Effects of Hydrophobic Surface Patterning on Boiling Heat Transfer and Critical Heat Flux of Water at Atmospheric Pressure

Abstract

The effects of hydrophilic/hydrophobic surface patterning on critical heat flux (CHF) and heat transfer coefficient (HTC) were studied using custom-engineered testing surfaces. Patterning was created over a sapphire substrate and tested in a pool boiling facility in MITs Reactor Hydraulics Laboratory. The hydrophilic and hydrophobic matrices were created using layer by layer deposition of 50 nm thick SiO2 nanoparticles and monolayer thickness fluorosilane, respectively. Ultraviolet ozone patterning was then used with chrome-printed masks to create the desired geometric features. Hexagon, ring, star, and mixed patterns were tested to determine their abilities to affect CHF and HTC through prevention of bubble pinning at high heat fluxes. During testing, an infrared camera was used to measure the surface temperature distribution as well as locate nucleation sites for data analysis. It was found that CHF values were enhanced over the bare sapphire values by approximately 90% for hexagons, 60% for stars, 65% for rings, and 50% for mixed patterns. Contrary to expectations, patterning did not seem to affect the HTC values significantly. Although patterning did improve CHF performance over bare heaters, both CHF and HTC were found to be statistically similar to those for unpatterned, uniformly hydrophilic surfaces.