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

Lead Research Organisation: Imperial College London
Department Name: Chemical Engineering

Abstract

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.

Planned Impact

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.

Publications

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Mameli M (2022) Innovations in pulsating heat pipes: From origins to future perspectives in Applied Thermal Engineering

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Alagumalai A (2022) Nano-engineered pathways for advanced thermal energy storage systems in Cell Reports Physical Science

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Moran H (2021) LASER-BASED DIAGNOSTICS OF SLUG FLOW BOILING IN A HORIZONTAL PIPE in Interfacial Phenomena and Heat Transfer

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Ajaev V (2021) PREFACE TO SPECIAL ISSUE: HEAT TRANSFER,WAVES, AND VORTEX PHENOMENA IN TWO-PHASE FLOWS in Interfacial Phenomena and Heat Transfer

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Dirker J (2021) Influence of inlet vapour quality perturbations on the transient response of flow-boiling heat transfer in International Journal of Heat and Mass Transfer

 
Description The preliminary results from this project have revealed the dynamics of a single vapour bubble from its inception to lift-off from the test substrate exposed to subcooled flow boiling conditions. The growing vapour bubble was found to be hemispherical in shape during the initial time instances immediately after its inception. However, as the vapour bubble grows bigger in size, the inertial forces due to the flow of the fluid around it tend to slide the vapour bubble away from the nucleation site. During the sliding process, the vapour bubble continues to increase in size due to its interaction with the heated substrate. Advanced laser-based measurement techniques are currently being developed to study the hydrodynamic and thermal interaction between the vapour bubble, the test substrate and the bulk fluid flow.
Exploitation Route 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.
Sectors Aerospace, Defence and Marine,Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology

 
Title BONSAI facility 
Description The experimental facility is designed to elucidate on the fundamental physics of two-phase heat transfer process under various flow boiling regimes. This is realized by making spatio-temporally resolved measurements of various flow field variables like temperature and velocity using state-of-the-art light-based diagnostic techniques like two-colour laser-induced fluorescence (2C-LIF), particle image velocimetry (PIV), and infra-red (IR) thermometry. The experimental facility is optically accessible from all four directions and uses a thin sapphire glass coated with Indium-Tin-Oxide (ITO) for a test substrate. The conductive ITO coating is used to heat the fluid to boiling conditions at specific heat flux values which is one of the control parameters of the experiments. Other control parameters such as mass flow rate, inlet vapour fraction, and temperature of fluid in the inlet of test section are appropriately controlled by specific devices like mass flow controller and inline tubular pre-heaters etc. 
Type Of Material Improvements to research infrastructure 
Year Produced 2021 
Provided To Others? Yes  
Impact The result of this work is expected to provide fundamental insights about various heat transfer mechanisms that occurs during flow boiling in mini/micro channels. This is then used to develop a roadmap or strategy to enhance heat transfer rates. The experimental data will also be used for benchmarking numerical simulations of flow boiling to be undertaken by collaborators from the University of Nottingham. 
 
Title LaVision DaVIS model 
Description Time-resolved dynamic codes in MATLAB for laser-induced fluorescence, particle image velocimetry and infra-red thermometry measurements. 
Type Of Material Computer model/algorithm 
Year Produced 2021 
Provided To Others? No  
Impact The Licensed software "DaVIS" from LaVision is used to operate/control the laser systems, record the images from multiple cameras, and pre-process and extract the images to MATLAB compatible files for post-processing. Time-resolved dynamic codes in MATLAB for post-processing of images obtained through laser-induced fluorescence, particle image velocimetry and infra-red thermometry measurements. 
 
Title Post-processing MATLAB scripts 
Description MATLAB-based post-processing scripts will be used to quantify various integral heat transfer parameters. 
Type Of Material Computer model/algorithm 
Year Produced 2021 
Provided To Others? No  
Impact The model can be used to quantify various integral heat transfer parameters. 
 
Description Collaboration on BONSAI project 
Organisation Brunel University London
Country United Kingdom 
Sector Academic/University 
PI Contribution BONSAI features 6 interconnected work packages (WP) with well-defined deliverables and milestones; Professor Christos Markides will be responsible for overall coordination and project management (WP1). Experimental activities at Imperial College London will be undertaken by RA (Dr. Suryanarayan Lakshminarayanan) & PhD (Mr. Zengchao Chen), who will apply complementary approaches, focusing on different flow aspects and applying a suite of different measurement techniques (2-colour Laser-induced fluorescence, Particle Image Velocimetry and high-speed videography). Matching numerical simulations will be conducted by Professor Mirco Magnini and Professor Omar Matar to compare with experiments and perform parametric studies. The new database will be merged with existing ones from Brunel University London.
Collaborator Contribution Develop unique experimental capabilities and advanced optical-diagnostic methods capable of high-spatiotemporal-resolution, simultaneous measurements of interfacial, wall and bulk-flow quantities, and of important global features of boiling flows in small/microchannels. Apply the methodology developed to produce a detailed map of, spatiotemporal phase, velocity, and temperature information.
Impact NA. Since the facility is currently being developed in Imperial College London, no experiments have been conducted so far. However, benchmarking experiments are expected to tentatively commence from late March 2022.
Start Year 2020
 
Description Collaboration on BONSAI project 
Organisation University of Nottingham
Country United Kingdom 
Sector Academic/University 
PI Contribution BONSAI features 6 interconnected work packages (WP) with well-defined deliverables and milestones; Professor Christos Markides will be responsible for overall coordination and project management (WP1). Experimental activities at Imperial College London will be undertaken by RA (Dr. Suryanarayan Lakshminarayanan) & PhD (Mr. Zengchao Chen), who will apply complementary approaches, focusing on different flow aspects and applying a suite of different measurement techniques (2-colour Laser-induced fluorescence, Particle Image Velocimetry and high-speed videography). Matching numerical simulations will be conducted by Professor Mirco Magnini and Professor Omar Matar to compare with experiments and perform parametric studies. The new database will be merged with existing ones from Brunel University London.
Collaborator Contribution Develop unique experimental capabilities and advanced optical-diagnostic methods capable of high-spatiotemporal-resolution, simultaneous measurements of interfacial, wall and bulk-flow quantities, and of important global features of boiling flows in small/microchannels. Apply the methodology developed to produce a detailed map of, spatiotemporal phase, velocity, and temperature information.
Impact NA. Since the facility is currently being developed in Imperial College London, no experiments have been conducted so far. However, benchmarking experiments are expected to tentatively commence from late March 2022.
Start Year 2020
 
Description Collaboration on micro-channel heat exchangers 
Organisation Hubbard Products
Country United Kingdom 
Sector Private 
PI Contribution This collaboration is at very early stages. We intend to support the R&D activities of Hubbard in the area of micro-channel heat exchangers for refrigeration equipment, by providing proved correlations for heat transfer from our experimental and numerical work.
Collaborator Contribution Hubbard Products will ensure the results and outputs from the work in BONSAI will find applications in industry, in particular refrigeration equipment.
Impact On-going collaboration has just started.
Start Year 2022