Heat and mass transfer during the melting process of a porous frost layer on a vertical surface

Abstract

An important problem in the refrigeration industry is the formation and removal of frost layers on sub-freezing air coolers. The frost layer, a porous structure of ice and air, directly diminishes the performance and efficiency of the entire cooling system by presenting resistances to air flow and heat transfer in the air cooler. To return the system to pre-frosted performance the layer must be removed through a defrost cycle. The most common defrost cycle uses heat applied at the heat exchanger surfaces to melt the frost. Current methods of defrosting are inherently inefficient, with the majority of the heat being lost to the surrounding environment. Most studies have concentrated on the formation of the frost layer, and not the melting phenomena during the defrost cycle. In this study, direct measurements and a fundamental model to describe the melting process of a frost layer on a vertical heated surface are presented. The experimental facility provides the first direct measurements of heat and mass transfer during defrost. The measurements confirmed the multistage nature of defrost. The multistage model characterized the different thermal and mass transport processes that dominate each stage. The first stage is dominated by sensible heating of the frost layer. Both the experiment and model showed that heat and mass transfer through sublimation during the initial stages are insignificant, accounting for less than 1% of the total energy transfer. The second stage of defrost is dominated by the melting of the frost layer. The melt rate model generally predicts the front velocity within 25% of the velocity determined using the digital image analysis technique. Higher heat transfer rates resulted in faster melt velocity, and thus shortened defrost times. Evaporation of the melt liquid from the surface dominates the final stage. The heat transfer model for this stage predicts the heat transfer coefficient within ±25% of the experiment. The overall defrost efficiency was found to be primarily dependent on the initial frost thickness, with thicker layer having less heat lost to the ambient space and a higher efficiency.