This dissertation focuses on various aspects of frost growth on louvered folded fins in outdoor microchannel heat exchangers used in air source heat pump systems. In this research, the effects of surface temperature, fin geometries, surface coatings and environmental parameters such as air humidity and velocity on the thermal and hydraulic performance of the microchannel heat exchangers under frosting condition were investigated. The overall aim was to provide some guidelines about frost behavior of microchannel heat exchangers and isolate and quantify the effect of geometry, surface coatings and surface temperature. A set of empirical correlations were developed to predict frost thickness, air face velocity drop and heat transfer coefficients for various fin geometries. The approach taken was to perform laboratory experiments on small scale coils and sample fins that were cut out from commercially available heat exchangers and to replicate their operating conditions in laboratory. A good estimation of the fin surface temperature was achieved with the methodology developed in the present study. Experimental data of frost weight, local frost thickness, air pressure drop across the coils, time of frost-defrost cycles and heat transfer rates were recorded for heat exchangers operating in actual transient frosting conditions. Data showed that the frosting time and the frost growth rates depend mainly on the local fin surface temperature while water retention and surface coatings could have secondary and minor effects. Some geometries performed better in frosting condition than others such as geometries with lower fin density that delayed the blockage of the air flow. The fin length and fin depth had minor effects on frosting performance. It was found that air humidity has a significant effect on rate of frost formation while air velocity seemed to have a small effect on frost formation.Mechanical & Aerospace Engineerin
