| Abstract |
Boiling phenomena are central to heating and cooling duties in many industries, such as cooling and refrigeration, power generation, and chemical manufacture. Limitations to boiling heat transfer arise through surface dry-out at high heat flux, leading to localised hot-spots on heat transfer surfaces and larger equipment requirements. Whilst this is a significant problem for many industries, it becomes even more of an issue when dealing with small-scale systems, such as those used for cooling of microelectronics, where failure to remove heat effectively leads to localised overheating and potential damage of components. Spatially non-uniform and unsteady dissipative heat generation in such systems is detrimental to their performance and longevity. The effective heat exchanger area is of order sq. cm, with heat fluxes of order MW/sqm. This requires a transformative, step-change, beyond the current state-of-the-art for cooling heat fluxes between 2-15 MW/sqm at local "hot spots" to prevent burn out. A number of attempts have already been made to extend the upper boundary for the heat flux through alteration of surface characteristics with the aim of improved nucleation of vapour bubbles, bubble detachment, and subsequent rewetting of the surface by liquid. Despite the progress made, previous work on surfaces for pool- (and potentially flow-) boiling does not involve a rational approach for developing optimal surface topography. For instance, nucleate boiling heat transfer (NBHT) decreases with increasing wettability, and the designer must consider the nucleation site density, associated bubble departure diameter, and frequency related to the surface structure and fluid phase behaviour. For high surface wettability, the smaller-scale surface structure characteristics (e.g. cavities) can act as nucleation sites; for low wettability, the cavity dimensions, rather than its topology, will dominate. Therefore, characterising surfaces in terms of roughness values is insufficient to account for the changes in the boiling curve: the fluid-surface coupling must be studied in detail for the enhancement of NBHT and the critical heat flux. EMBOSS brings together a multi-disciplinary team of researchers from Brunel, Edinburgh, and Imperial, and six industrial partners and a collaborator (Aavid Thermacore, TMD ltd, Oxford Nanosystems, Intrinsiq Materials, Alfa Laval, CALGAVIN, and OxfordLasers) with expertise in cutting-edge micro-fabrication, experimental techniques, and molecular-, meso- and continuum-scale modelling and simulation. The EMBOSS framework will inform the rational design, fabrication, and optimisation of operational prototypes of a pool-boiling thermal management system. Design optimality will be measured in terms of materials and energy savings, heat-exchange equipment efficiency and footprint, reduction of emissions, and process sustainability. The collaboration with our partners will ensure alignment with the industrial needs, and will accelerate technology transfer to industry. These partners will provide guidance and advice through the project progress meetings, which some of them will also host. In addition, Alfa Laval will provide brazed heat exchangers as condensers for the experimental work, Intrinsiq will provide copper ink for coating surfaces and Oxford nanoSystems will provide nano-structured surface coatings. The project will integrate the challenges identified by EPSRC Prosperity Outcomes and the Industrial Strategy Challenge Fund in Energy (Resilient Nation), manufacturing and digital technologies (Resilient Nation, Productive Nation), as areas to drive economic growth. |
