Porosity effect of bio-coated surfaces on flow boiling heat transfer of HFE-7000

Surface modification has emerged as a prominent approach for providing ultra-high heat flux cooling and enhancing boiling heat transfer of particularly dielectric fluids, where inherent limitations such as low thermal conductivity and latent heat deteriorate the performance. Despite the large number of methodologies in surface modification for enhanced boiling heat transfer, a considerable proportion of surface modification techniques necessitate access to cleanroom facilities or involve protracted procedures, including the utilization of environmentally hazardous materials. This study presents flow boiling heat transfer results of HFE-7000 in a rectangular high aspect ratio minichannel with a coated surface using proposed environmentally friendly and economical microbial bio-coating (Saccharolobus solfataricus P2 bio-coatings). The boiling heat transfer characteristics of bio-coated surfaces, which were optimized in terms of coating structure and durability against the fluid flow by the dip mixed coating method, were explored. Flow boiling experiments were performed on coated surfaces at heat fluxes ranging from 5.4 to 50.9 W/cm², mass flux of 500 kg/m²s, inlet subcooling of 10 ℃, and atmospheric pressure. Bubble dynamics and flow boiling patterns were obtained using a high-speed camera. The bio-coated surface offered significant heat transfer enhancement compared to the bare silicon sample by offering more active nucleation sites and showing resistance to vapor film/dry spot formation on the surface by providing a porous structure. This porous architecture not only increases the density of nucleation sites but also enables capillary-driven rewetting, which sustains thin liquid films and delays dry-out at high heat fluxes. The coated surface achieved the highest heat transfer coefficient, with a maximum enhancement of 50 %, and enhanced the critical heat flux (by a factor of about 1.5 relative to the plain surface) by stabilizing two-phase flow and promoting rewetting.