Sandwich panel structures based upon highly porous, periodic cellular metal (PCM) structures have attracted significant interest for load supporting structural applications. Cellular metals with open cell topologies are also attractive heat exchange media where dissipation of high intensity heat in relatively small spaces is required. Consider a typical application where a high heat flux is deposited on one of the face sheets of a sandwich panel structure. Corrugated or prismatic core structures are widely used to dissipate heat because they provide ample opportunity to conduct the heat from the hot face sheet into the web structure. This results in a temperature gradient through the thickness of the core. If a coolant flows through the core, heat transfer at the metal – coolant interface occurs and this heat is transported away from the webs raising the average temperature of the coolant and reducing that of the metal core. Figure 1 shows several examples of copper based, multifunctional heat exchanger sandwich structures.

The performance of a sandwich panel heat exchanger can be characterized by the heat removed under constant fluid flow velocity and by the pressure difference required to propagate the coolant at that velocity through the structure. These can be quantified by two topology dependent dimensionless parameters. The Nusselt number, Nu_H, characterizes the heat removed from the structure and is defined by:

where h is the heat transfer coefficient between the metal webs and the coolant, H is a characteristic dimension (e.g the cell or core height) and k_f the thermal conductivity of the coolant. The friction factor, f, defined by:

where U_m is the mean coolant velocity at the inlet of the heat exchanger, and Δ P/L is the pressure drop per unit length. The coolant velocity can also be expressed in dimensionless form by the Reynolds number of the flow defined as:

where   ρ_f  and  μ_f   are the coolant density and viscosity.

Any open cell metal structure that allows a coolant flow to pass through the pore structure can be used as a heat exchange medium. Both stochastic (metal foams and sintered powders) and the periodic topology structures shown in Fig. 1 have been made from a wide variety of metals and their heat exchange performance explored. Metal foams with open cells have inferior load supporting capability compared to periodic structures of the same weight because their deformation under mechanical loading is dominated by cell wall bending as opposed to cell wall stretching in most periodic structures. However, they provide a high thermal conductivity path for thermal transport, possess a very high surface area for heat transfer to a cooling fluid and provide a contiguous (though tortuous) path for coolant flow through the structure. Periodic cellular structures have anisotropic pore structures. For instance, prismatic structures have one low friction flow direction, where pyramidal lattices have two, and the 3D Kagome and tetrahedral topologies have three easy flow directions. Textile and co-linear structures have one very easy flow direction while flow in others lies between that of the lattices and prismatic structures. Their thermal characteristics are therefore orientation dependent, leading to optimization opportunities.

Numerous studies that have sought to characterize the cross flow heat exchange performance of the various cellular structures. Several recent attempts have been made to compare them in terms of the dimensionless metrics described above as a function of the flow velocity expressed in the form of the dimensionless Reynolds number based upon H defined as the core thickness used for measurements. Figure 2 shows a recent comparison of the friction factor and Nusselt number of aluminum, copper and iron base alloy foams, copper textile structures, copper prismatic structures and various truss structures measured in the easiest flow direction. The metrics for various reference configurations are also shown including Moody’s result for an empty channel (a panel with the core removed), a corrugated duct and a louvered fin structure. The most promising structures have a high Nusselt number and low friction factor at the coolant velocity of interest (set by the input thermal flux, the coolant, the required operating temperature and the available fluid pumping capacity).

Several observations can be deduced from these preliminary efforts to compare structures. If heat removal and friction factor are equally important, Louvered fins and prismatic structures have the best performance because of their low frictional losses. As friction becomes less of a constraint, or the need to remove heat dominates, the textile structures and smaller cell size diamond prismatic structures become interesting because they have a large area of contact with the hot face sheet and present a high surface area to the flow. There are important caveats yet to be resolved for multifunctional structures which must support loads and enable heat transfer to a cross flow. The optimum heat exchanger structures balance the conduction of heat through webs and trusses (maximized by increasing the relative density) against the need to create easy flow paths (maximized by minimizing relative density). The optimum is material selection and application specific but lies in the 10 - 20% relative density range. Light weight sandwich panel optimizations for bending typically result in structures with much lower core relative densities (around 2%).

One approach to resolving this dilemma is to increase the thermal conductivity of the cellular structure. This then enables the conductive transport of the thermal flux using smaller cross sectional area webs or trusses. Heat pipes and plates offer a novel potential route. Preliminary results are encouraging. An additional advantage of the approach arises when hollow truss structures are utilized since these also possess exceptional strength compared to their solid equivalent relative density counterparts.