Abstract: The frost growth characteristics on the outdoor heat exchanger of an air source heat pump(ASHP)under different face velocity conditions were investigated experimentally.The dynamic frost thickness on the fins and the frost accumulation mass on the evaporator were measured,and the growth process of the frost layer was observed through a microscope.The experimental results indicate that the decrease in the face velocity of the outdoor heat exchanger accelerates the growth of the frost layer,the frosting duration reduces almost linearly with the decrease in the face velocity,further more,the lower the relative humidity is,the faster the frosting duration decreases.Therefore,the decrease in the face velocity of the outdoor heat exchanger will deteriorate the average performance of the ASHP unit in the frost/defrost period.The observation to the frost layer through a microscope shows that the acceleration of the frost layer growth under low face velocity condition is mainly attributed to the morphological variation of the frost crystal comparing with that under the high face velocity condition,which is resulted from the decrease in the wall temperature of the evaporator.Under the low face velocity condition,the column-shaped ice crystals grow mainly in its length in whole frosting period,which is similar with the form of the frost crystal under the low temperature and high face velocity conditions.The total frost accumulation mass reduces with the decrease in the face velocity,therefore,the average frost density is fallen down.

Key words: air source heat pump, face velocity, frost growth characteristics, frosting experiment

摘要： 对不同迎面风速条件下空气源热泵系统室外换热器表面霜层生长特性进行了实验研究,测量了翅片表面动态霜层厚度、换热器结霜量,显微观察了霜晶生长过程。实验结果表明,迎面风速的降低使得空气源热泵机组室外换热器表面霜层厚度加速增长,结霜周期随迎面风速的下降呈近乎线性地减小,而且相对湿度越低,结霜周期下降的速度越快;因此,减小室外换热器迎面风速将恶化空气源热泵机组结霜/除霜周期中的平均性能。对霜晶形态的显微观察发现,低迎面风速工况下霜层厚度增长速度加快的原因是由于空气源热泵蒸发器壁面温度降低造成的霜晶形态的改变,翅片表面柱状冰晶始终在高度方向快速生长,这种现象与低环境温度工况下翅片表面霜晶生长形态类似。换热器总结霜量随迎面风速的减小而下降,造成霜层平均密度降低。

关键词: 空气源热泵, 迎面风速, 霜层生长特性, 结霜实验