Separate effects of surface roughness, wettability and porosity on boiling heat transfer and critical heat flux and optimization of boiling surfaces

Abstract

The separate effects of surface wettability, porosity, and roughness on critical heat flux (CHF) and heat transfer coefficient (HTC) were examined using carefully-engineered surfaces. All test surfaces were prepared on nanosmooth indium tin oxide - sapphire heaters and tested in a pool boiling facility in MIT's Reactor Thermal Hydraulics Laboratory. Roughness was controlled through fabrication of micro-posts of diameter 20[mu]m and height 15[mu]m; intrinsic wettability was controlled through deposition of thin compact coatings made of hydrophilic SiO₂ (typically, 20nm thick) and hydrophobic fluorosilane (monolayer thickness); porosity and pore size were controlled through deposition of layer-by-layer coatings made of SiO₂ nanoparticles. The ranges explored were: 0 - 15[mu] for roughness (Rz), 0 - 135 degrees for intrinsic wettability, and 0 - 50% and 50nm for porosity and pore size, respectively. During testing, the active heaters were imaged with an infrared camera to map the surface temperature profile and locate distinct nucleation sites. It was determined that wettability can play a large role on a porous surface, but has a limited effect on a smooth non-porous surface. Porosity had very pronounced effects on CHF. When coupled with hydrophilicity, a porous structure enhanced CHF by approximately 50% - 60%. However, when combined with a hydrophobic surface, porosity resulted in a reduction of CHF by 97% with respect to the reference surface. Surface roughness did not have an appreciable effect, regardless of the other surface parameters present. Hydrophilic porous surfaces realized a slight HTC enhancement, while the HTC of hydrophobic porous surfaces was greatly reduced. Roughness had little effect on HTC. A second investigation used spot patterning aimed at creating a surface with optimal characteristics for both CHF and HTC. Hydrophobic spots (meant to be preferential nucleation sites) were patterned on a porous hydrophilic surface. The spots indeed were activated as nucleation sites, as recognized via the IR signal. However, CHF and HTC were not enhanced by the spots. In some instances, CHF was actually decreased by the spots, when compared to a homogenous porous hydrophilic surface.