| A major problem faced by air source heat pump (ASHP) is the deposition and progressive build up of frost on the outdoor heat exchanger surface. The frost not only increases the thermal resistance between the air and the refrigerant, but also reduces the airflow rate through the evaporator. As a result, the coefficient of performance (COP) is reduced. In this paper the dynamical performanc of ASHP under frosting condition is analyzed by numerical simulation and experimental study. The deeper knowledge and better understanding of the effects of the frost on the performance is of immense importance to the defrosting control and the heat pump system optimization.In this paper, based on steady distributed parameter model for ASHP, a dynamic distributed parameter model is presented by regarding the frost accumulation as the quasi-dynamic process. A simulation program has been developed by using finite difference method to predict the dynamic performance of the refrigeration system under frosting conditions. The split two-phase flow model is used in the heat exchange models. In the capillary tube model the homogeneous model is employed and the delay of the flash point and the choked flow are included. In the system model the decrease of the air mass flow and the arrangement of the heat exchange tubes are considered. By solving the system model, the dynamic characteristics of heat pump under frosting condition and the frost distribution on the surface of the evaporator are obtained.In order to verify the validity of the mathematical model and calculation method, an experimental study has been conducted in the enthalpy difference test set-up. The indoor air temperature is kept at 20℃. The outdoor air temperature is controlled to vary from -15℃to 7℃and the relative humidity from 40% to 90%. The transient heating capacity, COP, frosting quality and wall temperature distribution along the tubes of the evaporator are measured. Based on the experimental results, the influence of the outdoor air temperature and the relative humidity on frost accumulation and the performance of ASHP are analyzed. The frost formation conditions are discussed and a frosting zone chart is presented, which is benefit to the defrosting control. The numerical and experimental results are in good agreement, which demonstrates that the simulation model and calculation method are feasible. |
