36 research outputs found

    Improved gas turbine diagnostics towards an integrated prognostic approach wiht vibration and gas path analysis

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    The degradation of a gas turbine engine in operation is inevitable, leading to losses in performance and eventually reduction in engine availability. Several methods like gas path analysis and vibration analysis have been developed to provide a means of identifying the onset of component degradation. Although both approaches have been applied individually with successes in identifying component faults; localizing complex faults and improving fault prediction confidence are some of the further benefits that can accrue from the integrated application of both techniques. Although, the link between gas path component faults and rotating mechanical component faults have been reported by several investigators, yet, gas path fault diagnostics and mechanical fault diagnostics are still treated as separated toolsets for gas turbine engine health monitoring. This research addresses this gap by laying a foundation for the integration of gas path analysis and vibration to monitor the effect of fouling in a gas turbine compressor. Previous work on the effect of compressor fouling on the gas turbine operation has been on estimating its impact on the gas turbine’s performance in terms of reduction in thermal efficiency and output power. Another methodology often used involves the determination of correlations to characterize the susceptibility and sensitivity of the gas turbine compressor to fouling. Although the above mentioned approaches are useful in determining the impact of compressor fouling on the gas turbine performance, they are limited in the sense that they are not capable of being used to access the interaction between the aerodynamic and rotordynamic domain in a fouled gas turbine compressor. In this work, a Greitzer-type compression system model is applied to predict the flow field dynamics of the fouled compressor. The Moore-Greitzer model is a lumped parameter model of a compressor operating between an inlet and exit ii duct which discharges to a plenum with a throttle to control the flow through the compression system. In a nutshell, the overall methodology applied in this work involves the interaction of four different models, which are: Moore-Greitzer compression system model, Al-Nahwi aerodynamic force model, 2D transfer matrix rotordynamic model and a gas turbine performance engine model. The study carried out in this work shows that as the rate of fouling increases, typified by a decrease in compressor massflow, isentropic efficiency and pressure ratio, there is a corresponding increase in the vibration amplitude at the compressor rotor first fundamental frequency. Also demonstrated in this work, is the application of a Moore-Greitzer type compressor model for the prediction of the inception of unstable operation in a compressor due to fouling. In modelling the interaction between the aerodynamic and rotordynamic domain in a fouled gas turbine compressor, linear simplifications have been adopted in the compression system model. A single term Fourier series has been used to approximate the resulting disturbed flow coefficient. This approximation is reasonable for weakly nonlinear systems such as compressor operating with either an incompressible inlet flow or low Mach number compressible inlet flow. To truly account for nonlinearity in the model, further recommendation for improvement includes using a second order or two-term Fourier series to approximate the disturbed flow coefficient. Further recommendation from this work include an extension of the rotordynamic analysis to include non-synchronous response of the rotor to an aerodynamic excitation and the application of the Greitzer type model for the prediction of the flow and pressure rise coefficient at the inlet of the compressor when fouled

    Complex oscillations with multiple timescales - Application to neuronal dynamics

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    The results gathered in this thesis deal with multiple time scale dynamical systems near non-hyperbolic points, giving rise to canard-type solutions, in systems of dimension 2, 3 and 4. Bifurcation theory and numerical continuation methods adapted for such systems are used to analyse canard cycles as well as canard-induced complex oscillations in three-dimensional systems. Two families of such complex oscillations are considered: mixed-mode oscillations (MMOs) in systems with two slow variables, and bursting oscillations in systems with two fast variables. In the last chapter, we present recent results on systems with two slow and two fast variables, where both MMO-type dynamics and bursting-type dynamics can arise and where complex oscillations are also organised by canard solutions. The main application area that we consider here is that of neuroscience, more precisely low-dimensional point models of neurons displaying both sub-threshold and spiking behaviour. We focus on analysing how canard objects allow to control the oscillatory patterns observed in these neuron models, in particular the crossings of excitability thresholds

    Model predictive control of gas compression station in off-shore production platforms

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    Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia de Automação e Sistemas, Florianópolis, 2016.Uma plataforma off-shore normalmente produz petróleo bruto e gás natural. O gás é tratado para a remoção da humidade e sua pressão e sua temperatura são modificadas de acordo com sua aplicação final. Parte do gás é direcionado para a linha de exportação de gás para ser comercializado. Muitas vezes o gás é utilizado por poços que operam com elevação por gaslift. O gás natural também é usado em turbinas para gerar eletricidade. Um sistema de compressão de gás é uma parte importante de uma unidade de produção off-shore de petróleo. O tipo de compressor mais usado em um sistema de compressão de gás é o compressor centrífugo. Uma falha do compressor pode fazer com que uma unidade de produção completa seja desligada. Os compressores centrífugos têm limites operacionais muito restritos e são muito sensíveis a mudanças na vazão de entrada de gás ou nas propriedades do mesmo. O compressor pode entrar em surge, que é uma condição operacional instável caracterizada pelo fluxo reverso de gás dentro do compressor e que pode acontecer quando a vazão de entrada de gás é muito baixa. Um compressor centrífugo que opera em surge não comprimirá o gás corretamente, causando danos permanentes à máquina. O procedimento normal utilizado quando se detecta a ocorrência de surge é parar o compressor. Geralmente, os compressores centrífugos são instalados com um controle regulatório que inclui a prevenção de surge. No entanto, mudanças bruscas na vazão de entrada de gás e na composição do gás são conhecidas por fazer com que o compressor centrífugo pare com frequência. Esta dissertação propõe um controlador MPC que reduz o consumo de energia do sistema de compressão e melhora sua proteção contra surge. Este trabalho também apresenta a modelagem de uma estação de compressão real composta de dois compressores de três estágios. Com base na análise do comportamento do sistema e da relação dinâmica entre as entradas e saídas do sistema, são propostas e testadas duas formulações de MPC diferentes. Para ajustar o controlador MPC foi aplicada a técnica de ajuste satisfatório, melhorando o desempenho do controlador.Abstract : An offshore oil production unit normally produces crude oil and natural gas. The gas is treated for removal of moisture and its pressure and temperature are conditioned to its target application. Part of the gas is directed to the gas export line for sales. Often it is used by wells operating with gas lift. Natural gas is also used in turbines to generate electricity. A gas compression system is an important part of an offshore oil production unit. The most important type of equipment used in a gas compression system is the centrifugal compressor. A compressor failure may cause a complete production unit shut down. Centrifugal compressors have a limited operational range and are very sensitive to changes in the gas flow rate or in its properties. Compressor surge is an unstable operational condition characterized by reverse flow inside the compressor and it can happen when the gas flow rate is too low. A centrifugal compressor operating in surge mode will not compress the gas as required and the machine could be damaged permanently. The normal procedure used when surge is detected is to stop the compressor. Usually centrifugal compressors are installed with a regulatory control that includes the avoidance of surge. But abrupt changes in gas flowrate and gas composition are known to cause centrifugal compressor to stop the production operations too often. This dissertation proposes a MPC controller that reduces the energy consumption of the compression system and improves its protection against surge. This work also presents the modeling of a real compression station composed of two three-stage compressors. Based on the analysis of the system?s behavior and the dynamic relation between inputs and outputs, two different MPC formulations are proposed and tested. To tune the MPC controller the satisficing tuning technique is applied, improving the controller?s performance

    Hydroacoustic modelling and numerical simulation of unsteady operation of hydroelectric systems

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    Hydropower represented in 1999 19% of the world electricity production and the absolute production is expected to grow considerably during the next 30 years. Francis turbines play a major role in the hydroelectric production due to their extended range of application. Due to the deregulated energy market, hydroelectric power plants are increasingly subjecting to off design operation, start-up and shutdown and new control strategies. Consequently, the operation of Francis turbine power plants leads to transients phenomena, risk of resonance or instabilities. The understanding of these propagation phenomena is therefore paramount. This work is a contribution to the hydroacoustic modelling of Francis turbine power plants for the investigation of the aforementioned problematic. The first part of the document presents the modelling of the dynamic behavior and the transient analysis of hydroelectric power plants. Therefore, the one-dimensional model of an elementary pipe is derived from the governing equations, i.e. momentum and continuity equations. The use of appropriate numerical schemes leads to a discrete model of the pipe consisting of a T-shaped equivalent electrical circuit. The accuracy in the frequency domain of the discrete model of the pipe is determined by comparison with the analytical solution of the governing equations. The modelling approach is extended to hydraulic components such as valve, surge tanks, surge shaft, air vessels, cavitation development, etc. Then, the modelling of the Francis, Pelton and Kaplan turbines for transient analysis purposes is presented. This modelling is based on the use of the static characteristic of the turbines. The hydraulic components models are implemented in the EPFL software SIMSEN developed for the simulation of electrical installations. After validation of the hydraulic models, transient phenomena in hydroelectric power plants are investigated. It appears that standard separate studies of either the hydraulic or of the electrical part are valid only for design purposes, while full hydroelectric models are necessary for the optimization of turbine speed governors. The second part of the document deals with the modelling and analysis of possible resonance or operating instabilities in Francis turbine power plants. The review of the excitation sources inherent to Francis turbine operations indicates that the draft tube and the rotor-stator interaction pressure fluctuations are of the major concern. As the modelling of part load pressure fluctuations induced by the cavitating vortex rope that develops in the draft tube at low frequencies is well established, the focus is put on higher frequency phenomena such as higher part load pressure fluctuations and rotorstator interactions or full load instabilities. Three hydroacoustic investigations are performed. (i) Pressure fluctuations identified experimentally at higher part load on a reduced scale model Francis turbine are investigated by means of hydroacoustic simulations and high speed flow visualizations. The resonance of the test rig due to the vortex rope excitation is pointed out by the simulation while the special motion and shape of the cavitating vortex rope at the resonance frequency is highlighted by the visualization. A description of the possible excitation mechanisms is proposed. (ii) A pressure and power surge measured on a 4 × 400 MW pumped-storage plant operating at full load is investigated. The modelling of the entire system, including the hydraulic circuit, the rotating inertias and the electrical installation provides an explanation of the phenomenon and the related conditions of apparition. A non-linear model of the full load vortex rope is established and qualitatively validated. (iii) The rotor-stator interactions (RSI) are studied in the case of a reduced scale pump-turbine model. An original modelling approach of this phenomenon based on the flow distribution between the stationnary and the rotating part is presented. The model provides the RSI pressure fluctuation patterns in the vaneless gap and enables to predict standing waves in the spiral case and adduction pipe. The proposed one-dimensional modelling approach enables the simulation, analysis and optimization of the dynamic behavior of hydroelectric power plants. The approach has proven its capability of simulating properly both transient and periodic phenomena. Such investigations can be undertaken at early stages of a project to assess the possible dynamic problems and to select appropriate solutions ensuring the safest and optimal operation of the facility

    Understanding Inquiry, an Inquiry into Understanding: a conception of Inquiry Based Learning in Mathematics

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    IBL (Inquiry Based Learning) is a group of educational approaches centered on the student and aiming at developing higher-level thinking, as well as an adequate set of Knowledge, Skills, and Attitudes (KSA). IBL is at the center of recent educational research and practice, and is expanding quickly outside of schools: in this research we propose such forms of instruction as Guided Self-Study, Guided Problem Solving, Inquiry Based Homeschooling, IB e-learning, and particularly a mixed (Inquiry-Expository) form of lecturing, named IBLecturing. The research comprises a thorough review of previous research in IBL; it clarifies what is and what is not Inquiry Based Learning, and the distinctions between its various forms: Inquiry Learning, Discovery Learning, Case Study, Problem Based Learning, Project Based Learning, Experiential Learning, etc. There is a continuum between Pure Inquiry and Pure Expository approaches, and the extreme forms are very infrequently encountered. A new cognitive taxonomy adapted to the needs of higher-level thinking and its promotion in the study of mathematics is also presented. This research comprises an illustration of the modeling by an expert (teacher, trainer, etc.) of the heuristics and of the cognitive and metacognitive strategies employed by mathematicians for solving problems and building proofs. A challenging problem has been administered to a group of gifted students from secondary school, in order to get more information about the possibility of implementing Guided Problem Solving. Various opportunities for further research are indicated, for example applying the recent advances of cognitive psychology on the role of Working Memory (WM) in higher-level thinking

    Influence of cavity flow on turbine aerodynamics

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    In order to deal with high temperatures faced by the components downstream of the combustion chamber, some relatively cold air is bled at the compressor. This air feeds the cavities under the turbine main annulus and cool down the rotor disks ensuring a proper and safe operation of the turbine. This thesis manuscript introduces a numerical study of the effect of the cavity flow close to the turbine hub on its aerodynamic performance. The interaction phenomena between the cavity and main annulus flow are not currently fully understood. The study of these phenomena is performed based on different numerical approaches (RANS, LES and LES-LBM) applied to two configurations for which experimental results are available. A linear cascade configuration with an upstream cavity and various rim seal geometries (interface between rotor and stator platform) and cavity flow rate available. A rotating configuration that is a two stage turbine including cavities close to realistic industrial configurations. Additional losses incurred by the cavity flow are measured and studied using a method based on exergy (energy balance in the purpose to generate work)

    Benelux meeting on systems and control, 23rd, March 17-19, 2004, Helvoirt, The Netherlands

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    Book of abstract

    Compositional Taylor model based validated integration

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    Validated integration is a family of methods that compute enclosures for sets of initial conditions in the Initial Value Problems. The Taylor model based validated integration methods use truncated Taylor series to approximate the solution to the Initial Value Problem and often give better results than other validated integration methods. Validated integration methods, and especially Taylor model based ones, become increasingly more impractical as the number of variables in the system get higher. In this thesis, we develop techniques that mitigate the issues related to the dimension of the system in Taylor model based validated integration methods. This is done by taking advantage of the compositional structure of the problem when possible. More precisely, the main contribution of this thesis is to enable computing an enclosure to a higher dimensional system by using enclosures for smaller lower dimensional subsystem that are contained in the larger system. The techniques called shrink wrapping and preconditioning are used in the Taylor model based validated integration to improve accuracy. We also analyse these techniques from a compositional viewpoint and present their compositional counterparts. We accompany compositional version of the Taylor model based validated integration with implementation of our tool CFlow* and experiments using our tool. The experimental results show performance gains for some systems with non-trivial compositional structure. This work was motivated by interest in formally analysing biological systems and we use biological systems examples in a number of our systems
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