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In this article, we design, demonstrate, and characterize a 3-D printed package-level polymer jet impingement cooling solution on a $23\times23$ mm2 thermal test chip. The experimental hardware results for a nozzle pitch of 2 mm show that, with 1-kW power dissipation, at a coolant (deionized (DI) water) flow rate of 3 liters per minute (LPM), the measured average chip temperature increase is ~65 °C with a cooler pressure drop of 0.15 bar between the inlet and outlet connections. It is also shown that bare die cooling without lid [and thermal interface material (TIM)] shows better cooling performance than the lidded package. Second, an advanced 3-D printed manifold with an additional flow redistribution structure is demonstrated. The experimental results show that the improved design achieves a better chip temperature uniformity compared to the reference design, showing a reduction of the chip temperature gradient with a factor of 4 and 2.3 for a flow rate of 0.5 and 3 LPM, respectively, while no significant impact on the cooler pressure drop was measured. The numerical modeling studies predict an additional 15.4% thermal performance improvement, by reducing the nozzle pitch from 2 to 1 mm, for a flow rate of 3 LPM.

The neutral atoms in the plasma edge of nuclear fusion devices are typically modeled using either a fluid or kinetic approach. The kinetic approach is most accurate, but it has two main disadvantages. First, the usual solution of the high-dimensional kinetic equation using Monte Carlo techniques introduces statistical noise, which hampers the convergence of the coupled plasma-neutral model. Second, the computational time strongly increases for highly collisional regimes. For these reasons, deterministic fluid neutral models remain an attractive alternative, in particular for the highly collisional conditions where their accuracy is expected to be high. In recent years, efforts have been undertaken to improve the agreement between the fluid and kinetic approach by introducing consistent transport coefficients and consistent boundary conditions in the fluid models. In this work, these so-called advanced fluid neutral models are further enhanced by introducing different strategies to cope with the high heterogeneity of the ion-neutral collisionality encountered in realistic plasma-edge geometries, namely isotropic neutral flux limiters and an automated selection criterion for the optimal neutral boundary conditions. The validity of the resulting fluid neutral models is thoroughly assessed for various representative simulation cases with different geometries, divertor collisionalities, and wall materials, including, for the first time, simulations in a realistic ITER plasma edge geometry. Strong quantitative agreement between the fluid and kinetic models is achieved for cases with highest divertor collisionality.

This article presents the design, fabrication, experimental characterization, and modeling analysis of the chip-level hotspot targeted liquid impingement jet cooling for high-power electronics. The hotspot targeted jet impingement cooling concept is successfully demonstrated with a chip-level jet impingement cooler with a 1-mm nozzle pitch and 300- $\mu \text{m}$ nozzle diameter fabricated using high-resolution stereolithography (additive manufacturing). The computational fluid dynamics (CFD) modeling and experimental analysis show that the improved hotspot targeted cooler design with fully open outlets can reduce the on-chip temperature difference by 70% compared with the full array cooler at the same pumping power of 0.03 W. The local heat transfer coefficient can achieve $15\times 10^{4}$ W/m2 K with a local flow rate per nozzle of 40 mL/min, requiring a pump power of 0.6 W. The benchmarking study proves that the hotspot targeted cooling is much more energy-efficient than uniform array cooling, with lower temperature difference and lower pump power.

ispartof: CONTRIBUTIONS TO PLASMA PHYSICS vol:62 issue:5-6 status: published

ispartof: Contributions To Plasma Physics vol:62 issue:5-6 status: published

In this work, the friction factor for steady periodically developed flow through micro-and mini-channels with periodic arrays of offset strip fins is analyzed. The friction factor is studied numerically on a unit cell of the array for Reynolds numbers ranging from 1 to 600, and fin height-to-length ratios below 1. It is shown that the friction factor correlations from the literature, which primarily focus on larger conventional offset strip fin geometries in the transitional flow regime, do not predict the correct trends for laminar flow in micro-and mini-channels. Therefore, a new friction factor correlation for micro-and mini-channels with offset strip fin arrays is constructed from an extensive set of numerical simulations through a least-squares fitting procedure. The suitability of this new correlation is further supported by means of the Bayesian approach for parameter estimation and model validation. The correlation predicts an inversely linear relationship between the friction factor and the Reynolds number, in accordance with our observation that a strong inertia regime prevails over nearly the entire range of investigated Reynolds numbers. Yet, through a more detailed analysis, also the presence of a weak inertia regime and a transitional regime is identified, and the transitions from the strong inertia regime are quantified by means of two critical Reynolds numbers. Finally, the new correlation also incorporates the asymptotic trends that are observed for each geometrical parameter of the offset strip fin array, and whose origins are discussed from a physical perspective., This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Physics of Fluids 33, 103610 (2021) and may be found at https://aip.scitation.org/doi/abs/10.1063/5.0067652 49 pages, 18 figures, 3 tables

The chip-level bare die direct liquid impingement jet cooling is regarded as a highly efficient cooling solution for high-performance applications. Furthermore, it shows potential to be integrated inside the chip package. In previous studies, we demonstrated this cooling concept using a prototype fabricated with mechanical micromachining using polymers. With the improvement of fabrication resolution, additive manufacturing or 3-D printing technology enables the fabrication of low-cost polymer microjet coolers with complex internal 3-D geometries and allows easy customization of the cooler design. In this paper, a chip-level impingement jet cooler with a $4 \times 4$ jet array and 575- $\mu \text{m}$ nozzle diameter is fabricated with high-resolution stereolithography, and assembled directly on the top of the bare die. The modeling study shows that the pressure drop in the 3-D printed cooler is reduced by 24% compared to the micromachined (MM) cooler with the same nozzle dimensions thanks to an improved more complex internal geometry. The fabrication quality and tolerances of the printed cooler are evaluated using optical measurements, scanning acoustic microscope (SAM) inspection, and cross-sectional analysis, showing a measured average nozzle diameter of 575 $\mu \text{m}$ compared to the designed nozzle diameter of 600 $\mu \text{m}$ . Moreover, the 3-D computational fluid dynamics (CFD) simulations used in this paper are experimentally validated by chip temperature measurements. The achieved minimal thermal resistance of 3-D printed $4 \times 4$ cooler is 0.16 cm $^{2}\cdot \text{K}$ /W for a flow rate of 1000 mL/min. The benchmarking study shows the cooler size can be reduced by a factor of 6.5 by using the 3-D printing technology compared to the MM cooler.

The electrolytic neutral pickling is a process used in the stainless steel industry to remove the oxide layer formed during annealing. In this process the stainless steel strip is pulled through a tank containing a sodium sulfate solution and a set of alternating electrodes, which indirectly polarize the strip surface and cause oxide dissolution under the influence of an electric current. State-of-the-art models describing this process assume concentration-independent electrochemical kinetics, which leads to the conclusion that homogeneous chemical reactions have a negligible influence on the final current density distribution. The present work revisits these conclusions by assessing the impact of concentration-dependent Butler-Volmer kinetics. Concentration distributions within the diffusion layer are estimated using a reduced boundary layer model for the ionic species at each electrode surface, each of these concentrations being two-way coupled with the electric potential in the bulk. Finally, the effects of different electrolyte compositions, homogenous chemical reactions, and flow conditions on the current density at the electrode surfaces are evaluated and discussed.

Functionally graded materials (FGMs) are a means to remove discrete material interfaces which lead to high local stress concentrations, such as the tungsten–copper (W–Cu) interface of the current ITER monoblock design. This paper employs adjoint-based optimization methods to identify the highest potential reduction of stresses that could be reached with these materials, while ensuring that the local temperature does not exceed the material temperature operational window. The cheap sensitivity evaluation inherent to the adjoint approach enables the optimization of the detailed 3D material distribution. Furthermore, a novel optimization method based on an augmented Lagrangian formulation is proposed that allows accurate treatment of the material temperature window constraints. The temperature and stresses are modelled by the steady heat conduction and Navier’s equation, respectively. We compare the results of different optimization formulations, with cost functions based on the von Mises stress and corresponding yield criterion and considering different values of the stress free temperature. To assess the performance under off-design conditions, two optimized designs were chosen and compared to the ITER and flat tile (FT) design, which consists of a copper block protected by a tungsten layer on top. The optimized designs lead to a factor 2–4 decrease in maximal stress near the original W–Cu interface of the FT design and a factor 10 decrease in yield criterion measure near the cooling duct. Under off-design conditions, they realized a factor 2–10 decrease in yield criterion in the upper part of the monoblock. This confirms numerically that FGMs can lead to significant design improvements. Finally, the inclusion of the material temperature operation window constraints leads to a decrease of 30–55 vol% W compared to the unconstrained cases, thus profoundly influencing the final design. The stress free temperature was found to have a comparably weaker influence on the final design with differences of 5–30 vol% W.

Type I Edge Localized Modes (ELMs) occur naturally in H-mode plasmas, the operational regime envisaged for ITER, and lead to high temporal heat load peaks on plasma-facing components that might induce surface melting and deteriorated material properties due to recrystallization. ELM mitigation techniques are being developed that either attempt to fully suppress the ELM, e.g. using resonant magnetic perturbations (RMP), or to trigger the ELM at higher frequencies, intending to reduce its energy content and hence the heat load peak. In this paper, the effect of the increased ELM frequency achieved by ELM mitigation techniques on the ITER tungsten divertor monoblock thermal and recrystallization behavior is numerically analyzed by solving the unsteady heat conduction equation when repeatedly exposing the monoblock to ELM-like heat loads under ITER baseline burning plasma operating conditions. The impinging heat load is based on a set of empirical relations readily found in literature and the recrystallization fraction is computed using an empirical data fit which is based on the non-isothermal JMAK equation as function of temperature and time. Uncertainties such as the statistical spread on empirical relations and the unknown inter-ELM heat load, as well as the effect of the ELM frequency, are quantified by a parameter scan. Adopting the recently developed concept of a ‘recrystallization budget’ to determine the allowed monoblock heat loads [G. De Temmerman et al., PPCF 60 (2018) 0044018], it was found that ELM frequencies between 25 – 50 Hz are allowed for inter-ELM heat loads between 9 – 6 MW m − 2 , respectively, when assuming that the scaling for the peak ELM target energy density in [T. Eich et al., NME 12 (2017) 84-90] holds. This provides a new limit for ELM mitigation strategies in ITER.

Energy dissipation in the plasma edge is key for future tokamaks. The potential of neon as radiating seeding species in disconnected double null (DDN) configuration is assessed in EAST discharges in high confinement mode (H-mode). As the separation between the two separatrices in the studied DDN discharges is minimum 1.5 cm, the configuration is effectively a single null configuration, and the benefits of the double null topology are minimal. Neon seeding, on the other hand, has a favourable effect: both the target heat flux and the divertor temperature decrease more than five-fold with increased seeding rate in high-recycling conditions. Interpretive edge plasma simulations with SOLPS-ITER in support of ongoing transport analysis are presented. For the unseeded case the numerical results agree with the experimental data within a factor two for the target temperature conditions and measured neutral pressures in the active divertor. The key for achieving good agreement is a suitable selection of coefficients for anomalous transport and neutral conductances between the upper cryopump and the main chamber.

This article deals with the problem of finding the best topology, pipe diameter choices, and operation parameters for realistic district heating networks. Present design tools that employ non-linear flow and heat transport models for topological design are limited to small heating networks with up to 20 potential consumers. We introduce an alternative adjoint-based numerical optimization strategy to enable large-scale nonlinear thermal network optimization. In order to avoid a strong computational cost scaling with the network size, we aggregate consumer constraints with a constraint aggregation strategy. Moreover, to align this continuous optimization strategy with the discrete nature of topology optimization and pipe size choices, we present a numerical continuation strategy that gradually forces the design variables towards discrete design choices. As such, optimal network topology and pipe sizes are determined simultaneously. Finally, we demonstrate the scalability of the algorithm by designing a fictitious district heating network with 160 consumers. As a proof-of-concept, the network is optimized for minimal investment cost and pumping power, while keeping the heat supplied to the consumers within a thermal comfort range of 5 %. Starting from a uniform distribution of 15 cm wide piping throughout the network, the novel algorithm finds a network lay-out that reduces piping investment by 23 % and pump-related costs by a factor of 14 in less than an hour on a standard laptop. Moreover, the importance of embedding the non-linear transport model is clear from a temperature-induced variation in the consumer flow rates of 72 %. ispartof: Applied Energy vol:280 status: published

This work presents, for the first time, a package- level, bare die liquid jet impingement 3D polymer microfluidics heatsink fabricated using 3D printing, or additive manufacturing for large die size and high-power applications. The heatsink achieves a chip temperature increase of 17.5°C at a chip power of 285 W for a coolant flow rate of 3.25 LPM, demonstrating that 3D printing enables the design for low-cost, high efficiency direct on-chip microfluidic heatsink with complex internal 3D manifold liquid delivery channels. The measurement results show that the jet impingement cooling performance can be successfully described using a unit cell approach, allowing an easy scaling of the thermal performance for arbitrary die size applications. Long term thermal tests of 1000h show a constant thermal performance and no degradation of the cooler material.

Monte Carlo (MC) simulations of the full kinetic equation for the neutral particles in the plasma edge become computationally costly for reactor-relevant regimes. To accelerate the simulations, we propose a hybrid fluid-kinetic approach that is based on a micro-macro decomposition of the kinetic equation. This leads to a macro/fluid model with kinetic corrections that follow from an MC simulation of the micro/kinetic part. We distinguish three hybrid models with different underlying fluid equations: (i) a pure pressure-diffusion equation with equal neutral and ion temperatures ( T n = T i ); (ii) a continuity and parallel momentum equation with pressure-diffusion transport retained in the directions perpendicular to the magnetic field lines, with T n = T i ; and (iii) the same model as (ii), but with a separate neutral energy equation ( T n ≠ T i ). To facilitate the future integration in more complete plasma edge codes, we neglect some kinetic correction terms. Hence, the hybrid model is not exactly equivalent to the full kinetic equation. We assess the hybrid performance on the basis of the reduction of the CPU time compared to an MC simulation of the full kinetic equation for the same statistical error on a certain plasma source. This is done for a high recycling slab case. Only the models with parallel momentum equation ((ii)-(iii)) are able to significantly reduce the CPU time. However, due to the incomplete kinetic corrections there is a remaining hybrid-kinetic discrepancy that mainly pops up in the ion energy source from the model with energy equation (iii).

ispartof: CONTRIBUTIONS TO PLASMA PHYSICS vol:60 issue:5-6 status: published

© 2018 The Authors We elaborate a hybrid fluid-kinetic model for neutral particles in the plasma edge. A macro/fluid model with kinetic corrections in the closure terms of the fluid moment equations is solved in the entire domain. The kinetic corrections follow from a Monte Carlo simulation of the micro/kinetic part. We compare two hybrid models with different underlying fluid approximations: (i) a pure pressure-diffusion equation; and (ii) a continuity and parallel momentum equation with pressure-diffusion transport only retained in the directions perpendicular to the magnetic field lines. We asses their performance for a high recycling slab case with fixed background plasma. Compared to a Monte Carlo simulation of the full kinetic equation, the statistical error on the particle source is reduced with 40% and 47% for respectively the hybrid model without and with the parallel momentum equation for the same computational time. For the parallel momentum source there is only a reduction for the model with parallel momentum equation (about 69%). Unfortunately, the statistical error on the ion energy source slightly increases for both hybrid models. ispartof: NUCLEAR MATERIALS AND ENERGY vol:18 pages:201-207 status: published

Jet impingement cooling with distributed outlet configuration is regarded as an efficient cooling solution for high performance systems. This work develops the Nusselt number Nu ¯ f - R e d and pressure drop k -factor correlations for microscale multi-jet impingement cooling with alternating feeding and draining jets. An extensive design of experiments (DOE) of unit cell modeling-based simulations is used for the correlation fitting. In order to understand the flow and thermal behavior, the impact of single variables including the inlet diameter, outlet diameter, jet-to-target distance and nozzle length is studied systematically. On the other hand, this work firstly shows the predictive model with k - R e d , including the impact of nozzle length t/L, cavity height H/L and inlet diameter ratio d i /L. Finally, the extracted correlations are experimentally validated, both with experimental data of different jet impingement demonstrators, as well as with the literature experimental data.