Numerical Analysis of Mist-Cooled High Power Components in Cabinets

The heat dissipation capacity of air-cooled computing and telecommunications cabinets is limited by acoustic noise and fan reliability considerations. The present work quantifies the potential for thermal management of a sealed cabinet using an evaporating mist introduced upstream of the high-power electronic components. The proposed concept consists of droplets of mist being dispersed in the air flowing through a heat sink. The evaporated mist is condensed at the outlet of the circuit packs and recycled back to the inlet. The flow and heattransfer characteristics of mist flows in a representative heat sink inside the cabinet are explored through numerical analysis of the coupled mass, momentum and energy transport equations for an evaporating two-phase mixture. The effect of droplet size and the mist loading fraction on the heat sink temperature reduction is computed and parametrically analyzed. The results reveal significant insights into the complex transport processes associated with mist flows. Mist cooling is shown to offer significant promise as a feasible thermal management solution for telecommunications cabinets and data centers. INTRODUCTION The waste heat densities in telecommunications central offices and data centers have significantly increased in the past decade, which has led to significant interest in the development of novel high-heat-load thermal management solutions. A typical telecommunications central office or data center consists of rows of vertically arranged cabinets, each containing multiple shelves. Microelectronics and photonics components are mounted on circuit packs which are housed in the shelves. The currently employed thermal management strategy consists of heat removal from the shelves by drawing in cold air using fans. Air cooling technologies in their current form are not adequate to manage the increasing heat loads in next-generation circuit packs used in the telecommunications cabinets. Furthermore, the heat-flux densities in circuit packs are highly non-uniform, but present air cooling-based approaches do not lend themselves to site-specific thermal management. Approaches for increasing heat dissipation by increasing fan speed are severely limited by acoustic noise and fan reliability considerations. Novel thermal management solutions are needed to manage the thermal loads associated with nextgeneration telecommunications central offices and data centers, which are projected to dissipate upwards of 100,000 W/m of cabinet footprint area by 2010 [1]. Spray cooling has been explored by a number of investigators [2-7] as a high-heat-flux thermal management tool. Mist impingement cooling [8-13] is a variant of spray cooling wherein a two-phase mixture of finely-dispersed liquid droplets in air is sprayed onto the hot surface. It should be noted that the high heat dissipation capacity of spray cooling-based technologies is primarily a result of direct contact of the cooling fluid with the hot surface and subsequent evaporation of the liquid at the surface. A practical limitation associated with the use of spray-cooling based techniques is the extensive piping and plumbing needed to direct the fluid to the hot component in the circuit pack (and the evaporated fluid to the condenser) which may not always be feasible. Bahadur et al. [14] introduced a novel concept of a recirculating, two-phase, evaporating-condensing mist flow for thermal management of next-generation telecommunications central offices. The cooling scheme consisted of introducing finely dispersed liquid droplets in the airstream (subsequently referred to as mist) entering the circuit pack as shown in Figure Proceedings of the ASME 2009 Int rPACK Conference IPACK2009 July 19-23, 20 9, San Francisco, California, USA