Study of onset of frost and crystals growth on hydrophilic and hydrophobic flat surfaces

Abstract

Air-source heat pump systems extract heat directly from the cold outdoor ambient and reject heat to the warm indoor environments of residential and commercial buildings. During their winter operation, the outdoor coil often accumulates frost on its surface. Frost acts as an insulator and blocks air passages, reducing the heat transfer rate and increasing the pressure drop of air passing through the coil. Defrost cycles are periodically executed in between the heating times to melt the ice, drain the water from the outdoor coil, and free accumulated frost before the heating service can start again. Unfortunately, too many defrost cycles penalize the efficiency of the heat pumps. Currently, most research in frost mitigation focuses on superhydrophobic surfaces, lubricant impregnated surfaces, and nanostructured surfaces. Some studies proposed surface types that would lower ice adhesion such that droplets removal was promoted before freezing. However, the mitigation effects of these surfaces can be sensitive to experimental conditions and surface structure. Additionally, in circumstances where frost formation cannot be prevented due to the operating conditions, the challenge of predicting frost nucleation and growth rate is further complicated by transient flow conditions with combined heat and mass transfer phenomena to moving frost boundaries. This dissertation presents new data of freezing time, droplet diameter, and droplet shape with different surface wettability during initial droplet icing. Water condensation and icing formed on the flat plates for convective channel flows. Four surfaces with different wettability were investigated under two test conditions. The contact angle ranged from less than 10 degree (i.e., superhydrophilic) to over 109 degree (i.e., hydrophobic). Two surfaces shared similar contact angle but had different coating components. Because frost nucleation was partially a stochastic phenomenon subjected to many variables that were difficult to control and replicate even in a laboratory setting, frost tests with identical environmental and surface temperature conditions were repeated several times in order to gather meaningful averages for the freezing time and to quantify the magnitude of potential variability in the frost nucleation time and droplets size due to the surface wettability characteristics. The new data presented in this paper are used to inform and validate physics based frost models, which can predict the nucleation features and actual frost formation time for coated fin structures of heat exchangers. With the continually repeat experiment test for each test plate, the freezing time is not always consistent at the same test condition. Contamination particles suspended in the air and trapped on the top surface of the test plate could increase the uncertainty of the actual freezing time. Investigating the inside energy change and phase change phenomenon of single water droplets could explain the experiment results. Combining the contamination particles with the Classic Nucleation Theory (CNT) will be used to predict the freezing time in this dissertation. The effects of contamination particles for changing the critical Gibbs energy and critical ice embryo size were discussed in this model section. Contamination particles accelerate the freezing process by enlarge the critical radius of ice embryo in the water droplet. Comparing the inconsistent freezing time results from the experiment data, the model would theoretically explain the phenomenon and give a way for onset of freezing time prediction with hydrophilic and hydrophobic flat surfaces.