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CFA - Tours 2006
37
Acoustique
&
Techniques n° 45
a scrolled radial configuration. Consequently, a range of
scaled radial microfans has been designed and fabricated at
Stokes. Initial characterisation results demonstrate superior
performance characteristics to equivalent axial microfans.
Microchannel coolers
The Stokes Institute, in conjunction with the Tyndall Institute
in Ireland, have successfully produced relatively large
microchannels – hydraulic diameters from 255 to 317µm
– in thermoset plastic and silicon using three different
methods: Precision Sawing, Deep Reactive Ion Etching
(DRIE) and Wet Etching. Mechanical sawing produced near
rectangular channels in two types of thermoset plastic, with
SU-8 photoresist used as an adhesive to bond glass covers
over the channels. Glass covers were anodically bonded over
the DRIE and Wet Etched silicon channels. Full fabrication
details are presented in Eason et al [13], and sections of the
channels are illustrated in figure 5.
Eason et al [14] describes a modular system for testing
each sample using identical inlet and outlet manifolds,
pressure tappings, pumping system, temperature sensors
and instrumentation. The channels – occupying a 16×30mm
area array – were tested using the same inlet and outlet
manifolds. Figure 6 shows measured pressure flow and heat
transfer behaviour for the channels, with other reported data
from the literature. Comparisons with theoretical values, as
calculated from macro scale theory, were also drawn. Heat
transfer data from the channels correlates well with literature
given that entrance effects are ignored in the comparison.
Predicted pressure drops were also found to compare well
to experimental values with no deviation from classical
hydrodynamics theory outside a 95% confidence interval
(see Fig. 6 next page).
A key challenge in the practical implementation of microchannel
coolers is the pumping of the fluid, specifically due to the
high pressure drop associated with the channels. Garimella
and Singhal [15] indicate that valveless, piezoelectric and
electroosmotic pumps appear to be most appropriate
for thermal management applications. Contemporary
developments in surface coatings may offer opportunities to
enhance the performance of microchannel coolers, moreover,
specifically through a reduction in pressure drop. In this regard,
Stokes are currently collaborating with Bell Laboratories on
the adaptation of nano-structured surfaces – as described in
Krupenkin et al [16] – to microchannel geometries in order to
provide control of hydrophobicity and pressure drop.
Energy Considerations
The thermal management of today’s large scale electronic
systems represents a substantial energy cost. Shah et al [17]
indicate power densities of over 3kW/m2 for data centres
within 3 years, outlining a 10,000m2 centre containing five
thousand 10kW computing racks which could feature power
dissipation of order 50MW. A conventional air conditioning
cooling scheme would consume an extra 20 MW which, at
an assumed energy cost of
$100/MWh, would incur $18
million per annum for cooling
alone. Moreover, concerns have
been expressed with reference
to sustainability aspects of
thermal management: Bar-
Cohen and Iyengar [18] adapted
a ‘least energy’ approach to the
optimisation of air cooled heat
sinks which considers not only
thermal performance, but also
energy consumption associated
with material extraction and
fabrication. There is clear
Fig. 4 : Outlet axial and radial flow vectors, full scale and
1/3rd scale axial fans at 0.061 specific speed
Fig. 5 : Microchannel geometries
Thermal Management of Electronic Systems: Emerging Technologies and Acoustic Challenges
Gestion thermique des systèmes électroniques : Technologies naissantes et défis acoustiques