Background Heat Flux Research Articles

Research on background heat flux of the thermal balance test system based on solar simulator

Abstract In the thermal balance test system based on the solar simulator, the background heat flux of the test system can cause additional heating to the spacecraft, reducing the effectiveness of the thermal test. Hence, it is necessary to conduct in-depth research on the impact of background heat flux through experimental and numerical methods. This paper first measures the background heat flux of the thermal test system based on the KM6 large-scale solar simulator experimentally, which is 35W/m2. Then, a simulation model of the thermal balance test system is established to clarify that the background heat flux originates from the cone chamber and collimating reflector of the solar simulator system. The result provides technical support for achieving a high-precision simulation of the space external heat flux based on the solar simulator.

Experimental and numerical investigation of a single-phase microchannel flow under axially non-uniform heat flux

The design of microchannel based heat sinks for advanced electronic component continues to receive research interest, especially with the ever-increasing density of advanced chips with concomitant increase in heat generation. One aspect of microchannel heat sink design objectives is the optimum employment of non-uniform heat dissipation through the channel length. Two dominant schools in the performance optimization of non-uniform heat sinks are: 1) entropy minimization and 2) thermal resistance minimization. These two approaches usually result in contradictory conclusions: one recommending the use of increasing heat flux, the other recommending a decreasing heat flux. The current study seeks to explain this discrepancy by experimentally and numerically studying the heat transfer mechanisms in a single microchannel tube under the effect of non-uniform heat flux. This study presents an experimental framework capable of generating a wide range of pre-specified heat flux profiles over a single microchannel, mimicking actual heat flux profiles observed in thermo-electronic devices. We employ three flux profiles: 1) hotspots located at different locations with uniform background heat flux, 2) linearly ascending heat flux, and 3) linearly descending heat flux. The results show good agreement between numerical simulations and experimental measurements and provide insights into the microchannel tube's fundamental heat transfer mechanisms. Based on these insights, the study provides design guidelines to enhance microchannel heat sink performance under non-uniform axial heat fluxes. Finally, the discrepancy between entropy and heat resistance minimization approaches is explained.

A new buoyancy instability in galaxy clusters due to streaming cosmic rays

ABSTRACT Active Galactic Nuclei (AGN) are believed to provide the energy that prevents runaway cooling of gas in the cores of galaxy clusters. However, how this energy is transported and thermalized throughout the Intracluster Medium (ICM) remains unclear. In recent work, we showed that streaming cosmic rays (CRs) destabilize sound waves in dilute ICM plasmas. Here, we show that CR streaming in the presence of gravity also destabilizes a pressure-balanced wave. We term this new instability the CR buoyancy instability (CRBI). In stark contrast to standard results without CRs, the pressure-balanced mode is highly compressible at short wavelengths due to CR streaming. Maximal growth rates are of order (pc/pg)β1/2ωff, where pc/pg is the ratio of CR pressure to thermal gas pressure, β is the ratio of thermal to magnetic pressure, and ωff is the free-fall frequency. The CRBI operates alongside buoyancy instabilities driven by background heat fluxes, i.e. the heat-flux-driven buoyancy instability (HBI) and the magneto-thermal instability (MTI). When the thermal mean free path lmfp is ≪ the gas scale height H, the HBI/MTI set the growth rate on large scales, while the CRBI sets the growth rate on small scales. Conversely, when lmfp ∼ H and (pc/pg)β1/2 ≳ 1, CRBI growth rates exceed HBI/MTI growth rates even on large scales. Our results suggest that CR-driven instabilities may be partially responsible for the sound waves/weak shocks and turbulence observed in galaxy clusters. CR-driven instabilities generated near radio bubbles may also play an important role redistributing AGN energy throughout clusters.

Controlled generation of a vapor bubble representative of nucleate boiling conditions using transient focused laser heating

Nucleate boiling is a phenomenon of significant importance in a broad range of industries. Increasing boiling performance parameters could lead to more efficient power plants and better electronics thermal management. However, difficulties associated with studying this extremely complex phenomenon have prevented a meaningful progress in the area of boiling heat transfer enhancement. In this paper, we implement a laser-based controlled bubble generation technique to enable accurate phenomenological studies of the boiling process. We present details of the transient focused-laser heating mechanism used to nucleate a microscale vapor embryo on the boiling surface. We present high-speed optical imaging data showing how this vapor embryo grows into a bubble using electrically applied background heat flux. Unlike the currently available artificial bubble generation approaches, which either generate unphysical bubbles or are extremely difficult to implement, we show that the laser-nucleated controlled single bubble demonstrates bubble ebullition characteristics closely representative of naturally occurring bubbles in nucleate pool boiling.

Kinetic Landau-fluid closures of non-Maxwellian distributions

New kinetic Landau-fluid closures, based on the cutoff Maxwellian distribution, are derived. A special static case is considered (the frequency ω=0). In the strongly collisional regime, our model reduces to Braginskii's heat flux model, and the transport is local. In the weak collisional regime, our model indicates that the heat flux is non-local and recovers the Hammett–Perkins model while the value of the cutoff velocity approaches to infinity. We compare the thermal transport coefficient χ of Maxwellian, cutoff Maxwellian and super-Gaussian distribution. The results show that the reduction of the high-speed tail particles leads to the corresponding reduction of the thermal transport coefficient χ across the entire range of collisionality, more reduction of the free streaming transport toward the weak collisional regime. In the collisionless limit, χ approaches to zero for the cutoff Maxwellian and the super-Gaussian distribution but remains finite for Maxwellian distribution. χ is complex if the cutoff Maxwellian distribution is asymmetric. The Im(χ) approaches to different convergent values in both collisionless and strongly collisional limit, respectively. It yields an additional streaming heat flux in comparison with the symmetric cutoff Maxwellian distribution. Furthermore, due to the asymmetric distribution, there is a background heat flux q0 though there is no perturbation. The derived Landau-fluid closures are general for fluid moment models, and applicable for the cutoff Maxwellian distribution in an open magnetic field line region, such as the scape-off-layer of Tokamak plasmas, in the thermal quench plasmas during a tokamak disruption, and the super-Gaussian electron distribution function due to inverse bremsstrahlung heating in laser-plasma studies.

Effects of Thin Film Heat Spreader on Hot Spots Mitigation in Heat Sinks

Abstract The effects of graphene platelets and diamond-based thin film heat spreaders on maximum temperature of integrated electronic circuits were investigated. A fully three-dimensional conjugate heat transfer analysis was performed to investigate the effects of thin film material and thickness on the temperature of a hot spot and temperature uniformity on the heated surface of the integrated circuit when subjected to forced convective cooling. Two different materials, diamond and graphene, were simulated as materials for thin films. The thin film heat spreaders were applied to the top wall of an array of micro pin fins having circular cross sections. The integrated circuit with a 4 × 3 mm footprint featured a 0.5 × 0.5 mm hot spot located on the top wall, which was also exposed to a uniform background heat flux of 500 W cm−2. A hot spot uniform heat flux of magnitude 2000 W cm−2 was centrally situated on the top surface over a small area of 0.5 × 0.5 mm. Both isotropic and anisotropic properties of the thin film heat spreaders made of graphene platelets and diamond were computationally analyzed. The conjugate heat transfer analysis also incorporated thermal contact resistance between the thin film and the silicon substrate. It was found that isotropic thin film heat spreaders significantly reduce the hot spot temperature and increase temperature uniformity, allowing for increased thermal loads. Furthermore, it was found that thickness of the thin film heat spreader does not have to be greater than a few tens of microns.

Cascaded Multicore Vapor Chambers for Intrapackage Spreading of High-Power, Heterogeneous Heat Loads

A cascaded multicore vapor chamber (CMVC) is designed for dissipating heat from high-flux hotspots simultaneously with a high-total-power background. Current thermal management strategies rely on spreading high local heat fluxes by conduction in the lid of electronics packages. Embedding vapor chambers (VCs) within the lid is an attractive option to directly address intrapackage hotspots. We investigate the design of intralid VCs, for a generic device having a total heat load of 476 W having a background heat flux of 0.75 W/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , with hotspots of 8 W/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> over a 1-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area. A conventional VC design, having a single vapor core, will require a thick evaporator wick to avoid the capillary limit for large total power. The necessity for a thick wick then imposes a large thermal conduction resistance when the VC is exposed to high heat flux hotspots. The proposed CMVC architecture aims to address this limitation. The cascaded architecture comprises a bottom-tier VC having an array of multiple small vapor cores for spreading heat from the small hotspots. These small vapor cores have short paths of liquid return to the evaporator, such that they can handle their footprint heat load while using thin wicks, resulting in a low hotspot thermal resistance. Furthermore, local dampening of the hotspots by the bottom tier then reduces the thermal conduction resistance across the necessarily thick wick in the top tier. Hence, the cascaded architecture has the potential to significantly reduce the overall thermal resistance, relative to a single tier. To substantiate this design rationale, experiments are performed to illustrate that the resistance of a commercial VC can be significantly reduced by interfacing the heat source with an intermediate heat spreader. Reduced-order models are then used to understand the effect of the wick properties (porosity and particle size) and geometric parameters on the thermal performance of the CMVC for the representative power map. The optimal CMVC design offers a thermal resistance (0.66 K/W) that is significantly lower compared to a conventional single-core VC (1.76 K/W) owing to a reduction in the conduction resistances across the internal wicks. That parametric optimization results demonstrate that the thermal resistance of the CMVC is more sensitive to the wick porosity compared to the particle diameter. Furthermore, there exists a wide range of wick properties and vapor core sizes for which near-optimum thermal performance can be attained, which is particularly attractive from the standpoint of flexibility in design and manufacturing.

Single Phase Liquid Cooling of High Heat Flux Devices With Local Hotspot in a Microgap With Nonuniform Fin Array

Abstract Single-phase liquid cooling in microchannels and microgaps has been successfully demonstrated for heat fluxes of ∼1 kW/cm2 for silicon chips with maximum temperature below 100 °C. However, effectively managing localized hotspots in heterogeneous integration, which refers to the integration of various components that achieve multiple functionalities, entails further thermal challenges. To address these, we use a nonuniform pin-fin array. Single phase liquid-cooling performance of four silicon test chips, thermal design vehicles (TDVs), each with a nonuniform pin-fin array, are experimentally examined. We evaluate multiple combinations of hotspot and background heat fluxes using four background heaters aligned upstream to downstream, and one additional hotspot heater located in the center. We examine the thermal performance of cylindrical fin-enhanced TDVs and hydrofoil fin-enhanced TDVs, both with two designs: one with increased fin density around the hotspot only, and another with increased fin density spanning the entire width of the channel. The resulting heat flux ratio of the localized hotspot to background heaters varies from 1 to 5. TDVs with spanwise increased hydrofoil fin density (spanwise hydrofoil) exhibit the best thermal performance with 6% to 14% lower hotspot temperature than others. TDVs with spanwise increased cylindrical fin (cylindrical spanwise) maintain a balance between hotspot cooling performance and pressure drops. In general, as the temperature of the hotspot remains around 70 °C with a heat flux of 625 W/cm2, the nonuniform fin-enhanced microgaps appears to be a promising hotspot thermal management approach. The pressure drop of hydrofoil spanwise chip is highest among all the cases.

Influence of hydrogels embedding positions on automatic adaptive cooling of hot spot in fractal microchannel heat sink

To solve the failure of electronic chips caused by local hot spot, the thermoresponsive shrinkage hydrogels are embedded into fractal microchannel heat sink to regulate mass flow rate, which brings more fluid cooling hot spot. The influence of hydrogels embedding positions on adaptive cooling in fractal microchannel heat sink is investigated by numerical simulation. The fractal microchannel heat sink embedded with hydrogels in first and secondary level branch simultaneously (SFMHS 1 + 2) and fractal microchannel heat sink embedded with hydrogels only in first level branch (SFMHS 1) are suitable for dealing with high background heat flux. And the fractal microchannel heat sink embedded with hydrogels in secondary level branch (SFMHS 2) is proper to manage low background heat flux. Moreover, SFMHS 1 + 2 has the highest pumping power, followed by SFMHS 2, while SFMHS 1 consumes the lowest pumping power. When the heat flux of hot spot is 400 W/cm2 and the background heat flux is 25 W/cm2, the temperature increment of hot spot in SFMHS 1 and SFMHS 2 are 5.23 K and 3.24 K lower than that of SFMHS 1 + 2, respectively. Besides, SFMHS 1 + 2 still has capability in cooling hot spot when SFMHS 1 cannot detect the hot spot or SFMHS 2 loses its ability in cooling hot spot. That is to say, the fractal microchannel heat sink embedded with hydrogels in first and secondary level branch simultaneously can deal with a wider range of heat flux.

Experimental Study of a Single Microchannel Flow Under Nonuniform Heat Flux

Abstract Nonuniform heat fluxes are commonly observed in thermo-electronic devices that require distinct thermal management strategies for effective heat dissipation and robust performance. The limited research available on nonuniform heat fluxes focus mostly on microchannel heat sinks while the fundamental component, i.e., a single microchannel, has received restricted attention. In this work, an experimental setup for the analysis of variable axial heat flux is used to study the heat transfer in a single microchannel with fully developed flow under the effect of different heat flux profiles. Initially, a hot spot at different locations, with a uniform background heat flux, is studied at different Reynolds numbers, while varying the maximum heat fluxes in order to compute the heat transfer in relation to its dependent variables. Measurements of temperature, pressure, and flow rates at a different locations and magnitudes of hot spot heat fluxes are presented, followed by a detailed analysis of heat transfer characteristics of a single microchannel under nonuniform heating. Results showed that upstream hotspots have lower tube temperatures compared to downstream ones with equal amounts of heat fluxes. This finding can be of importance in enhancing microchannel heat sinks effectiveness in reducing maximum wall temperatures for the same amount of heat released, by redistributing spatially fluxes in a descending profile.

Characterization of hierarchical manifold microchannel heat sink arrays under simultaneous background and hotspot heating conditions

A hierarchical manifold microchannel heat sink array is fabricated and experimentally characterized for uniform heat flux dissipation over a footprint area of 5 mm × 5 mm. A 3 × 3 array of heat sinks is fabricated into the silicon substrate containing the heaters for direct intrachip cooling, eliminating the thermal resistances typically associated with the attachment of a separate heat sink. The heat sinks are fed in parallel using a hierarchical manifold distributor that delivers flow to each of the heat sinks. Each heat sink contains a bank of high-aspect-ratio microchannels; five different channel geometries with nominal widths of 15 μm and 33 μm and nominal depths between 150 μm and 470 μm are tested. The thermal and hydraulic performance of each heat sink array geometry is evaluated using HFE-7100 as the working fluid, for mass fluxes ranging from 600 kg/m2 s to 2100 kg/m2 s at a constant inlet temperature of 59 °C. To simulate heat generation from electronics devices, a uniform background heat flux is generated with thin-film serpentine heaters fabricated on the silicon substrate opposite the channels; temperature sensors placed across the substrate provide spatially resolved surface temperature measurements. Experiments are also conducted with simultaneous background and hotspot heat generation; the hotspot heat flux is produced by a discrete 200 μm × 200 μm hotspot heater.Heat fluxes up to 1020 W/cm2 are dissipated under uniform heating conditions at chip temperatures less than 69 °C above the fluid inlet and at pressure drops less than 120 kPa. Heat sinks with wider channels yield higher wetted-area heat transfer coefficients, but not necessarily the lowest thermal resistance; for a fixed channel depth, samples with narrower channels have increased total wetted areas owing to the smaller fin pitches. During simultaneous background and hotspot heating conditions, background heat fluxes up to 900 W/cm2 and hotspot fluxes up to 2700 W/cm2 are dissipated. The hotspot temperature increases linearly with hotspot heat flux; at hotspot heat fluxes of 2700 W/cm2, the hotspot experiences a temperature rise of 16 °C above the average chip temperature.

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 &amp;gt;5 kW/cm2, background heat flux &amp;gt;1 kW/cm2, and “miniaturization” of device within a substrate thickness of &amp;lt;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 substrate, advanced packaging and manufacturing concepts, and integration of TEC of GaN devices to nanoscale. The “structural reliability” is a concern and we have reported the same in terms of mean time to failure (cycles) of SAC305 (96.5% tin, 3% silver, 0.5% cu) solder joint by application of Engelmaier's failure model and evaluation of stresses in the structure. The mathematical equation of failure model incorporates the failure phenomena of fatigue and creep in addition to the dwell time, average solder temperature, and plastic strain accumulation. The approach to this problem is a nonlinear finite element analysis technique, which incorporates thermal, mechanical, and thermoelectric boundary conditions.

Multi-Objective Optimization of Micro Pin-Fin Arrays for Cooling of High Heat Flux Electronics with a Hot Spot

ABSTRACTThis article presents fully three-dimensional conjugate heat transfer analysis and a multi-objective, constrained optimization to find sizes of pin-fins, inlet water pressure, and average speed for arrays of micro pin-fins used in the forced convection cooling of an integrated circuit with a uniformly heated 4 × 3 mm footprint and a centrally located 0.5 × 0.5 mm hot spot. Sizes of micro pin-fins having cross sections shaped as circles, symmetric airfoils, and symmetric convex lenses are optimized to completely remove heat due to a steady, uniform heat flux of 500 W cm−2 imposed over the entire footprint (background heat flux) and a steady, uniform heat flux of 2000 W cm−2 imposed on the hot spot area only (hot spot heat flux). The two simultaneous objectives are to minimize maximum substrate temperature and minimize pumping power, while keeping the maximum temperature constrained below 85°C and removing all of input thermal energy by convection. The design variables are the inlet average velocity and size of the pin-fins. A response surface is generated for each of the objectives and coupled with a genetic algorithm to arrive at a Pareto frontier of the best trade-off solutions. Numerical results show that, for a specified maximum temperature, optimized arrays with pin-fins having symmetric convex lens shapes create the lowest pressure drop, followed by the symmetric airfoil and circular cross-section pin-fins. An a posteriori three-dimensional stress–deformation analysis incorporating hydrodynamic and thermal loads shows that Von-Mises stress for each pin-fin array is significantly below the yield strength of silicon, thus, confirming structural integrity of such arrays of micro pin-fins.

Studying lower crustal flow beneath Mead basin: Implications for the thermal history and rheology of Venus

Mead, the largest crater on Venus, has low topographic relief at the surface and at the crust-mantle boundary. Due to high surface temperatures, viscous deformation could play an important role in crustal structure. Using the finite element method, we simulate the long-term viscoelastic deformation of Mead crater and investigate the role of lower crustal flow in the evolution of the surface and subsurface topography. We examine the thermal states that allow this evolution to occur and determine the background heat flux. Our study constrains the background heat flux in the vicinity of Mead basin to 55–90mWm−2. This surface heat flow is generally higher than the average Venusian global values suggested by recent thermal models. In addition by applying hydrous and anhydrous creep rheological parameters, we demonstrate that the Venus's interior is rheologically dry and that the crust near Mead could be relatively high in plagioclase.

Embedded single phase microfluidic thermal management for non-uniform heating and hotspots using microgaps with variable pin fin clustering

The presence of variable heat fluxes, such as localized hotspots in integrated circuit (IC) architectures poses a key challenge for thermal management of existing (2D) and emerging three-dimensionally (3D) stacked chips. The use of conventional microchannel or uniform pin fin arrays for microfluidic cooling do not provide adequate surface area for heat transfer in the vicinity of regions of concentrated high power (hotspots), and therefore significant temperature gradients might arise in such zones. In the present investigation, the concept of using a single microfluidic loop for the combined and efficient cooling of hotspot and moderate power (background) areas is proposed, experimentally demonstrated, and supported by a comprehensive numerical model. Two different thermal device vehicles (TDVs) are considered for a range of operating conditions, in which the surface area is locally increased by clustering a dense array of pin fins in the hotspot region for one configuration, while for the other the clustering is uniform in the spanwise direction. De-ionized (DI) water is used as the coolant through a silicon (Si) microgap with 200μm spacing; the hotspot heat flux is varied from 250 to 750W/cm2, while the background heat flux is fixed at 250W/cm2. Results indicate the capability of the proposed designs to keep the maximum temperature of the combined device below 65°C for an inlet water temperature of 21.3°C, with moderate temperature gradients and pressure drop. In addition, a robust computational fluid dynamics/heat transfer (CFD–HT) model capable of predicting spatially resolved temperature fields arising from heterogeneous heating is validated with relevant experimental data. Detailed benchmark simulations are provided, so they can be reproduced and used for reference in numerical studies with variable pin fin densities. The described methodology represents a cost-effective thermal modeling technique that may be applicable to virtually any type of heat flux distribution or power map, and IC architecture.

Using the viscoelastic relaxation of large impact craters to study the thermal history of Mars

We simulate the long-term deformation of Martian craters and investigate the role of lower crustal flow in the evolution of surface and subsurface topography. Using the finite element method and a viscoelastic rheological model, we model the deformation of more than 30 large craters and Quasi-Circular Depressions (QCDs), in the diameter range of ∼200–500km, in both the Northern Lowlands and Southern Highlands. We determine the most appropriate background heat fluxes that produce the current topography beneath the impacts at the crust–mantle boundary (ranges from 40 to ∼90mW m−2). Our study shows that a higher background heat flux leads to more relaxation at the surface and subsurface. By applying various viscous creep parameters for hydrous and anhydrous rheologies, we demonstrate that Mars's interior is wet to a certain degree, which is consistent with other estimates. Since craters and QCDs are distributed fairly equally on the surface of the Red Planet, this study provides a less regionally biased picture of the thermal history of early Mars than in previous studies. Based on our results, the ancient average background heat flux in the Northern Lowlands was higher than that of the Southern Highlands, which could indicate that whatever process formed the crustal dichotomy had a thermal signature at least through the middle Noachian.

Micro Satellite Thermal Balance Testing: Orbit Heat Flux Simulation Method and Verification

Comparing to conventional satellites, micro satellite features smaller envelop size, lower cost, and shorter R&D period. At the same time, while micro satellite generally runs within the LEO, it is more susceptible to the earth IR radiation. To simulate the orbital heat flux efficiently for the micro satellite, in this paper the thermal balance testing respectively using infrared cage and solar simulator are performed. The resulting differences between the two simulating approaches are examined with care. Conclusions can be drawn from analysis: in thermal balance testing of micro satellite, in terms of simulating orbital heat flux, the results of two simulators are close enough to consider that it is feasible to choose infrared cage as orbital heat flux simulator, given thermal coating material of typical IR emittance and solar absorptance ratio. Results demonstrate that by using infrared cage simulator, the actual temperature is about 5°C higher than theoretical value. The background heat flux interference stays stable at 17% of intensity of simulated orbital heat flux, and the value could be referred to in the thermal mathematical model modification.

A Review of Two-Phase Forced Cooling in Three-Dimensional Stacked Electronics: Technology Integration

Three-dimensional (3D) stacked electronics present significant advantages from an electrical design perspective, ranging from shorter interconnect lengths to enabling heterogeneous integration. However, multitier stacking exacerbates an already difficult thermal problem. Localized hotspots within individual tiers can provide an additional challenge when the high heat flux region is buried within the stack. Numerous investigations have been launched in the previous decade seeking to develop cooling solutions that can be integrated within the 3D stack, allowing the cooling to scale with the number of tiers in the system. Two-phase cooling is of particular interest, because the associated reduced flow rates may allow reduction in pumping power, and the saturated temperature condition of the coolant may offer enhanced device temperature uniformity. This paper presents a review of the advances in two-phase forced cooling in the past decade, with a focus on the challenges of integrating the technology in high heat flux 3D systems. A holistic approach is applied, considering not only the thermal performance of standalone cooling strategies but also coolant selection, fluidic routing, packaging, and system reliability. Finally, a cohesive approach to thermal design of an evaporative cooling based heat sink developed by the authors is presented, taking into account all of the integration considerations discussed previously. The thermal design seeks to achieve the dissipation of very large (in excess of 500 W/cm2) background heat fluxes over a large 1 cm × 1 cm chip area, as well as extreme (in excess of 2 kW/cm2) hotspot heat fluxes over small 200 μm × 200 μm areas, employing a hybrid design strategy that combines a micropin–fin heat sink for background cooling as well as localized, ultrathin microgaps for hotspot cooling.

On-Chip Power Generation Using Ultrathin Thermoelectric Generators

Thermoelectric generators (TEGs) can significantly improve the net power consumption and battery life of the low power mobile devices or high performance devices by generating power from their waste heat. Recent advancements also show that the ultrathin thermoelectric devices can be fabricated and integrated within a micro-electronic package. This work investigates the power generation by an ultrathin TEG embedded within a micro-electronic package considering several key parameters such as load resistance, chip heat flux, and proximity of the TEG to chip. The analysis shows that the power generation from TEGs increases with increasing background heat flux on chip or when TEGs are moved closer to the chip. An array of embedded TEGs is considered in order to analyze the influence of multiple TEGs on total power generation and conversion efficiency. Increasing the number of TEGs from one to nine increases the useful power generation from 72.9 mW to 378.4 mW but decreases the average conversion efficiency from 0.47% to 0.32%. The average power generated per TEG gradually decrease from 72.9 mW to 42.0 mW when number of TEGs is increased from one to nine, but the total useful power generated using nine TEGs is significant and emphasize the benefits of using embedded TEGs to reduce net power consumption in electronics packages.

Local measurement of flow boiling heat transfer in an array of non-uniformly heated microchannels

As electronics packages become increasingly thinner and more compact due to size, weight, and performance demands, the use of large intermediate heat spreaders to mitigate heat generation non-uniformities are no longer a viable option. Instead, non-uniform heat flux profiles produced from chip-scale variations or from multiple discrete devices are experienced directly by the ultimate heat sink. In order to address these thermal packaging trends, a better understanding of the impacts of non-uniform heating on two-phase flow characteristics and thermal performance limits for microchannel heat sinks is needed. An experimental investigation is performed to explore flow boiling phenomena in a microchannel heat sink with hotspots, as well as non-uniform streamwise and transverse peak-heating conditions spanning across the entire heat sink area. The investigation is conducted using a silicon microchannel heat sink with a 5×5 array of individually controllable heaters attached to a 12.7mm×12.7mm square base. The channels are 240μm wide, 370μm deep, and separated by 110μm wide fins. The working fluid is the dielectric fluorinert liquid FC-77, flowing at a mass flux of approximately 890kg/m2s. High-speed visualizations of the flow are recorded to observe the local flow regimes. Despite the substrate beneath the microchannels being very thin (200μm), significant lateral conduction occurs and must be accounted for in the calculation of the local heat flux imposed. For non-uniform heat input profiles, with peak heat fluxes along the streamwise and transverse directions, it is found that the local flow regimes, heat transfer coefficients, and wall temperatures deviate significantly from a uniformly heated case. These trends are assessed as a function of an increase in the relative magnitude of the nonuniformity between the peak and background heat fluxes.