589 research outputs found

    Prédiction de la génération des pertes des écoulements compressibles anisothermes appliquée aux distributeurs hautes pressions de turbine avec les simulations aux grandes échelles

    Get PDF
    Afin d'améliorer l'efficacité des moteurs aéronautiques, une des solutions envisagées par les industriels est d'augmenter la température d'entrée de la turbine. Cependant, ces hautes températures induisent de fortes contraintes thermiques sur les pales de turbine ce qui réduit leur durée de vie. Pour surmonter ces problèmes thermiques, des systèmes de refroidissement efficaces sont nécessaires. Afin d'évaluer la performance de ces systèmes, une prédiction précise de la température de paroi des pales de turbine et des pertes générées par ces systèmes est requise. Profitant de l'opportunité de récents développements d'outils de prédiction haute-fidélité, cette thèse financée par Safran Helicopter Engines à travers le projet FUI CASCADE, a pour but de valider la prédiction de la température de paroi des pales de turbine refroidie et des pertes générées par ces systèmes avec la Simulation aux Grandes Echelles (SGE). Pour atteindre ces objectifs, différentes configurations académiques et industrielles refroidies par film de refroidissement ont été simulées et étudiées. Les résultats obtenus dans cette thèse montrent que la SGE est capable de prédire l'aérodynamique et l'environnement thermique pour de tels systèmes. Pour faciliter l'utilisation de la SGE dans l’industrie et limiter le coût CPU lié à la résolution de l'écoulement dans le système de refroidissement des pales, un modèle de jets de refroidissement a été proposé et évalué dans ce travail. Les résultats montrent que ce modèle permet de reproduire l'aérodynamique des jets de refroidissement et la température de paroi des pales sans mailler le système de refroidissement. Pour évaluer les pertes dans ce contexte, l’approche Second Law Analysis (SLA) est adoptée. Contrairement aux bilans de température et pression totales, cette approche donne directement accès aux champs de perte 3D qui sont construits à partir des termes sources de l’entropie résolus sur le maillage. Ainsi, le mécanisme de génération de perte peut être localement étudié et ne requière pas de procédure de moyenne contrairement aux modèles de perte 1D. Ces champs de perte sont décomposés en deux contributions : une contribution aérodynamique et une contribution thermique liée au mélange chaud-froid. L'étude de ces champs montre que les pertes aérodynamiques sont principalement générées dans les régions de fort cisaillement (couche limite et de mélange) alors que les pertes de mélange sont générées dans les films de refroidissement et dans le sillage des pales. Des analyses avancées des champs de perte mettent en évidence que les fluctuations turbulentes dominent la génération des pertes pour ces systèmes. Ce dernier résultat met en évidence les bénéfices de l'approche Second Law Analysis pour prédire les pertes à partir des champs obtenus avec la SGE. En effet et contrairement aux approches RANS, les contributions turbulentes des pertes sont directement résolues sur le maillage avec la SGE et ne requiert aucune stratégie de modélisation. La principale conclusion de cette thèse est que l'approche Second Law Analysis couplée avec la SGE est une méthodologie très prometteuse et pertinente pour la prédiction des écoulements et des pertes pour les futurs designs de pale de turbine industriel

    Heuristic optimization of electrical energy systems: Refined metrics to compare the solutions

    Get PDF
    Many optimization problems admit a number of local optima, among which there is the global optimum. For these problems, various heuristic optimization methods have been proposed. Comparing the results of these solvers requires the definition of suitable metrics. In the electrical energy systems literature, simple metrics such as best value obtained, the mean value, the median or the standard deviation of the solutions are still used. However, the comparisons carried out with these metrics are rather weak, and on these bases a somehow uncontrolled proliferation of heuristic solvers is taking place. This paper addresses the overall issue of understanding the reasons of this proliferation, showing a conceptual scheme that indicates how the assessment of the best solver may result in the unlimited formulation of new solvers. Moreover, this paper shows how the use of more refined metrics defined to compare the optimization result, associated with the definition of appropriate benchmarks, may make the comparisons among the solvers more robust. The proposed metrics are based on the concept of first-order stochastic dominance and are defined for the cases in which: (i) the globally optimal solution can be found (for testing purposes); and (ii) the number of possible solutions is so large that practically it cannot be guaranteed that the global optimum has been found. Illustrative examples are provided for a typical problem in the electrical energy systems area – distribution network reconfiguration. The conceptual results obtained are generally valid to compare the results of other optimization problem

    Automated Grid Generator for MHD Flow Simulations Made With the Gems Code

    Get PDF
    Numerical simulations for Hypersonic Vehicle Power System (HVEPS) Project were based on a multi-domain, general Navier-Stokes Solver Code called the GEMS® Code. The GEMS® Code was in the process of being extended to solve for plasma flows with both self-induced as well as externally-applied magnetic fields. GEMS® is also capable of simulating both laminar and turbulent flow in unbounded as well as ducted flows. For application to the ducted Plasma flows generated experimentally in the HVEPS program, GEMS® was set up to calculate turbulent flow in 1-3D (dimensional) duct geometries in general and in 2D, 3D ones for purposes of numerical simulation in HVEPS Project. To model a real small scale experiment for Conductivity and MHD plasma channel flows it was decided to consider turbulent flow in both 2D and 3D duct geometries and to compare the results to experimental data obtained in HVEPS, as well as with numerical results from other known codes, such as the Mach 2 Code in 2D duct geometry. Accurate MHD channel flow simulations should require only 3D calculations, since MHD power generation is a completely three-dimensional phenomenon. In 2002, the latest generation GEMS® code (GEMS – General Equations and Mesh Solver), for CFD problems was created by Dr. Ding Li and Dr. Charles Merkle. This code can run in 1D, 2D and 3D as options and will be mentioned below. The problem attacked by the author of this thesis was to prepare a numerical method to generate appropriate and acceptable computational domains with acceptable grid formats that provide for convergence of numerical simulations made with GEMS® Code. In numerical modeling the HVEPS facility at UTSI the following computational domains were required: • a combustor chamber area; • a supersonic nozzle; • an adaptor fitted to the nozzle (ceramic ring as nozzle extension); • a straight conductivity channel with 6 dielectrics rings; • a conical MHD plasma channel with 6 dielectric rings; • an air surrounding nozzle and channel; • a channel extension as option for grid generation algorithm. In addition to the variety of computational domains modeled, boundary layers required near the walls must be adequately resolved by the computational grids. Flexibility of grid generation by UTGRID®, which will be discussed later, in section 3.9, boundary layer issues, allows choosing of appropriate grid aspect ratios to provide convergence of the solutions in boundary layer regions. Known grid generators are incapable of working in an automated mode, and the process of grid generation for complicated domains, like the diagonal wall MHD channel, is an extremely time-consuming and difficult task. The author developed a semi-automated process for creating the meshed domains for the CFD modeling in the HVEPS project. Thus, within the period 2002-2004 a new generation code, UTGRID®, for automatic and flexible grid generation was created by the author, which and links the set of the following programs together: • GEMS® (Created by Dr. Ding Li and Dr. Charles Merkle) • PGRID® (Created by Dr. Ding Li) • UTGRD® (Created by the author of this thesis) • GAMBIT™ (a commercial product associated with FLUENT®

    Metaheuristic optimization of power and energy systems: underlying principles and main issues of the 'rush to heuristics'

    Get PDF
    In the power and energy systems area, a progressive increase of literature contributions containing applications of metaheuristic algorithms is occurring. In many cases, these applications are merely aimed at proposing the testing of an existing metaheuristic algorithm on a specific problem, claiming that the proposed method is better than other methods based on weak comparisons. This 'rush to heuristics' does not happen in the evolutionary computation domain, where the rules for setting up rigorous comparisons are stricter, but are typical of the domains of application of the metaheuristics. This paper considers the applications to power and energy systems, and aims at providing a comprehensive view of the main issues concerning the use of metaheuristics for global optimization problems. A set of underlying principles that characterize the metaheuristic algorithms is presented. The customization of metaheuristic algorithms to fit the constraints of specific problems is discussed. Some weaknesses and pitfalls found in literature contributions are identified, and specific guidelines are provided on how to prepare sound contributions on the application of metaheuristic algorithms to specific problems

    Piezoelectric-silicone structure for vibration energy harvesting: testing and modelling

    Get PDF
    Mechanical vibrations from heavy machines, building structures or the human body can be harvested and directly converted into electrical energy. In this paper the potential to effectively harvest mechanical vibrations and locally generate electrical energy using a novel piezoelectric-rubber composite structure is explored. Piezoelectric lead zirconate titanate (PZT) is bonded to silicone rubber to form a cylindrical composite-like energy harvesting device which has the potential to structurally dampen high acceleration forces and generate electrical power. The device was experimentally load tested and an advanced model was verified against experimental data. While an experimental output power of 57 μW/cm3 was obtained, the advanced model further optimises the device geometry via a relative optimisation approach. The proposed energy harvesting device generates sufficient electrical power for structural health monitoring and remote sensing applications, while also providing structural damping properties for low frequency mechanical vibrations

    Metaheuristic Optimization of Power and Energy Systems: Underlying Principles and Main Issues of the `Rush to Heuristics'

    Get PDF
    In the power and energy systems area, a progressive increase of literature contributions that contain applications of metaheuristic algorithms is occurring. In many cases, these applications are merely aimed at proposing the testing of an existing metaheuristic algorithm on a specific problem, claiming that the proposed method is better than other methods that are based on weak comparisons. This ‘rush to heuristics’ does not happen in the evolutionary computation domain, where the rules for setting up rigorous comparisons are stricter but are typical of the domains of application of the metaheuristics. This paper considers the applications to power and energy systems and aims at providing a comprehensive view of the main issues that concern the use of metaheuristics for global optimization problems. A set of underlying principles that characterize the metaheuristic algorithms is presented. The customization of metaheuristic algorithms to fit the constraints of specific problems is discussed. Some weaknesses and pitfalls that are found in literature contributions are identified, and specific guidelines are provided regarding how to prepare sound contributions on the application of metaheuristic algorithms to specific problems
    • …
    corecore