Metal Foams: A Design Guide

Chapter 13
Thermal management and heat transfer
13.1 Introduction
The thermal conductivities of metal foams (see Table 4.1(b)) are at least an
order of magnitude greater than their non-metallic counterparts, so they are
generally unsuited for simple thermal insulation though they can provide some
fire protection. The thermal conductivities of closed-cell foams are, however,
lower than those of the fully dense parent metal by a factor of between 8
and 30, offering a degree of fire protection in, for instance, an automobile
bulkhead between engine and passenger compartment. More important, opencell
metal foams can be used to enhance heat transfer in applications such
as heat exchangers for airborne equipment, compact heat sinks for power
electronics, heat shields, air-cooled condenser towers and regenerators (Antohe
et al., 1996; Kaviany, 1985). The heat-transfer characteristics of open-cell
metal foams are summarized in this chapter.
Examples of the use of metfoams for thermal management can be found in
the case studies of Sections 16.4, 16.5, 16.6 and 16.8.
Figure 13.1 illustrates a prototypical heat-transfer configuration. Heat
sources are attached to thin conducting substrates between which is bonded
a layer of open-celled foam of thickness b and length L. A fluid is pumped
at velocity vf through the foam, entering at temperature T0 and exiting at
temperature Te. An idealization of the foam structure is shown below: the
relative density is Q D c/ s and the diameter of the cell edges is d. Thelocal
heat-transfer coefficient at the surface of a cell edge is h.
There are three guiding design principles.
1. High conductivity ligaments are needed that transport the heat rapidly into
the medium: the preference is for metals such as Cu or Al.
2. A turbulent fluid flow is preferred that facilitates high local heat transfer
from the solid surface into the fluid.
3. A low-pressure drop is needed between the fluid inlet and outlet such that
the fluid can be forced through the medium using a pumping system with
moderate power requirements making low fluid viscosity desirable.
Heat transfer to the fluid increases as either the ligament diameter, d,
becomes smaller or the relative density, / s, increases because the internal