Simulating the Performance of a Heat Exchanger During Frosting

摘要

Factors affecting frost distribution are explored using a finite element model, developed and validated usinga full-scale 8-row heat exchanger in a wind tunnel. The heat exchanger is typical of the type used in supermarketdisplay cases; so face velocities and air inlet temperatures were varied from 0.5-2.3 m/s and 0 to -20 ??C,respectively, and inlet humidities from 70-90%. In order to focus on frost distribution, the prototype was designedto have a simple geometry and single-phase refrigerant to provide maximum certainty on parameters not directlyrelated to frost. Measured and predicted total and sensible heat transfer agreed within RMS 6% and 8%,respectively, over the range of operating conditions. For latent heat, there was more scatter due to frost nonuniformitiesinduced by the experimental apparatus. The simulation model was used to illustrate how the point ofmaximum frost thickness moved from the front to the rear of the heat exchanger, depending on face velocity, inlethumidity and fin surface temperature. Heat transfer and pressure drop were calculated from standard correlations,with fin thickness and tube diameter increasing as a function of frost thickness. The model was further extended tosimulate the performance of the heat exchanger under the effect of a fan curve. A comparison is made between DXand indirect refrigeration system performance with respect to capacity, pressure drop and air flow variations underfrosting conditions.