## 4 Conclusions  
In this paper, the effect of working pressure around the cooling surface on frosting is investigated through a frosting visualization test rig. The frost crystal morphologies and frost layer characteristics under different working pressures around the cooling surface are studied with partial pressure of water vapor, temperature of humid air, and temperature of cooling surface. The main conclusions are as follows.  
1. The vertical growth of frost crystals and the lateral distribution of frost crystals are strongly influenced by working pressure around the cooling surface. In a lower pressure environment, the frost crystals grow faster in the vertical direction and are more densely distributed in the lateral direction. The lateral growth habits of frost crystal are a primary function of air temperature and partial pressure of water vapor.
2. The frost crystal morphologies are more various with the variation of influencing parameters in a lower pressure environment. Frost crystals transform from needle to feather shape with the rising partial pressure of water vapor. A feather-columnar-needle shape transition of frost crystals is presented with hotter air temperature. In addition, the needle frost crystals lose their sharp point and become a clear columnar with a higher temperature of the cooling surface. However, under the higher pressure condition, only insignificant frost crystal morphologies shift between the needle and columnar shape with the variation of other parameters.
3. Under low working pressure around the cooling surface, the absolute differences of frost thickness, frost accumulation and density with different environmental parameters are larger than those at high pressures. Under different working pressures around the cooling surface, the dominating factor in the growth of the frost layer is presented by the partial pressure of water vapor. The dimensionless correlation equations of the frost thickness within a certain range on the temperature of humid air, temperature of cooling surface, partial pressure of water vapor, and working pressure around the cooling surface are proposed. The overall relative error is within \(\pm\) 25%.  
Conceptualization, T.L. and X.L.; methodology, T.L. and X.L.; validation, X.L. and X.D.; formal analysis, X.L. and X.D.; investigation, X.L., M.Q. and S.Y.; resources, M.Q. and S.Y.; data curation, S.Y. and X.L.; writing--original draft preparation, X.L.; writing--review and editing, T.L., X.L. and Y.H.; visualization, T.L. and X.L.; supervision, T.L. and Y.H.; project administration, T.L.; funding acquisition, T.L. All authors have read and agreed to the published version of the manuscript.  
Figure 14: Comparison of test and correlated data for dimensionless frost thickness.  
**Funding:** This project is supported by the National Natural Science Foundation of China (51976150), the Fundamental Research Funds for the Central Universities, and the Youth Innovation Team of Shaanxi Universities.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** The study did not involve humans.  
**Conflicts of Interest:** The authors declare no conflict of interest.