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Droplet Heat Transfer on Micro-Post Arrays with Hydrophobic and Hydrophilic Characteristics

[+] Author and Article Information
Abdullah Al-Sharafi

Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
alsharafi@kfupm.edu.sa

Bekir Sami Yilbas

Department of Mechanical Engineering & Centre of Excellence for Renewable Energy, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
bsyilbas@kfupm.edu.sa

Haider Ali

Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
haiali@kfupm.edu.sa

1Corresponding author.

ASME doi:10.1115/1.4039013 History: Received April 17, 2017; Revised January 03, 2018

Abstract

Heat transfer analysis for a water droplet on micro-post arrays is carried out while mimicking the environmental conditions. Since the micro-post arrays spacing size alters the state of the hydrophilicity of the surface, the size of the micro-post arrays spacing is varied and the resulting heat transfer characteristics are analyzed. Spreading rate of water droplet on the micro-post arrays is considered and the adhesion force for the pinning of the water droplet on the micro-post arrays is presented. Temperature and flow fields are predicted and the predictions of flow velocity inside the water droplet are validated through the particle image velocimetry (PIV). The Nusselt number variation for various sizes of the micro-post arrays is obtained for two droplet volumes. It is found that reducing the solid fraction of micro-post array beyond ?s = 0.25, the Cassie & Baxter state of the surface changes to the Wenzel state; in which case, hydrophobic characteristics changes to hydrophilic characteristics for the water droplet. Heat transfer from the droplet bottom gives rise to development of the buoyancy and the Marangoni currents, which in turn generate two counter rotating circulation cells. The center of circulation cells moves further in the droplet upper part for the hydrophobic droplet case. The Nusselt number attains high values for the hydrophobic droplet at micro-post array spacing size b = 10 ?m and hydrophobic droplet at spacing size b = 50 ?m due to fin effects of the micro-post arrays.

Copyright (c) 2018 by ASME
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