Flow Boiling and Condensation of Mixtures in Microscale

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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.