Thermal-hydraulic and drainage performance of microchannel heat exchangers for refrigeration and heat pump systems under frosting, defrosting, and wet conditions: A comprehensive review

Currently, microchannel heat exchangers (MCHXs) are widely employed in refrigeration and heat pump systems due to their advantages of being more efficient, smaller in size, lower refrigerant charge, and cheaper price. However, when used as evaporators, MCHXs have the issues of a faster frosting rate and frost melt water, condensate that is more difficult to remove than fin-and-tube heat exchangers. This review paper focuses on the research progress on the thermal-hydraulic and drainage performance of MCHX under frosting, defrosting, and wet conditions from the year 2000-2022. First, experimental research on the frosting and drainage characteristics of MCHX is summarized and analyzed in periodic frosting and defrosting conditions. Then, three typical methods for improving the frosting and defrosting performance of MCHX are introduced. Moreover, empirical correlations and simulation models developed to predict the frosting and defrosting characteristics of MCHX are discussed. Uneven frost layer distribution should be considered in the frosting model and the effect of drainage characteristics on defrosting perfor-mance should be considered in the defrosting model. The thermal-hydraulic and drainage per-formance of MCHX under wet conditions is also reviewed. First, methods for testing real-time condensate retention are introduced. Then, the parameters affecting the thermal-hydraulic and drainage characteristics of the MCHX are analyzed. Based on the above research, two methods for improving the performance of MCHX under wet conditions are introduced. Empirical correlations and numerical models for predicting thermal-hydraulic performance under wet conditions are classified and compared. A novel one-dimensional numerical model based on the fin theory combined with the moving boundary technique can accurately capture the actual dehumidifi-cation state with a small computational cost. The main conclusions regarding the thermal -hydraulic and drainage performance of MCHX based on the open literature are presented. Finally, some research initiatives and vital improvement measures are suggested for future studies. This review aims to serve as a reference guide for the thermal-hydraulic and drainage characteristics studies of MCHX under frosting, defrosting, and wet conditions.