Onset of Boiling, Heat Transfer, and Flow Patterns of Flow Boiling on the Superhydrophobic Porous Copper Surface in a Microchannel

To explore the effect of microstructured porous surface on enhancing the heat transfer of flow boiling in a microchannel, the microporous copper surface was fabricated with microscale pores in ranges of 1-5 mu m, which presented super-hydrophobicity. Subcooled flow boiling experiments were carried out to study the hydraulic and thermal transport performance in a single narrow rectangular microchannel. High-speed flow visualizations were conducted coupled with instrumental measurements to illustrate the effects of heat flux and mass flux on heat transfer performance and flow patterns for originally hydrophilic bare copper surface and superhydrophobic microporous structured surface. The onset of boiling (ONB) characteristics of both test surfaces was compared with predictive correlations with a good agreement and verified by the captured flow pattern images. Benefit from the superhydrophobic wettability provided by its microscale porous structures and a large number of potential nucleation sites, the required wall superheats, and imposed heat fluxes of onset of boiling both decreased for the modified surface. The flow patterns on the two surfaces varied, which resulted in the different trends of heat transfer coefficient (HTC) with mass fluxes and heat fluxes. Because of the strengthened bubble departure process, the enhancement of the porous surface compared to the original bare surface gradually increased with mass fluxes. The average two-phase heat transfer coefficients of the superhydrophobic porous copper surface were enhanced for up to 74.84%, due to the earlier onset of boiling, higher frequency of bubble nucleation, and strengthened boiling intensity.