Boiling in Microchannels: integrated design of closed-loop cooling system for devices operating at high heat fluxes

Lead Research Organisation: Brunel University London
Department Name: Mech. Engineering, Aerospace & Civil Eng


Current developments and future trends in small-scale devices used in a variety of industries such as electronic equipment and micro-process and refrigeration systems, place an increasing demand for removing higher thermal loads from small areas. In some cases further developments are simply not possible unless the problem of providing adequate cooling is resolved. The progression from air to liquid and specifically flow boiling to transfer the high heat fluxes generated is thus the only possible way forward. Evaporative cooling can, not only transfer these loads but also offer greater temperature uniformity since the working fluid can be (in a carefully designed system) at a constant saturation temperature. The consideration of microchannel flow boiling processes has been made possible by developments in microfabrication techniques both in metals and substances such as silicon. However, there still remain fundamental fluid flow and heat transfer related questions that need to be addressed before a wider use of these micro heat exchangers is possible in industry. The specific challenges that will be researched - both fundamental and practical in nature - include flow instabilities and mal-distribution which are the result of interaction between the system manifolds and the external circuit. These can lead to flow reversal and dry-out in the heat exchanger with subsequent drastic reduction in heat transfer rates. The understanding of the fundamental physical phenomena and their relevance to industrial designs is one of the focal points and constitutes one of the major challenges of the proposed research. The effect of other parameters such as inlet sub-cooling, which again relates not only to the micro-heat exchanger itself but also to the overall design, will be addressed along with material/surface characteristics through the use of both metallic and silicon microchannels.

The work proposed will include carefully contacted detailed experiments measuring relevant parameters such as local heat flux, temperature and pressure combined with flow visualization through industrially available and purposely developed and manufactured sensors. The research teams will not only develop or adapt advanced instruments for accurate measurements at these small scales but also develop new three-dimensional numerical tools capable of capturing the extremely complex physical phenomena at, for example the triple-line (vapour-liquid-solid). These techniques will not only help elucidate the current phenomena but can find wide application in similar research, both in thermal and biomedical flows.

The proposal brings together two teams of academics working both in microfabrication/sensors and two-phase flow supported by industry (Thermacore, Selex Galileo, Sustainable Engine Systems and Rainford Precision) to tackle some of the key fundamental challenges that will enable a wider adoption of this cooling method hence meeting current and future needs in the industry. The proposed research will also have a wider impact on energy conservation and environmental footprint trough, for example, more efficient thermal management of data/supercomputing centres around the world that can lead to a reduction in energy consumption and reuse of heat that would otherwise be rejected.

Planned Impact

A range of cooling techniques has already been considered for cooling devices operating at fluxes above 0.5 MW/m2, the practical limit for air cooling, such as liquid convection, pool boiling, liquid jet impingement or sprays, heat pipes or capillary pumped loops, thermo-acoustic refrigeration and thermoelectric devices. None has reached a state of commercial maturity and many technological difficulties still arise at heat fluxes above 1 MW/m2. Heat sinks comprising many microchannels in parallel, connected by manifolds, offer the advantage of a high ratio of heat transfer area to base area in contact with the heat source. Evaporative cooling offers the prospect of much higher heat fluxes, due to the higher heat transfer coefficients associated with bubble formation and growth in microchannels, and greater regulation and uniformity of temperature because the coolant is at its saturation temperature.

The research groups at BU and UoE have long-term experience in heat transfer at the microscale and have worked together before in this area. The access to high quality micromachining and microfabrication facilities, including clean room facilities, permits integrated production of cooling systems and microelectronic devices. The links with Thermacore, who are specialists in advanced cooling technologies, Sustainable Engine Systems (SES) who specialise in extremely high surface/volume ratio heat exchangers and Selex, who use advanced cooling systems, provides the research partnership with avenues for exploitation of their research outcomes. This project will open up a range of applications in enhanced thermal control of power plants, cooling of electronics, heat pipes for space and earth applications, chemical micro-reactors, micro-heaters and refrigeration devices and thermal control for biological detection devices. The possible use of manifold designs developed in this project with Selective Laser Melting micro-heat exchangers (SES) in applications such as heat engines is also an important opportunity. In some of the application areas above, the proposed research developments will allow step changes in thermal processes and facilitate or enable new designs and product development and in some others, benefits such as lightness and compactness, rapid heat and mass transport, extremely precise control of the process conditions, high performance (i.e. high throughput per unit hardware volume), cost economies through mass production and availability for distributed or mobile applications.

The UK currently has a commercial presence in the production of power electronics and in providing thermal solutions for power management. One estimate for the global market for Electronic Thermal Management is that it will reach $8.6 billion by 2015. For the UK to remain a significant international player in this area there needs to be a strong research base. Our proposal not only provides new knowledge into the physical phenomena of two-phase flow at the micro scale but also direct benefit, in the form of enhanced cooling technologies that can be applied by the industrial partners supporting this proposal.

The proposal will also lead to the training of researchers who can contribute to this area in the future. This includes training in advanced experimental techniques and numerical methods that can be used in a variety of projects/applications. In addition, the immediate interaction with our industrial partners will add direct value to the training of the PDRA and offer both them and the more experienced academics, increased insight into the industrial needs, constraints and avenues for application of state-of-the-art research. Without expertise in adequate cooling technologies, the future growth of microsystems in the UK will be hampered with the danger that innovative designs are shelved through the inability to fabricate cooling systems or that commercialisation takes place outside the UK where cooling solutions are available.
Description The research verified clearly the possibility of designing an integrated thermal management system consisting of a multi - microchannel evaporator and a water-cooled micro scale condenser. The system was capable of transferring more that 1MW/m2, while operating at low mass flux and low - near atmospheric - pressure. Higher heat flux values are possible at higher mass flux and system pressure. The project also verified that evaporative cooling, not only offers the prospects of cooling high heat flux devices, but also allows operation at uniform temperature of the substrate to be cooled. The results, and transferable knowledge gained during this project, can enable industry to design and adopt thermal systems capable of removing high heat transfer rates that prevail (and in cases form a restriction for further developments) in a wide range of applications namely, cooling of electronics and high-power semiconductor devices, chemical micro-reactors, fuel cells, laser diode arrays, micro-heaters and refrigeration devices.
The research continued with a PhD student funded by the Government of Iraq. The student completed his studies in 2019 and returned to his university in Iraq as Assistant Professor. The PDRA returned to his position, now Associate Professor at Zagazig University, but remained a close collaborator on this (and other projects) in his capacity of a Visiting Professor at Brunel. He has now returned to Brunel to work as a PDRA on a new EPSRC funded project, EP/S019502/1.
See section on Narrative Impact on collaboration with industry and academia based on the work of this project. Research findings from this project and others related, were presented to TMD Ltd and a new project was funded directly by the company looking at designs for meeting high heat flux values while rejecting heat through air-cooled condensers. This has applications in the aerospace industry. A laboratory-based prototype was designed and tested and the results presented to TMD in January 2020.
Exploitation Route Current developments and future trends in small-scale devices used in a variety of industries place an increasing demand for removing increasingly higher thermal loads from small areas. In some cases, further developments are simply not possible unless the problem of providing adequate cooling is resolved. Microchannel heat sinks, directly placed on the heat source, offer the advantage of high ratio of heat transfer area to base area. These heat sinks, used as evaporators and condensers, can offer the possibility of dissipating very high heat fluxes due to the higher heat transfer coefficients and possible temperature regulation of the heat generating device.
The findings of this research can be taken forward by design engineers seeking to provide integrated cooling systems (microchannel evaporators and condensers) to remove the high thermal loads encountered, enabling efficient and safe operation of these devices and further advances in this area.
Sectors Aerospace, Defence and Marine,Electronics,Energy,Manufacturing, including Industrial Biotechology,Transport

Description This work formed the basis for Thermacore (now Boyd Corporation) and Brunel to collaborate further including Oxford nanosystems (OnS) in the use of enhanced surfaces in two phase heat transfer (Electronics Cooling Via Hi Performance Heat Pipes) supported by Innovate UK (47335-33120). We also discussed extending this know-how in designing high heat flux thermal management systems with other companies and possibly enabling our partners to increase their customer base. Work with Oxford nanosystems continued supported by an Innovate UK grant, plus a current EPSRC grant (EP/P004709/1) to verify the effectiveness of their coating in enhancing heat transfer rates in compact heat exchangers originally with the contribution of SWEP and currently with Alfa Laval (heat exchanger manufacturers). This can have beneficial output for both OnS and the companies involved. At the same time, the research outcomes from this project formed the basis of collaboration with Queen Mary and Edinburgh involving industrial partners to implement enhanced two-phase techniques including fluid mixtures in industrial micro thermal management systems (EP/N011112/1). This work also enabled the Brunel team to support further new R&D work in Industry. In particular, TMD Ltd is now supporting work at Brunel on a 4-year programme, aimed at designing thermal management systems for the aerospace industry capable of removing high heat fluxes under extreme ambient conditions on the ground and in the air. OnS is involved to coat the microchannel heat exchangers for additional improved performance. Furthermore, a new grant was obtained to complete fundamental and high impact research in this area (Brunel, Imperial, Nottingham plus industrial Partners - EP/T033024/1). The research is due to start in May 2021 and involves new experiments at Brunel and the use of a large database (now including data collected under this grant - EP/K011502- and the subsequent work of the PhD student funded by the Government of Iraq, plus data from our partners of the Virtual International Inst. of Two-Phase Flow and Heat transfer (VIR2AL)). The data will be processed through Machine Learning techniques to provide generic design equations for industry on integrated closed-loop cooling systems (combining micro evaporators and condensers) for high heat flux devises, expanding the objectives of the original grant.
First Year Of Impact 2017
Sector Aerospace, Defence and Marine,Electronics,Energy,Manufacturing, including Industrial Biotechology
Impact Types Economic

Description Industry
Amount £60,000 (GBP)
Organisation TMD Technologies Limited 
Sector Private
Country United Kingdom
Start 10/2016 
End 09/2020
Description Responsive Mode
Amount £1,218,088 (GBP)
Funding ID EP/N011112/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2016 
End 02/2020
Description Thermodynamic Efficient Heat Exchangers
Amount £287,376 (GBP)
Funding ID 132152 
Organisation Innovate UK 
Sector Public
Country United Kingdom
Start 06/2016 
End 05/2017
Title Flow pattern Transition Models and Correlations for Flow Boiling in Mini Tubes 
Description Model predicting flow patterns that prevail in small to micro scale passages. Presented in Mahmoud M.M. and Karayiannis T.G., Experimental Thermal and Fluid Science, Vol 70, pp. 270-282, 2016. 
Type Of Material Computer model/algorithm 
Year Produced 2016 
Provided To Others? Yes  
Impact The model has just been published. We expect the international community to benefit as it enables direct prediction of flow patterns in mini tubes. This will help in the efforts of the international community to understand the fundamental mechanisms of flow boiling in micro passages but also assist in the design of practical micro scale heat exchangers using flow boiling. 
Title Heat transfer correlation for flow boiling in small to micro tubes 
Description A model predicting heat transfer rates in small to micro passages. Available in Mahmoud, M. M. and Karayiannis, T.G., Int. Journal Heat and Mass Transfer, 66, pp 553-574, 2013. 
Type Of Material Computer model/algorithm 
Year Produced 2013 
Provided To Others? Yes  
Impact Increase standing of our group. Provide a fairly accurate model predicting heat transfer rates - useful both for comparisons with other research groups but very importantly, as a design tool for small scale heat exchangers using flow boiling. 
Title Microscale heat transfer and pressure drop data 
Description During the course of the project we up-graded our data bank containing results of local and average heat transfer coefficient and pressure drop as a function of operating parameters (heat flux, mass flux, system pressure) for two fluids, see link: This can now be used by the national and international academic community to compare and help develop more accurate correlations for heat transfer rates and pressure drop in microchannels, thus leading to better designs in thermal management systems for high heat flux devices. 
Type Of Material Database/Collection of data 
Year Produced 2014 
Provided To Others? Yes  
Impact This has just been made up-dated, after further addition of data. We are not expecting researchers from other institutions to begin using our databases. 
Description Oxford nanosystems 
Organisation Oxford nanoSystems Ltd
Country United Kingdom 
Sector Private 
PI Contribution To provide accurate data on the effect of coatings produced by Oxford nanosystems on heat transfer rates. This will enable the company to improve marketability of their coating technique for the heat exchanger industry.
Collaborator Contribution To provide coating materials plus time to carry out coating processes. Attend meetings (CEO plus scientific officer)
Impact Too early to estimate from this particular project. Oxford nanosystems benefitted from our collaboration through previous work funded consultancy projects and by Innovate UK.
Start Year 2013
Description Rainford Precision Machines 
Organisation Rainford Precision Machines
Country United Kingdom 
Sector Private 
PI Contribution Information on practical use of micro machining tools in precision manufacturing.
Collaborator Contribution Support on the use of micro machining tools
Impact Too early for results. Note: Support was provided for project (EP/K011502/1) with positive indirect results, i.e. for the company the ability to verify that their products work under micromachining conditions and for the university to have support and advice in the manufacture of micro metallic multichannel heat exchangers.
Start Year 2012
Description SES Ltd 
Organisation Sustainable Engine Systems Ltd
Country United Kingdom 
Sector Private 
PI Contribution Make available data, as needed, on performance of high heat flux micro scale heat exchangers using flow boiling for comparative purposes.
Collaborator Contribution Help develop expertise in design applications of micro scale heat exchangers
Impact Indirect positive outcomes: Help familiarise research team with practical needs and aspects of design of high heat flux thermal management systems. Sustainable Engine Systems will be able to explore the possibility of enhancing the possible heat fluxes further by using Selective Laser Melting to modify the surface of the microchannels, directly utilizing knowledge from this project of the effect of surface characteristics.
Start Year 2013
Description Selex ES 
Organisation Selex ES
Department SELEX Galileo Ltd
Country United Kingdom 
Sector Private 
PI Contribution Provide data/design recommendations on high heat flux thermal management systems for specific applications relevant to company products.
Collaborator Contribution Offer advice on practical aspects of design of high heat flux small scale thermal management systems.
Impact Indirect positive benefits: Help familiarise research team with practical aspects of design of high heat flux thermal management systems where cooling is required at more than one location. Knowledge from this project led to a partnership with TMD for actual design of thermal management systems (current PhD studentship). Brunel and TMD are working on the development of a high heat flux cooling system for the aerospace industry using flow boiling in micro passages that can operate under extreme ambient temperatures (on the ground in hot climates and in the air).
Start Year 2013
Description Thermocore Europe Ltd 
Organisation Thermacore Europe
Country United Kingdom 
Sector Private 
PI Contribution To provide data on heat transfer with mixtures in microscale heat exchangers as they become available.
Collaborator Contribution Advice on practical design/applications/market possibilities of flow boiling in microchannels
Impact Design of a practical thermal management system for high heat flux devices. This collaboration is still active and Thermacore is partner to a new EPSRC grant. They also worked with Brunel and Oxford nanosystems (OnS) on heat pipes funded by Innovate UK.
Start Year 2006
Description Industrial Visit 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Industry/Business
Results and Impact Present research findings to Oxford nanSystems. The company continues to expand and supports research programmes in our group with mutual benefit.
Research findings provided by our group is used by the company to enhance possibilities of finding new customers/use of their products.
Year(s) Of Engagement Activity 2020