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In the current research work, a computational analysis of a high-speed centrifugal compressor stage for turbocharger applications is presented. A detailed investigation about the interactions between backswept impeller and downstream vaneless and vaned diffusers is carried out. ' A unshrouded backswept impeller with splitters was combined with a vaneless diffuser or a number of different designs of vaned diffusers. The CFD solver CFX-TASCow was used. The three-dimensional Reynolds- Averaged Navier-Stokes equations are solved and a pressure correction method is employed to solve the system of equations. A steady simulation and analysis of the interactions between the impeller and the vaneless diffuser is carried out, emphasis is focused on the comparisons of the different interactions at different conditions regarding the flow structures at different radius ratios, effect of rotational speed, mass flow rate and impeller tip clearance. The predicted results were also compared with the available experimental results in terms of radial Velocity, tangential Velocity and flow angle. In general, the predicted results show a reasonable agreement with the experimental data. A steady state simulation and analysis regarding the interaction between the impeller and various vaned diffusers is carried out. For the interface between the rotational impeller outlet and the stationary vaned diffuser inlet, the stage averaging condition is used. A detailed comparison between the predicted and the available experimental data is performed in terms of static pressure rise, total pressure ratio, choking mass flow and efficiency characteristics, and very good agreement is accomplished. In addition, detailed flow distributions are compared, assessed and critically analysed, regarding different number of diffuser vanes, rotational speed, gap between the leading edge of the vaned diffuser and impeller tip, mass flow rate. Emphasis is focused on the steady state study of the effect of the number of diffuser vanes on the stage operating range. Further more, unsteady simulation and analysis regarding the interactions between backswept impeller and downstream vaned diffusers is carried out. In the unsteady simulation, a geometry scaling method is used to modify the diffuser geometry to the nearest integer pitch ratio while keeping the throat area, flow direction and area ratio unchanged in order to deal with the unequal pitch ratio problems which exist in the unsteady simulation. The unsteady investigation was undertaken regarding different number of diffuser vanes, rotational speed, gap between the leading edge of the vaned diffuser and impeller tip, mass flow rate and impeller tip clearance. The detailed interactions at different conditions are compared, assessed and analysed. The studies focus on the analyses of the effect of the different interactions on the stage operating range, peak efficiency, total pressure ratio, level of unsteadiness, flow structures, flow angle or incidence angle, etc. In addition, the' predicted results are compared with available experimental data and a quite good agreement is achieved although the geometry is scaled. On the other hand, a detailed investigation on the differences between the time averaged unsteady simulation results and steady simulation results was performed at different conditions. The comparisons were carried out regarding static pressure, total pressure, speed, flow angle (or incidence angle) and isentropic efficiency. The investigation confirms that unsteady simulation is still quite important, since some of the steady state simulation results are still not similar to the time averaged ones. Designers should take into account the influence of the unsteadiness on the flow fields when they employ the steady state model in the design process.

A computational study was performed on a vaned radial diffuser using the MEFP (The Moore Elliptic Flow Program) flow code. The vaned diffuser studied by Dalbert et al. was chosen as a test case for this thesis. The geometry and inlet conditions were established from this study. The performance of the computational diffuser was compared to the test case diffuser. The CFD analysis was able to demonstrate the 3D flow within the diffuser. An inlet conditions analysis was performed to establish the boundary conditions at the diffuser inlet. The given inlet flow angles were reduced in order to match the specified mass flow rate. The inlet static pressure was held constant over the height of the diffuser. The diffuser was broken down into its subcomponents to study the effects of each component on the overall performance of the diffuser. The diffuser inlet region, which comprises the vaneless and semi-vaneless spaces, contains the greatest losses, 56%, but the highest static pressure rise, 54%. The performance at the throat was also evaluated and the blockage and pressure recovery were calculated. The results show the static pressure comparison for the computational study and the test case. The overall pressure rise of the computational study was in good agreement with the measured pressure rise. The static pressure and total pressure loss distributions in the inlet region, at the throat, and in the exit region of the diffuser were also analyzed. The flow development was presented for the entire diffuser. The 3D flow calculations were able to illustrate a leading edge recirculation at the hub, caused by an inlet skew and high losses at the hub, and the secondary flows in the diffuser convected the high losses. The study presented in this thesis demonstrated the flow development in a vaned diffuser and its subcomponents. The performance was evaluated by calculating the static pressure rise, total pressure losses, and throat blockage. It also demonstrated current CFD capabilities for diffusers using steady 3D flow analysis. / Master of Science

L'amélioration de la performance des turbomachines fonctionnant loin de leur point d'adaptation passe par la maîtrise des phénomènes instationnaires qui se produisent dans ces différents organes. L'étude présentée ici se concentre sur les interactions entre une roue de ventilateur centrifuge et son diffuseur. Elle vise, par des mesures de pression sur les différentes parois du diffuseur, ainsi que par des sondages dans les canaux inter-aubages à analyser finement ces phénomènes instationnaires et notamment les décollements fluctuants apparaissant sur les aubes à sur débit, et à mesurer leur influence sur la performance du diffuseur. Dans un second temps, un dispositif de contrôle passif de ces décollements utilisant des générateurs de vortex est proposé. Plusieurs configurations sont testés et leurs résultats sont comparés / The performance improvement of turbomachinery operating at off-design conditions can be achieved by the understanding of unsteady phenomena which are occuring in its components. The present study is focussing on the interaction between a centrifugal impeller fan and its vaned diffuser. It aims at analysing precisely these unsteady phenomena (and especially the fluctuating separated region identified on the vanes wall) and their consequences on the diffuser performance by three holes probe and unsteady pressure measurements. In a second step, devices for a passive control of the separation are introduced. Several configurations are tested and their results are compared

<p>  </p> <p>As aviation fuel costs and consumption have continued to rise over recent decades, gas turbine engine manufacturers have sought methods to reduce fuel burn. Manufacturers plan to achieve this by reducing the specific fuel consumption of the machine by increasing the bypass ratio through a reduction of the diameter of the engine core. This presents an opportunity for implementing a centrifugal compressor as the final stage of the high-pressure compressor. The vaned diffuser in a centrifugal compressor stage maintains an integral role in determining the extents of the operating range as well as conditioning the flow for the downstream combustor. Thus, it is critical to understand the aerodynamics and performance of the vaned diffuser across the entire compressor operating range.</p> <p>This investigation focused on seven compressor operating points at the stage’s design corrected speed, which ranged from choked flow to the minimum mass flow rate before rotating stall. Steady-state and unsteady performance data were acquired to study the aerodynamics at each operating point as well as the steady-state performance of the vaned diffuser. Laser Doppler velocimetry, high-frequency pressure transducers, and additive manufacturing techniques were all implemented to acquire data in the research compressor.</p> <p>Unsteady velocity measurements were acquired in the vaneless space and were used to quantify the change in diffuser inlet incidence as the stage mass flow rate changes. The impeller exit jet and wake were compared at each operating point to understand the effect of these flow structures on the spanwise incidence profile. Steady-state performance metrics for the vaned diffuser were compared with the change in incidence to assess the effect of incidence on performance. Maximum static pressure recovery and minimum total pressure loss occurred at the maximum incidence operating point. </p> <p>The chordwise static pressure distribution is critical for health monitoring of the polymer, additive manufactured diffuser vanes. Steady-state and unsteady pressure measurements were acquired along the diffuser vane surface to assess the change in the aerodynamic lift force across the compressor operating range as well as the static pressure differential across the vane leading edge. The maximum unsteady lift on the diffuser vanes was up to 34% greater than the steady-state lift force. Unsteady static pressure differentials across the diffuser vane leading edge were similar to the steady-state values, but they were marginally greater across the entire examined operating range. These unsteady pressure measurements were acquired with high-frequency response pressure transducers installed along the diffuser vane surfaces. These transducers were also used to study the rotating stall and surge behavior of the investigated centrifugal compressor stage. This centrifugal compressor stage exhibits a spike-type rotating stall pattern at the onset of stage instability, which rapidly evolves into full flow reversal with several surge cycles. During these surge cycles, the diffuser vane leading edges are subject to a 20 psid static pressure differential. </p> <p>A computational model was used to predict the compressor flow at three different operating points. This model utilized the BSL-EARSM turbulence model, and it included surface roughness and an experimentally measured shroud thermal profile. The model accurately predicted the diffuser inlet flow angles near the shroud, but it predicted more radial flow near midspan. The diffuser vane leading edge static pressure differential was predicted within 1 psid at higher aerodynamic loading conditions. The differences between the computationally predicted and experimentally measured flow are attributed to difficulties associated with modelling the rate of mixing within the flow.</p>

L’étude effectuée au cours de cette thèse a permis de caractériser numériquement les instabilités d’origine aérodynamique rencontrées dans un compresseur centrifuge dessiné par Turbomeca. Ce compresseur est composé d’une roue directrice d’entrée, d’un rouet centrifuge, d’un diffuseur radial et de redresseurs axiaux. Le module expérimental, dénommé Turbocel, sera accueilli au LMFA courant 2016. Le contenu de cette étude repose donc exclusivement sur des résultats numériques dont certains sont cependant comparés à des résultats expérimentaux partiels obtenus par Turbomeca sur une configuration proche. _ Le fonctionnement du compresseur est analysé à différentes vitesses de rotation, à partir de simulations RANS et URANS menées avec le code elsA. Du point de vue de la méthodologie, deux points importants sont à retenir :- Du fait du caractère transsonique de l’écoulement dans le rouet et le diffuseur radial à haut régime de rotation, les simulations RANS stationnaires ne permettent pas d’accéder à une description satisfaisante des phénomènes physiques. Cela est dû à l’utilisation d’un plan de mélange aux différentes interfaces rotor-stator qui a pour effet d’empêcher les ondes de choc de remonter à l’amont, et qui affecte tant la physique de l’écoulement que l’étendue de la plage de fonctionnement stable.- En-dessous d’un certain débit, les calculs URANS sur période machine révèlent que le comportement de l’étage n’obéit plus à la périodicité spatio-temporelle mono-canal. Une plage instable est alors obtenue à toutes les iso-vitesses simulées. A bas régime de rotation, une autre plage stable existe lorsque le compresseur est suffisamment vanné. L’étage retrouve alors une périodicité spatio-temporelle, à condition d’étendre le domaine de calcul dans le stator à deux canaux inter-aubes. En ce qui concerne les limites de stabilité de Turbocel, différentes évolutions sont décrites selon la vitesse de rotation considérée :- A haut régime de rotation, une basse fréquence commence à émerger près du point de rendement maximal et son intensité ne fait qu’augmenter jusqu.au pompage.- A bas régime, une signature basse fréquence comparable se manifeste près du point de rendement maximal mais disparaît passé un certain vannage, et n’est donc présente que sur une plage de débit délimitée. La seconde zone stable peut alors être numériquement parcourue jusqu.au pompage proprement dit. La signature basse fréquence est imputée à l’instauration d’une recirculation dans l’inducteur qui une fois établie est quasi-stationnaire. Les résultats numériques mettent en évidence que la source d’instabilité sévère sur Turbocel provient du diffuseur aubé. En fonction du point de fonctionnement, ce composant adopte des comportements différents, entre lesquels une certaine continuité existe, et ses performances chutent progressivement lorsque le débit diminue. Au final, les domaines de stabilité de l’étage de compression peuvent être reliés au type d’écoulement qui se développe dans le diffuseur radial, et apparaissent dictés par le diffuseur semi-lisse à haut régime de rotation. Enfin, afin d’étendre les plages de fonctionnement stable, une stratégie de contrôle basée sur l’aspiration de couche limite dans le diffuseur aubé a également été déterminée dans le cadre de cette thèse. Son évaluation fera l’objet d’études ultérieures sur Turbocel. / The present study aims at characterizing the aerodynamic instabilities involved in a centrifugal compressor designed by Turbomeca, by means of numerical simulation. This compressor is composed of inlet guide vanes, a centrifugal impeller, a radial vaned diffuser and axial outlet guide vanes. The test module, named Turbocel, will be delivered to the LMFA in 2016. Thus, the results presented in this manuscript are only based on CFD, although some of them are compared to experimental results obtained by Turbomeca on a close configuration.RANS and URANS simulations are performed for several rotational speeds, using the elsA software.Two methodological key points are to be emphasized:- As the flow in both the impeller and the radial diffuser is transonic at high rotational speed, steady RANS simulations cannot provide a satisfactory description of the physical phenomena taking place. This can be explained by the use of the mixing plane approach which prevents shock waves to extend upstream the rotor-stator interfaces, and which impacts the flow field predicted as well as the prediction of the stable operating range.- Below a given massflow rate, URANS simulations covering the spatial period of the compressor prove that the stage behavior does not obey to the single passage spatio-temporal periodicity anymore. An unstable operating range then appears at all the simulated rotational speeds. At low rotational speed, another stable range is however obtained if the compressor is further throttled’ A new periodicity arises on this massflow range, provided that the stator domain is extended to two neighboring blade passages. Concerning the stability domains of Turbocel, different evolutions are obtained depending on the rotational speed:- At high rotational speed, a low frequency phenomenon starts to develop near the peak efficiency point and its intensity keeps increasing until surge happens.- At low rotational speed, a low frequency signature also appears near the peak efficiency point, but it then vanishes when the compressor is further throttled, so that only a restricted operating range exhibits this instability. It then gives rise to a second stable operating range which can be described numerically, ending with surge itself. The low frequency signature is attributed to the enhancement of a flow recirculation in the inducer which, once fully established, is quasi-steady. The numerical results underline that the source of severe instability in the compressor comes from the vaned diffuser. Depending on the operating point, this component can adopt different behaviors, between which a relative continuity exists, and its performances decrease when the massflow rate decresases. The overall stage performances prove that at high rotational speed, the global stability is driven by the semi-vaneless diffuser and depends on the flow developing in the radial diffuser. Finally, in order to extend the stable operating range of the compressor, a flow control strategy based on boundary layer suction has also been determined in the diffuser. Its impact on the performances of Turbocel will be deeply studied later on.

The subject of this diploma thesis is engineering design of the sediment conveyer. Conveyer is instrumental to total emptying of sediment silo that is situated in a sewage treatment plant. Proposed construction has to fulfill the specified functional requirements and parameters. Parts of solution are also necessary analytical and FEM calculations.

This work will be focused on issue of a jet engine. The thesis will be divided into search retrieval part and computational part. In the search retrieval part it will focus on different configurations of jet engines as well as areas of their use. The main part of the thesis will however focus on a calculations where a turbine, compressor and an exhaust nozzle will be designed in order to give a thrust of approximately 1kN. Next step will be determination of an engine charcteristic that will give us a preview on how the engine performance will look like in off-design modes.