Formação de geada em superfícies frias

Abstract

Frost formation is one of the largest problems in some types of refrigeration equipment and heat pumps and occurs by the progressive deposition and ice accumulation on surfaces of heat exchangers when their surface is below zero, leading to significant problems in operation of evaporators. Many of the existing researches concentrate on understanding the characteristics of the frost layer and their growth on flat plates. The objective of this work was to predict the behavior of frost from a physical model formed by a system of nonlinear partial differential equations that are solved in a coupled manner. The boundary is considered mobile in order to represent the growth of the frost in each time of evaluation. The frost layer is treated as a porous medium and the local volume averaging technique was used for resolution. It was identified that the best formulation for the Nusselt number is the one considering the frost roughness, representing the heat transfer by convection in a better way. Evaluating the effective diffusivity expression, it was observed the extreme dependence of the assumed diffusion resistance factor value in relation to the thickness and density of frost, and the choice had great importance for validation of simulation with the experimental data found on literature. The assumed initial thickness had influence from certain value and the initial density influenced in a proportional way as it was increased.