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Second Thermal and Fluids Engineering  Conference

ISSN: 2379-1748
ISBN: 978-1-56700-430-4

ROUGHNESS EFFECT ON POOL BOILING HEAT TRANSFER OF WATER ON HYDROPHOBIC SURFACES

Jinsub Kim
Extreme Mechanical Engineering Research Division, Korea Institute of Machinery and Materials, Daejeon, 34103, Korea

Seongchul Jun
Mechanical Engineering Department, University of Texas at Dallas, 800 W. Campbell Rd., Richardson, Texas 75080

Jung Ho Lee
Extreme Mechanical Engineering Research Division, Korea Institute of Machinery and Materials, Daejeon, 34103, Korea

Seung-Moon You
Mechanical Engineering Department, University of Texas at Dallas, 800 W. Campbell Rd., Richardson, Texas 75080

Abstract

The present study was aimed to investigate the effect of surface roughness on pool boiling of water on hydrophobic surfaces. The copper surfaces were polished with sandpapers to have different surface roughness ranging from Ra = 0.052 μm to Ra = 1.54 μm, and coated with polytetrafluoroethylene (PTFE) thin films with the thickness of approximately 50 nm. Static and dynamic contact angles both parallel and normal to the scratches were measured for all surfaces tested and the averaged static contact angles varied from 122° (at the smooth surface) to 133° (at the roughest surface). At low heat flux (below 10 kW/m2), rough surfaces (Ra > 1 μm) showed larger number of active nucleation sites and better boiling performance, which resulted from the combination of increased number of cavities and higher contact angles. However, as heat flux increases, rough surfaces exhibited deterioration of boiling performance due to partial dryout, resulting in the earlier transition to the film boiling compared with smooth surfaces (Ra < 1 μm). As a result, smooth surfaces showed better boiling performance than rough surfaces at the high heat flux above 20 kW/m2.

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