RANS and LES Simulations at Partial Load in Francis Turbines: Three-Dimensional Topology and Dynamic Behaviour of Inter- Blade Vortices

International audienceHydraulic machines are designed to operate in flow conditions close to the best efficiency point. However, to respond to the increasing demand for flexibility mainly due to the integration of renewable energy in the electric grid, the operating range of Francis turbines has to be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the flow field is characterized by the appearance of inter-blade vortices in the runner. In these off-design operating conditions and due to these phenomena, dynamic stresses level can increase, and potentially lead to fatigue damage of the mechanical structure of the machine. The objective of this paper is to present investigations on the dynamic behaviour of the inter-blade vortices and their impact on the runner by using numerical simulations. Computations were performed with different turbulence modelling approaches to assess their relevance and reliability: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). Computations aimed to better understand the emergence condition of the inter-blade vortices. The analysis showed that vortices can be generated due to poor inlet adaptation at part load, however other vortices can also be due to a local backflow in the runner. The competition between these both phenomena leads to various topologies of the inter-blade vortices. The numerical results were compared to experimental visualizations performed on scaled model as well as to previous numerical studies results. The impact of these inter-blade vortices on the runner were also investigated by considering the pressure fluctuations induced on the blades. The dynamic loading on the blade has to be known in order to evaluate the lifetime of the runner by mechanical analysis. Different operating conditions have been simulated to understand how the pressure fluctuations depend on the operating conditions. The localization of the pressure fluctuations and their consequences on the frequency signature of the torque fluctuations have been analyzed. This article is presenting a part of the work presented at the 29th IAHR Symposium on Hydraulic Machinery and Systems, Kyoto, 2018 [1], and presents another vortex topology and a comparison of LES results of several operating conditions

RANS and LES simulations at partial load in Francis turbines: Three-dimensional topology and dynamic behaviour of inter-blade vortices

Hydraulic machines are designed to operate in flow conditions close to the best efficiency point. However, to respond to the increasing demand for flexibility mainly due to the integration of renewable energy in the electric grid, the operating range of Francis turbines has to be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the flow field is characterized by the appearance of inter-blade vortices in the runner. In these off-design operating conditions and due to these phenomena, dynamic stresses level can increase, and potentially lead to fatigue damage of the mechanical structure of the machine. The objective of this paper is to present investigations on the dynamic behaviour of the inter-blade vortices and their impact on the runner by using numerical simulations. Computations were performed with different turbulence modelling approaches to assess their relevance and reliability: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). Computations aimed to better understand the emergence condition of the inter-blade vortices. The analysis showed that vortices can be generated due to poor inlet adaptation at part load, however other vortices can also be due to a local backflow in the runner. The competition between these both phenomena leads to various topologies of the inter-blade vortices. The numerical results were compared to experimental visualizations performed on scaled model as well as to previous numerical studies results. The impact of these inter-blade vortices on the runner were also investigated by considering the pressure fluctuations induced on the blades. The dynamic loading on the blade has to be known in order to evaluate the lifetime of the runner by mechanical analysis. Different operating conditions have been simulated to understand how the pressure fluctuations depend on the operating conditions. The localization of the pressure fluctuations and their consequences on the frequency signature of the torque fluctuations have been analyzed. This article is presenting a part of the work presented at the 29th IAHR Symposium on Hydraulic Machinery and Systems, Kyoto, 2018 [1], and presents another vortex topology and a comparison of LES results of several operating conditions

Caractérisation spatio-temporelle des tourbillons générés proches de la soupape d'un moteur à combustion interne

L'aérodynamique des moteurs à combustion interne est soumise à de larges fluctuations cycle à cycle qui rentrent dans le bilan des émissions polluantes. Le développement des outils de diagnostiques optiques permet désormais de mesurer et d'étudier ces fluctuations, notamment dans des géométries réelles de moteur. Dans le cadre d'une analyse de ces fluctuations dans la phase d'admission, phase durant laquelle se structure l'écoulement, nous étudions la dynamique d'un jet de soupape sur un banc d'essai stationnaire avec une différence de pression constante aux bornes du montage et une levée de soupape fixe. Des mesures de vitesses résolues temporellement ont été réalisées par PIV à une cadence de 7,5 kHz. Une analyse de type POD (Décomposition Orthogonale en Modes Propres) des champs instantanés de vorticité permet de caractériser l'évolution spatio-temporelle des tourbillons présents dans les couches de cisaillement du jet, et d'analyser certains aspects de la dynamique de l'écoulement à la soupape

Variabilité aérodynamique d'un moteur à combustion interne pendant la phase d'admission (Vers l'origine des fluctuations cycliques)

La connaissance des mouvements aérodynamiques dans la chambre de combustion est un enjeu majeur pour optimiser le rendement des moteurs à combustion interne, et réduire ainsi les émissions polluantes. Les travaux présentés ici analysent les fluctuations cycle à cycle du champ aérodynamique pendant la phase d'admission d'un moteur 4 temps dans le but de discriminer les principaux phénomènes influents sur cette variabilité au point mort bas. Deux étapes permettent une approche progressive de la complexité de l'écoulement (turbulence, confinement, géométrie déformable). Dans une première étape, le système d'admission est caractérisé dans une configuration à géométrie fixe (banc volute). Des mesures de vitesse effectuées par imagerie de particules (TRPIV à une cadence de 10kHz) mettent en évidence différents types d'instationnarités : un mécanisme de battement du jet lié au confinement de l'écoulement et la génération de structures tourbillonnaires dans les couches de cisaillement. Les caractéristiques spatio-temporelles de ces instationnarités sont quantifiées par des techniques avancées de traitement du signal (POD). Elles sont en bon accord avec celles déduites d'une simulation numérique des grandes échelles (LES) réalisée dans la même configuration. Dans une seconde étape, l'écoulement est caractérisé sur banc moteur monocylindre optique. Les résultats montrent que les fluctuations acoustiques et aérodynamiques en amont de la soupape influent très peu sur la structure et la variabilité de l'écoulement dans la chambre de combustion. Les fluctuations du champ aérodynamique au point mort bas sont principalement liées au battement du jet de soupape au cours de la phase d'admission.Knowledge of airflow in the combustion chamber is of great interest to improve the performance of internal combustion engines, thereby reducing emissions. Present work analyzes cycle to cycle fluctuations of aerodynamics flow during the intake part of a 4-stroke engine in order point out main phenomena leading to cycle to cycle fluctuations at the low break-even point. Two stages allow a gradual approach to the complexity of the flow (turbulence, confinement, moving geometry). In a first step, the admissions system is characterized on a steady flow test bench. High speed particle image velocimetry measurements (TR-PIV) highlight different types of instationnarities: the flapping of the valve jet and the generation of vortex structures in shear layers. The spatial and temporal characteristics of these instationnarities are quantified by advanced signal processing (POD). They are in good agreement with those derived from a Large Eddy Simulation (LES) carried out in the same configuration. In a second step, the flow is studied using an optical motored engine. Results show that acoustic fluctuations and aerodynamic variability upstream the valve have very little influence on the structure and variability of the flow into the combustion chamber. Fluctuations in the aerodynamic field at the low break-even point are primarily related to the flapping valve jet during the intake stroke.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

RANS and LES simulations at partial load in Francis turbines: Three-dimensional topology and dynamic behaviour of inter-blade vortices

Hydraulic machines are designed to operate in flow conditions close to the best efficiency point. However, to respond to the increasing demand for flexibility mainly due to the integration of renewable energy in the electric grid, the operating range of Francis turbines has to be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the flow field is characterized by the appearance of inter-blade vortices in the runner. In these off-design operating conditions and due to these phenomena, dynamic stresses level can increase, and potentially lead to fatigue damage of the mechanical structure of the machine. The objective of this paper is to present investigations on the dynamic behaviour of the inter-blade vortices and their impact on the runner by using numerical simulations. Computations were performed with different turbulence modelling approaches to assess their relevance and reliability: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). Computations aimed to better understand the emergence condition of the inter-blade vortices. The analysis showed that vortices can be generated due to poor inlet adaptation at part load, however other vortices can also be due to a local backflow in the runner. The competition between these both phenomena leads to various topologies of the inter-blade vortices. The numerical results were compared to experimental visualizations performed on scaled model as well as to previous numerical studies results. The impact of these inter-blade vortices on the runner were also investigated by considering the pressure fluctuations induced on the blades. The dynamic loading on the blade has to be known in order to evaluate the lifetime of the runner by mechanical analysis. Different operating conditions have been simulated to understand how the pressure fluctuations depend on the operating conditions. The localization of the pressure fluctuations and their consequences on the frequency signature of the torque fluctuations have been analyzed. This article is presenting a part of the work presented at the 29th IAHR Symposium on Hydraulic Machinery and Systems, Kyoto, 2018 [1], and presents another vortex topology and a comparison of LES results of several operating conditions