Date of Award

Summer 2019

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Jamil Khan

Abstract

The primary focus of this research is to provide a validated model for a comprehensive understanding of hydrophobic-­‐hydrophilic condensation on patterned-­‐ hybrid surface. Establishing the model requires the modeling of fully dropwise condensation (DWC) before applying modifications to evaluate heat transfer performance of patterned-­‐hybrid condensation surface.

The model for fully DWC consists of defining expressions for heat transfer through a single drop, maximum radius of the drop and drop-­‐size distribution which all are primarily based on the work in the literature. In this work the author utilized a proposed modified version of the simulation for drop-­‐size distribution for fully DWC, the proposed drop-­‐size distribution is capable of defining population density as a function of the contact angle, hydrophobic region width and hydrophilic region width. The new expression is derived from simulating the growth process of drops by direct condensation and coalescence for various hydrophobic and hydrophilic region widths as well as for various contact angles. Evaluating the overall heat transfer performance for the proposed model involves modeling of hydrophobic and hydrophilic regions in addition to the interaction between the regions due to merging drops to the hydrophilic region. Results for fully dropwise condensation shows that heat flux increases with total temperature difference. In addition, an optimum contact angle that corresponds to the maximum heat flux exists at approximately 132 degrees. In this study a parametric analysis is performed on hydrophobic-­‐hydrophilic hybrid condensation surface that shows the influence of hydrophobic region width, hydrophobic contact angle, hydrophilic region width and total temperature difference. Results show that the integrated hydrophobic-­‐hydrophilic surface performs better and has the advantage over fully DWC for certain cases. In general, decreasing hydrophilic region width shows a better enhancement factor, which is defined as the ratio of hybrid heat flux to that for fully DWC. Furthermore, enhancement factor of greater than unity exists when hydrophobic region contact angle is less than approximately 120 degrees, depending on hydrophilic region width. In addition, with respect to hydrophobic region width, an optimum condition exists for given total temperature difference and hydrophilic region width. Overall, hybrid surface heat flux there is greater enhanced for relatively lower temperature difference, lower hydrophobic contact angle and lower hydrophilic region width at the optimized hydrophobic region width.

The proposed model utilizes two correlations, derived from simulation work, to define a modified drop-­‐size distribution with respect to drop-­‐maximum-­‐radius-­‐to-­‐region-­‐ width ratio. When the ratio is less than unity, it implies that sweeping and merging of drops are the main mechanism for surface renewal. For ratio equal to unity, surface renewal is achieved by merging only. The model accepts a range for hydrophobic and hydrophilic region width and contact angle for total temperature difference range as inputs. It provides results for hybrid surface heat flux and evaluates heat transfer performance for input data relative to fully dropwise condensation surface. The proposed model can be used to design optimum condenser surfaces that can be used in a variety of industrial applications, such as condensers in Rankine cycle power plant, an condensers for vapor-­‐compression refrigeration cycle.

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