Theoretical study on the critical heat and mass transfer characteristics of a frosting tube

A theoretical study was conducted to investigate the critical heat and mass transfer characteristics on a frosting tube. The parameters, Critical Frost Thickness (CFT) and Neutral Frost Thickness (NFT) were presented for the first time. The heat transfer was found to increase before the point CFT, and decrease to the initial value of a non-frosted tube at the point NFT. The findings provide a reasonable explanation for the heat transfer increase for a fin-tube heat exchanger at the early stages of frost formation. By theoretical study, the values of CFT and NFT were impacted by the following five parameters, the air temperature Ta, humidity φ, velocity V, tube wall temperature Tw and tube outside diameter d2. The influences of these parameters on the CFT and NFT were further discussed. By sensitivity analysis, the CFT and NFT were found to be more sensitive to the parameters of φ, V and d2.

► Critical and Neutral Frost Thickness, CFT and NFT were presented for a frosting tube.
► CFT: Before which, the frost layer enhances the heat transfer rather than depress it.
► NFT: Before which, the frost layer does not execute negative impact on heat transfer.
► The values of CFT and NFT are concluded to be impacted by five parameters.
► By sensitivity analysis, the main factors to impact the CFT and NFT were discussed.