Heat and Mass Transfer of the Droplet Vacuum Freezing Process Based on the Diffusion-controlled Evaporation and Phase Transition Mechanism.

Zhijun Zhang,Jingxin Gao,Shiwei Zhang

doi:10.1038/srep35324

Abstract

A frozen phase transition model is developed to investigate the heat and mass transfer of a single water droplet during the vacuum freezing process. The model is based on the diffusion-controlled evaporation mechanism and phase transition characteristics. The droplet vacuum freezing process can be divided into three stages according to the droplet states and the time order. It includes the evaporation freezing stage, the isothermal freezing stage and the sublimation freezing stage. A numerical calculation is performed, and the result is analysed. The effects of the vacuum chamber pressure, initial droplet diameter and initial droplet temperature on the heat and mass transfer characteristics at each stage are studied. The droplet experiences supercooling breakdown at the end of the evaporation freezing stage before the isothermal freezing stage begins. The temperature is transiently raised as a result of the supercooling breakdown phenomenon, whose effects on the freezing process and freezing parameters are considered.

Highlights

At the droplet surface[9]
The effects of the vacuum chamber pressure, initial droplet temperature and initial droplet diameter on the freezing parameters during the droplet vacuum freezing process are studied by using the frozen phase transition model in this paper
The model is based on the diffusion-controlled evaporation mechanism and phase transition characteristics

Summary

OPEN Heat and Mass Transfer of the Droplet Vacuum Freezing Process

A frozen phase transition model is developed to investigate the heat and mass transfer of a single water droplet during the vacuum freezing process. The effects of the vacuum chamber pressure, initial droplet diameter and initial droplet temperature on the heat and mass transfer characteristics at each stage are studied. The behaviour of water droplets sprayed out of a snow gun was modelled to investigate the heat and mass transfer processes of water droplets during the phase transition from the supercooled liquid phase to the liquid–solid phase and to the solid phase. The improvement of the frozen phase transition model is that the influence of the transient temperature rise effect on the freezing process and freezing parameter change is considered

Results and Discussion

Conclusions

Pa Ta

The vapour pressure variation of the droplet surface is expressed by

The heat release for the phase transition is determined by

Additional Information