Every type of current meter is different and has its proper characteristics. Knowing the performance of a current meter is essential in order to use it properly either for field or laboratory measurements (such as in the Offshore Technology Research Center wave basin). A study of the MAVS (Modular Acoustic Velocity Sensor) in a wave basin is a first step essential for later deployment in real studies. This thesis is based on data obtained from different series of laboratory measurements conducted in the OTRC wave basin. The objective of the first part of the study was to characterize the MAVS frequency response using benchmarks such as tow tests or wave tests. These benchmarks allowed us not only to characterize the sensor but also to eventually correct some of the measurement distortions due to flow blockage, vortex shedding, or vibrations of the mounting structure, for example. After the preliminary study was done, we focused on the potential use of the MAVS in the OTRC wave basin. Indeed, in the case of a study of a scale model in the wave basin, the stresses applied to the model have to be accurately known. In the case of current-induced loads, this includes contributions from both the mean flow and the turbulence. Thus, after correcting the values measured by the MAVS, a mapping of the current jet was executed to determine its three-dimensional structure in the wave basin. Knowing the structure of the current in the OTRC wave basin, it was then possible to define a domain in which the current can be considered uniform with a certain tolerable error. This domain of uniformity will allow us to validate the use of the OTRC wave basin to study large models such as FPSOs (Floating Production, Storage and Offloading Units).

Ce travail de thèse s'inscrit dans le cadre d'un projet de recherche industriel visant à améliorer les méthodes et les outils de simulation du comportement thermohydraulique et vibratoire dans les générateurs de vapeur des centrales nucléaires, en présence de colmatage. Le colmatage des générateurs de vapeur est un phénomène de déposition de matières au niveau d'interstices, appelés passages foliés, qui perturbe la libre circulation de l'écoulement appartenant au circuit secondaire. L'objectif de ces travaux de thèse est de disposer d'un modèle permettant de simuler ce phénomène dans l'intégralité du générateur de vapeur à l'échelle du sous-canal du faisceau de tubes. Le modèle proposé se décompose en deux étapes au niveau de chaque passage folié : une étape de déposition des particules, et une étape de consolidation du dépôt par précipitation d'espèces solubles. Une méthode inverse d'assimilation de données a été développée pour ajuster ce modèle sur les différentes observations issues du retour d'expérience disponible à EDF. Les résultats de simulation sont comparables aux données mesurées sur sites d'exploitation pour les générateurs de vapeur étudiés. L'impact du colmatage sur l'écoulement se caractérise par l'apparition de survitesses en partie supérieure du générateur de vapeur à l'origine d'instabilités vibratoires des tubes, et par la diminution du taux de recirculation. Par ailleurs, les résultats de simulation confirment qu'une augmentation du pH dans l'ensemble du circuit secondaire semble une solution intéressante pour atténuer le phénomène de colmatage. Ce remède est actuellement envisagé sur le parc nucléaire français. / In nuclear power plants, corrosion product deposits in the secondary side of steam generators may result in tube support plate flow blockage. Flow blockage is a deposit at the inlet of tube support plate flow holes. It may induce high velocity zones in the secondary flow, then vibrations and tube cracks in some cases. The main objective of this work is to model and simulate this deposit phenomenon in the whole steam generator. A new deposit model has been also developed and implemented in the frame of THYC. THYC is the EDF's reference code for the modelling of 3D two-phase thermal-hydraulic phenomena at the subchannel scale. The deposit model is defined by two main steps : particle deposition, and strengthening process due to soluble species precipitation in the pores of particle deposits. It is calibrated on blockage rates observed in steam generators using an inverse method also developed in this work. The relevance of this model is tested by comparing the simulation results to the actual levels of flow blockage observed in some nuclear plants. The main impact of flow blockage on the secondary flow is localized at the upper tube support plate and may induce tube vibrations. Moreover the simulation results underline the pH dependence of flow blockage phenomenon. A pH elevation of the secondary flow is one of the remedies currently considered on EDF's nuclear fleet.

Submitted by Pedro Silva Filho (pfsilva@ipen.br) on 2017-11-17T17:27:31Z No. of bitstreams: 0 / Made available in DSpace on 2017-11-17T17:27:31Z (GMT). No. of bitstreams: 0 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Os elementos de combustível de um reator nuclear de pesquisa tipo MTR (\"Material Testing Reactor\") são, em sua grande maioria, formados por placas de combustível revestidas com alumínio contendo no cerne silicileto de urânio (U3Si2) disperso em matriz de alumínio. Essas placas possuem espessura da ordem de milímetros e comprimentos muito maiores em relação à sua espessura. Elas são dispostas paralelamente no conjunto que forma o elemento combustível, de maneira a formar canais entre elas com poucos milímetros de espessura, por onde escoa o fluido de refrigeração (água leve ou água pesada). Essa configuração, associada à necessidade de um escoamento com altas vazões para garantir o resfriamento das placas em operação, pode gerar problemas de falhas mecânicas das placas de combustível devido às vibrações induzidas pelo escoamento nos canais e, consequentemente, acidentes de proporções graves no caso de velocidade crítica que possa gerar o colapso das placas. Embora não haja ruptura das placas de combustível durante o colapso, as deflexões permanentes excessivas das placas podem causar bloqueio do canal de escoamento no núcleo do reator e levar ao superaquecimento nas placas. Para este trabalho, foram desenvolvidas uma bancada experimental com capacidade para altas vazões volumétricas (Q=100 m3/h) e uma seção de testes que simula um elemento combustível do tipo placa com três canais de resfriamento. A seção de testes foi construída com placas de alumínio e acrílico e foi instrumentada com sensores de deformação, sensores de pressão, um acelerômetro e um tubo de pitot. As dimensões da seção de testes foram baseadas nas dimensões do Elemento Combustível do Reator Multipropósito Brasileiro (RMB), cujo projeto está sendo coordenado pela Comissão Nacional de Energia Nuclear - CNEN. Os experimentos realizados alcançaram o objetivo de chegar à condição de velocidade crítica de Miller com o colapso das placas. A velocidade crítica foi atingida com 14,5 m/s levando a consequente deformação plástica das placas que formam o canal do escoamento. O canal central na entrada da seção de testes apresentou uma abertura de 3 mm em seu centro, causando um grande bloqueio do escoamento nos canais laterais. Este comportamento foi v constatado visualmente durante a desmontagem da seção de testes, ilustrado e discutido na análise de resultados apresentado neste trabalho. O bloqueio dos canais também foi observado por meio de gráficos de queda de pressão e por gráficos das deformações da entrada, centro e saída das placas contra a velocidade média da seção de testes. Observou-se uma queda da resistência hidráulica da seção de testes devido ao aumento da seção transversal de escoamento no canal central e um aumento exponencial das deformações quando da ocorrência da velocidade crítica. Comparativamente, o valor experimental obtido para velocidade crítica na seção de testes foi da ordem de 85% do valor obtido por cálculo com a expressão teórica de Miller. Os experimentos realizados permitiram um melhor entendimento da interação fluido estrutura em elementos de combustível tipo placa como: valores de frequências de vibrações naturais, instabilidade fluido elástica e desenvolvimento de técnicas para a detecção de valores de velocidade crítica. / Tese (Doutorado em Tecnologia Nuclear) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP / CNPq:481193/2012-0

This research aims to assess the causes of inefficient and unstable operation of centrifugal compressors and turboexpanders in process gas applications in order to provide a solution for performance restoration and enhancement. It encompasses thermodynamic and flow evaluations to examine the efficiency and operating range improvement options of new units. Besides, this work is complemented by a technoeconomic analysis to provide a rounded outcome from these studies. In order to achieve the desired objectives, a novel integrated approach has been developed to assess the design and performance of multi-stage centrifugal compressors. The proposed systematic methodology involves five basic elements including evaluation of compressor selection, compressor sizing and casing structure, performance prediction at the design and off-design conditions, modelling of efficiency and head deterioration causes; and stage design evaluation. This will contribute towards evaluating the geometrical parameters of the new units’ designs at the early preliminary design phase, and thus, will be useful to identify the options for efficiency and operating range enhancements. For installed units, this approach can be implemented to assess the cause of inefficient and unstable operation by assessing the available operation data. A method was developed to predict the performance curve of multi-stage centrifugal compressor based on a stage stacking technique. This approach considers the advantages of Lüdtke and Casey-Robinson methods with an incorporation of a methodology for compressor selection and sizing to generate more accurate results. To emphasize the validity of the developed model, it has been evaluated for both low and high flow coefficient applications. The obtained results show a significant improvement in the estimated efficiency, pressure ratio, shaft power and operating range as compared with the existing methods. The centrifugal compressor is designed to run under various operating conditions and different gas compositions with the primary objective of high efficiency and reliability. Therefore, a new iterative method has been developed to predict the equivalent compressor performance at off-design conditions. This technique uses the performance parameters at design conditions as a reference point to derive the corresponding performance characteristics at numerous suction conditions with less dependency on the geometrical features. Through a case study on a gas transport centrifugal compressor, it was found that the developed approach can be applied for design evaluation on the expected variation of working conditions, and for the operation diagnosis of installed units as well. Furthermore, a parametric study has been conducted to investigate the effect of gas properties on the stage efficiency, surge margin, and compressor structure. The obtained results support the need for considering the gas properties variation when the off-design performance is derived. To evaluate the impact of internal blockage on the performance parameters, this study proposed an approach to model the effect of non-reactive deposits, which has been qualified using four operation cases and the obtained results are compared with the internal inspection findings from the stage overhauling process. This also covers the influential aspects of flow blockage on the technical and economic values. Since the main challenge here is to analyze the process gas composition in real time, the influences of the non-reactive deposits have been compared with the effect of the unanticipated gas composition change. Subsequently, it has turned out that the pressureratio parameter is not enough to assess the possibility of flow blockage and unexpected gas properties change. Moreover, it was observed that the stage discharge pressure was more sensitive to the fouled aftercooler comparing with suction and internal blockage. However, the effect of contaminated aftercooler on the surge point and discharge pressure and temperature of the upstream stage was found greater than its impact on the shaft power. Thus, a substantial surge margin reduction was detected when the first stage was operating with a fouled aftercooler comparing with the measured reduction as a result of unanticipated gas properties change. Furthermore, a larger pressure ratio drop was measured in the case of liquid carryover which revealed a more significant impact of the two phases densities difference comparing with the gas volume fraction (GVF) effect. The possibility of hydrate formation has been assessed using hydrate formation temperature (HFT) criteria. Additionally, this research highlights a number of challenges facing the selection of typical centrifugal stage design by assessing the contribution of design characteristics on the operating efficiency and stable flow range. Besides, an empirical-based-model was established to select the optimum impeller and diffuser configurations in order to make a compromise decision based on technical and economic perspective. It was concluded that there is no absolute answer to the question of optimum rotor and stator configuration. The preliminary aerothermodynamic evaluation exposed that the selection of the optimum impeller structure is governed by several variables: stage efficiency, pressure loss coefficient, manufacturing cost, required power cost, resonance frequency and stable operating range. Hence, an evaluation is required to compromise between these parameters to ensure better performance. Furthermore, it was argued throughout this study that the decision-making process of the typical stage geometrical features has to be based upon the long-term economic performance optimization. Thus, for higher long-term economic performance, it is not sufficient to select the characteristics of the impeller and diffuser geometry based on the low manufacturing cost or efficiency improvement criterion only. For turboexpanders, a simple and low cost tool has been developed to determine the optimum turboexpander characteristics by analysing the generated design alternatives. This approach was used in designing a turboexpander for hydrocarbon liquefaction process. Moreover, since the turboexpanders are expected to run continuously at severe gas conditions, the performance of the selected turboexpander was evaluated at different inlet flow rates and gas temperatures. It has turned out that designing a turboexpander with the maximum isentropic efficiency is not always possible due to the limitations of the aerodynamic parameters for each component. Therefore, it is necessary to assess the stage geometrical features prior the construction process to compromise between the high capital cost and the high energetic efficiency.

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