Flow Boiling and Condensation of Mixtures in Microscale

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.

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