Martine Baelmans

The EIRENE and B2-EIRENE Codes

Abstract The EIRENE neutral gas transport Monte Carlo code has been developed initially for TEXTOR since the early 1980s. It is currently applied worldwide in most fusion laboratories for a large variety of different purposes. The main goal of code development was to provide a tool to investigate neutral gas transport in magnetically confined plasmas. But, due to its flexibility, it also can be used to solve more general linear kinetic transport equations by applying a stochastic rather than a numerical or analytical method of solution. Major applications of EIRENE are in connection with plasma fluid codes, in particular with the various versions of the B2 two-dimensional plasma edge fluid code. The combined code package B2-EIRENE was developed, again initially for TEXTOR applications, in the late 1980s. It too has become a standard tool in plasma edge science. It is currently mainly used for divertor configurations, such as by the ITER central team, to assist the design of the ITER divertor. Both the EIRENE and B2-EIRENE concepts are introduced and illustrated with sample applications.

Database analysis of errors in large-eddy simulation

A database of decaying homogeneous, isotropic turbulence is constructed including reference direct numerical simulations at two different Reynolds numbers and a large number of corresponding large-eddy simulations at various subgrid resolutions. Errors in large-eddy simulation as a function of physical and numerical parameters are investigated. In particular, employing the Smagorinsky subgrid parametrization, the dependence of modeling and numerical errors on simulation parameters is quantified. The interaction between these two basic sources of error is shown to lead to their partial cancellation for several flow properties. This leads to a central paradox in large-eddy simulation related to possible strategies that can be followed to improve the accuracy of predictions. Moreover, a framework is presented in which the global parameter dependence of the errors can be classified in terms of the “subgrid activity” which measures the ratio of the turbulent to the total dissipation rate. Such an analysis allows one to quantify refinement strategies and associated model parameters which provide optimal total simulation error at given computational cost.

B2-EIRENE simulation of ASDEX and ASDEX-Upgrade scrape-off layer plasmas

The 2D multifluid edge code B2 coupled with the 3D neutral gas Monte Carlo code EIRENE is being used for edge interpretation and model validation on the axisymmetric poloidal divertor experiments ASDEX and ASDEX-Upgrade. For this purpose B2 was significantly improved especially by a fully implicit treatment of the topological cuts appearing in X-point configurations, and a reasonably accurate handling of inclined target plates. A fast, automatic grid generator has been developed, which allows direct implementation of experimental MHD equilibria into B2-EIRENE. Typical ASDEX and ASDEX-Upgrade simulations are presented and discussed.

Thermal challenges in next generation electronic systems - summary of panel presentations and discussions

The presentations made, as well as the discussions, in the panels at the workshop, Thermal Challenges in Next Generation Electronic Systems (THERMES), are summarized in this paper. The panels dealt with diverse topics including thermal management roadmaps, microscale cooling systems, numerical modeling from the component to system levels, hardware for future high performance and Internet computing architectures, and transport issues in the manufacturing of electronic packages. The focus of the panels was to identify barriers to further progress in each area that require the attention of the research community.

A CONTINUUM MODEL FOR AIRFLOW, HEAT AND MASS TRANSFER IN BULK OF CHICORY ROOTS

This article presents a transient 3D CFD model of heat and mass transfer in bulks of chicory roots. The model consists of the system of conservation equations of momentum, energy, and mass for the air phase, and the energy and mass for the product phase without considering the internal gradient in temperature and moisture in the product phase. The interaction between the airflow and the porous media is described by an Ergun-type equation based on experimental data. Heat of respiration is included in the model as an empirically derived function of temperature. A finite volume code is used to solve the model equations. The results show a good agreement between the model and the experiments. Differences between predicted and measured weight loss only amount to a maximum 10% after the initial cooling period. The non-ideal experimental conditions (high velocity, low relative humidity of the air), the various sizes of the products, the small scale of the porous region compared to the size of the product, and the estimation of transfer correlations contribute to the observed differences between experiment and simulation. The model can be applied to study the cooling process in an industrial cold store to find the optimal process settings to improve product quality and reduce product weight loss.

Sensitivity analysis of initial condition parameters on the transitional temporal turbulent mixing layer

This paper aims at determining the most influential inlet turbulence parameters on the downstream transitional mixing region. To this end, a stochastic method is developed to generate a divergence-free random velocity field with a prescribed energy distribution in physical and wave-number space. In addition, predetermined integral length scales can be established. Ten direct numerical simulations of a temporally evolving transitional turbulent mixing layer are examined in detail. Simulation results show a large disparity in mean and instantaneous turbulent quantities, mainly effected by the energy spectrum, the integral length scale and the divergence freeness of the initial field.

Optimality of the dynamic procedure for large-eddy simulations

We present a database analysis to obtain a precise evaluation of the accuracy limitations associated with the popular dynamic eddy-viscosity model in large-eddy simulation. We consider decaying homogeneous isotropic turbulence at two different Reynolds numbers, i.e., Rel = 50 and 100. The large-eddy simulation errors associated with the dynamic model are compared with those arising in the “static” Smagorinsky model. A large number of systematically varied simulations using the Smagorinsky model provides a detailed impression of the dependence of the total simulation error on sid the spatial resolution and siid the resolution of the subgrid dissipation length. This error behavior also induces an “optimal refinement trajectory” which specifies the particular Smagorinsky parameter, in terms of the spatial resolution, for which the total error is minimal. In contrast, the dynamic model gives rise to a self-consistently determined “dynamic trajectory” that represents the dependence of the dynamic coefficient on the spatial resolution. This dynamic trajectory is compared with the optimal refinement trajectory as obtained from the full database analysis of the Smagorinsky fluid. It is shown that the dynamic procedure in which the top-hat test filter is adopted, predicts values for the eddy viscosity as function of resolution and Reynolds number, which quite closely follow the main trends established in the optimal refinement trajectory. Furthermore, a sensitivity analysis, including dependency on test-filter width and filter shape, is discussed. Total simulation errors, due to interacting discretization, and modeling errors associated with the dynamic procedure may be a factor 2 higher compared to the optimum; still the dynamic procedure represents one of the very few self-contained and efficient error-reduction strategies when increasing the spatial resolution.

Solder parameter sensitivity for CSP life-time prediction using simulation-based optimization method

Finite element modeling (FEM) is widely used for estimating the solder joint reliability of electronic packages. However, the solder properties are strongly process and geometry dependent. Even for the same type of solder, measurements conducted by different people at different locations show different results, due to differences in application conditions, benching etc. Those differences may lead to differences in constitutive equations and/or the parameter values. Therefore the effect of the solder parameter variation and parameter sensitivity should be taken into account before a reliable solder fatigue prediction can be made. In this research, a simulation based optimization method is used to investigate the sensitivity of the chosen solder parameters for the solder fatigue prediction using an inelastic strain criterion.

Effect of Flow Pulsation on the Heat Transfer Performance of a Minichannel Heat Sink

Heat sinks with liquid forced convection in microchannels are targeted for cooling electronic devices with a high dissipated power density. Given the inherent stability problems associated with two-phase microchannel heat transfer, this paper investigates experimentally the potential for enhancing single-phase convection cooling rates by applying pulsating flow. To this end, a pulsator device is developed which allows independent continuous control of pulsation amplitude and frequency. For a single minichannel geometry (1.9 mm hydraulic diameter) and a wide range of parameters (steady and pulsating Reynolds number, Womersley number), experimental results are presented for the overall heat transfer enhancement compared to the steady flow case. Enhancement factors up to 40% are observed for the investigated parameter range (Reynolds number between 100 and 650, ratio of pulsating to steady Reynolds number between 0.002 and 3, Womersley number between 6 and 17). Two regimes can be discerned: for low pulsation amplitude (corresponding to a ratio of pulsating to steady Reynolds number below 0.2), a small heat transfer reduction is observed similar to earlier analytical and numerical predictions. For higher amplitudes, a significant heat transfer enhancement is observed with a good correspondence to a power law correlation. This work establishes a reference case for future studies of the effect of flow unsteadiness in small scale heat sinks.

Flow modeling in air-cooled electronic enclosures

Computational fluid dynamics (CFD) has indisputably become a very useful tool in the early design phase of electronics systems. Its success, however, does not guarantee accurate predictions in all situations. Therefore, this paper focuses on typical, but complex, flow features where the predictive capability of CFD is still rather poor. Examples are given for enclosures with forced convection cooling. Different aspects of system level modeling are analyzed: fan modeling and induced swirling flow, pressure loads and friction forces induced by screens and flow aspects in between printed circuit boards (PCBs) or more specifically around in line positioned electronic components. All numerical results are thereby compared to experimental data.