Showing papers by "Avram Bar-Cohen published in 2017"

Gen 3 “Embedded” Cooling: Key Enabler for Energy Efficient Data Centers

Abstract: The achievement of high computational efficiency in the operation of data centers, server farms, and high-performance computers is intimately tied to the development and application of advanced thermal management techniques, capable of addressing high power dissipation, on-chip hotspots, and high heat density in chip stacks. The current Gen-2 remote cooling paradigm is shown to be incapable of meeting these challenges and to have exacerbated the demise of the venerable Moore’s law. It is argued that completion of the “inward migration” of thermal packaging technology, through the development and application of Gen-3 embedded cooling techniques, utilizing local on-chip microfluidic and thermoelectric heat extraction, can enable highly efficient computational platforms.

Gravity effects in microgap flow boiling

Abstract: Increasing integration density of electronic components has exacerbated the thermal management challenges facing electronic system developers. The high power, heat flux, and volumetric heat generation of emerging devices are driving the transition from remote cooling, which relies on conduction and spreading, to embedded cooling, which facilitates direct contact between the heat-generating device and coolant flow. Microgap coolers employ the forced flow of dielectric fluids undergoing phase change in a heated channel between devices. While two-phase microcoolers are used routinely in ground-based systems, the lack of acceptable models and correlations for microgravity operation has limited their use for spacecraft thermal management. Previous research has revealed that gravitational acceleration plays a diminishing role as the channel diameter shrinks, but there is considerable variation among the proposed gravity-insensitive channel dimensions and minimal research on rectangular ducts. Reliable criteria for achieving gravity-insensitive flow boiling performance would enable spaceflight systems to exploit this powerful thermal management technique and reduce development time and costs through reliance on ground-based testing. In the present effort, the authors have studied the effect of evaporator orientation on flow boiling performance of HFE7100 in a 218 pm tall by 13.0 mm wide microgap cooler. Similar heat transfer coefficients and critical heat flux were achieved across five evaporator orientations, indicating that the effect of gravity was negligible.

Two-phase flow regimes in a U-shaped diabatic manifolded-microgap channel

Abstract: Embedded cooling — an emerging thermal management paradigm for electronic devices — has motivated further research in compact, high heat flux, cooling solutions. Reliance on phase-change cooling and the associated two-phase flow of dielectric refrigerants allows small fluid flow-rates to absorb large heat loads. Our previous research as well as work of others has shown that dividing chip-scale microchannels into parallel arrays of channels with novel manifold designs can produce very high chip-scale heat transfer coefficients with low pressure drops. In such manifolded microchannel coolers, the coolant typically flows at relatively high velocities through U-shaped microgap channels, producing centripetal acceleration forces on the fluid that can be several orders of magnitude larger than gravity. The impact of such large accelerations on the two-phase flow regimes in microgap channels, and their associated transport rates, are not well understood. Moreover, because the channels are very small and optically inaccessible, the flow regimes occurring in such manifolded microchannels have yet to be imaged and documented. The present effort analyzes the effects of such high centripetal acceleration on two-phase flow characteristics, including flow morphology. Since the available literature deals almost exclusively with macroscale and miniscale channels, the differences between macroscale and microscale two-phase flows are identified and discussed. The paper also shows, using dimensionless numbers to characterize the prevailing two-phase flow regimes, that results of previous U-channel visualizations in miniscale geometries, approximately an order of magnitude larger in every geometrical dimension than the microgap channels of interest, can provide insight into the flow regimes occurring in very high-performance microgap channels.

Thin Thermally Efficient ICECool Defense Semiconductor Power Amplifiers

Abstract: Traditional power electronics for military and fast computing applications are bulky and heavy. The “mechanical design” of electronic structure and “materials” of construction of the components have limitations in performance under very high temperature conditions. The major concern here is “thermal management.” To be more specific, this refers to removal of high-concentration hotspot heat flux >5 kW/cm2, background heat flux >1 kW/cm2, and “miniaturization” of device within a substrate thickness of <100 μm. We report on the novel applications of contact-based thermoelectric cooling (TEC) to successful implementations of high-conductivity materials - diamond substrate grown on gallium nitride (GaN)/AlGaN transistors to keep the hotspot temperature rise of device below 5 K. The requirement for smarter and faster functionality along with a compact design is considered here. These efforts have focused on the removal of higher levels of heat flux, heat transfer across interface of junction and substr...

Integration of micro-contact enhanced thermoelectric cooler with a FEEDS manifold-microchannel system for cooling of high flux electronics

Abstract: Two-phase microchannel cooling has demonstrated substantial performance enhancement for thermal management of high-power electronics, offering remarkable heat removal capability without imposing high pumping power penalties. However, similar to other bulk cooling methods, this method alone too has difficulty in addressing remediation of local hotspots. Thermoelectric coolers, on the other hand, are scalable and perfectly suited for localized cooling. Thus in this paper, we report our work on integration of a micro-contact enhanced TEC with FEEDS (thin-Film Evaporation and Enhanced fluid Delivery System) manifold-micro channel system. Combining these two thermal management schemes into a single system can provide effective heat removal over the entire electronic chip surface. Integration of these two methods, however, poses several challenges, including hermetic sealing, wiring of the TEC, excessive joule heating in electrical traces, and thermal/electrical short-circuits. Thus, the aim of this study was to integrate an optimized, 3 mm × 0.8 mm TEC into a FEEDS manifold-microchannel system to create a reliable high flux cooling mechanism on a silicon or silicon carbide chip for cooling of 5kW/cm2 hotspot and 1kW/cm2 background heat fluxes. The manufacturing, integration configuration, and assembly of the system are discussed in this paper. A numerical model of the system is built and simulated using the commercial finite-element analysis software ANSYS. Preliminary numerical results demonstrated that with 30 °C temperature rise at the SiC chip's background surface, less than 35 °C hotspot temperature rise with respect to the coolant fluid temperature (110 °C) can be achieved.

Reducing thermal coupling using fluid cooled low-k interposers

Abstract: The thermal isolation capabilities of fluid cooled via-enhanced glass interposers are investigated with resistive-heater test chips and IR thermography. Previous studies have shown that the degree of thermal coupling between adjacent devices hosted on via-enhanced low conductivity interposers is lower than those on silicon interposers, but at the cost of increased chip-to-ambient thermal resistance. By integrating the enhanced low-k interposer with an embedded microgap cooler, much higher chip power can be dissipated while maintaining an acceptable temperature rise.

Microgap cooling of via-enhanced glass interposers

Abstract: A series of single-phase water microgap cooling experiments (gap height: 200 μm) are conducted on via arrays in 400 μm thick glass interposers. Surface temperature rise is compared to trials run with bulk Si of the same thickness. The results show that the copper vias are necessary to control the temperature rise of the glass substrate, and that while the via-enhanced interposers do exhibit a larger thermal resistance than silicon, they also provide the desired increase in lateral thermal isolation. As flow rates within the gap are increased (approaching Re=1600), the penalty associated with constraining the flow of heat to the footprint of the via array is mitigated, owing to the reduction in the thermal resistance attributable to the convection boundary.

Flow boiling performance in smooth and internally-grooved tube U-bend cold plates

Abstract: This paper outlines evaporative flow boiling results in custom smooth and internally-grooved tube cold plates with U-bends. Heat transfer data, captured with an infrared camera, is analyzed for liquid-vapor HFE-7100 flow in horizontal 9mm channel diameter cold plates, with mass fluxes from 75–250 kg/m2s, heat fluxes from 0–18 kW/m2, and vapor qualities approaching unity. This data is compared to a recently developed internally-grooved tube diabatic flow regime map and associated heat transfer coefficient correlation by Sharar and Bar-Cohen. The internally-grooved tube model is shown to predict the current data set satisfactorily, however, the traditional Wojtan et al. smooth tube model has limited applicability at low mass flux due to flow regime transition after the U-bends.