Ferdinand Meyer

Bio: Ferdinand Meyer is an academic researcher from ETH Zurich. The author has contributed to research in topics: Strouhal number & Vortex shedding. The author has an hindex of 1, co-authored 1 publications receiving 23 citations.

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.

A novel method of energy efficient hotspot-targeted embedded liquid cooling for electronics: An experimental study

Abstract: The shift to multicore microprocessor architecture is likely to result in higher coolant flow requirements and thus exacerbate the problem of increasing data center energy consumption, also with respect to hotspot elimination. We present and experimentally prove a novel concept, for embedded, hotspot-targeted and energy efficient cooling of heterogeneous chip power landscapes. The rationally distributed, embedded microstructures presented here are able to adapt the heat transfer capability to a steady but non-uniform chip power map by passively throttling the flow in low heat flux areas. For the industrially acceptable limit on pressure drop of approximately 0.4 bar, the hotspot-targeted embedded liquid cooling (HT-ELC) designs are evaluated against a conservatively chosen conventional embedded liquid cooling (C-ELC) design and existing heat sinks in the literature. For an average steady-state heat flux of 150 W/cm 2 in core areas (hotspots) and 20 W/cm 2 over the remaining chip area (background), the chip temperature variation is reduced from 10 °C under the conventional cooling to 4 °C under the current hotspot targeted heat sink – a reduction of 57%. For heat fluxes of 300 and 24 W/cm 2 , the temperature variation is reduced by 30%. We show that the HT-ELC designs consume less than 0.3% of total chip power as pumping power to achieve this thermal performance, which the C-ELC design cannot match under all feasible levels of pumping power. Moreover, the HT-ELC designs achieve at least 70% improvement over the existing hotspot targeted heat sinks in terms of normalized chip temperature non-uniformity, without the need for any additional system level complexity, reducing reliability risks.

Microvortex-enhanced heat transfer in 3D-integrated liquid cooling of electronic chip stacks

Abstract: The cooling of three-dimensional electronic chip assemblies (stacks) is one of the most serious challenges facing the electronics industry as it moves toward fabrication approaches combining speed with energy efficiency. Here we show that by generating vortical microscale flows taking advantage of the inherent presence of Through Silicon Vias (TSV) in 3D integrated liquid (water) cooling of chip stacks, both large heat transfer enhancement as well as significantly better temperature uniformity can be accomplished. The approach is demonstrated experimentally in heat sinks consisting of a microcavity confining micropin fin arrays, mimicking TSV. Flow fluctuations and vortex shedding were triggered at specific Reynolds numbers, which are functions of the pin geometries and level of confinement. The resulting heat transfer enhancement due to the vortex-induced fluctuations and mixing, yields local Nusselt number increases up to 230% thereby reducing the chip temperature non-uniformity almost by a factor of three. The vortex shedding also induces a pressure drop increase. Remarkably, the effective improvement in the thermal performance due to vortex shedding, even after factoring in the rise in pumping power, reaches a peak value of 190%. Analysis of instantaneous liquid temperature signatures of shed microvortices using micron-resolution laser-induced fluorescence (μLIF), proved them to be the reason for both the elimination of liquid hotspots and the exceptional augmentation in heat transfer. These findings have important implications in the design of the new generation of integrated, out of plane electronics cooling with liquids.

Pitch and aspect ratio effects on single-phase heat transfer through microscale pin fin heat sinks

Abstract: Heat transfer and pressure drop of single-phase liquid flow is characterized in eight micro pin fin heat sinks with varied pitch and aspect ratios. The pins are diamond shaped with respect to the flow and have transverse pitch-to-diameter ( S T / D h ) and aspect ( H pin / D h ) ratio variations in the range of 1.7–3.0 and 0.7–3.2, respectively. The fluid used is PF-5060 over a Reynolds numbers (based on pin fin hydraulic diameter) range of 8–1189. Flow visualization is performed on all the heat sinks and flow transition into unsteady vortex shedding is observed only in those with specific pitch and aspect ratios. Flow visualization reveals upstream propagation of the onset of vortex shedding along the length of heat sink with an increase in Reynolds number. The existence of vortex shedding in micro pin fin heat sinks affects the prediction error of heat transfer correlations in literature. To address this gap, together with data from a prior study using liquid nitrogen [1], separate correlations are developed to predict Nu in the steady and unsteady regimes. The resulting correlation for the unsteady regime shows significantly decreased dependency of Nusselt on the Prandtl number compared to the non-vortex-shedding condition.

Pressure drop and friction factor of a rectangular channel with staggered mini pin fins of different shapes

Abstract: Experiments were performed on de-ionized water as working fluid, flowing across staggered mini pin fins of the same height and transverse spacing but with different pin density and different shapes of circular, elliptical, square, diamond and triangle, in a rectangular channel. The volume flow rate, pressure difference and temperature at the inlet and outlet were measured for the channel with different pin fin shapes at various Reynolds number ( Re ) in the range of 50 – 1800 to obtain the friction factor. The results showed that the friction factor for all the fins decreased with the increase of Re . At low Re ( Re (>300), the friction factor caused by eddy dissipation is the main part. In the intermediate range of Re (100–300), there is a transition. At different flow regimes, the shape and the fin density affects f differently. For laminar flow, the channel with triangle pin fins of the smallest density has the minimum f value while the elliptical one of the largest density has the maximum f value. On the contrary, for turbulent flow the channel with triangle pin fins has the maximum f while the elliptical one has the minimum f . Comparisons were made between experimental data and existing correlations, and results showed that there were large deviations between them. The existing correlations for the friction factor cannot correctly describe the whole flow range including laminar, transitional and turbulent zones, and new correlations are needed.

Intracellular delivery of mRNA to human primary T cells with microfluidic vortex shedding.

Abstract: Intracellular delivery of functional macromolecules, such as DNA and RNA, across the cell membrane and into the cytosol, is a critical process in both biology and medicine. Herein, we develop and use microfluidic chips containing post arrays to induce microfluidic vortex shedding, or μVS, for cell membrane poration that permits delivery of mRNA into primary human T lymphocytes. We demonstrate transfection with μVS by delivery of a 996-nucleotide mRNA construct encoding enhanced green fluorescent protein (EGFP) and assessed transfection efficiencies by quantifying levels of EGFP protein expression. We achieved high transfection efficiency (63.6 ± 3.44% EGFP + viable cells) with high cell viability (77.3 ± 0.58%) and recovery (88.7 ± 3.21%) in CD3 + T cells 19 hrs after μVS processing. Importantly, we show that processing cells via μVS does not negatively affect cell growth rates or alter cell states. We also demonstrate processing speeds of greater than 2.0 × 106 cells s−1 at volumes ranging from 0.1 to 1.5 milliliters. Altogether, these results highlight the use of μVS as a rapid and gentle delivery method with promising potential to engineer primary human cells for research and clinical applications.