The effects of surface energy on phase change of water vapor at initial stage of frost growth were studied to find an effective method of restraining frost growth.The mechanism of restraining frost growth by low energy surface(bigger contact angle) was analyzed based on crystal growth theory.Then,the phase change of water vapor and the process of frost growth on the copper and wax energy surfaces were observed using microscope.The results indicate that it is difficult for wax surface(low energy surface),on which there are still water droplets at 100 s,to form critical embryo,so frost growth can be restrained in a way.Water formation,droplet growth,ice formation and dendritic ice growth processes happen on both surfaces,ordinally.But the ice beads,with larger average diameter and sparse distribution on the wax surface,form later(at about 300 s) than that on the copper surface,and the dendritic ice also appears later.All of these support that ice crystal formation and dendritic crystal growth at initial stage of frost growth can be retarded on the low energy surface.
The influence of inlet temperature of cooling water on the solidification of paraffin outside the coil tube was investigated.Numerical equations were set up to calculate the heat transfer inside the heat exchanger.The calculation of temperature fields of water and paraffin agreed well with experimental results.The temperature fields of paraffin near to the tube were different with different inlet temperatures of water,while the temperature fields far away from the tube were almost the same.The distribution of dimensionless temperature fields of paraffin was similar at different inlet temperatures of water,except for the region near to the phase interface.The interface moved faster at a lower inlet temperature of water.The moving rate of the interface slowed down at first,and became faster again when the interface was discontinuous.The results can provide the guidance for choosing the right inlet temperature of water for an appropriate solidification rate of paraffin.
