Experimental Methods for the Analysis of Frost Nucleation and Frost Growth on Hydrophilic and Hydrophobic Coated Flat Plate Surfaces in Forced Convection Channel Flows

Abstract

An experimental system was designed and built to explore the effect of coated surfaces on frost formation. The experimental system consists of a wind tunnel test apparatus, dew point sensors, thermocouples for temperature measurements, pressure sensors and other devices. The experimental system is equipped with a computerized data acquisition and storage system. The temperature and humidity of the experimental system is computer controlled. The system is suitable to study the condensation and frosting effects on various coated and uncoated surfaces.

The formation of frost and ice decreases the energy efficiency of refrigeration systems and of air-source heat pump systems. This work presents experimental data of frost nucleation and frost growth on cold flat plates operating in frosting conditions with air forced convective flow. The plates surfaces had different wettability: from an uncoated aluminum surface with fine finish roughness and contact angle of 75°, to hydrophobic (θ ≈ 110-116°) and hydrophilic (θ ≈ 19-29°) coatings. An experimental technique was developed in order to mimic actual field type operating conditions of convective channel flow experienced by fin structures of heat exchangers of typical air-source heat pump systems. The surface wettability characteristics affected the elapsed time and the initial thickness of the frost before the droplets froze on the test plates. They also affected the starting point for the phase changeover from crystal growth to frost growth. The hydrophobic surface had early phase changeovers and high thresholds of the frost thickness before switching to the frost growth phase due to the presence of large droplets on the surfaces before they froze into ice beads. Fine-finished aluminum surface and hydrophilic coating had delayed phase changeovers and low frost thicknesses with respect to the other surfaces.

During pull-down tests, nucleation on the hydrophilic surface occurred quickly, leaving less time for disc-like shaped water droplets to continue to grow in the radial and vertical directions before freezing. Frost thicknesses followed “S” shaped like profiles due to transitions of droplets to ice beads, to crystals growth, and to initial frost accretion on the top of the ice beads. The wettability characteristics of the surfaces affected the elapsed time at which these changeovers occurred and the thresholds of the frost thickness when switching to the frost growth phase.