Proceedings of the
Second International Energy 2030 Conference,
November 4-5, 2008, Abu Dhabi, U.A.E.
Thermal Design for Sustainability of Air Cooled Heat Sinks
Avram Bar-Cohen
University of Maryland, USA
Madhusudan Iyengar
International Business Machines, USA
Abstract
Extended surfaces of fins, or so-called heat sinks, are in common use in the electronic industry and serve to
extend the thermal capability of convective cooling with air. The use of passive, natural convection cooled heat
sinks offers substantial advantages in cost and reliability, but is often accompanied by relatively low heat
transfer rates, and significant added mass. The use of fan driven forced air cooling facilitates high performance,
compact, and lighter heat sink designs, but at the increased burden of pumping power. Thus, the substantial
material stream and energy consumption rate associated with the cooling of electronic equipment using air
cooled heat sinks, lends urgency and importance to the "perfection" of these fin arrays. The financial constraints
at work in the electronic industry make it essential that specific cooling requirements be achieved with the
lowest cost solution. It is anticipated that in large volume production, as worldwide energy costs escalate, the
“least-energy” solution will nearly always provide the lowest cost solution.
The least-energy optimization of natural and forced-convection plate-fin heat sinks is described.
Emphasis is placed on the use of a Coefficient of Performance, COPT, relating cooling capability to the
energy invested in the formation, fabrication, and operation of the heat sink. It is shown possible to
determine the heat sink geometry, which maximizes the value of COPT, for each operating condition and
cooling mode. For forced convection cooling, the most favorable distribution of invested energy - between
heat sink formation/fabrication and operation - can also be found. Although optimum natural convection
heat sinks can deliver highly reliable, noise-free, operation, the COPT values for forced convection cooled
heat sinks are found to far exceed the values associated with passive cooling. The thermal analysis of the
natural convection rectangular plate-fin array has been carried out using a previously developed model by
Iyengar and Bar-Cohen [1], which utilizes the composite Nusselt number correlation developed by Bar-
Cohen and Rohsenow [2] to calculate the fin average heat transfer coefficient. The forced convection
results are obtained using a well-validated, semi-analytical model developed by Holahan et al. [3]. In
order to concretize the benefits of such "least-energy" heat sink designs, the proposed modeling and
optimization techniques will be applied to an advanced heat sink configuration, considered suitable for the
cooling of a high-end microprocessor. Thus, many of the results are derived for an aluminum plate-fin
heat sink on a 10cm × 10cm base, and 5cm fin height, operating at an excess temperature of 25 K.
