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8
e
CFA - Tours 2006
35
Acoustique
&
Techniques n° 45
Conventional schemes offer many advantages – cost, flexibility,
no limitations in terms of cooling fluid – however a penalty
is incurred due to relatively high thermal resistances from
the source to the ultimate energy sink. Cooling technologies
deployed at source can offer enhanced thermal performance.
Some examples are as follows:
Small-scale air movers :
Rotary or oscillatory devices have
been reported in the literature. Microfans are covered in detail
in this paper, and oscillatory fans driven by piezoelectric
actuators have been developed – for example, Burmann et
al [2]. These small-scale air movers can be used to enhance
local heat transfer rates, as package-level coolers, or to act
as system-level fans for portable devices.
Microjets/microspray :
A number of authors have reported
microjets for chip-level liquid cooling – Wang et al [3], for
example. Micro-spray nozzles generate 50-100mm droplets of
a dielectric fluid which, impinging on the textured back surface
of a chip, can remove fluxes of over 100 W/cm2.
Heat pipes: Numerous references cite the development of
micro-scale heat pipes – Lee et al [4], for example. Micro-
scale hydraulic diameters have been realised, with modest
power densities. Contemporary research is addressing
microfabricated structures to enhance capillary pressure,
and the integration of heat pipes in silicon, Gillot et al [5].
Microchannel coolers :
The formation of channels in silicon
to facilitate liquid cooling dates back to the early 1980’s,
however widespread application is not yet evident. High heat
fluxes – of order 103 W/cm2 – are feasible, yet a number
of practical impediments remain – in particular related to
pumping requirements. Microchannel characteristics are
reported in this paper. With reference to liquid-cooled cold-
plates, Garimella et al [6] note the evolution of meso-scale
components as an enabler for smaller scale devices.
Thermoacoustic cooling :
The adaptation
of thermoacoustic engines for thermal
management has been made feasible by
today’s microfabrication techniques. A high-
frequency – 4 to 24 kHz – thermoacoustic
device for electronics cooling is reported by
Symko et al [7]. Operation as both acoustic
cooler and prime mover is described, and
avenues for further improvement – specifically
smaller scale, higher frequency – are discussed
in Abdel-Rahman et al [8].
Micro thermoelectric cells :
Small scale
thermoelectric cells have been realised
– daSilva and Kaviany [9], for example.
Thermoelectric cells are more suitable for
thermal control than for cooling, however,
due to their high thermal resistance, high
current requirement, and low coefficient of
performance.
A key feature of these point-of-source
technologies is that their deployment has
generally been facilitated by recent advances
in micro-manufacturing. Micro-fabrication
techniques such as surface and bulk silicon
micromachining, laser micromachining,
polymer micro-embossing and Electro-
Discharge Machining (EDM) have enabled the creation of
the small scale (~100mm) features at the core of these
technologies; moreover, surface structuring has allowed the
enhancement of effective area, and the control of parameters
such as radiation properties, friction factor and – for objects
in contact – thermal interface resistance. Contemporary
development of microfabrication is particularly vibrant,
offering rich possibilities for the further evolution of thermal
management technologies.
The next sections of this paper cover some point-of-source
thermal management solutions under investigation at the
author’s Institute. Energy aspects of point-of-source cooling
technologies are then considered, with specific reference
to their sink temperatures. In the context of large-scale
electronic systems, heat transfer paths from source to sink
with greatly reduced thermal resistance open opportunities
for efficient cooling schemes or – potentially – the recovery of
energy. Finally, the paper outlines three challenges in thermal
management for the acoustics community: the minimisation
of noise from sources such as fans and hard disk drives; the
simulation of acoustic noise in electronic systems; and the
application of thermoacoustic phenomena.
Thermal Management Solutions
This section comprises a review of recent research at the
Stokes Institute into point-of-source thermal management
solutions: small-scale fans; and microchannel coolers.
Small-scale fans
A programme of research is in progress at Stokes on the
design, fabrication and performance characterisation of small-
scale fans for thermal management. Two main application
Fig. 1: Thermal management: conventional air
cooling and point-of-source technologies
Thermal Management of Electronic Systems: Emerging Technologies and Acoustic Challenges
Gestion thermique des systèmes électroniques : Technologies naissantes et défis acoustiques