Adrian Renfer

Bio: Adrian Renfer is an academic researcher from ETH Zurich. The author has contributed to research in topics: Vortex shedding & Reynolds number. The author has an hindex of 5, co-authored 5 publications receiving 125 citations.

Experimental investigation into vortex structure and pressure drop across microcavities in 3D integrated electronics

Abstract: Hydrodynamics in microcavities with cylindrical micropin fin arrays simulating a single layer of a water-cooled electronic chip stack is investigated experimentally. Both inline and staggered pin arrangements are investigated using pressure drop and microparticle image velocimetry (μPIV) measurements. The pressure drop across the cavity shows a flow transition at pin diameter–based Reynolds numbers (Re d ) ~200. Instantaneous μPIV, performed using a pH-controlled high seeding density of tracer microspheres, helps visualize vortex structure unreported till date in microscale geometries. The post-transition flow field shows vortex shedding and flow impingement onto the pins explaining the pressure drop increase. The flow fluctuations start at the chip outlet and shift upstream with increasing Re d . No fluctuations are observed for a cavity with pin height-to-diameter ratio h/d = 1 up to Re d ~330; however, its pressure drop was higher than for a cavity with h/d = 2 due to pronounced influence of cavity walls.

Computational modeling of vortex shedding in water cooling of 3D integrated electronics

Abstract: This work aims at understanding the flow and heat transfer through a microcavity populated with micropins, representing a layer of a 3D integrated electronic chip stack with integrated cooling. The resulting vortex shedding behavior and its effect on the heat removal is analyzed in the Reynolds number (Re) range from 60 to 450. The lateral confinement, expressed as the ratio of diameter to lateral distance between two cylinders’ centers, is varied between 0.1 and 0.5; the longitudinal confinement (diameter to longitudinal distance between two cylinders’ centers) between 0.25 and 0.5; and vertical confinement (diameter to microcavity height ratio) between 0.1 and 0.5. For a single pin, as the lateral confinement is increased, the Strouhal number (St) and the shedding frequency increase by up to 100%. The thermal performance represented by the spatiotemporal averaged Nusselt number (Nu), based on the average pin surface and fluid temperatures, is also enhanced by over 30%. A direct relationship between Nu and the shedding frequency was found. For a row of pins, Nu in the vortex shedding regime was found to be up to 300% higher compared to the steady case. A decrease in the longitudinal confinement, tested with rows of pins (either with 50 or 25 pins) in the streamwise direction, led to an upstream migration of the vortex shedding location and in more homogeneous but higher wall temperatures. This coincided with a drastic reduction of pressure losses and a 30% Nu enhancement for the same pumping power. Finally, the vertical confinement is also investigated with 3D simulations around a single cylinder. With increasing Re and vertical confinement, the wake becomes strongly three-dimensional. For a given Re, the increase of vertical confinement naturally shows a suppression or even a complete elimination of the vortex shedding due to a strong end-wall effect. Our results shed light on the effects of confinement on vortex shedding and related heat transfer in the integrated cooling of 3D chip stacks.

Vortex shedding from confined micropin arrays

Abstract: The hydrodynamics in microcavities populated with cylindrical micropins was investigated using dynamic pressure measurements and fluid pathline visualization. Pressure signals were Fourier-analyzed to extract the flow fluctuation frequencies, which were in the kHz range for the tested flow Reynolds numbers (Re) of up to 435. Three different sets of flow dependent characteristic frequencies were identified, the first due to vortex shedding, the second due to lateral flow oscillation and the third due to a transition between these two flow regimes. These frequencies were measured at different locations along the chip (e.g. inlet, middle and outlet). It is established that vortex shedding initiates at the outlet and then travels upstream with increase in Re. The pathline visualization technique provided direct optical access to the flow field without any intermediate post-processing step and could be used to interpret the frequencies determined through pressure measurements. Microcavities with different micropin height-to-diameter aspect ratios and pitch-to-diameter ratios were tested. The tests confirmed an increase in the Strouhal number (associated with the vortex shedding) with increased confinement (decrease in the aspect ratio or the pitch), in agreement with macroscale measurements. The compact nature of the microscale geometry tested, and the measurement technique demonstrated, readily enabled us to investigate the flow past 4,420 pins with various degrees of confinements; this makes the measurements performed and the techniques developed here an important tool for investigating large arrays of similar objects in a flow field.

Waste heat recovery in supercomputers and 3D integrated liquid cooled electronics

Abstract: Ever increasing device density in electronic chips is beneficial for enhancing their computing efficiency. However, it also introduces severe challenges with respect to cooling solution which are indispensible for ensuring a reliable chip operation. Such steady miniaturization will soon render the traditional air cooling strategies futile and make the switching to liquid cooling inevitable. Superior thermal properties and ubiquitous availability make water the most suitable candidate as coolant. Building up on the reported studies in the literature, which have already established the feasibility of water as coolant, here we show that the superior thermal properties of water make it possible to cool electronic chips and data centers using hot water with inlet temperature up to 60°C. The concept is demonstrated through measurements on a copper made scalable manifold microchannel heat sink and a hot water cooled data center prototype. The high exergetic efficiency achieved using hot water cooling should make it possible to reuse the heat otherwise discarded in data centers and therefore improve the overall system efficiency and lower the carbon foot print of the data centers. Finally, the encouraging results are used to model water cooling of 3D chip stacks using an interlayer integrated cooling approach. The model results are compared with measurements on a model simulator and good agreement is found, which lays the ground work for realizing a model based optimization of integrated cooling structures for 3D chip stacks.

Analysis of conjugated heat transfer in micro-heat exchangers via integral transforms and non-intrusive optical techniques

Abstract: Purpose – The purpose of this paper is to employ the Generalized Integral Transform Technique in the analysis of conjugated heat transfer in micro-heat exchangers, by combining this hybrid numerical-analytical approach with a reformulation strategy into a single domain that envelopes all of the physical and geometric sub-regions in the original problem. The solution methodology advanced is carefully validated against experimental results from non-intrusive techniques, namely, infrared thermography measurements of the substrate external surface temperatures, and fluid temperature measurements obtained through micro Laser Induced Fluorescence. Design/methodology/approach – The methodology is applied in the hybrid numerical-analytical treatment of a multi-stream micro-heat exchanger application, involving a three-dimensional configuration with triangular cross-section micro-channels. Space variable coefficients and source terms with abrupt transitions among the various sub-regions interfaces are then defined...

Thermal management and temperature uniformity enhancement of electronic devices by micro heat sinks: A review

Abstract: The electronic equipment developing towards miniaturization and high integration is facing the danger of high heat flux and non-uniform temperature distribution which leads to the reduction of life and reliability of electronic devices. The micro heat sinks have gained significant attention in heat dissipation of electronic devices with a high heat flux due to its large heat transfer surface to volume ratio, compact structure and outstanding thermal performance. In this review, we present the advantages and shortcomings of thermal enhancement technologies in different structural micro heat sinks. Moreover, the non-uniform temperature distribution which includes the temperature rising along the flow direction and hotspots, especially, the random hotspot with high heat flux, has been the serious issues in the thermal management of electronic devices. They are the main challenges for the efficient operation and service life of electronic components. Thus, it is urgent to develop an effective and economic process in automatic adaptive cooling of random hotspots. The purpose of this article is to introduce the existing thermal enhancement technologies in micro heat sinks and the reduction of non-uniform temperature distribution. Finally, the barriers and challenges for the developments of thermal management of electronic devices by micro heat sinks are discussed, and the future directions of the research topic are provided.

Aquasar: A hot water cooled data center with direct energy reuse

Abstract: We report the energy and exergy efficiencies of Aquasar, the first hot water cooled supercomputer prototype. The prototype also has an air cooled part to help compare the coolants's performances. For example, a chip/coolant temperature differential of only 15 °C was sufficient for chip cooling using water. The air cooled side, however, required air pre-cooling down to 23 °C and a chip/coolant temperature differential of 35 °C. Whereas extra exergy was expended for air pre-cooling, the higher thermal conductivity and specific heat capacity of water enabled coolant temperatures to be safely raised to 60 °C. Using such hot water not only eliminated the need for chillers, it also opened up the possibility of heat reuse. The latter was realized by using the hot water from Aquasar for building heating. A heat recovery efficiency of 80% and an exergetic efficiency of 34% were achieved with a water temperature of 60 °C. All these results establish hot water as a better coolant compared to air. A novel concept of economic value of heat was introduced to evaluate different reuse strategies such as space heating and refrigeration using adsorption chillers. It was shown that space heating offers the highest economic value for the heat recovered from data centers.

Effect of volume concentration and temperature on viscosity and surface tension of graphene–water nanofluid for heat transfer applications

Abstract: In the present study, the effect of volume concentration (0.05, 0.1 and 0.15 %) and temperature (10–90 °C) on viscosity and surface tension of graphene–water nanofluid has been experimentally measured. The sodium dodecyl benzene sulfonate is used as the surfactant for stable suspension of graphene. The results showed that the viscosity of graphene–water nanofluid increases with an increase in the volume concentration of nanoparticles and decreases with an increase in temperature. An average enhancement of 47.12 % in viscosity has been noted for 0.15 % volume concentration of graphene at 50 °C. The enhancement of the viscosity of the nanofluid at higher volume concentration is due to the higher shear rate. In contrast, the surface tension of the graphene–water nanofluid decreases with an increase in both volume concentration and temperature. A decrement of 18.7 % in surface tension has been noted for the same volume concentration and temperature. The surface tension reduction in nanofluid at higher volume concentrations is due to the adsorption of nanoparticles at the liquid–gas interface because of hydrophobic nature of graphene; and at higher temperatures, is due to the weakening of molecular attractions between fluid molecules and nanoparticles. The viscosity and surface tension showed stronger dependency on volume concentration than temperature. Based on the calculated effectiveness of graphene–water nanofluids, it is suggested that the graphene–water nanofluid is preferable as the better coolant for the real-time heat transfer applications.

Thermo-fluid analysis of micro pin-fin array cooling configurations for high heat fluxes with a hot spot

Abstract: Effect of micro pin-fin shapes on cooling of high heat flux electronic chips with a single hot spot was investigated numerically. Hydrothermal performances of different micro pin-fin shapes were evaluated. Circular shape, hydrofoil shape, modified hydrofoil shape, and symmetric convex shape were the cross section shapes used for micro pin-fins. All cooling configurations had the same staggered arrangements for micro pin-fins. An electronic chip with a 2.45 × 2.45 mm footprint having a hot spot of 0.5 × 0.5 mm at its centre was used for simulations. Uniform heat flux of 2000 W cm−2 was applied at the hot spot. The rest of the chip was exposed to 1000 W cm−2 uniform heat load. The cross section area of the circular shape and hydrofoil shape micro pin-fins was kept the same to have a fair comparison. Convex and hydrofoil shape designs showed significant reduction in the required pumping power as well as the maximum required pressure. In the last case, the height of micro pin-fins was increased from 200 μm to 400 μm to remove 100% of the total heat load via convection, and at the same time keep the maximum temperatures within an acceptable range.

Sustainable Cloud Data Centers: A survey of enabling techniques and technologies

Abstract: Cloud computing services have gained tremendous popularity and widespread adoption due to their flexible and on-demand nature. Cloud computing services are hosted in Cloud Data Centers (CDC) that deploy thousands of computation, storage, and communication devices leading to high energy utilization and carbon emissions. Renewable energy resources replace fossil fuels based grid energy to effectively reduce carbon emissions of CDCs. Moreover, waste heat generated from electronic components can be utilized in absorption based cooling systems to offset cooling costs of data centers. However, data centers need to be located at ideal geographical locations to reap benefits of renewable energy and waste heat recovery options. Modular Data Centers (MDC) can enable energy as a location paradigm due to their shippable nature. Moreover, workload can be transferred between intelligently placed geographically dispersed data centers to utilize renewable energy available elsewhere with virtual machine migration techniques. However, adoption of aforementioned sustainability techniques and technologies opens new challenges, such as, intermittency of power supply from renewable resources and higher capital costs. In this paper, we examine sustainable CDCs from various aspects to survey the enabling techniques and technologies. We present case studies from both academia and industry that demonstrate favorable results for sustainability measures in CDCs. Moreover, we discuss state-of-the-art research in sustainable CDCs. Furthermore, we debate the integration challenges and open research issues to sustainable CDCs.