Abstract

As we seek to address the escalating demand for energy and water, superhydrophobic surfaces have gained the attention of researchers due to their potential to boost condensation heat transfer rates and improve efficiency across various applications. One way to significantly improve dropwise condensation heat transfer rate is to improve drop mobility. Water-repelling properties of superhydrophobic surfaces can help in reducing the size of condensed drops as they depart the surface, thus increasing drop refresh rates by increasing drop mobility. Accurate models of retention forces between drops and superhydrophobic surfaces are needed for predicting drop dynamics including drop mobility. Once drop dynamics are known, models for drop-size distribution and heat transfer to a single drop can be used to predict overall heat transfer rates. Therefore, an understanding of drop retention forces, drop-size distribution and single drop heat transfer are crucial for modeling heat transfer rates during dropwise condensation on SH surfaces. This work addresses these topics in an effort to understand and influence heat transfer rates at SH surfaces. First, contact angle distribution and base area shapes of various drop sizes were experimentally measured across a wide range of solid fractions on inclined microstructured SH surfaces. Results indicate that the base area shape can be approximated using two ellipses with different aspect ratios, and the contact angle distribution is best described by a sigmoid function with physically relevant parameters. Measured contact angle distribution and base area shape were used to calculate the retention force for a range of microstructure solid fractions. Second, this research demonstrates the effectiveness of using nanostructured and two-tiered superhydrophobic surfaces for improving drop mobility. Enhanced drop mobility is directly linked to enhanced heat transfer rates as it helps to reduce the thermal resistance for heat transfer from the environment to the surface. Surfaces with tunable geometry are investigated to study the effect of factors like surface pitch, solid fraction and feature size on drop mobility. The behavior and size distribution of condensate drops are shown to depend on the interstitial gap of the surface feature. Experimental data on drop-size distributions for surfaces experiencing coalescence-induced jumping are obtained. Finally, factors such as nucleation site density, maximum drop size, heat transfer to individual drops, and surface feature characteristics are examined to study their effect on the overall heat transfer rate in the third part of this research. Studies include the development of a modified single drop heat transfer model for two-tiered surfaces along with predicted drop-size distribution to determine overall heat transfer rates in a shear flow environment. Predicted heat transfer rates are then compared with measured heat transfer rates in channel flow. Results show that heat flux values are approximately 10 times larger on nanostructured and two-tiered SH surfaces as compared to smooth hydrophobic and microstructured SH surfaces.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

https://lib.byu.edu/about/copyright/