Metal Foams: A Design Guide

232 Metal Foams: A Design Guide
heat flux, back pressure and fluid flow rate. Some results for a representative
density ( / s D 0.3) are plotted in Figure 17.16. The benefits of the cellular
metal can only be utilized if the fan/blower assembly is capable of operating at
back pressures of order 0.1 atm (10 kPa), while delivering flow rates about one
l/s. Upon comparison with Figure 17.12, it is apparent that these heat fluxes
substantially exceed those normally associated with forced air convection.
A corollary of the heat flux is that there must be a temperature rise in the
cooling air.
Peak power density (MW/m 2 )
14
12
10
8
6
4
2
0
H s = 10 mm
b hs = 3 cm
b si = 0.5 cm
ρ/ρ s = 0.3
0.1 1 10
Back pressure, ∆p* (kPa)
Figure 17.16 Peak power density as a function of back pressure
Device design issues
To take full advantage of the heat transfer capabilities of the cellular metal,
the thermal design of the device must establish nearly isothermal conditions:
that is, minimal thermal resistance between the electronics and the heat sink
surface. This can be achieved by combining a high thermal conductivity
aluminum nitride dielectric with a copper planar micro-heat-pipe, fully integrated
with the heat sink by brazing (in order to exclude high thermal resistance
at the interfaces Ł ). Simulated temperatures for this scenario indicate that the
required isothermality can be realized: albeit that the associated manufacturing
requirements are stringent and yet to be demonstrated.
17.9 Applications under consideration
Skin-stiffened structures become more weight-efficient if the skin itself is a
sandwich structure. This issue is discussed in some detail in Chapter 10, in
Ł Applications of polymer bonding and thermal greases would violate these requirements.