Flow Boiling and Condensation of Mixtures in Microscale

Lead Research Organisation: Queen Mary, University of London
Department Name: Sch of Engineering and Materials Science

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.

Publications

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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 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.
Sectors Aerospace, Defence and Marine,Agriculture, Food and Drink,Energy,Environment,Manufacturing, including Industrial Biotechology,Transport

 
Description 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 Convers, 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.
Sector Education,Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology
Impact Types Economic

 
Title Heat transfer and pressure drop database during condensation of steam-alcohol mixtures in microchannels 
Description A large, accurate database based on our novel inverse method for heat transfer and pressure drop during condensation of steam-ethanol and steam-butanol mixtures in microchannels have been obtained. A large, accurate database for condensation heat transfer of steam-ethanol and steam-butanol mixtures on a horizontal smooth tube have been obtained. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Impact Part of the data has been published in international conferences. The data will be published in more conferences and international journals.