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research-article

Mathematical model for dropwise condensation on a surface with wettability gradient

[+] Author and Article Information
Manjinder Singh

Department of Mechanical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India-110016
mez138469@iitd.ac.in

Sasidhar Kondaraju

School of Mechanical Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar, Orrisa, India-751013
sasidhar@iitbbs.ac.in

Supreet Singh Bahga

Department of Mechanical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India-110016
bahga@mech.iitd.ac.in

1Corresponding author.

ASME doi:10.1115/1.4039014 History: Received May 05, 2017; Revised January 04, 2018

Abstract

We present a mathematical model for dropwise condensation heat transfer on a surface with wettability gradient. We adapt well-established population balance model for dropwise condensation on inclined surfaces to model dropwise condensation on a surface with wettability gradient. In particular, our model takes into account the effect of wettability gradient and energy released during drop coalescence to determine the drop departure size. We validate our model with published experimental data of dropwise condensation heat flux and drop size distribution. Based on various experimental studies on drop motion, we also propose a mechanism that explains how the energy released during drop coalescence on a surface with wettability gradient and in a condensation environment aids drop motion. The mechanism correctly explains the shift of center of mass of two coalescing drops on a surface with wettability gradient towards the drop on high wetting region. Using the model, we analyze the effect of wettability gradient on the dropwise condensation heat flux. Our model predictions show that the optimal choice of wettability gradient is governed by differential variations in population density and heat transfer through a drop with change in wettability of the surface. We also demonstrate that contact angle at which there is maximum heat transfer through a drop varies with thickness of coating layer leading to change in optimal wettability gradient.

Copyright (c) 2018 by ASME
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