Experimental investigation of frost and defrost performance of microchannel heat exchangers for heat pump systems

The cycle frosting and defrosting performance of two types of microchannel heat exchangers were investigated. All the processes were observed using a CCD camera to better understand the cycle frost mechanism. Ice blockage formed in the fin root gaps of the horizontal-tube sample because of water retention. Cycle operation increased the blockage severity until the fin space was completely blocked. The amount of water retained and its impact on frosting time, pressure drop, and capacity were investigated. With increasing water retention, frosting time decreased, air pressure-drop and capacity could not return to the initial value after each defrosting time. Approximately 800 g of water was retained on the heat exchanger after four operating cycles, causing the ice blockage that shortened the effective operating time by 40 min compared with that of the vertical-tube sample at the end of the test. At the beginning of the fifth frost cycle, air pressure-drop had reached thrice the initial pressure drop, even when no frost was on the surface. The capacity decreased by 27% compared with the initial value. However, the vertical-tube sample exhibited no obvious water retention on the surface; as such, pressure drop and capacity experienced a similar degradation process during each cycle. The distribution of ice crystals on the fin surface was also studied, and the frosting process was divided into three periods: initial, developing, and fully grown. With increasingly serious water retention, frost only formed at the fin front-end surface, and could only reach the initial period because the ice blockage rapidly increased the pressure drop, thereby causing the defrosting process.

► Cycle frosting performances of two microchannel heat exchangers were studied.
► The vertical-tube sample with less water retention had better performance.
► Water retention impact on frosting time and performance was studied quantitatively.
► Growing of ice crystals had three periods, observed by image processing method.
► The impact of water retention on the frost crystal formation was analyzed.