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CFA - Tours 2006
39
Acoustique
&
Techniques n° 45
hard disks. In high-power applications such as servers and
telecommunications switches, the minimisation of noise
generated by fans has become a critical challenge for
designers – see Kaivola and Avikainen [20], for example.
The noise spectrum of a fan is typically broadband, but with
strong peaks associated with bearings, vortex shedding from
blades, and their harmonics. In addition, the spectrum varies
as a function of rotational speed – the faster the speed, the
louder the fan noise, and the stronger its distinct tones –
and the operating point of the fan. In this regard, the higher
the pressure differential across the fan, the higher the noise
level. Fan manufacturers have made significant progress
in creating quieter products but, nevertheless, it is usually
necessary for designers to minimise noise through the use
of speed control, for example, or the application of sound
absorbent material. Extensive testing is customary because
typical electronic systems are geometrically complex, and it
is notably difficult to anticipate how sound may be amplified
or attenuated by the structures and cavities of the system’s
cabinet. Moreover, the interaction of airflow with system
features may generate specific tones, and multiple fans
may create ‘beating’ effects. It is clear that acousticians can
contribute through the formulation of design guidelines, and
through the development of noise control techniques – the
deployment of sound absorbent materials, for example, or
active techniques for noise cancellation.
Simulation tools for acoustic noise
A range of simulation tools is available to assist with mechanical
design aspects of electronic systems – computational fluid
dynamics solvers for the thermal and airflow phenomena
associated with cooling, and finite element packages for
analysing mechanical and thermomechanical issues. There
is no commercial simulation tool tailored for acoustic noise
in electronic systems at present, however, and an opportunity
is evident for a computationally-efficient, easy-to-use software
package to support designers in dealing with acoustic
phenomena. In this context, it is instructive to consider thermal
simulation packages such as Flotherm which utilise ‘compact
models’ in order to achieve easy model building and swift
numerical convergence.
For example, perforated plates or filters – common features of
electronic systems – are modelled simply as flow resistances
using empirically-determined loss coefficients. In this manner,
designers can select from a library of ‘smart parts’ such as
fans, heat sinks, components and printed circuit boards in
order to create a thermal model of an electronic system.
A similar approach may be appropriate for an acoustic
noise solver. In this regard, sources such as fans could be
characterised in terms of noise level and spectral content,
with ‘compact models’ used to represent the interaction of
the acoustic field with structures such as circuit boards, heat
sinks, perforated plates, filters and enclosures. It is also
necessary, however, to consider noise generated by the
interaction of airflow and system structures such as grilles or
perforated plates. In this regard, mean air velocities in typical
systems are low – of order 1-4m/s – but airflow is generally
unsteady and spatially-varying, particularly downstream
from cooling fans. There is much scope for applied research
in identifying and implementing computationally-efficient
modelling approaches for sound generation and propagation
in electronic systems.
Thermoacoustic phenomena
Apart from vapour compression refrigeration for air chilling,
there are some applications for heat pumps in contemporary
electronic systems – in particular, thermoelectric modules
based on Peltier cells are commonly used to achieve
temperature control for photonic devices such as laser diodes.
Thermoacoustic devices configured as acoustic coolers
potentially offer advantages over heat pumping techniques
such as vapour compression cycles or thermoelectric
modules. Specifically, thermoacoustic devices feature low
power consumption because their relative effectiveness is
approximately twice that of thermoelectric cells. Moreover, they
obviate the environmental threat associated with refrigerant
fluids and, significantly, the simplicity of their structure offers
the potential for low cost manufacture and high reliability.
Finally, thermoacoustic devices configured as prime movers
to dissipate heat in the form of sound could, for ultrasonic
operation, facilitate point-of-source cooling for microelectronic
devices – an intriguing, if difficult, possibility.
These three challenges – noise minimisation; simulation;
and the application of thermoacoustic phenomena – offer
rich opportunities for acousticians in the application area of
thermal management.
Conclusions
A range of emerging point-of-source thermal management
technologies has been outlined, from small-scale air movers
and microchannel coolers, to heat pipes and thermoacoustic
engines. Recent research at the Stokes Institute has been
reviewed :
- Pressure-flow characteristics of a 6mm diameter axial fan
have been presented, featuring a maximum delivery of 0.263
m3/hr, and mean outlet velocities of order 3-4 m/s. Small-
scale radial fans are currently under development.
- The fabrication and performance characterisation of
microchannels for package-level cooling has been reported.
Relatively large channels – hydraulic diameters from 255 to
317µm – have been created in silicon and thermoset plastic.
Pressure-flow characteristics and heat transfer data correlate
well with classical theory for macro-scale geometries.
Energy aspects of point-of-source cooling technologies have
also been considered, with specific reference to their sink
temperatures. Heat dissipation paths with greatly reduced
thermal resistance may facilitate efficient cooling schemes or
– potentially – the recovery of energy to drive cooling systems.
Finally, three challenges in thermal management have been
outlined which represent opportunities for acousticians: noise
minimisation; simulation; and the application of thermoacoustic
phenomena.
Acknowledgments
The author particularly acknowledges his collaborator Pierrick
Lotton of Laboratoire d’Acoustique de l’Université du Maine
(LAUM), and the contributions of colleagues Ronan Grimes,
David Quin, Kieran Hanly, Cormac Eason and Tara Dalton. The
work reported in this paper has been enabled by the financial
support of Enterprise Ireland, and assisted by Science
Foundation Ireland under grant number 03/CE3/I405.
Thermal Management of Electronic Systems: Emerging Technologies and Acoustic Challenges
Gestion thermique des systèmes électroniques : Technologies naissantes et défis acoustiques