No description available.

Cette thèse porte sur la caractérisation expérimentale et la simulation numérique d’une configurationde jet rond en impact peu rencontrée dans la littérature : un jet chauffé issu d’une conduitepleinement développée à un haut nombre de Reynolds (ReD = 60 000) impacte normalement uneparoi située à trois diamètres en aval. Le premier volet de ce travail est dédié à la génération d’unebase de donnée expérimentale à l’aide de plusieurs moyens de mesure, avec pour objectif de caractériserà la fois la dynamique et la thermique de l’écoulement. Les techniques complémentaires devélocimétrie laser à franges (LDV) et vélocimétrie par image de particules (S-PIV) ont été mises àprofit pour la caractérisation du champ de vitesse et du tenseur de Reynolds tandis que les champsde température moyenne et fluctuante ont été mesurés à l’aide d’un fil froid. Enfin, les échangesthermiques au niveau de la paroi ont été obtenus par la méthode inverse de thermographie en facearrière (ThEFA). En plus de fournir une base de donnée très complète nécessaire à la validation dessimulations numériques, ces mesures ont également permis de mettre en évidence l’organisation àgrande échelle de l’écoulement, avec la présence de grandes structures tourbillonnaires dont la fréquencede passage correspond au mode colonne du jet libre et qui s’approchent de la paroi d’impactaux alentours du second maximum observé dans la distribution des échanges pariétaux. Le secondvolet concerne les simulations numériques visant à reproduire la configuration expérimentale. Deuxapproches ont été évaluées : l’approche RANS pour quantifier la pertinence des modèles utilisés parles industriels et l’approche LES, plus coûteuse, mais donnant accès aux propriétés instationnaireset tridimensionnelles de l’écoulement. Les simulations RANS ont montré que les modèles reconnuscomme les plus performants pour ce type de configuration sont incapables de prévoir correctementle niveau des échanges pariétaux. Ils sont, en revanche, bien reproduits par la simulation LES. Lesdonnées obtenues ont été mises à profit pour mieux comprendre les mécanismes liés à l’apparitiondu second maximum. Cette analyse a mis en avant le rôle des « points chauds ». Seuls certains d’entreeux ont pu être reliés à la présence de régions « décollées » tandis que la majorité est associée à desstructures allongées dans la direction de l’écoulement. / This thesis is dedicated to the experimental characterisation and the numerical simulations ofa round impinging jet configuration seldom dealt with in the literature : a heated jet issues from apipe fully developed pipe at a high Reynolds number (ReD = 60 000) and normally impinges a platelocated three diameters downstream. The first part of this work is directed towards the generationof an experimental database by means of several measurement techniques in order to characteriseboth the dynamical and thermal flow features. The complementary techniques of laser Doppler velocimetry(LDV) and particle image velocimetry (S-PIV) allowed for the velocity and Reynolds tensorfield characterisation. The mean and fluctuating temperature fields were measured through cold-wirethermometry. Finally, the plate heat transfer distribution was obtained through the inverse methodof « rear face thermography » (ThEFA). The gathered data not only provided a comprehensive databasenecessary to validate numerical simulations but also permitted to highlight the large-scale floworganisation, with the presence of large vortices shedding at the free jet preferred mode and closelyapproaching the plate in the vicinity of the secondary peak observed in the heat transfer distribution.The second part of this thesis focuses on the numerical simulations aiming at reproducing the experimentalconfiguration. Two approaches were evaluated : the RANS approach in order to quantifythe relevance of industrial turbulence models and the Large-Eddy Simulation, more expensive, butproviding the 3D unsteady flow features. The RANS simulations showed that the models recognisedas the most efficient for this kind of configuration are unable to correctly predict the heat transferlevels. They are, on the other hand, well reproduced by the LES. The generated data allowed for betterunderstanding of the mechanisms leading to the secondary peak. This analysis highlighted theprominent role of the "hot spots", where only some of them can be related to « separated » regions,while the majority are associated with streamwise elongated structures.

The complex multiphase flow physics of spray-swirl interaction in both reacting and non-reacting environment is of fundamental and applied significance for a wide variety of applications ranging from gas turbine combustors to pharmaceutical drug nebulizers. Understanding the intricate dynamics between this two phase flow field is pivotal for enhancing mixing characteristics, reducing pollutant emissions and increasing the combustion efficiency in next generation combustors. The present work experimentally investigates the near and far-field break-up, dispersion and coalescence characteristics of a hollow cone spray in an unconfined, co¬annular isothermal swirling air jet environment. The experiments were conducted using an axial-flow hollow cone spray nozzle having a 0.5 mm orifice. Nozzle injection pressure (PN = 1 bar) corresponding to a Reynolds number at nozzle exit ReN = 7900 used as the test setting. At this setting, the operating Reynolds number of the co-annular swirling air stream number (Res) was varied in four distinct steps, i.e. Res = 1600, 3200, 4800 and 5600. Swirl was imparted to the co¬axial flow using a guided vane swirler with blade angle of Ф=45° (corresponding geometric swirl number SG = 0.8). Two types of laser diagnostic techniques were utilized: Particle / Droplet imaging analysis (PDIA) and shadowgraph to study the underlying physical mechanisms involved in the primary breakup, dispersion and coalescence dynamics of the spray. Measurements were made in the spray in both axial and radial directions and they indicate that Sauter Mean Diameter (SMD) in radial direction is highly reliant on the intensity of swirl imparted to the spray. The spray is subdivided into two zones as function of swirl in axial and radial direction: (1) near field of the nozzle (ligament regime) where variation in SMD arises predominantly due to primary breakup of liquid films (2) far-field of the nozzle where dispersion and collision induced coalescence of droplets is dominant. Each regime has been analyzed meticulously, by computing probability of primary break-up, probability of coalescence and spatio-temporal distribution of droplets which gives probabilistic estimate of aforementioned governing phenomena. In addition to this, spray global length scale parameters such as spray cone angle, break-up length, wavelength of liquid film has been characterized by varying Res while maintaining constant ReN.

Dans cette thèse, nous analysons l’écoulement et les transferts thermiques convectifs dans des modèles géométriques d’échangeurs de chaleur multi-rangs de tubes à ailettes planes continues. Dans un premier temps, les phénomènes Aérauliques qui s’y développent sont mis en évidence par des mesures PIV et LDA. Une étude locale de la génération, du développement, de l’évolution spatiale etde la dissipation des enroulements tourbillonnaires produits dans l’échangeur est effectuée. Dans un second temps, des simulations numériques U-RANS sont réalisées et validées par comparaison de la structuration de l’écoulement et de la dynamique tourbillonnaire aux mesures expérimentales. Dansun troisième temps, l’influence de ces tourbillons sur le transfert thermique est mise en exergue. Les performances d’échange sont caractérisées selon une analyse de synergie entre le champ de vitesse, les gradients de vitesse et de température ainsi qu’en évaluant les irréversibilités thermoaérauliques produites dans l’écoulement. Dans un quatrième temps, une analyse de l’influence de la forme du tube sur les performances thermoaérauliques locales et globales de l’échangeur est effectuée selon le premier et le second principe de la Thermodynamique. Les transferts thermiques, les pertes visqueuses ainsique les taux de production d’entropie thermique et visqueuse dans le fluide sont évalués. Enfin une méthode d’optimisation géométrique globale basée sur l’analyse factorielle de TAGUCHI est utilisée pour sélectionner les paramètres les plus influents sur les performances thermoaérauliques globales dans l’optique d’une conception optimisée des surfaces d’échange pour une application à la climatisationferroviaire. / In this thesis, we analyze the flow and convective heat transfer in multi-row plain fin and tube heat exchangers. The aeraulic phenomena that occur in these devices are first highlighted by means of PIV and LDA measurements. A local study of horseshoe vortices production, development, spatial evolution and dissipation is therefore performed. Secondly, Unsteady RANS modeling of the flow is carried out by means of numerical simulations and the results are validated by comparing theflow structure and the vortex dynamics with experimental data. In a third step, the influence of these vortices on heat transfer is highlighted. The thermalhydraulic performances are characterized on the basis of an analysis of synergy between the velocity field, velocity gradients and temperature gradients.The thermal and viscous entropy rate generated in the flow are locally characterized. In a fourth step, an analysis of the influence of the tube pattern on thermalhydraulic performances is performed by considering the first and the second law of thermodynamics. The convective heat transfer and wallviscous friction losses are evaluated as well as thermal and viscous entropy production rates. Finally an overall geometrical optimization process based on the factorial analysis of TAGUCHI is used to select the major parameters that affect the thermalhydraulic performances aiming to optimize the design ofmultirow plain fin-and-tube heat exchangers for HVAC applications in rail transport.

No description available.

This report documents the experimental study for four different three-dimensional turbulent flows. The investigation focuses on near wall measurements in these flows. Several experimental techniques are used in the studies; however, the bulk of the investigation focuses on a three-orthogonal-velocity-component fiber-optic laser Doppler anemometer (3D-LDA) system. The control volume of the 3D-LDA is on the order of 50 micro-meter in size, or a y⁺ distance of around 2.3 units (using average values of U_&#964 and ν from the experiment). An auxiliary small boundary layer wind tunnel (auxiliary tunnel) and a low speed linear compressor cascade wind tunnel (cascade tunnel) are utilized in this study. One of four flow experiments is done in the auxiliary tunnel the other three are in the cascade tunnel. The first three-dimensional turbulent flow is a vortical flow created by two half-delta wing vortex generators. Near wall secondary flow features are found. The second flow is an investigation of the first quarter chord tip gap flow in the cascade tunnel. Strong three-dimensional phenomena are found. The third flow investigated is the inflow to the compressor cascade with the moving wall. The experiment records shear layer interaction between the upstream flow and moving wall. Finally the fourth flow investigated is the inflow to the compressor cascade with the moving wall with half-delta wing vortex generators attached. Phase-averaged data reveal asymmetrical vortex structures just downstream of the vortex generators. This is the first time any near wall data has been taken on any of these flows. / Master of Science

No description available.

In the first part of this dissertation, the phenomenon of self-sustained pressure os-cillations due to the flow past a circular, axisymmetric cavity, associated with inline gate valves, was investigated. In many engineering applications, such as flows through open gate valves, there exists potential for coupling between the vortex shedding from the up-stream edge of the cavity and a diametral mode of the acoustic pressure fluctuations. The effects of the internal pipe geometry immediately upstream and downstream of the shal-low cavity on the characteristics of partially trapped diametral acoustic modes were in-vestigated numerically and experimentally on a scaled model of a gate valve mounted in a pipeline that contained convergence-divergence sections in the vicinity of the valve. The resonant response of the system corresponded to the second acoustic diametral mode of the cavity. Excitation of the dominant acoustic mode was accompanied by pressure oscillations, and, in addition to that, as the angle of the converging-diverging section of the main pipeline in the vicinity of the cavity increased, the trapped behavior of the acoustic diametral modes diminished, and additional antinodes of the acoustic pressure wave were observed in the main pipeline. In addition to that, the effect of shallow chamfers, introduced at the upstream and/or downstream cavity edges, was investigated in the experimental system that con-tained a deep, circular, axisymmetric cavity. Through the measurements of unsteady pressure and associated acoustic mode shapes, which were calculated numerically for several representative cases of the internal cavity geometry, it was possible to identify the configuration that corresponded to the most efficient noise suppression. This arrangement also allowed calculation of the azimuthal orientation of the acoustic modes, which were classified as stationary, partially spinning or spinning. Introduction of shallow chamfers at the upstream and the downstream edges of the cavity resulted in changes of azimuthal orientation and spinning behaviour of the acoustic modes. In addition, introduction of splitter plates in the cavity led to pronounced change in the spatial orientation and the spinning behaviour of the acoustic modes. The short splitter plates changed the behaviour of the dominant acoustic modes from partially spinning to stationary, while the long split-ter plates enforced the stationary behaviour across all resonant acoustic modes. Finally, the evolution of fully turbulent, acoustically coupled shear layers that form across deep, axisymmetric cavities and the effects of geometric modifications of the cavity edges on the separated flow structure were investigated using digital particle image velocimetry (PIV). Instantaneous, time- and phase-averaged patterns of vorticity pro-vided insight into the flow physics during flow tone generation and noise suppression by the geometric modifications. In particular, the first mode of the shear layer oscillations was significantly affected by shallow chamfers located at the upstream and, to a lesser degree, the downstream edges of the cavity. In the second part of the dissertation, the performance of aortic heart valve pros-thesis was assessed in geometries of the aortic root associated with certain types of valve diseases, such as aortic valve stenosis and aortic valve insufficiency. The control case that corresponds to the aortic root of a patient without valve disease was used as a reference. By varying the aortic root geometry, it was possible to investigate corresponding changes in the levels of Reynolds shear stress and establish the possibility of platelet activation and, as a result of that, the formation of blood clots. / Graduate / 0541 / 0546 / 0548 / 0986 / alexbn024@gmail.com

In the first part of this dissertation, the phenomenon of self-sustained pressure os-cillations due to the flow past a circular, axisymmetric cavity, associated with inline gate valves, was investigated. In many engineering applications, such as flows through open gate valves, there exists potential for coupling between the vortex shedding from the up-stream edge of the cavity and a diametral mode of the acoustic pressure fluctuations. The effects of the internal pipe geometry immediately upstream and downstream of the shal-low cavity on the characteristics of partially trapped diametral acoustic modes were in-vestigated numerically and experimentally on a scaled model of a gate valve mounted in a pipeline that contained convergence-divergence sections in the vicinity of the valve. The resonant response of the system corresponded to the second acoustic diametral mode of the cavity. Excitation of the dominant acoustic mode was accompanied by pressure oscillations, and, in addition to that, as the angle of the converging-diverging section of the main pipeline in the vicinity of the cavity increased, the trapped behavior of the acoustic diametral modes diminished, and additional antinodes of the acoustic pressure wave were observed in the main pipeline. In addition to that, the effect of shallow chamfers, introduced at the upstream and/or downstream cavity edges, was investigated in the experimental system that con-tained a deep, circular, axisymmetric cavity. Through the measurements of unsteady pressure and associated acoustic mode shapes, which were calculated numerically for several representative cases of the internal cavity geometry, it was possible to identify the configuration that corresponded to the most efficient noise suppression. This arrangement also allowed calculation of the azimuthal orientation of the acoustic modes, which were classified as stationary, partially spinning or spinning. Introduction of shallow chamfers at the upstream and the downstream edges of the cavity resulted in changes of azimuthal orientation and spinning behaviour of the acoustic modes. In addition, introduction of splitter plates in the cavity led to pronounced change in the spatial orientation and the spinning behaviour of the acoustic modes. The short splitter plates changed the behaviour of the dominant acoustic modes from partially spinning to stationary, while the long split-ter plates enforced the stationary behaviour across all resonant acoustic modes. Finally, the evolution of fully turbulent, acoustically coupled shear layers that form across deep, axisymmetric cavities and the effects of geometric modifications of the cavity edges on the separated flow structure were investigated using digital particle image velocimetry (PIV). Instantaneous, time- and phase-averaged patterns of vorticity pro-vided insight into the flow physics during flow tone generation and noise suppression by the geometric modifications. In particular, the first mode of the shear layer oscillations was significantly affected by shallow chamfers located at the upstream and, to a lesser degree, the downstream edges of the cavity. In the second part of the dissertation, the performance of aortic heart valve pros-thesis was assessed in geometries of the aortic root associated with certain types of valve diseases, such as aortic valve stenosis and aortic valve insufficiency. The control case that corresponds to the aortic root of a patient without valve disease was used as a reference. By varying the aortic root geometry, it was possible to investigate corresponding changes in the levels of Reynolds shear stress and establish the possibility of platelet activation and, as a result of that, the formation of blood clots. / Graduate / 0541 / 0546 / 0548 / 0986 / alexbn024@gmail.com

Global renewable energy requirements rapidly increase with the transition to a fossil-free society. As a result, intermittent energy resources, such as wind- and solar power, have become increasingly popular. However, their energy production varies over time, both in the short- and long term. Hydropower plants are therefore utilized as a regulating resource more frequently to maintain a balance between production and consumption on the electrical grid. This means that they must be operated away from the design point, also known as the best-efficiency-point (BEP), and often are operated at part-load (PL) with a lower power output. Moreover, some plants are expected to provide a spinning reserve, also referred to as speed-no-load (SNL), to respond rapidly to power shortages. During this operating condition, the turbine rotates without producing any power. During the above mentioned off-design operating conditions, the flow rate is restricted by the closure of the guide vanes. This changes the absolute velocity of the flow and increases the swirl, which is unfavorable. The flow field can be described as chaotic, with separated regions and recirculating fluid. Shear layer formation between stagnant- and rotating flow regions can be an origin for rotating flow structures. Examples are the rotating-vortex-rope (RVR) found during PL operation and the vortical flow structures in the vaneless space during SNL operation, which can cause the flow between the runner blades to stall, also referred to as rotating stall. The flow structures are associated with pressure pulsations throughout the turbine, which puts high stress on the runner and other critical parts and shortens the turbine's lifetime. Numerical models of hydraulic turbines are highly coveted to investigate the detrimental flow inside the hydraulic turbines' different sections at off-design operating conditions. They enable the detailed study of the flow and the origin of the instabilities. This knowledge eases the design and assessment of mitigation techniques that expand the turbines' operating range, ultimately enabling a wider implementation of intermittent energy resources on the electrical grid and a smoother transition to a fossil-free society. This thesis presents the numerical study of the Porjus U9 model, a scaled-down version of the 10 MW prototype Kaplan turbine located along the Luleå river in northern Sweden. The distributor contains 20 guide vanes, 18 stay vanes and the runner is 6-bladed. The numerical model is a geometrical representation of the model turbine located at Vattenfall Research and Development in Älvkarleby, Sweden. The commercial software ANSYS CFX 2020 R2 is used to perform the numerical simulations. Firstly, the draft tube cone section of the U9 model is numerically studied to investigate the sensitivity of a swirling flow to the GEKO (generalized kω) turbulence model. The GEKO model aims to consolidate different eddy viscosity turbulence models. Six free coefficients are changeable to tune the model to flow conditions and obtain results closer to an experimental reference without affecting the calibration of the turbulence model to basic flow test cases. Especially, the coefficients affecting wall-bounded flows are of interest. This study aims to analyze if the GEKO model can be used to obtain results closer to experimental measurements and better predict the swirling flow at PL operation compared to other eddy viscosity turbulence models. Results show that the near-wall- and separation coefficients predict a higher swirl and give results closer to experimentally obtained ones. Secondly, a simplified version of the U9 model is investigated at BEP and PL operating conditions and includes one distributor passage with periodic boundary conditions, the runner and the draft tube. The flow is assumed axisymmetric upstream of the runner, hence the single distributor passage. Previous studies of hydraulic turbines operating at PL show difficulties predicting the flow's tangential velocity component as it is often under predicted. Therefore, a parametric analysis is performed to investigate which parameters affect the prediction of the tangential velocity in the runner domain. Results show that the model predicts the flow relatively well at BEP but has problems at PL; the axial velocity is overpredicted while the tangential is underpredicted. Moreover, the torque is overpredicted. The root cause for the deviation is an underestimation of the head losses. Another contributing reason is that the runner extracts too much swirl from the flow, hence the low tangential velocity and the high torque. Sensitive parameters are the blade clearance, blade angle and mass flow. Finally, the full version of the U9 model is analyzed at SNL operation, including the spiral casing, full distributor, runner and draft tube. During this operating condition, the flow is not axisymmetric; vortical flow structures extend from the vaneless space to the draft tube and the flow stalls between the runner blades. A mitigation technique with independent control of each guide vane is presented and implemented in the model. The idea is to open some of the guidevanes to BEP angle while keeping the remaining ones closed. The aim is to reduce the swirl and prevent the vortical flow structures from developing. Results show that the flow structures are broken down upstream the runner and the rotating stall between the runner blades is reduced, which decreases the pressure- and velocity fluctuations. The flow down stream the runner remains mainly unchanged.