The frost of the evaporator can greatly affect the performance of the air source heat pump. When humid air comes in contact with a cold surface maintained below the freezing temperature, a frost layer begins to grow. This paper experimentally studied the frosting process of a folded-louvered-fin, parallel flow evaporator in a heat pump central air-conditioning system under three conditions, in which three different diameter capillaries were used. The surface temperature distribution of the evaporator was measured by sixteen thermocouples which buried on the leeward side and mesoscale frost formation processes on its partial surface for three different test conditions were observed using a CCD camera. The experimental result showed that the surface temperature distribution of the parallel flow evaporator was non-uniform and frost formation generally began on some partial surface of the louvered fins where the surface temperature was lowest after the heat pump system had starting up for six minutes under condition B and C, while the fins began to frost after eight minutes under condition A. The non-uniform surface temperature distribution which was caused by the unequal distribution of the refrigerant flux in the microchannels of flat tubes led to uneven distribution of frost, furthermore, incomplete evaporation of refrigerant in some tubes may be a critical factor of folded-louvered-fins’ easily frost under relative high environmental temperature condition. The ice crystals distributions and frost thickness in frost period can be obtained by the digital image processing method in which the initial pictures can be converted into binary image. The results showed that in a thin layer near to fins’ surface, ice crystals were comparable high dense in the frost growing and full grown period of frost layer; the ice crystals rate was reduced in approximate linear curves with the increasing of the frost thickness (frost height), where the decreasing rate in the frost full grown period was slower than that in the frost growing period (In 0.1 mm frost thickness, the ice crystals rate had decreased to 58% in the frost growing period, while in the frost full grown period the ice crystals rate was 90%; and in 0.25 mm frost thickness, the ice crystals rate were 0%, 45% respectively.). It also can be found that the frost thickness increased as the time increasing and then finally reached 0.3mm, 0.35 mm, and 0.32mm respectively at three conditions. The frost thickness had reached 0.1mm, 0.05mm under condition B and C respectively in 6 minutes, however, concerning condition A, the frost began to grow after 8 minutes and frost thickness arrived only 0.05mm in 14 minutes.

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