Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Abstract
Factors affecting frost distribution are explored using a finite element model, developed and validated using
a full-scale 8-row heat exchanger in a wind tunnel. The heat exchanger is typical of the type used in supermarket
display 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 designed
to have a simple geometry and single-phase refrigerant to provide maximum certainty on parameters not directly
related 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 nonuniformities
induced by the experimental apparatus. The simulation model was used to illustrate how the point of
maximum frost thickness moved from the front to the rear of the heat exchanger, depending on face velocity, inlet
humidity 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 to
simulate the performance of the heat exchanger under the effect of a fan curve. A comparison is made between DX
and indirect refrigeration system performance with respect to capacity, pressure drop and air flow variations under
frosting conditions.Air Conditioning and Refrigeration Project 10
