Effect of Functional Surfaces with Gradient Mixed Wettability on Flow Boiling in a High Aspect Ratio Microchannel

Vahid Ebrahimpour Ahmadi,Ali Koşar,Khellil Sefiane,Abdolali Khalili Sadaghiani,Akam Aboubakri

doi:10.3390/fluids5040239

Vahid Ebrahimpour Ahmadi, Ali Koşar + Show 3 more

Open Access

https://doi.org/10.3390/fluids5040239

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Journal: Fluids	Publication Date: Dec 10, 2020
Citations: 18	License type: CC BY 4.0

Affiliation: Sabancı University, University of Edinburgh

Abstract

Flow boiling is one of the most effective phase-change heat transfer mechanisms and is strongly dependent on surface properties. The surface wettability is a crucial parameter, which has a considerable effect on the heat transfer performance, particularly in flow boiling. The contact angle determines the number of nucleation sites as well as bubble dynamics and flow patterns. This study introduces three new generation mixed wettability surfaces and compares them with a wholly hydrophobic surface reference sample, in flow boiling in a high aspect ratio microchannel. The mixed wettability substrates have five regions as fully Al2O3, (hydrophobic zone) region, three different patterned configurations with various A* values, and fully SiO2 (hydrophilic zone) region, where A* is defined as A Al2O3/A total (hydrophobicity ratio). Boiling heat transfer results were obtained for each surface at various wall heat fluxes and three different mass fluxes. According to the obtained results, significant enhancements in heat transfer (by up to 56.7%) could be obtained with biphilic surfaces compared to the reference sample (hydrophobic surface). Performed flow visualization proves that the tested biphilic surfaces enhance heat transfer by reducing the bubbly flow regime and extending the slug regime.

Highlights

Along with rapid developments in emerging technologies, such as high-power electronic devices and power generation systems, the increasing trend in generated heat has become a noticeable issue in such systems
Pool boiling curve shifted to larger wall superheats, which suggested decreased critical heat flux (CHF). These findings proved that the results drastically changed with surface wettability
De-ionized water was used as the working fluid in flow boiling experiments

Summary

Introduction

Along with rapid developments in emerging technologies, such as high-power electronic devices and power generation systems, the increasing trend in generated heat has become a noticeable issue in such systems. An efficient heat dissipation system can enhance the safety, performance and reliability of miniature systems involving high power density [1]. Major cooling techniques in thermal management are natural convection [2], forced convection utilizing mechanical air-handling equipment [3,4], or convection with phase change phenomena [1,5,6,7,8,9]. Boiling heat transfer is an effective cooling mechanism, where a considerable amount of heat can be dissipated through heating and vaporization (phase change process) of the coolant. Boiling heat transfer including both pool and flow boiling offers high heat transfer coefficients (HTC), which are caused by sensible and latent heating of the coolant [6].

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