Thermal management

Periodic Cellular Metal Heat Exchangers

Open-celled cellular metals have shown to be useful thermal management devices for high power density applications [1]. Much of the research and application of cellular metal heat exchangers to date has focused on stochastic topologies (i.e. metal foams). One major drawback of these materials is their cost, which can be as high as $6,000 per pound [2]. CMI has developed methods for manufacturing periodic cellular materials with comparable properties at much lower costs.

One type of periodic cellular metal, termed Woven Microtruss™, is constructed of several layers of wire mesh that have been metallurgically bonded. An example of a plain square weave copper heat exchanger (shown in both 0/90º and ±45º orientations) is shown in Figure 1. Manufacturing flexibility allows for the cells to be oriented in any manner, allowing the mechanical response of the sandwich structure to be tailored to specific loading applications. Layered hollow tubes and straight wires can be used instead of woven precursors. One advantage to this approach is the ability to flow fluids through some or all of the members in a cross flow. Microtruss™ cores are available in several different alloys of high purity copper, aluminum, and stainless steel for heat exchanger applications.

Figure 1: Orthogonal view of a Woven Microtruss™ periodic cellular metal copper heat exchanger shown in both 0/90º and ±45º orientations. (cell size: ~1.5mm).

Figure 2 shows the conceptual operation of a Microtruss™ passive heat exchanger. As a thermal load is applied to one of the surfaces of the panel, the metal wires of the core conduct the heat into the cellular core. A fluid (air, water, etc.) is then passed through the cellular core to extract the heat. The metallic bonding of the core to the facesheets, and among individual lamina, assists in the conduction of heat into the core of the material where it is extracted by the fluid. The flow around the wires creates localized turbulent flow in the region of the wires, while the overall flow through the cellular core remains laminar. Thus fluid contact made to the conducting wire is enhanced and thermal energy is better conveyed to the working fluid. While the open-celled topology of the core allows for fluid to flow vertically and laterally between cells, the preferential flow of the fluid is along the cells' axes. As all of the cells in a given row and column are co-linear, this allows for a principally straight-line flow of the working fluid and few head losses due to obstruction.

Figure 2: Schematic illustration of a Microtruss™ passive heat exchanger where heat, applied to the face of the panel, is conducted into the cellular core and extracted via the working fluid.

The pressure drop, average heat transfer, and overall thermal performance of Woven Microtruss™ passive heat exchangers have been experimentally investigated under steady-state forced convection conditions [2]. For the range of Reynolds number (based on unit cell) considered, fluid flow in the wire screen meshes is turbulent: that is, pressure loss coefficient in both cases is independent of coolant velocity. Comparisons made between copper Woven Microtruss™ and open-celled copper foams show that, thermally, the wire-screens perform as well as metal foams, since both have large surface area densities, Figure 3. However, the penalty on pumping power required is significantly smaller for wire-screens because of their periodic, rather than stochastic, structure. The overall thermal performance index (ratio of heat transfer to pressure drop) of wire-screens has a value approximately 3 times larger than that of copper foams with stochastic structure but similar pore sizes. Significant opportunities to maximize the heat transfer performance of periodic cellular metals by varying the pore fraction, anisotropy of the pores and metallic alloy used appear to exist. Such manipulation can be accomplished by selection of the appropriate wire mesh.

Figure 3: Experimental data for the friction factor and Nusselt number for a variety of stochastic and periodic heat exchanger concepts

References

[1] M.F. Ashby, A.G. Evans, N.F. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley. Metal Foams: A Design Guide. Butterworth-Heineman, 2000.
[2] J. Tian, T. Kim, T. J. Lu, H. P. Hodson, D.T. Queheillalt, H. N. G. Wadley. "The effects of topology upon fluid-flow and heat-transfer within cellular copper structures." Submitted for publication