Flow boiling of hfe-7000 in minichannels with structured surfaces

Flow boiling heat transfer of dielectric fluids in minichannels is one of the mostpromising cooling methods for electronic components. Although HFE-7000 offerssafe operation in low working pressure and ideal temperature for direct contact electroniccooling, its performance is restricted by low thermal conductivity and latentheat. Phase change heat transfer combined with surface modification is a popularapproach for offering ultra-high heat flux cooling. This thesis aims to enhanceheat dissipation and heat transfer performance of dielectric fluid in minichannels byincreasing the surface area using porous and pin fin configurations.In the first part of this study the primary objective was to dissipate high heat fluxes(> 400W/cm2) using flow boiling of a dielectric fluid HFE-7000 in a multi-channelmicro pin fin heat sink with 8 hotspots subjected to uniform heating. Flow boiling ofHFE-7000 at the mass flux of 500kg/m2s was investigated in a minichannel with across-section area of (14mm × 3 mm) which was divided into three subminichannelseach having staggered elliptical pin fins with a height of 3mm (No tip clearance) oneach hotspot and distribution pins at the inlet. The collected data from temperature,pressure sensors, and flow meters were used to measure the heat transfer performanceand pressure drop during experiments. Additionally, high-speed camera recordingswere used to analyse variations in the heat transfer coefficient along the length ofthe channel and to link them to flow patterns. The obtained results were discussedat total heat fluxes up to 480W/cm2. High heat dissipation and ultra-high heat fluxcooling could be achieved with the designed and developed heat sink despite of low thermophysical properties of HFE-7000 due to the presence of distribution pin finsat the inlet for suppressing boiling instabilities and early transition to stable annularflow thanks to elliptical micro pin fins along the heat sink.The second part of this thesis discusses the flow boiling heat transfer of HFE-7000in a rectangular high aspect ratio minichannel over a proposed environmentallyfriendly and economical microbial bio-coated surface (crenarchaeon Sulfolobus solfataricusP2 bio-coatings). The thermal-fluidic characteristics of optimized bio-coatedsurfaces in terms of coating structure and durability against the fluid flow by dipmixed coating method were explored. Flow boiling experiments were performedon a mixed-coated surface in heat fluxes ranging from 5.4 to 50.9 W/cm2. Bubbledynamics and flow boiling patterns were obtained using a high-speed camera. Biocoatedsurface proposed an excellent heat transfer enhancement compared to baresilicon sample by offering more active nucleation sites and showing resistance to vaporfilm formation by providing a porous structure. The obtained results indicatedthat coated surface showed the highest heat transfer coefficient with a maximum enhancementof 50% in the range of medium heat fluxes by stabilizing the two-phaseflow instabilities and rewetting process.