The characteristics of frost growth on parallel plates

Abstract

An experimental investigation was undertaken to characterize the effect of environmental conditions on frost growth on a vertical plate in a parallel flow geometry. Humid air was conditioned to have a dew point below 0°C and laminar flow prevailed. The test section was fabricated by using three cooling plates with individual insulators to minimize longitudinal conduction. It is known that frost formation on the heat exchanger surfaces seriously affects the performance of the system. The frost is dominantly formed in the inlet region of a heat exchanger. In order to understand the characteristics of frost growth in the entrance region, several experiments were carried out. The experimental parameters were plate temperature, air humidity, air temperature, air Reynolds number, location, and uncooled inlet length. The frosting conditions were limited to air temperatures from 5 to 15°C, air Reynolds numbers from 1600 to 2270, air humidity ratios from 0.00275 to 0.0037 kg _w /kg _a , and plate temperatures from −10 to −20°C. Frost growth toward the front of the plate was thicker and denser than toward the rear. In the low humidity conditions below 0°C dew point frost growth increased with decreasing plate temperature and increasing humidity. For laminar flow, the dew point below 0°C and non-cyclic frosting period, the frost thickness increased with increasing air temperature. This behavior can be explained by an increase of the transfer rate and a non-cyclic frosting without melting in a sublimation-ablimation process. The average growth thickness at three locations showed little dependence on the Reynolds numbers. However, there were only small differences in the front and rear plates. Frost thickness decreased with increasing uncooled inlet length. This result could be used to suppress frost growth in the plate finned tube heat exchanger.