Flow Boiling and Condensation of Mixtures in Microscale

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

Abstract

This proposal is for a joint project between internationally-leading, UK heat transfer research groups at the Universities of Edinburgh, Brunel and Queen Mary, London in collaboration with four industrial partners (Thermacore, Oxford Nanosystems, Super Radiator Coils and Rainford Precision) in the areas of micro-fabrication and thermal management.

Advances in manufacturing processes and subsequent use of smaller scale electronic devices operating at increased power densities have resulted in a critical demand for thermal management systems to provide intensive localised cooling. To prevent failure of electronic components, the temperature at which all parts of any electronic device operates must be carefully controlled. This can lead to heat removal rate requirements averaging at least 2 MW/m2 across the complete device, with peak rates of up to 10-15 MW/m2 at local 'hot spots'. Direct air cooling is limited to about 0.5 MW/m2 and liquid cooling systems are only capable of 0.7 MW/m2. Other techniques have not yet achieved heat fluxes above 1 MW/m2.

Boiling in microchannels offers the best prospect of achieving such high heat fluxes with uniform surface temperature. In a closed system an equally compact and effective condenser is required for heat rejection to the environment. At high heat flux, evaporator dry-out poses a serious problem, leading to localised overheating of the surface and hence potentially to burn out of electronic components reliant on this evaporative cooling. Use of novel mixtures, termed 'self-rewetting fluids', whose surface tension properties lend themselves to improved wetting on hot surfaces, potentially offers scope for enhanced cooling technologies.

In this project, two different aqueous alcohol solutions (one of which is self-rewetting) will be studied to ascertain whether they can provide the necessary evaporative and condensation characteristics required for a closed-loop cooling system capable of more than 2 MW/m2.

Researchers at the University of Edinburgh will study the fundamentals of wetting and evaporation/condensation of the mixtures to establish the optimum mixture concentrations and heat transfer surface coating for both evaporation and condensation, using advanced imaging techniques. At Brunel University London, applications of the fluids in metallic single and multi microchannel evaporators will be investigated. Researchers at Queen Mary University London will carry out experimental and theoretical work on condensation of the mixtures in compact exchangers. The combined results will feed into the design of a complete microscale closed-loop evaporative cooling system.

Thermacore will provide micro-scale heat exchangers and Oxford Nanosystems will provide structured surface coatings. Sustainable Engine Systems, Super Radiator Coils and will provide advice and represent additional ways of taking developments originating from this research to the market. Rainford Precision will provide Brunel University micro tools and support on their use in micromachining.

Planned Impact

This proposal, from three UK research groups in association with cutting-edge companies, will enhance the ability of UK industry to compete worldwide in state-of-the-art, CFC-free, cooling systems. The research team incorporates three leading research groups from the Universities of Edinburgh, Brunel and Queen Mary London along with four industrial partners (Thermacore, Oxford Nanosystems, Super Radiator Coils and Rainford Precision) in the areas of micro-fabrication and thermal management. It is in line with EPSRC strategic priorities in the Delivery Plan 2011-15 and 2015/16 update, addressing sub-themes in both Energy and Manufacturing the Future. In particular the proposal relates to Energy Efficiency (smaller overall systems for given loads), Fuel Cell Technology (compact H2 storage systems), nuclear fission and fusion programmes (training of the next generation of two-phase experts and cooling of ultra-high thermal devices) and Solar Energy (cooling of photovoltaics using microchannels). In relation to Manufacturing the Future the project aims to provide enabling technological advances referred to in Microsystems and Microelectronics Design.

In general, an increased industrial use of micro scale thermal heat sinks will provide higher thermal efficiencies while reducing material and fluid inventory hence enabling a reduction in energy use and emissions. In certain cases, relating to the product lines of our partners, the proposal can enable advances in technological products. Hence we expect this to contribute to the UK competiveness, growth and job creation in thermal management equipment and marketable know-how.

Through the use of novel fluid mixtures, there is the opportunity for significant gains to be made in closed-loop cooling systems, especially those required for thermal management of small scale devices, such as electronic chips and components where existing cooling technologies are fast becoming insufficient. In this area, there is a need for cooling technologies, capable of providing a uniform surface temperature, where an average of 2 MW/m2 has to be removed, with heat removal rates of up to 10-15 MW/m2 at local 'hot spots'. Current limitations to heat flux, even in the case of laboratory prototypes, are below 1 MW/m2. Given that the value of the thermal management industry is predicted to be $14.7B by 2019 1, this represents a major opportunity for the UK to grow its share of a lucrative market segment.

Product development in some areas is being limited by the need for such thermal solutions. Boiling in microchannels offers the best prospect for delivering such high heat fluxes, with the required uniform surface temperature. However, for such systems, dry out of the heated surface in such devices is still a major problem, leading to local overheating of the surface. Novel mixtures, termed 'self-rewetting fluids' offer the prospect of ultra-high heat flux in micro-scale devices, by improving the capability of the fluid to 'wet' and hence maintain a liquid film on the heated surface. For such cooling systems, recycling of coolant is essential, so a condenser is required to return the vapour to the liquid state: thus the research team will establish working parameters for both evaporation (cooling) and condensation for a complete working system.

Immediate impact through our partners is expected. Involvement of Oxford Nanosystems enables surface coatings to be chosen in association with the self-rewetting fluids for optimising the wettability of the liquid on the surface, critical for high heat flux applications. Rainford Precision offers support for the development of precision machining of microchannels and hence their ease of manufacture and subsequent use. Thermacore, and Super Radiator Coils provide routes to commercialise the cooling systems developed as a result of this work.

1 http://www.bccresearch.com/market-research/semiconductor-manufacturing/thermal-management-smc024j.html
 
Description Flow boiling in microchannels offers the best method for removing the high heat fluxes that prevail in electronic devices and form a bottle neck for further required increases in power densities. Use of novel mixtures, termed 'self-rewetting fluids', whose surface tension properties lend themselves to improved wetting on hot surfaces, potentially offers scope for enhanced performance at the evaporator and the required condenser for a fully integrated thermal management system.
The key findings of the project can be summarised below.
A comparison between ordinary mixtures (water-ethanol) and self-rewetting mixtures (water-butanol) was undertaken to elucidate fundamentals of Marangoni and solutal effects on the evaporation and wetting of mixtures inside a microchannel. Measurements of evaporation rates under variable applied power was carried out. The work has led to the quantification of the contribution of solutal capillary effects on the evaporation and wetting in ordinary mixtures as well as self-rewetting fluids.
The work on condensation quantified condensation rates in the ordinary mixtures (water-ethanol) as well as the self-rewetting fluids (water-butanol).The experiments have demonstrated the noticeable effect of the condensation/absorption of vapour on both a sessile and pendant drops. The most affected property was found to be dynamic wetting.
In flow condensation in microchannels, the experiments have not shown noticeable heat transfer enhancement through addition of small amount of ethanol and butanol (less than 1% by mass). In condensation on horizontal smooth tubes, the experiments have shown significantly high heat transfer enhancements - four times of steam-ethanol and ten times of steam-butanol mixtures (less than 1% by mass). This is thought to be attributable to the model of condensation prevailing in the confined space of microchannels.
In flow boiling, heat transfer was found to increase with the addition of small amount (5% v/v) of ethanol in water. This enhancement is found to depend on the applied heat flux. The first results with the addition of butanol indicated the possibility of a maximum enhancement at 2% v/v butanol in water by 130%. Increasing the butanol concentration further (4 and 6 %) resulted in heat transfer rates lower than that of pure water. We hope to be able to repeat these results verifying an optimum butanol concentration for maximum heat transfer enhancement. Also the mixture delayed the occurrences of dry-out, which can lead to low heat transfer rates and failure of a cooling system. The effect of surface coating provided by Oxford nanosystems were examined in an oxygen free copper multi microchannel heat exchanger and the results indicated up to 43% enhancement in the average heat transfer coefficient.
Exploitation Route The results of the project were published in 17 conference and journal papers by the academic partners and their teams. This topic was also covered in keynote lectures (e.g. 16th Int. Heat Transfer Conf, Beijing, 2018 and 15th Int. Conf. on Nanochannels, Microchannels and Minichannels, 10-13 June 2018, Dubrovnik, Croatia) as well as in presentations at an academia-industry workshop (Thermal Management Workshop, 31st May 2018, Brighton University)
Discussion with industrial partners for application are now well underway. A recent visit and presentation to Thermacore -Boyd Corporation in January 2020 initiated discussion on the possibility of further work. In particular, our industrial partners are interested in comparison trials between a water charged commercial heat pipe assembly and novel self-rewetting fluid assembly (ethanol or butanol water mixtures). The positive result on the effect of coatings on the flow boiling performance and the possible heat transfer enhancement was communicated to Oxford nanoSystems and was presented at TMD Technologies in January 2020. Representatives from the two companies have agreed to arrange to meet to discuss collaboration on cooling of electronic equipment with designs that include the coating provided by Oxford nanosystems. Thermacore will also consider coated surfaces for their ammonia systems. The academic team is supporting Oxford nanoSystems in developing their own testing facilities and providing data in the form of graphs to help them market their product.
The work completed under this project formed the basis for an additional EPSRC funded project (Brunel, UoE and Imperial College) entitled Enhanced Multiscale Boiling Surfaces (EMBOSS): From Fundamentals to Design (EP/S0195202). In this work, our experimental and computational techniques, spanning the scales from molecular to millimetres, will inform the rational design, fabrication, and optimisation of operational prototypes of pool-boiling thermal management systems. Our industrial partners include Thermacore, TMD ltd and Oxford nanoSystems as in the project described here.
Sectors Aerospace, Defence and Marine,Electronics,Energy,Environment

URL https://www.research.ed.ac.uk/portal/files/45797096/Flow_boiling_of_ethanol_water_in_square_minichannel.pdf
 
Description A total of 17 conference and journal papers were published by the academic partners and their teams related to the work of the project. Keynote lectures that covered this area included: 16th Int. Heat Transfer Conference, Beijing, 2018 and 15th Int. Conf. on Nanochannels, Microchannels and Minichannels, 10-13 June 2018, Dubrovnik, Croatia). a presentation was also made at an academia-industry workshop (Thermal Management Workshop, 31st May 2018, Brighton University). The academics have presented their work directly to industry during visits to discuss findings and plan further work. Examples: Thermacore -Boyd Corporation (regular communication and visit in January 2020) Initiated discussion on the possibility of further work. In particular, Thermacore is interested in comparison trials between a water charged commercial heat pipe assembly and novel self-rewetting fluid assembly (ethanol or butanol water mixtures). Oxford nanosystems (coatings) The positive result on the effect of coatings on the flow boiling performance and the possible heat transfer enhancement was communicated to Oxford nanoSystems on a regular basis. The benefits of using coatings (heat transfer enhancement) was also presented at TMD Technologies in January 2020. The academics facilitated an introduction and representatives from the two companies have agreed to arrange to meet to discuss collaboration on cooling of electronic equipment with designs that may include the coating provided by Oxford nanoSystems. Thermacore will also consider further and discuss with the academic team and Oxford nanoSystems coated surfaces for their ammonia systems. The academic team continues to support Oxford nanoSystems in developing their own testing facilities and provide data in the form of graphs to help them market their product. A meeting to discuss this and further work is scheduled for March 2020.
Sector Aerospace, Defence and Marine,Electronics,Energy,Environment,Transport
 
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 Super Radiator Coils 
Organisation Super Radiator Coils Limited
Country United States 
Sector Private 
PI Contribution Provide data on thermal performance of facility
Collaborator Contribution Test Condenser/Progress meetings
Impact Too Early for that
Start Year 2016
 
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