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

Lead Research Organisation: Brunel University London
Department Name: Mechanical and Aerospace Engineering


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.

Planned Impact

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.
Description The pool boiling process is one of the most effective heat transfer modes capable of transferring large amounts of heat with small temperature difference between the heated surface and the fluid. Therefore understanding of pool boiling heat transfer is central in the design of efficient thermal systems covering a range of applications, e.g. electronics cooling,cooling of nuclear reactors, refrigeration systems, chemical industries and power production. Enhancing the heat transfer rates will result in improved efficiencies, reduction of energy used and hence emissions and design of smaller plants for the same output.

A thorough review of fundamental aspects of pool boiling, including passive enhancement methods, was performed and published that will help future researchers and designers of heat transfer equipment in their surge for more thermally efficient systems.
Bubble growth rate is one of the most important parameters required for the development of accurate mechanistic nucleate boiling heat transfer models. It is also very important for understanding the hydrodynamic forces and the mechanism of bubble departure. An experimental study on bubble growth measurements in saturated pool boiling of deionized water on a plain copper surface at atmospheric and sub-atmospheric pressure was completed. The prevailing forces (surface tension and buoyancy forces at atmospheric pressure and buoyancy and liquid inertia forces at sub-atmospheric pressure) were discussed along with the mechanisms of bubble growth.

Prediction of the actual bubble growth rate is very important for the development of accurate models for bubble departure diameter and thus the heat transfer rates in nucleate boiling. An evaluation study of the existing homogeneous and heterogeneous bubble growth models using our new experimental data for bubble growth in saturated pool boiling of deionized water was subsequently performed. A critical review on bubble growth models in homogeneous and heterogeneous boiling was also carried out. It was found that homogeneous growth models achieved some partial agreement with the experimental data at some conditions and thus they should be used carefully in heterogeneous boiling. There was good agreement between some of the models that were suggested based on the assumption that bubble growth occurs due to evaporation from the superheated boundary layer around the bubble. The models based on microlayer evaporation only could not explain the experimental data, i.e. partial agreement at some conditions. The model that predicted the data very well at all conditions was the "relaxation boundary layer" model by Van Stralen. This model was generalized in the current study by suggesting two new empirical models for the departure diameter and departure time.
Exploitation Route Designers can understand better pool boiling heat transfer and the possibilities of enhancement. They will be able to use the knowledge in the design of more efficient thermal systems.
New empirical models for the departure diameter and departure time are suggest that to help generalise models predicting bubble growth rate.
Sectors Aerospace

Defence and Marine





Description The recommendations on the use of passively enhanced heat transfer surfaces in pool boiling made in this study will be very useful in the design of more effective heat exchangers resulting in more efficient thermal systems. This will result in savings in energy use, increased possibilities of using renewable energy sources where the temperature differentials may be small (e.g. solar, geothermal, energy from waste) and a reduction in emissions. The fundamental work on the pool boiling mechanisms and bubble growth models will enable more accurate heat transfer predictions and improve component and overall plant designs.
First Year Of Impact 2023
Sector Aerospace, Defence and Marine,Chemicals,Education,Electronics,Energy,Environment
Impact Types Societal


Description Boiling Flows in Small and Microchannels (BONSAI): From Fundamentals to Design
Amount £514,287 (GBP)
Funding ID EP/T033045/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2021 
End 06/2024
Description Alfa Laval - Heat Exchanger Manufacturer 
Organisation Alfa Laval AB
Country Sweden 
Sector Private 
PI Contribution Too early to say yet
Collaborator Contribution Too early to say yet
Impact Too early to say yet
Start Year 2019
Description CALGAVIN 
Organisation CalGavin
Country United Kingdom 
Sector Private 
PI Contribution Too early to say
Collaborator Contribution Too early to say
Impact To early to say
Start Year 2019
Description Oxford Lasers 
Organisation Oxford Lasers Ltd
Country United Kingdom 
Sector Private 
PI Contribution Too early to say
Collaborator Contribution Too early to say
Impact Too early to say
Start Year 2019
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 TMD Technologies Ltd 
Organisation TMD Technologies Limited
Country United Kingdom 
Sector Private 
PI Contribution Too early to say
Collaborator Contribution Too early to say
Impact Too early to say
Start Year 2019
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; Talk on Enhanced Boiling Surfaces 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Industry/Business
Results and Impact Presentation and discussion at Aavid, Thermal Division of Boyd Corporation, Ashington, Northumberland. Fundamental concepts on boiling heat transfer, leading to practical aspects of designing new surfaces for enhanced heat transfer were presented by the Brunel team and discussed with technical staff of Aavid. The company is one of the Partners on the project and this meeting was part of our plan to engage with our partners, keeping them informed on progress.
Year(s) Of Engagement Activity 2019