frosting process mechanism of outdoor unit evaporator under haze condition and to improve the defrosting control mode according to the new requirements is the key to avoid the problem of "mist defrosting" in heat pump units.The effect of PM2.5 on the performance of heat pump unit with frosting characteristics of evaporator was obtained by comparing experiments,and the distribution map of frosting affected degree was drawn for the establishment of a new ASHP system defrosting control strategy under air pollution conditions.Firstly,the schematic diagram of the experimental device and the construction of the key components for testing the frosting characteristics of the PM2.5 on the outdoor finned tube evaporator and the performance of the heat pump unit were described.According to the schematic diagram of the experimental device,the experimental platform of fine particles affecting frosting process was built.The key components of the experimental system were introduced.Under the condition of no pollution,mild pollution,moderate pollution and serious pollution,experiments were carried out to test the heat transfer rate,head-on wind speed and system COP of outdoor finned tube evaporator during frosting.The promotion effect of PM2.5 on the frosting process of outdoor finned tube evaporator was verified.It was pointed out that the air moderate pollution(the lower limit of PM2.5 concentration range is146 ?g/m3)was the critical range for the frosting process of outdoor finned tube evaporator.Then,the influence mechanism of PM2.5 on frosting process of outdoor finned tube evaporator was studied.To quantitatively analyze the affected degree of frosting,the evaluation index based on the impedance of air side tube section of evaporator: frosting rate ratio(FRR)was established,and it was pointed out that the FRR was the same in different stages of frosting.Based on the analysis of the experimental results under different temperature and humidity conditions,it was considered that the frosting zone could be divided into two regions.In the Region I where the moisture content of the air was very low,the effect of PM2.5 on thefrosting was weakened,and the FRR was positively correlated with the moisture content of the air.While in Region II with a relatively high moisture content of air,the FRR was positively correlated with the mass ratio of PM2.5 to moisture.Finally,through a wide range of comparative experiments,the influence degree of frosting at different temperature and humidity operating points in the ASHP frosting area was measured,the distribution map of frosting influence degree was drawn.And the reasons leading to the "false defrosting" operation of the unit and the limitations of the existing defrosting control strategy were pointed out.The necessity and feasibility of considering the concentration of air pollutants in the defrosting control process of the ASHP system were put forward,and a new defrosting control mode of the ASHP system was put forward accordingly.The research of this subject is of great significance for understanding the influence mechanism of PM2.5 on the frosting process of outdoor finned tube evaporator under haze weather conditions,and provides a theoretical basis for solving the problem of wrong defrosting operation of ASHP units under the condition of "haze-frosting coupling ".