The effect of surface wettability on frost growth and densification on flat plates

Wettability;frost;hydrophobic;hydrophilic

El Cheikh, Amne

关键词: Wettability;    frost;    hydrophobic;    hydrophilic;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/46878/Amne_El%20Cheikh.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In addition to important factors like the surrounding air temperature and humidity, and surface temperature, frost growth and densification rates on cold, flat surfaces subject to forced convective heat and mass transfer depend on surface wettability and cooling rate during the condensation phase. If the cooling rate is high during the condensation phase, then the droplet distribution at incipient freezing is dominated by small droplets and a narrow size range. Conversely, if the cooling rate is low during the condensation phase, then the droplet distribution at incipient freezing follows established stationary distributions with a very wide range of droplet sizes. In the present study, the impact of the surface wettability is experimentally and analytically investigated for different cooling rates in the condensation phase. Aluminum substrates, with coatings that have undergone conversion by plasma-polymerization to alter their wettability, are used to create surfaces that range from completely wetting to hydrophobic, with advancing contact angles as high as 110o. The coated aluminum plates are then subjected to frosting conditions in wind tunnel experiments. The cooling rate during the condensation phase affects the frost thickness and density for hydrophilic and hydrophobic surfaces alike, with the low-cooling-rate condensation phase resulting in a thicker and denser frost layer—with differences exceeding 20% for thickness and density. When the frost growth is preceded by a high-cooling-rate condensation phase, the wettability effect on frost growth rate and densification is small. However, when frost growth is preceded by a low-cooling-rate condensation phase, a significantly denser frost layer grows on the hydrophilic surface. The impact of the surface wettability for a low-cooling-rate condensation phase remains significant even after 2 hours of frosting. Repeated cycles of defrost and re-frost show the same behavior.As part of the effort to more fully understand wettability effects on frost growth, a mathematical model is developed. The coupled heat and mass diffusion equations are solved numerically, with special attention directed toward the formulation of the initial and boundary conditions, and to modeling heat and mass transport within the frost layer. Unlike previous models reported in the literature, the current formulation does not erroneously assume saturation at the frost-air interface, nor does it require specification of the super-saturation ratio. Because it is extremely difficult to measure super-saturation at the frost-air interface, prescribing it as a boundary condition vitiates the utility of the model. In the current approach, the heat flux at the frost-substrate interface is specified to close the model. This approach results in a model with much more utility and flexibility. Initial conditions for frost density and thickness are obtained from the droplet distribution on the surface during the condensation phase, providing the first link between wettability and frost growth. The model assumes that the steady-state droplet distribution is achieved on the surface before freezing occurs; therefore, it is only applicable for a low-cooling-rate condensation phase. The model results for frost thickness and density are in good agreement with experimental data for surfaces with varying contact angles. This result makes it clear that the wettability effect on frost growth is primarily due to the distribution of condensate at incipient freezing. Interestingly, adapting the model to use initial conditions commonly adopted in the literature results in fairly good agreement with experiments conducted with a high-cooling-rate condensation phase. This result suggests that previous experiments have been predominately conducted with a high-cooling-rate condensation phase, obfuscating the effects of wettability.