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.