Effect of Length Scales on the Boiling Enhancement of Structured Copper Surfaces

Boiling heat transfer can be substantially altered with the addition of surface structures. While significant enhancements in critical heat flux (CHF) and heat transfer coefficient (HTC) have been demonstrated using this approach, fundamental questions remain about the nature of enhancement and the role of structure length scale. This work presents a systematic investigation of structures from 100' s of nanometers to several millimeters. Specifically, copper substrates were fabricated with five different microchannel geometries (characteristic lengths of 300 mu m to 3 mm) and four different copper oxide nanostructured coatings (characteristic lengths of 50 nm to 50 mu m). Additionally, twenty different multiscale structures were fabricated coinciding with each permutation of the various microchannels and nanostructures. Each surface was tested up to CHF during pool boiling of saturated water at atmospheric conditions. The nanostructured coatings were observed to increase CHF via surface wicking, consistent with existing models, but decrease HTC due to the suppression of the nucleation process. The microchannels were observed to increase both CHF and HTC, generally outperforming the nanostructured coatings. The multiscale surfaces exhibited superior performance, with CHF and HTC values as high as 313 W/cm(2) and 461 kW/m(2) K, respectively. Most importantly, multiscale surfaces were observed to exhibit the individual enhancement mechanisms seen from each length scale, namely, increased nucleation and bubble dynamics from the microchannels and wicking-enhanced CHF from the nanostructures. Additionally, two of the surfaces tested here exhibited uncharacteristically high HTC values due to a decreasing wall superheat at increasing heat fluxes. While the potential mechanisms producing this counterintuitive behavior are discussed, further research is needed to definitively determine its cause.