Effect of coating layer thickness for boiling heat transfer on micro porous coated surface in confined and unconfined spaces

Attributed to its high heat transfer coefficient, evaporating cooling involving the use of micro heat exchangers is considered a possible thermal management solution for cooling of high heat flux electronic devices. The boiling heat transfer in micro heat exchangers is generally confined in a very narrow space. The heat transfer characteristics are indeed different from those of conventional unconfined boiling. This work provides an experimental analysis of the boiling heat transfer of methanol on plain and micro porous coated surfaces inside confined space. Five micro porous coating surfaces with thicknesses of 81, 109, 150, 182 and 225 μm were tested in confined and unconfined spaces. Effects of space confinement, coating layer thickness and heat flux on the heat transfer coefficient and critical heat flux were discussed. The micro porous coating layer provides large amount of active nucleation sites that significantly enhances heat transfer coefficients up to a factor of 4.5 in comparing to that on plain surface. But the porous coating layer also exerts higher bubble leaving resistance and coating layer thermal resistance. The combination of these three effects brought about an optimum coating layer thickness for micro porous surfaces in both confined and unconfined spaces.