Marc Hodes

Gas-assisted evaporative cooling of high density electronic modules

Reliable operation of advanced microelectronic components in three-dimensional packaging configurations necessitates the development of cooling systems capable of removing high heat fluxes and very high heat densities. A recently patented thermal management technique, using high velocity flow of a liquid-gas mixture in the narrow channels between populated substrates, appears to provide such a thermal transport capability. A prototype, high packaging density module, relying on this approach, has been successfully operated and a research study, focusing on the heat transfer rates attainable with this technique in a single, asymmetrically-heated channel has been completed. This paper begins with a description of this gas-assisted evaporative cooling approach, its advantages in thermal packaging of microelectronics, and its implementation in a prototype high-performance computer module. Attention is then paid to theoretical considerations in the flow of gas-liquid-vapor mixtures in narrow parallel plate channels and to the design and operation of an appropriate experimental apparatus. Next, experimental results for the wall temperature, heat transfer coefficients, and pressure drops are presented and compared to theoretical predictions. The paper concludes with a discussion of the thermal packaging potential of this novel thermal management technique. >

On one-dimensional analysis of thermoelectric modules (TEMs)

A novel framework for the one-dimensional analysis of a thermoelectric module (TEM) in which controlled and uncontrolled sides rather than cold and hot sides of it are defined is introduced. Next, heat conduction in a TEM is considered within this framework. Then, the operating modes of a TEM (cooling, generation, etc.) are defined and a means to compute the operating mode from a minimal set of operating parameters is provided. Refrigeration mode is considered in depth to illustrate the application of the analysis framework. Finally, the analysis is extended to TEMs subjected to boundary conditions of the third kind. Novel aspects of the analysis are indicated in the Conclusions.

Electrically tunable superhydrophobic nanostructured surfaces

In this work, we discuss dynamic electrical control of the wetting behavior of liquids on nanostructured surfaces spanning the entire possible range from superhydrophobic behavior to nearly complete wetting. It is demonstrated that a droplet of liquid can be reversibly switched between the superhydrophobic “rolling ball” state and the hydrophilic immobile droplet state by the application of electrical voltage and current. The nature of the transition mechanism is studied both experimentally and theoretically. The reported results provide novel methods of manipulating liquids at microscale.

On the Potential of Galinstan-Based Minichannel and Minigap Cooling

Galinstan, a gallium, indium, and tin eutectic, may be exploited for enhanced cooling of microelectronics because of its favorable thermophysical properties. A careful evaluation of its cooling potential, however, has not been undertaken. Provided here is a first-order model to compute the total (i.e., caloric plus conjugate conduction and convection) thermal resistance of galinstan-based heat sinks. Geometrically optimized minichannel heat sinks with rectangular channels for surface area enhancement and minigap, i.e., single parallel-plate channel, heat sinks are considered. Direct liquid cooling of a microprocessor die is envisioned. Therefore, the flow channels within the heat sinks are 302- μm tall, the pressure drop prescribed across them is 214 kPa, and their streamwise length is varied from 5 to 20 mm. The calculations suggest that galinstan is a better coolant than water in such configurations, reducing thermal resistance by about 40%.

Optimal Pellet Geometries for Thermoelectric Refrigeration

For a thermoelectric module (TEM) operating in refrigeration mode, the cooling flux provided by it, its coefficient of performance and its operating voltage per unit footprint are key parameters. They are provided here as a function of the temperatures on both sides of a TEM, current flow through it, electrical contact resistance at its interconnects and the geometry, material properties and packing density of its pellets. The pellet heights which maximize TEM performance (i.e., cooling flux they can accommodate for a specified temperature difference) or efficiency (i.e., coefficient of performance) at a specified performance are derived. Moreover, it is shown how to tune the operating current and voltage of a TEM by adjusting the cross-sectional area of its pellets. The analysis is performed in both the absence and presence of electrical contact resistance at the interconnects in a TEM. Implications of the results are placed in context for representative conditions

Isoflux Nusselt Number and Slip Length Formulae for Superhydrophobic Microchannels

We analytically and numerically consider the hydrodynamic and thermal transport behavior of fully developed laminar flow through a superhydrophobic (SH) parallel-plate channel. Hydrodynamic slip length, thermal slip length and heat flux are prescribed at each surface. We first develop a general expression for the Nusselt number valid for asymmetric velocity profiles. Next, we demonstrate that, in the limit of Stokes flow near the surface and an adiabatic and shear-free liquid–gas interface, both thermal and hydrodynamic slip lengths can be found by redefining existing solutions for conduction spreading resistances. Expressions for the thermal slip length for pillar and ridge surface topographies are determined. Comparison of fundamental half-space solutions for the Laplace and Stokes equations facilitate the development of expressions for hydrodynamic slip length over pillar-structured surfaces based on existing solutions for the conduction spreading resistance from an isothermal source. Numerical validation is performed and an analysis of the idealized thermal transport behavior suggests conditions under which superhydrophobic microchannels may enhance heat transfer.

Optimized Thermoelectric Refrigeration in the Presence of Thermal Boundary Resistance

Thermoelectric refrigerators (TEMs) offer several advantages over vapor-compression refrigerators. They are free of moving parts, acoustically silent, reliable, and lightweight. Their low efficiency and peak heat flux capabilities have precluded their use in more widespread applications. Optimization of thermoelectric pellet geometry can help, but past work in this area has neglected the impact of thermal and electrical contact resistances. The present work extends a previous 1-D TEM model to account for a thermal boundary resistance and is appropriate for the common situation where an air-cooled heat sink is attached to a TEM. The model also accounts for the impact of electrical contact resistance at the TEM interconnects. The pellet geometry is optimized with the target of either maximum performance or efficiency for an arbitrary value of thermal boundary resistance for varying values of the temperature difference across the unit, the pellet Seebeck coefficient, and the contact resistances. The model predicts that when the thermal contact conductance is decreased by a factor of ten, the peak heat removal capability is reduced by at least 10%. Furthermore, when the interconnect electrical resistance rises above a factor of ten larger than the pellet electrical resistance, the maximum heat removal capability for a given pellet height is reduced by at least 20% and the maximum coefficient of performance at low Ku-infin,u/(NK) values is reduced by at least 50%.

Optimal Design of Thermoelectric Refrigerators Embedded in a Thermal Resistance Network

Thermoelectric refrigeration offers advantages (e.g., no moving parts) over other refrigeration technologies. However, because maximum performance (i.e., heat load for a specified temperature drop below ambient temperature or vice versa) and efficiency (i.e., coefficient of performance) are relatively low, it is important to realize them. It is shown that the cross-sectional area of the semiconductor pellets in a thermoelectric module (TEM) operating in refrigeration mode does not affect its performance or efficiency, but may be sized to tune its operating current and voltage. Then, a procedure is provided to determine the height of the pellets which maximizes performance. Next, it is shown that a range of pellet heights accommodates a specified performance below the maximum one and a procedure is provided to compute that corresponding to maximum efficiency. A thermal resistance boundary condition is applied between the interface in a TEM where Peltier cooling occurs and the control point where it maintains the temperature of a component or medium below ambient temperature. Thermal resistance boundary conditions are also applied between the control point and its local ambient and the interface in a TEM where Peltier heating occurs and its local ambient. The analysis is generalized by using flux-based quantities where applicable and it accounts for the electrical contact resistance at the interconnects in a TEM. Implementation of the optimization procedures are illustrated and the ramifications of the results are discussed.

Design of Complex Structured Monolithic Heat Sinks for Enhanced Air Cooling

The design and characterization of monolithic heat sinks, which can take the form of complex structures, is reported. The designs were conceived to augment heat transport for enhanced air cooling by exploiting clearly identified physical mechanisms, i.e., by streaming the flow through a 2-D array of polygonal ducts, by introducing flow-obstacle-induced local mixing, and by exploiting hydrodynamic instabilities to sustain flow unsteadiness. Fabrication of these unconventional designs was achieved by 3-D printing plastic patterns and converting them into monolithic copper structures by investment casting. A direct simulation approach aided by analytical solutions and experimental validation was undertaken to quantify fluid flow and heat transfer parameters. This paper concludes by quantifying the performance enhancement of the proposed heat sink geometries relative to a conventional longitudinally finned heat sink. On an equal pumping power basis, finned foam and slotted hexagonal heat sinks outperform conventional parallel plate finned heat sinks. On the other hand, the parallel plate heat sinks are better for pressure drop less than 20 Pa and slotted honeycombs are better for higher pressure drops (>;20 Pa).

Energy savings achievable through liquid cooling: A rack level case study

Largely due to cooling power consumption needed for their thermal management, data centers consume over 2% of the electricity produced in the United States. A fundamental understanding of data center energy management is both environmentally and economically important. This paper develops a model of data center energy utilization for thermal management based on thermo-fluid first principles. The model is applied to a single rack system that can be used as an experimental platform for its validation. Models are developed for conventional air-cooling and hybrid cooling, where microprocessors are liquid cooled and other components are air cooled. The air-cooling model considers a single chiller with a variable speed compressor and an intermediate air-to-water heat exchanger. The hybrid model considers cold plates rather than air-cooled heat sinks on the microprocessors and includes an additional chiller. Compiled results quantify the power consumption of the two cooling schemes over ranges of key variables, such as ambient temperature. For example, the air-cooling model shows a 10% reduction in energy consumption for thermal management when the chiller setpoint is raised by 5°C, and the water flow rate pumped through the air-to-liquid heat exchanger is increased to accommodate this change. Hybrid model results show that energy consumption decreases as much as 40% over the parametric ranges investigated relative to air-cooling. The models can be utilized for future system design and trade-off studies. These models do not include variations in air flow rates or power consumption by the server as part of the sensitivity study.