Enhancing Heat Transfer in Porous Metals through Optimisation of Flow Resistance

Lead Research Organisation: University of Liverpool
Department Name: Management School


Thermal management has become a critical issue in electronics because of increasing volumetric power densities and the harsh environments in which they are deployed. Active cooling is often required for high rates of heat dissipation because conventional passive cooling techniques are inadequate. Porous metal has been demonstrated to be highly efficient and cost effective in heat dissipation by forced fluid cooling. A main problem impeding its wider application is the high pumping power required to move the working fluid through the cooling device due to its large resistance to fluid flow.

This project sets out to address the scientific and technical issues in thermal applications of porous metals manufactured by the space holder methods, which have distinctive porous structure and unique heat transfer behaviour. The aims of the research are to understand the mechanistic relationships between flow resistance, heat transfer and pore structure and to develop technologies to create tailored porous metal structures for significantly enhanced heat transfer performance with minimised flow resistance. A combination of manufacturing, properties characterisation, modelling and process development will be carried out to identify the fundamental structural properties underpinning the thermal fluid behaviour in porous metals, to quantify their effects on heat transfer coefficient and fluid flow resistance, and to design and create heterogeneous porous structures for a step change in overall active cooling performance.

The global market for thermal management products is more than $10 billion with an annual growth rate of 6.8%. UK has a significant share in this market and is one of the leaders in developing new materials and technologies for active cooling devices for electronics. This project will provide scientific understanding and technical development underpinning the design and manufacture of a promising class of porous metals that are currently being developed by industry for thermal management applications. This research will ensure that UK maintains the leading position in this niche field. This research will also benefit the research and development of non-thermal porous products for environmental and energy applications, e.g., sound absorbers, porous electrodes and catalyst supports, where flow resistance has a deterministic effect.

Planned Impact

Thermal management of microelectronics is an important issue in many branches of industry, such as computer, renewable energy and telecommunications. The global market for thermal management products is more than $10 billion in 2014, with an annual growth rate of 6.8%. Thermal management hardware accounts for 84% of the total thermal management market. Porous metals have been demonstrated to be a competitive microfluidic material for use in compact heat exchangers for active cooling, passive cooling and heat recovery.

This project tackles key scientific issues and technical challenges in the design and manufacture of porous metals for thermal management applications. Mechanistic understanding of the fluid flow and heat transfer in porous metals will provide a scientific basis for optimising the design of the porous metal compact heat exchangers for significant enhanced heat transfer performance. The techniques developed for creating heterogeneous structures, if successful, can result in a step change in the overall heat exchanger performance, i.e., high heat transfer efficiency coupled with low pumping power requirement.

The industrial communities involved in thermal management, especially the industrial project partners involved in this project, will benefit directly from this research. The new technologies developed in this project for manufacturing heterogeneous porous structures can be exploited by porous metal manufacturers and process developers. The principles established in this research can be applied to the design and manufacture of compact heat exchangers for active cooling and waste heat recovery, making significant improvement to the heat transfer efficiency.

The manufacturers and developers of porous metal products for non-thermal applications will also benefit from the research. For example, the performance of sound absorbers, porous electrodes and porous catalyst supports all depends on their resistance to fluid flow. The knowledge accrued from this research is transferrable to these areas.

This research will likely generate new patents in low-cost technologies for manufacturing porous metals with heterogeneous structures. We have an outstanding track record in commercial exploitation of university research. We will work with the industrial partners to commercialise any technologies and products arisen from the project, ensuring that UK maintains the leading position in this niche field.
Description A new sintering process was developed for manufacturing copper microchannels and a patent application was made. The process provides a low cost technology for manufacturing multilayers of directional channels. The resultant microchannels combine the benefits of good heat transfer performance of porous copper and low pressure drop of conventional machined copper microchannels. They offer great potential for use as high-performance compact heat exchangers, e.g. in cooling electronic devices.

The heat transfer performance of porous copper manufactured by the Lost Carbonate Sintering (LCS) process was investigated experimentally and numerically. The unique porous structure was found to be represented well by a 3D geometric model based on the face-centred-cubic arrangement of spheres linked by cylinders. The effects of structural characteristics (porosity, pore size and gradient structures) on the heat transfer coefficient and pressure drop were studied. The fluid flow resistance was found to decrease with increasing porosity and decreasing pore size. Maximum heat transfer performance was observed at an optimum porosity of 60%, where heat removal by conduction and convection is balanced. The permeability, form drag coefficient and heat transfer coefficient of the LCS porous copper are in the ranges of 10-300 micron^2, 0.01-0.15 micron^-1 and 10-25 kW/m^2 K, respectively, showing excellent overall heat transfer performance.

Micro-particle image velocimetry was used to identify the flow regimes and quantify the velocity field at pore scale in microporous media. Pressure-drop measurements and particle image velocimetry measurements were conducted simultaneously to evaluate the flow regimes and flow behaviours. Four different flow regimes were found. The spatial distribution and fluctuation of the velocity in all regimes were investigated. Combining velocity and pressure measurements provided direct evidence that underpins the transitions between the different flow regimes.

The tortuosity of porous copper was measured by a diffusion method using a diaphragm cell. The tortuosity of porous metals manufactured by space holder methods is in the range of 1.3-1.8, decreasing with porosity and increasing with pore size. The direct diffusion method proves to be more accurate than the indirect methods commonly used.
Exploitation Route This research has demonstrated that the porous copper manufactured by the Lost Carbonate Sintering (LCS) process has good overall heat transfer performance. It has an excellent heat transfer coefficient, while a main drawback is its high flow resistance. The newly developed sintered microchannels reduced the flow resistance significantly compared to LCS porous structure without compromising the heat transfer coefficient. These materials have great potential for use as compact heat exchangers and active cooling devices, especially in high-power-density electronics. Manufacturers and designers of thermal management devices and solutions are sought to develop and commercialise these technologies.

The simultaneous velocity and pressure measurements of fluid flow in micro-porous media proves to be a powerful method to investigate fluid behaviours, especially the transitions between flow regimes. The combination of micro-particle image velocimetry and conventional pressure measurement can be applied to investigating other types of porous materials. The diaphragm cell method for tortuosity measurements has been demonstrated to be more effective and accurate than other common methods, e.g. the acoustic absorption method, and can be implemented with ease to any porous materials.
Sectors Aerospace, Defence and Marine,Electronics,Manufacturing, including Industrial Biotechology

Title Article and method 
Description A method of providing an article having a set of directional channels, including a first directional channel, therein is described. The method comprises preparing a mixture including particles comprising a first material and a first binding agent. The method comprises providing an article precursor by surrounding a pattern comprising a second material with the mixture. The method comprises heating the article precursor thereby coalescing the particles to provide the article. The method comprises removing the pattern by reacting the second material to form a gaseous product, thereby providing the set of directional channels in the article, wherein the set of directional channels corresponds with the removed pattern. Such an article is also described. 
IP Reference GB1811899.2 
Protection Patent application published
Year Protection Granted 2018
Licensed No
Impact Several companies have expressed interest in exploiting the invention commercially.