Experimental and Analytical Investigation of Early Frost Growth on Surfaces with Varying Wettability under Typical Heat Pump Operating Conditions

Abstract

Frost formation on the fins of the outdoor evaporators of heat pump systems during winter months is a naturally occurring phenomenon with adverse effects on heat pump operation. The frost impedes heat transfer by adding an extra layer of thermal resistance to the fins, and it increases the air-side pressure drop by blocking the airflow through the fins. Over the past few decades, researchers have investigated using surface coatings with different wettability to inhibit frost growth passively. Some of these studies showed promising results, yet other studies' results were inconclusive or even contradictory. At environmental conditions typical of heat pump operation, frost formation generally occurs in three distinct steps: liquid droplet growth and freezing, crystal growth, and frost layer growth. Surface wettability, often expressed by the contact angle of water on the surface, primarily affects the first step in the process: droplet growth and freezing. This research aims to enhance the understanding of surface wettability's effects on frost growth, emphasizing the first two stages of the frosting process. Experiments were performed to investigate early frost growth behavior on a bare aluminum surface, a hydrophobic surface, and a hydrophilic surface. Droplet freezing time, frozen droplet diameter, droplet surface area coverage and distribution, droplet coalescence rate, frost layer thickness, and frost layer density were compared for several sets of test conditions on all three surfaces. It was observed that both surface contact angle and test conditions had significant effects on droplet geometry at the time of freezing. It was also observed that environmental parameters governed droplet freezing time at typical heat pump operating conditions, while the contact angle effects were inconclusive. A semi-empirical model was developed and implemented in a simulation tool to describe frost growth behavior on various surface wettability types. The model is a multi-stage model and includes all three steps of the droplet growth process. The model’s droplet growth and crystal growth stages were developed as part of this research, then were coupled to a previously-existing frost layer growth model. It was initially developed to predict average frost properties on flat plates in convective airflow. However, it was also extended to predict the frost layer's changing behavior in the direction of airflow. It also accounted for multiple wettability types on the same surface and was extended for use in parallel plate channels, which mimic folded-flat fins of heat exchangers. This model's principles can be extended to many other surface geometries. They could be useful in investigating early frost growth for heat exchanger fin configurations where visualization and measurement of frost properties are difficult.