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Effect of Wettability on Pool Boiling Incipience in Saturated Water

[+] Author and Article Information
Jinsub Kim

Multi-Scale Heat Transfer Lab, Department of Mechanical Engineering, University of Texas at Dallas, Richardson, TX 75080, USA
jxk150430@utdallas.edu

Seongchul Jun

Multi-Scale Heat Transfer Lab, Department of Mechanical Engineering, University of Texas at Dallas, Richardson, TX 75080, USA
sxj142030@utdallas.edu

Jungho Lee

Dept. of Extreme Thermal Systems, Korea Institute of Machinery and Materials, Daejeon, Korea
jungho@kimm.re.kr

Seong Hyuk Lee

School of Mechanical Engineering, Chung-Ang University, Seoul, Korea
shlee89@cau.ac.kr

Seung M. You

Multi-Scale Heat Transfer Lab, Department of Mechanical Engineering, University of Texas at Dallas, Richardson, TX 75080, USA
you@utdallas.edu

1Corresponding author.

J. Heat Transfer 138(8), 080910 (Jul 08, 2016) (1 page) Paper No: HT-16-1197; doi: 10.1115/1.4033815 History: Received April 13, 2016; Revised June 02, 2016

Abstract

Three different copper surfaces - bare, Al2O3 nano-coated, and Polytetrafluoroethylene (PTFE) coated - are prepared and tested to examine the effect of wettability on the pool boiling incipience in saturated water at 1 atm. A copper surface is coated with Al2O3 particles ranging 25~43 nm in diameter by immersing the surface in Al2O3/ethanol nanofluid (1g/l) and boiled for 3 min. SEM image in Fig. 1 shows the coated Al2O3 nanoparticles on the copper surface, together with the reference bare surface. PTFE coating is also applied to the copper surface using spin coating method with the mixture of Dupont AF 2400 particles and 3M FC-40 solvent. The final coating thickness of the PTFE coating is estimated to be 30 nm. The three surfaces exhibit different static contact angles, 78° (bare), 28° (nano-coated), and 120° (PTFE coated) in Fig. 2, respectively. Wettability affects the boiling incipience heat flux where initial bubble nucleation starts: 15 kW/m2 for the bare surface; 30 kW/m2 for the nano-coated surface; and 2.5 kW/m2 for the PTFE coated surface. Captured images from the high speed camera at 2,000 fps show significantly different bubble shapes and departure frequencies in Fig. 3. During the bubble growth, advancing contact angles are captured and shown qualitatively and found consistent with their static angle measurements for the sessile droplet observed at each surface. The larger bubble is generated on the nano-coated surface compared to that of the bare surface because improved wetting makes promising cavities flood and thus incipience is delayed, resulting in higher superheat. The single bubble life cycle appears to be much longer on the PTFE coated surface due to the increase of the contact angle which becomes hydrophobic (> 90°), resulting in lower bubble departure frequency. Successive tests at the same heat flux of 30 kW/m2 confirmed that life cycle on the PTFE coated surface (88.5 ms) is consistently longer than that on the bare surface (16.5 ms) and nano-coated surface (20 ms).

Copyright © 2016 by ASME
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