The system-level packaging of the IBM z13™ supports the implementation of a new drawer-based Central Processor Complex (CPC). Departing from previous IBM z Systems™ designs, the introduction of distributed land-grid-array (LGA) attached single-chip modules (SCMs) required new mechanical, power, and cooling designs to address specified performance requirements and to provide enhanced reliability, availability, and serviceability (RAS) attributes. Building upon the designs created for the IBM zEnterprise® BC12 (zBC12), new CPC drawer and frame mechanical designs were created to significantly increase overall packaging density. Similar to its predecessor, the IBM zEnterprise EC12 (zEC12), the z13 utilizes water-cooling of the processors, but in contrast to the single input and return flow used to cool the multi-chip module (MCM) in the zEC12, the z13 accomplishes its processor cooling using a flexible hose internal manifold design that provides parallel input and return fluid flow to each SCM. The use of flexible hose also enabled SCM field replacement, new to high-end IBM z Systems. A new internal cooling loop unit and an updated external (building-chilled) modular water-conditioning unit were designed utilizing customized water delivery manifold systems to feed the common CPC drawer design. Revised power delivery and service control structures were also created to address the distributed nature of the z13 system design.

Systems for energy conversion, heat rejection, and sensing and control often incorporate heat exchange devices. Recent developments sin microfabrication and assembly methods have led to significant miniaturization of these systems. Miniaturized heat exchange devices have commonly utilized microchannel flow passages. This chapter reviews the fundamental flow and heat-transfer phenomena in microchannels, the most common numerical and experimental characterization techniques, microfabrication methods, and the application of microchannels in thermal management of hi-h heat flux electronics. The range of channel hydraulic diameters covered in this chapter is From a few micrometers to a few millimeters where the larger diameter transport characteristics become valid.

One of the promising liquid cooling techniques for microelectronics is attaching a microchannel heat sink to, or directly fabricating microchannels on, the inactive side of the chip. A stacked microchannel heat sink integrates many layers of microchannels and manifold layers into one stack. Compared with single-layered microchannels, stacked microchannels provide larger flow passages, so that for a fixed heat load the required pressure drop is significantly reduced. Better temperature uniformity can be achieved by arranging counterflow in adjacent microchannel layers. The dedicated manifolds help to distribute coolant uniformly to microchannels. In the present work, a stacked microchannel heat sink is fabricated using silicon micromachining techniques. Thermal performance of the stacked microchannel heat sink is characterized through experimental measurements and numerical simulations. Effects of coolant flow direction, flow rate allocation among layers, and nonuniform heating are studied. Wall temperature profiles are measured using an array of nine platinum thin-film resistive temperature detectors deposited simultaneously with thin-film platinum heaters on the backside of the stacked structure. Excellent overall cooling performance (0.09°C∕Wcm2) for the stacked microchannel heat sink has been shown in the experiments. It has also been identified that over the tested flow rate range, counterflow arrangement provides better temperature uniformity, while parallel flow has the best performance in reducing the peak temperature. Conjugate heat transfer effects for stacked microchannels for different flow conditions are investigated through numerical simulations. Based on the results, some general design guidelines for stacked microchannel heat sinks are provided.

The laminar flow characteristics inside slightly tapered silicon microchannels in the hydraulic diameter range of 53–112μm are investigated. Velocity profiles for planes located at different channel depth are measured using micro resolution particle image velocimetry (micro-PIV). It is revealed that the location of the maximum velocity deviates from the mid-plane along the depth direction due to the wall taper. Numerical simulations are also carried out to examine the effects of the sidewall angle on flow and heat transfer. Large deviation in the velocity profile and consequent significant degradation in heat transfer are observed for considerably tapered microchannels.

Steady, laminar flow and heat transfer, inside a rectangular microchannel with a dimpled bottom surface, are numerically studied using FLUENT 5.5. The microchannel is 50 μm deep and 200 μm wide. The dimples are placed in a single row of along the bottom wall with a pitch of 150 μm. The dimple depth is 20 μm, and the dimple print diameter is 98 μm. Fully developed periodic velocity and temperature boundary conditions are used at the inlet and outlet of one unit cell of the dimpled micro-channel. The numerically predicted, laminar, micro-scale flow characteristics are compared to results from macro-scale studies with turbulent flow. Some of the secondary flows into and out of the dimple, and the recirculating flow within the dimple are similar for both situations. However, the type and number of vortex structures shed from the dimples are significantly different for the laminar-micro-scale flow and turbulent-macro-scale flow. Heat transfer enhancements are present for both situations, but they are somewhat smaller in the laminar-micro-scale flow. In the latter case, these augmentations (relative to a channel with smooth walls) are present both on the bottom-dimpled surface, and on the sidewalls of the channel, and are mostly due to shear layer reattachment, secondary flows produced near the edges of the dimples, and thermal boundary layer thinning. The pressure drop penalties in the laminar-micro-scale flow are either equivalent to, or less than values produced in smooth channels with no dimples.

Increasing circuit density and adherence to Moore’s law is driving advanced cooling systems for the next generation microprocessors. One method receiving considerable study is that of microchannel heat exchangers in silicon substrate. These very fine channels in the heat exchanger provide a greatly enhanced convective heat transfer rate and have been shown to be able to meet the demands of the cooling challenge for microprocessors for many generations to come. While the thermal performance has been demonstrated, the design methodology and analysis for fluid structures at this size scale remains difficult. This paper reviews the use of CFD analysis for the design and optimization of microchannel heat exchangers. These results are compared with classical approaches to the same design and demonstrate the need for CFD analysis. Errors using the standard correlations and methods are demonstrated through the results of an optimization study on microchannels. The effects of entrance lengths, spatial variation of the Nusselt number, and temperature dependencies are considered. The literature has widely varying reports on comparisons between experimental results and corresponding theoretical results in microchannel heat exchangers. Experimental validation of the CFD analysis has also been performed and demonstrates that current CFD techniques are actually well suited to heat exchanger designs of this size.

We demonstrate a prototype chip-scale microfluidic cooling scheme. CMOS compatible processes allow the monolithic integration of the microchannel heat sink into the backside of a Si chip at low temperature (≤260°C). At the front side of a chip, fine pitch area-array solder bumps are fabricated by electroplating for high-density electrical I/O interconnection, while a peripheral array of micro polymer pipes are fabricated as thermal-fluidic I/O interconnects. The resulting “microfluidic flip chip” can be bonded onto a liquid-cooled board substrate using conventional flip-chip assembly processes. The cooling liquid can, therefore, be transferred into a Si chip directly from the board-level manifolds to alleviate the thermal interface issues. This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.

Microchannel heat sinks feature a high convective heat transfer coefficient, which is particularly beneficial to high-end electronics cooling. There are some issues to be addressed before these can be commercially implemented, among which pressure drop penalty and temperature non-uniformity are critical. Recently, a stacked microchannel heat sink has been proposed to address these two issues. Stacked microchannels provide larger flow passage, so that for a fixed heat load the required pressure drop is significantly reduced. One unique feature of the stacked microchannel heat sink is that individual layers populated with parallel microchannels can be stacked independently. As a beneficial result, flexible control over the flow direction and flow rate can be harnessed to achieve better temperature uniformity and the lowest silicon temperature. The present study conducts numerical study of heat transfer inside stacked microchannels with different flow arrangements including parallel, counter-flow, and serial. For the serial arrangement both top feeding and bottom feeding are considered. The predicted heat removal performance is compared with single layer microchannels that have the same effective flow area. It has been identified that counter-flow arrangement has the best overall performance for temperature uniformity, while parallel flow has the best performance in reducing the peak temperature. This can be explained by the detailed heat transfer information obtained through the conjugate numerical study.

A novel heat sink based on a multilayer stack of liquid cooled microchannels is investigated. For a given pumping power and heat removal capability for the heat sink, the flow rate across a stack of microchannels is lower compared to a single layer of microchannels. Numerical simulations using a computationally efficient multigrid method [1] were carried out to investigate the detailed conjugate transport within the heat sink. The effects of the microchannel aspect ratio and total number of layers on thermal performance were studied for water as coolant. A heat sink of base area 10 mm by 10 mm with a height in the range 1.8 to 4.5 mm (2–5 layers) was considered with water flow rate in the range 0.83×10−6m3/s (50 ml/min) to 6.67×10−6m3/s (400 ml/min). The results of the computational simulations were also compared with a simplified thermal resistance network analysis.

With smaller inlet flow velocity, a micro-channel stack requires less pumping power to remove a certain amount of heat than a single-layered micro-channel, because it provides a larger heat transfer area. A simple thermal resistance network model was developed to evaluate the overall thermal performance of a stacked micro-channel heat sink. Based on this simple model, in this study, a single objective minimization of overall thermal resistance is carried out using genetic algorithms. The aspect ratio, fin thickness and the ratio of channel width to fin thickness are the variables to be optimized, subject to constraints of maximum pressure drop (4 bar) and maximum volumetric flow rate (1000 ml/min). During the optimization, the overall dimensions, number of layers and pumping power (product of pressure drop and flow rate) are fixed. The study indicates that reduction in thermal resistance can be achieved by optimizing the channel configuration. The effects of number of layers in the stack, pumping power per unit area, and the channel length are investigated.

A model on pressure drop in oil-gas-water three-phase slug flow has been set up, which takes account of grown-proceeding of falling liquid film around Taylor bubbles in slug flow. Based on the experimental study of oil-gas-water slug flow, the numerical simulations of this model agree well with experimental data under the condition of low pressure. The model is an effective method, which can be used for pressure drop of oil-gas-water three-phase vertical upwards slug flow in which the continuous liquid phase is water phase.