| In this paper the dynamical performance of the air source heat pump (ASHP)under frosting condition was investigated and experimentally as well as experimentalstudy of the defrosting process of the ASHP unit. The deeper knowledge and betterunderstanding of the effects of the frost on the performance and the process of defrostis of immense importance to the defrosting control and the heat pump systemoptimization.Regarding the frost accumulation on the outdoor heat exchanger as aquasi-dynamic process, the dynamic distributed parameter model of the refrigerationsystem was presented, and the dynamic performance of the ASHP unit under frostingconditions was predicted by using finite difference method, and the dynamicperformance of the ASHP unit under frosting conditions. The split two-phase flowmodel was used in the heat exchange models. In the capillary tube model thehomogeneous assumption was employed and the delay of the flash point and thechoked flow were included. In the system model the decrease of the air flow rate dueto frosting of outdoor heat exchanger, and the interactional effect of the frostingprocess and cycle parameters of the refrigeration system were considered.A test set-up was established in the psychrometric rooms to investigate thedynamic performance under frosting conditions and defrosting characteristics of anASHP unit. The transient heating capacity, COP, frosting accumulation, frostthickness and wall temperature distribution along the tubes of the evaporator weremeasured. Based on the experimental results, the effects of the outdoor environmentalparameters and the face velocity of the evaporator on the frost accumulation, frostthickness, total heating capacity, total COP and the defrosting performance wereanalyzed. The experimental results indicate that the frost accumulation mass increaseslinearly with the frosting time, but the growth rate of the frost thickness could bedivided into three stages: initial stage, constant speed stage and sharply increasingstage. At the meantime, it was found that the frost formation rate is maximized whenthe outdoor air temperature is about 0-3℃ for different outdoor air relative humidity,and the effect of the relative humidity on the growth rate of frost thickness was morethan on that of the frost accumulation. The total heating capacity, total COP, defrosting lossand defrosting time increase with the decrease of the outdoor air humidity. At the same time,experimental results indicate that the defrosting loss and defrosting time are minumium when theface velocity of the evaporator is 1.3 m/s for the tested ASHP unit smallest, the reason is that thefrost accumulation and density is smallest under it.
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