BOiliNg flows in SmAll and mIcrochannels (BONSAI): From Fundamentals to Design

BONSAI is an ambitious 3-year research project aimed at investigating the fundamental heat and mass transfer features of boiling flows in miniaturised channels. It combines cutting-edge experiments based on space/time-resolved diagnostics, with high-fidelity interface-resolving numerical simulations, to ultimately provide validated thermal-design tools for high-performance compact evaporators. The proposed project assembles multidisciplinary expertise of investigators at Imperial College London, Brunel University London, and the University of Nottingham, with support from 3 world-leading research institutes: Alan Turing Institute, CERN (Switzerland) and VIR2AL; and 11 industry partners: Aavid Boyd Thermacore, Alfa Laval, CALGAVIN, HEXAG&PIN, HiETA, Hubbard/Daikin, IBM, Oxford nanoSystems, Ricardo, TMD and TTP.

The recent trend towards device miniaturisation driven by the microelectronics industry has placed an increasing demand on removing higher thermal loads, of order of MW/m2, from areas of order cm2. In some applications (e.g. refrigeration) new 'green' refrigerants are needed, but in small volumes due to flammability or cost, while in others (e.g. batteries for EV and other applications) non-uniform or unsteady heat dissipation is highly detrimental to performance and lifetime. Flow boiling in multi-microchannel evaporators promises to meet such challenging requirements with low fluid volumes, also allowing better temperature uniformity and smaller pumping power, in systems that go well beyond the current state-of-the-art. Due to significant industrial (heat exchange) and environmental (efficient energy use) interest, the understanding of boiling heat transfer has improved in recent years, with focus on flow pattern transitions and characteristics, pressure drop, and heat transfer performance. However, our current understanding is simply insufficient to facilitate the wider use of these micro-heat-exchangers in industry, which remains unexploited.

BONSAI has been tailored specifically to address the fundamental phenomena underlying boiling in miniaturised devices and their relevance to industrial design. The challenges to be addressed include the impact of channel shape and surface characteristics on flow instabilities, heat transfer and pressure drop, and the relationship between the time-dependent evolution of the liquid-vapour interface, thin liquid-film dynamics, flow field, appearance of dry vapour patches, hot spots, and local heat transfer characteristics. The extensive experimental/numerical database generated will be exploited via theoretical and novel machine-learning methods to develop physics-based design tools for predicting the effects of industrially-relevant thermohydraulic parameters on system performance. The collaboration with our partners will ensure alignment with industrial needs and accelerate technology transfer to industry. In addition, HiETA will provide Metal Additive Manufacturing heat sinks that will be assessed against embossing technologies as ways of mass-producing microchannel heat exchangers, Oxford nanoSystems will provide nano-structured surface coatings, and IBM will support visits to their Research Labs focussed on efficient parallelisation of the numerical solver and scale-out studies.

The proposed research will not only enable a wider adoption of two-phase thermal solutions and hence the meeting of current and future needs across industrial sectors, but also will lead to more efficient thermal management of data-centres with associated reduction in energy consumption and carbon footprint, and the recovery and reuse of waste heat that is currently being rejected. This will constitute an important step towards meeting the UK's emission targets by 2050. Additionally, BONSAI will integrate with EPSRC Prosperity Outcomes of Delivery Plan 2016-20 and enable technological advances in relation to the Manufacturing the Future theme, contributing to a Productive and Resilient Nation.

The UK has a strong commercial presence in the production of power electronics and thermal solutions for power management. It is estimated that the global heat-exchanger market will reach £18B by 2023 from £12B in 2018, driven by the fast growth of manufacturing, chemicals, and construction industries. BONSAI will have a direct impact on these markets by allowing heat exchangers to operate with much greater efficiency, thus enabling step-change reductions to the exchange surfaces, heat-sink volumes, and thus material costs, reduced quantity of coolant or refrigerant in target systems, higher thermal uniformity, and lower pumping power.

The assessment of additive manufacturing techniques and of their impact on evaporator performance will provide new avenues for the mass-production of thermally-efficient thermal devices. Data-centres currently consume about 1% of the UK's total electricity, and it is estimated that by 2020 they will emit as much CO2 as the airline industry. On-chip two-phase cooling may reduce the necessary pumping power to circulate the coolant and allow the reuse of data-centre waste heat in secondary applications, thus reducing their carbon footprint. BONSAI will also enable next-generation heating/cooling technologies with low-volume eco-friendly fluids, reducing energy and emissions, and battery technologies via efficient thermal management, in line with UK government strategic investments (e.g. £246M Faraday challenge).

Thermal engineers, consultants and operators who are involved in the analysis, design, manufacture, decision-making and operation of heat-exchange systems will benefit significantly from game-changing technologies that will enable them to develop smarter, more environmentally-friendly, reliable and cost-efficient designs. This will be effected by providing new insight into fundamental flow and heat transfer mechanisms and impact of the flow regime; best channel configurations to reduce the risk of flow instabilities, maldistribution and optimise pressure drop and heat transfer; invaluable space/time-resolved data of dramatic need for model validation and design-code development; validated theoretical and computational models implementing the most accurate descriptions of the underlying phenomena, amongst other. Due to the ubiquitous utilisation of two-phase flows in industrial processes, the knowledge gained with the proposed research will be widely transferable, e.g. to the pharmaceutical industry (improving lab-on-a-chip devices), renewable energy sector (evaporative cooling of fuel cells, PV, batteries), oil-and-gas industry (dynamics of slug and annular flows), as well as biomedical applications (cleansing medical tools using capillary forces).

BONSAI will produce skilled UK-based experts in heat transfer, phase change, multiphase flows, experiments, numerical simulation, data management and computer science, who will engage in R&D and other contributions to UK industry, public education and social development. These opportunities will enable effective professional competencies towards future long-term employability. The numerical solver developed during the project will set new standards on the simulation of thin film flows and boiling. The experimental setup will inspire other experimental research, and multidisciplinary skills of staff and lab technicians will be enhanced. Private and public sector researchers will be able to undertake related proof-of-concept tests that will improve the Universities' track record and public visibility.

The involvement of industry will ensure that our objectives remain focused on aspects of practical importance, while still answering fundamental questions and enhancing the understanding of essential phenomena in miniature channels. The UK's economic competitiveness stands to be enhanced through this collaborative project, with the potential to attract international investment from other companies leading to new joint industrial projects.