Active Heat Transfer Enhancement in Air Cooled Heat Sinks Using Integrated Centrifugal Fans ⇑ Wayne L

International Journal of Heat and Mass Transfer 82 (2015) 189–205 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt Active heat transfer enhancement in air cooled heat sinks using integrated centrifugal fans ⇑ Wayne L. Staats , J.G. Brisson Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA article info abstract Article history: The enhancement of convective heat transfer in an air-cooled heat sink using integrated, interdigitated Received 16 August 2014 impellers was investigated. The experimentally investigated heat sink is representative of a subcompo- Received in revised form 26 October 2014 nent of an unconventional heat exchanger with a loop heat pipe, multiple parallel flat-plate condensers, Accepted 31 October 2014 and integrated, interdigitated centrifugal fans, designed to meet the challenges of thermal management Available online 1 December 2014 in compact electronic systems. The close integration of impeller blades with heat transfer surfaces results in a decreased thermal resistance per unit pumping power compared to conventional forced convection Keywords: heat sinks. Heat exchanger The fan performance (i.e. fan curve and power consumption) and heat transfer of a single integrated fan Heat sink Fan heat sink were experimentally characterized for 12 impeller designs and modeled in terms of dimension- Thermal management less correlations. Correlations were developed to give estimates of the dimensionless fan curve and the Convection enhancement dimensionless power curve based on the fan geometry. Additionally, a two-parameter correlation was Active enhancement developed to estimate the dimensionless heat flux based on the fan’s operating point. The heat transfer in the integrated fans was observed to be a function of the operating point (i.e. the rotational speed of the impeller and the flow rate of air), with only a weak direct dependence on the fan geometry. The insen- sitivity of the heat transfer performance to the impeller geometry greatly simplifies the design process of integrated fan heat sinks because the fan design can be optimized independently of the heat transfer performance. Finally, the heat transfer enhancement (compared to pressure-driven flow at the same flow rate) appears to be due to turbulent flow structures induced by the impeller. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction will be incapable of meeting the increasingly stringent demands of the ICs: ‘‘The high junction-to-ambient thermal resistance resulting 1.1. Motivation from an air-cooled heat sink provides inadequate heat removal capability at the necessary junction temperatures for ITRS projec- Modern electronics and computing power depend on effective tions at the end of this roadmap’’. thermal management to achieve high levels of performance. Con- Clearly, there is a broadly recognized need for more effective sidering the economic impact, it is not surprising that computer cooling — in particular, air cooling. The iNEMI roadmap also points users and manufacturers are interested in state-of-the-art thermal out another important issue in thermal management solutions: management technologies. The International Electronics Manufac- heat sinks are subject to volume constraints. Minimum volume tra- turing Initiative (iNEMI) roadmap [1] states that new processor ditionally comes at the expense of thermal performance. These vol- technology will demand improved cooling and high density pack- ume constraints are particularly evident in high end applications aging technologies. In addition, the International Technology Road- (e.g. telecommunications, radar, sensing and imaging), which can map for Semiconductors (ITRS) [2] predicts that the required have a larger (10Â) thermal duty than consumer CPUs, but still thermal resistance for high performance ICs will decrease from its be constrained to a similar volume envelope. Many such applica- 2009 level of 0.27 K/W to 0.08 K/W in 2024, a more than threefold tions have resorted to ‘‘exotic liquid-cooled manifolds, spray- reduction. This forecast predicts that current air cooling technology cooled enclosures, and vapor-compression refrigeration’’ due to the insufficient performance of the current state-of-the-art air cooling solutions [3]. With improvements in air cooling, simpler ⇑ Corresponding author at: Sandia National Laboratories, PO Box 969, MS 9052, Livermore, CA 94551, USA. Tel.: +1 925 294 3307; fax: +1 925 294 2595. heat sinks could replace these bulky and complex solutions. E-mail address: [email protected] (W.L. Staats). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.10.075 0017-9310/Ó 2014 Elsevier Ltd. All rights reserved. 190 W.L. Staats, J.G. Brisson / International Journal of Heat and Mass Transfer 82 (2015) 189–205 Cooling of electronics also has a surprisingly high energy cost, better than traditional air cooled heat sinks on a per-unit-volume which translates to monetary cost and environmental impact. Mei- basis. jer reported that data centers accounted for about 2% of worldwide This work focuses on the air flow in a heat sink developed at energy consumption in 2009, and half of this was devoted to cool- MIT to address the needs of high performance electronics. This ing [4]. Koomey estimates that the power use for data centers in heat sink, referred to as ‘‘PHUMP’’, consists of a loop heat pipe with 2010 accounted for 1.1–1.5% of worldwide electricity use, about multiple parallel condensers and integrated, interdigitated centrif- half of which was used for cooling [5]. Any improvements in the ugal fans [20–23]. The fan impellers drive air radially outwards efficiency of thermal management solutions would help to reduce across a multitude of parallel-plate condensers (Fig. 1). Heat trans- some of this energy use. fer into the bottom of the device causes the working fluid, water, to Finally, compact and efficient air cooling solutions are critical evaporate and travel up the vertical pipes into the parallel con- for enabling proliferation of new technologies such as solid-state densers (the condensers are described in detail by Peters [24] lighting. With increased adoption of these technologies and the and Hanks [21]). Heat is removed from the condensers by convec- continued pervasiveness of computing technology, any improve- tive heat transfer to the air flow. Fresh, cool air is drawn in through ments in air cooling have the potential to effect appreciable energy the inlet at the top. This configuration exploits the tremendous savings with associated financial and energy security benefits. heat transfer coefficient associated with evaporation to achieve a high heat flux at the evaporator base (the evaporator and system 1.2. Background literature operation are described in detail by Kariya [20]). Although the heat transfer coefficient from the condensers to the air is much lower Conventional finned heat sinks have been studied by many than in the evaporator, the large thermal power input from the investigators. Tuckerman and Pease [6] studied a microchannel evaporator is distributed among the parallel condensers. By paral- heat sink etched on silicon and minimized thermal resistance by lelizing the condensers, their overall thermal resistance is greatly choosing the appropriate channel width, fin width and aspect ratio reduced, analogous to the lower equivalent resistance of electrical subject to constraints on the geometry and pressure drop. Knight resistors in parallel. Finally, a practical advantage is that the device et al. [7] developed an analytical method to minimize thermal is self-contained, using the surrounding air as the heat exchange resistance of a heat sink with pressure driven flow in a closed fluid and therefore requiring no external fluid connections. finned channel by varying the geometry. Teertstra et al. [8] devel- The air flow design of the PHUMP also has advantages. First, a oped an analytical model to calculate the Nusselt number for a low profile motor can be used to drive the fans, which share a com- plate fin heat sink as a function of geometry and flow properties. mon shaft; this allows for a compact architecture that does not Bejan [9] illustrated a heat sink optimization using entropy gener- occupy much vertical space. Second, the fans actually enhance ation minimization (EGM), a method whereby the entropy produc- the convective heat transfer between the condenser surfaces and tion is quantified and ascribed to the various loss producing the air flowing through the device. Depending on the regime the mechanisms, and the free parameters are altered to minimize the fan is operating in, this heat transfer enhancement has been overall entropy generation rate (for an overview of the method, observed to be as high as 3Â the equivalent flow rate of pres- see [10]). Culham and Muzychka [11] presented a method similar sure-driven air flow. Finally, the integrated fan approach results to that of Bejan, where the geometry of plate fin heat sinks is opti- in a device whose thermal resistance is very low compared to con- mized by minimizing the entropy generation rate. Their method ventional heat sinks with a comparable volume. To illustrate this, also allowed for the incorporation of real fan performance data in the expected performance of an integrated fan heat sink that fits the model. Similarly, Khan et al. [12] applied EGM techniques to in a 102 mm cube (similar to the heat sink in Fig. 1) was compared optimize a pin fin heat sink. Bar-Cohen and Iyengar [13,14] pre- to twenty commercially available air-cooled heat sinks (tested by sented methods of optimizing a parallel plate heat sink. Their Page [25]). The thermal resistance and volume of these commercial methods accounted for the energy used in the manufacture of heat sinks are shown in Fig. 2. The commercial air-cooled heat the heat sink and the cooling energy used over the expected life sinks form a Pareto performance frontier, with the Sandia Cooler of the computer in which it resides; their optima sometimes dif- and PHUMP exceeding the frontier.

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