Enhanced Multiscale Boiling Surfaces (EMBOSS): From Fundamentals to Design

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.

EMBOSS addresses an urgent need to develop the next generation of enhanced surfaces for efficient cooling, utilising pool-boiling systems. It is directly applicable to the global heat exchanger and thermal management market, estimated to reach £14.2B by 2022, with the UK share exceeding its GDP by 2% in 2017. In the UK, power electronics has been identified as an important sector by EPSRC with recent R&D support totalling £44M. This includes the establishment of the National Power Electronics Centre (NPEC) as well £19M being committed by the then TSB. One of the drivers behind this investment is that 78,000 UK engineers are directly employed in PE, providing an excellent pathway to exploit any improvements in cooling performance. The UK is a key PE manufacturer with a £4B p.a. of PE product portfolio supporting £40B p.a. of PE-enabled systems, 90% of which are exported.
The EMBOSS research team has developed partnerships with Oxford nanoSystems, Intrinsiq Materials Ltd and Oxford lasers, who specialise in creating appropriate surface structures and surface coatings, along with Aavid Thermacore, Alfa Laval and CalGavin who produce heat transfer solutions and TMD Technologies, an end-user of advanced cooling systems for electronic components. The EMBOSS work will enhance the ability of UK industry to compete worldwide in state-of-the-art, CFC-free, cooling systems. The proposed project is in line with EPSRC strategic priorities in the Delivery Plan 2016/17-19/20, addressing primarily the themes of a Productive and Resilient Nation, though with implications also for the Connected Nation. In particular, the proposal relates to efficient transfer of energy along with production of reliable infrastructure.
EMBOSS will transform the manufacturing (and their standards) of heat transfer and thermal management surfaces, informed by industrial input, for increased UK competitiveness. Examples of EMBOSS impact include the creation of a platform for manufacturing optimised heat transfer surfaces, and a robust, accurate, efficient, multi-scale, and multi-physics, simulation platform, with reliable reduced-order, industry-ready models. The parallel development of significantly more efficient and compact cooling systems, and the UK's complete supply chain, provides the perfect opportunity to exploit the R&D in the UK, in order to drive the reduction in emissions and carbon footprints of processes. EMBOSS will also supply the next-generation of highly-skilled researchers in heat transfer, two-phase systems, coatings, and manufacturing techniques, which will be an asset to the UK economy.