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[en] The transport and slowing down of suprathermal electrons in a laser plasma was studied to estimate the preheating during a target implosion. The following processes are taken into account - diffusion in space and velocity due to Coulomb interactions with thermal electrons and ions, - interaction with the self-consistent electric-field which is calculated by using the diffusion approximation including a flux limitation for high density gradients, - Bremsstrahlung emission. The classical formulas in the Born approximation are used. The Coulomb diffusion is preponderant; the interactions with the electric field provide a coupling between suprathermal and thermal electrons. The Bremsstrahlung spectrum allows to test the numerical simulations (Monte-Carlo, multigroup diffusion) by measuring the X rays produced during the implosion
[fr]On etudie le transport et le ralentissement d'electrons suprathermiques dans un plasma cree par laser afin de pouvoir estimer l'influence du prechauffage qu'ils provoquent lors de l'implosion d'une cible. Les phenomenes physiques pris en compte sont les suivants: - ralentissement et deviation par interaction coulombienne avec les electrons et les ions thermiques, - interaction avec le champ electrique auto-consistant. Cecui-ci est calcule dans l'hypothese de la diffusion avec une limitation des flux pour les forts gradients de densite, - emission de rayonnement de freinage. On utilise les formules classiques calculees a l'approximation de Born. On montre la preponderance de la diffusion coulombienne; les interactions avec le champ electrique fournissant un couplage supplementaire entre les electrons suprathermiques et thermiques. Le spectre de rayonnement de freinage doit permettre de tester les simulations numeriques (methode Monte-Carlo, methode multigroupe) en permettant un raccordement a l'experience grace a la mesure des X produits durant l'implosion
[en] CFD simulations focusing on capturing dynamic fluctuations of the flow for three operating points were performed for a scale model of a high head Francis turbine. A mesh sensitivity study showed an influence of the near wall resolution, consequently a low Reynolds mesh with a SST turbulence model was used. Rotor/stator fluctuations are well reproduced with the full turbine simulation at all operating points. Velocity contours and average velocity profiles from LDV measurements in the draft tube confirm that the flow physics is generally well reproduced. Simplified approaches such as profile transform and Fourier transform underestimated the measured fluctuations. As full turbine simulations were time-consuming, a simulation with only the draft tube was performed at part load to predict the fluctuations in the draft tube cone. The SAS-SST turbulence model was able to capture the vortex rope behavior
[en] This paper presents various numerical setups for modelling Francis turbine startups involving moving meshes and variable runner speed in order to help define best practices. During the accelerating phase of the startup, the flow is self-similar between channels, thus making single sector configuration appropriate. Adding the draft tube improves the results by allowing pressure recovery midway during in the startup. At the speed no-load regime, a rotating stall phenomenon occurs and can only be capted with the full runner included in the simulation. Comparison with experimental data, such as runner speed and strain gauge measurements, generally shows good agreement
[en] To assess the life expectancy of hydraulic turbines, it is essential to obtain the loading on the blades, especially during transient operations known to be the most damaging. This paper presents a simplified CFD setup to model the startup phase of a Francis turbine while it goes from rest to speed no-load condition. The fluid domain included one distributor sector coupled with one runner passage. The guide vane motion and change in the angular velocity were included in a commercial code with user functions. Comparisons between numerical results and measurements acquired on a full-size turbine showed that most of the flow physics occurring during startup were captured.
[en] Hydro-Quebec has been using CFD to analyze the performance of its existing turbines for many years. Most of those analyses are based on the measurement of a single runner blade. However, due to manufacturing techniques, in-situ modifications or repairs, there are often small differences between individual blades of the same runner. The impact of this non uniformity was not known thus far and was often assumed to be negligible given the size of the runner. This paper highlights the impact of such differences by presenting the CFD analysis of various blades measured on the same runner. Two different geometries are used for demonstration: the AxialT model propeller and a 50-MW Francis turbine. In both cases, about 50% of the blades could not be considered as representative of the whole turbine and using them could lead to wrong conclusions regarding the turbine performance.
[en] This study reports the numerical predictions of flows over turbine blades, which include flow acceleration and deceleration. Two issues are addressed: (1) accurately predicting roughness effects, and (2) evaluating the performance of Reynolds-Averaged Navier-Stokes (RANS) simulations on moderately accelerating flows. For the present turbine surfaces, it is found that roughness correlations based on roughness surface slope better predict the roughness effects than both the correlations based on the moments of roughness height statistics and the IEC standard approach. It is shown that RANS simulations reproduce the flow evolution over rough-wall accelerating turbulent boundary layers, although, on a smooth wall, they fail to capture strong non-equilibrium flow behaviours. Finally, a hydraulic turbine simulation is performed to show the significant roughness impact on the total losses
[en] Some of the potentially most damaging continuous operating conditions for hydraulic turbines are the no-load (NL) conditions. At NL conditions the flow passes through the turbine without power generation, but with non-negligible flow rate, and therefore all the potential energy in the flow has to be dissipated. This takes place through a mechanism where the runner channels are partially pumping, thus generating large scale unsteady vortex structures which, by their nature, break down into smaller and smaller vortices until energy dissipation occurs at the smallest scales. This type of flow, dominated by its turbulent character, is inherently difficult to simulate by means of numerical methods since turbulence model and numerical dissipation have a major influence. The resulting dynamic loads on the runner are largely of stochastic nature, exciting a broad band of frequencies and thus, almost always interact with at least one deformation mode. The presented investigations are aimed at predicting the effect of the unsteady NL pressure loads on the fatigue life of a Francis turbine runner. A combination of computational fluid dynamics (CFD) and finite element analysis (FEA) methods has been employed. The results from transient CFD simulations are presented. Comparison of the results with prototype strain gauge measurements at no load conditions shows that the stochastic nature and the approximate range of the dynamic stresses can be predicted