The fluid flow in the mold during up-hill teeming is of great importance for the quality of the cast ingot and therefore the quality of the final steel products. At the early stage of the filling of an up-hill teeming mold, liquid steel enters, with high velocity, from the runner into the mold and the turbulence on the meniscus could lead to entrainment of mold flux. The entrained mold flux might subsequently end up as defects in the final product. It is therefore very important to get a mild and stable inlet flow in the entrance region of the mold. It has been acknowledged recently that swirling motion induced using a helix shaped swirl blade, in the submerged entry nozzle is remarkably effective to control the fluid flow pattern in both the slab and billet type continuous casting molds. This result in increased productivity and quality of the produced steel. Due to the result with continuous casting there is reason to investigate the swirling effect for up-hill teeming, a casting method with similar problem with turbulence. With this thesis we will study the effect of swirling flow generated through a swirl blade inserted into the entry nozzle, as a new method of reducing the deformation of the rising surface and the unevenness of the flow during filling of the up-hill teeming mold. The swirling blade has two features: (1) to generate a swirling flow in the entrance nozzle and (2) to suppress the uneven flow, generated/developed after flowing through the elbow. The effect of the use of a helix shaped swirl blade was studied using both numerical calculations and physical modelling. Water modelling was used to assert the effect of the swirling blade on rectifying of tangential and axial velocities in the filling tube for the up-hill teeming and also to verify the results from the numerical calculations. The effect of swirl in combination with diverged nozzle was also investigated in a similar way, i. e. with water model trials and numerical calculations. / QC 20101115

<p>Continuous caster moulds are the last and most important stage in the steelmaking process, where inclusions can either be generated or removed. With increasing casting speed using conventional immersion nozzles critical problems, such as unstable bulk mould flow have been noticed. Mould flux entrapment due to vortex and shearing action from the oscillating surface waves have become of particular concern. It is therefore necessary to have a calm inlet flow at the entrance of the mould. Recently, it has been acknowledged that a swirl blade placed at the upstream of the immersion nozzle effectively resolves the problems arising from unstable bulk mould flow. Therefore, to increase the knowledge of effect of swirling flow on the flow pattern in the mould, fundamental mathematical models of a billet mould equipped with a swirl blade in the nozzle have been developed. The model was used to study the effect of divergent angle of the immersion nozzle and mould aspect ratio on the flow field and temperature distribution inside billets moulds. Data from water model experiments were used to verify the mathematical model predictions. A fairly good agreement was found between physical modeling data and predictions, which ensured that the numerical model is reliable. Thereafter, the differences between square and round billet moulds were studied. Next, the effect of changing aspect ratio of the rectangular mould on the fluid flow and heat transfer, while keeping mould surface area constant, was studied. Two types of immersion nozzles, bottomless and conventional, were also analyzed during the research. The model moulds were changed gradually from a square billet with an aspect ratio of 1x1 to a rectangular billet with an aspect ratio of 3x1. First, the temperature and velocity distributions were calculated. Later, unsteady calculations were done to determine velocity fluctuations on the meniscus level for two types of nozzles and several moulds geometries.</p>

With strict environmental regulations, gas turbine emissions have been heavily constrained. This requires operating conditions wherein thermo-acoustic flame instabilities are prevalent. During this process the combustor acoustics and combustion heat release fluctuations are coupled and can cause severe structural damage to engine components, reduced operability, and inefficiency that eventually increase emissions. In order to develop an engine without these problems, there needs to be a better understanding of the physics behind the coupling mechanisms of this instability. Among the several coupling mechanisms, the “velocity coupling” process is the main focus of this thesis. The majority of literature has treated axisymmetric disturbance fields which are typical of longitudinal acoustic forcing and axisymmetric excitation of ring vortices. Two important non-axisymmetric disturbances are: (1) transverse acoustics, in the case of circumferential modes of a multi-nozzle annular combustor and (2) helical flow disturbances, seen in the case of swirling flow hydrodynamic instabilities. With significantly less analytical treatment of this non-axisymmetric problem, a general framework is developed for three-dimensional swirl-stabilized flame response to non-axisymmetric disturbances. The dynamics are tracked using a level-set based G-equation applicable to infinitely thin flame sheets. For specific assumptions in a linear framework, general solution characteristics are obtained. The results are presented separately for axisymmetric and non-axisymmetric mean flames. The unsteady heat release process leads to an unsteady volume generation at the flame front due to the expansion of gases. This unsteady volume generation leads to sound generation by the flame as a distributed monopole source. A sound generation model is developed where ambient pressure fluctuations are generated by this distributed fluctuating heat release source on the flame surface. The flame response framework is used to provide this local heat release source input. This study has been specifically performed for the helical flow disturbance cases to illustrate the effects different modes have on the generated sound. Results show that the effects on global heat release and sound generation are significantly different. Finally, the prediction from the analytical models is compared with experimental data. First, a two-dimensional bluff-body stabilized flame experiment is used to obtain measurements of both the flow and flame position in time. This enables a local flame response comparison since the data are spatially resolved along the flame. Next, a three-dimensional swirl-stabilized lifted flame experiment is considered. The measured flow data is used as input to the G-equation model and the global flame response is predicted. This is then compared with the corresponding value obtained using global CH* chemilumenescence measurements.

Pour des raisons évidentes d’opération et de sécurité, l’allumage est un problème essentiel dans les moteurs aéronautiques. La conception d’une chambre de combustion de turbine à gaz intègre de multiples objectifs contradictoires, l’un d’entre eux étant un allumage ou ré-allumage efficace des brûleurs. Parmi les paramètres dont disposent les ingénieurs dans la phase d’optimisation du design, le nombre de systèmes d’injection de carburant et leur espacement sont des points cruciaux qui doivent être fixés dès le début. En effet, de tels choix ont non seulement un impact sur le coût de fabrication et la taille de la chambre mais ils affectent aussi l’efficacité d’un moteur ainsi que ses caractéristiques d’allumage. Afin d’améliorer les connaissances relatives au processus l’allumage dans des moteurs réels, la recherche actuelle combine des expériences fondamentales de plus en plus complexes et des simulations numériques de haute fidélité. Ces actions se concentrent d’une part sur les premiers instants où le noyau de flamme apparaît et d’autre part sur la phase de propagation entre les différents brûleurs. Ces deux phases sont capitales mais restent difficiles à étudier simultanément. Le premier objectif de cette thèse vise à évaluer les modèles SGE sur un seul brûleur expérimental situé au CORIA (France) pour mettre en place une méthodologie fiable afin de réaliser numériquement une séquence d’allumage dans des conditions d’opération réelles et équivalentes aux premiers instants. Une telle étude met en jeu plusieurs phénomènes tels que les écoulement swirlés, l’allumage, l’extinction, la propagation de flamme et les interactions flamme/turbulence. Tous ces processus et mécanismes interagissent et augmentent de façon significative le niveau de difficulté, notamment pour modéliser la combustion turbulente d’un tel allumage. Ces modèles requièrent donc d’être évalués précisément. Ensuite, ce travail examine par la simulation numérique la phase de propagation en utilisant les expériences réalisées sur une chambre composée de plusieurs injecteurs. La comparaison des séquences d’allumage obtenues numériquement avec celles des données expérimentales montre que la SGE reproduit les bonnes tendances et s’avère prédictive. D’un point de vue global, les caractéristiques de propagation du front de flamme en direction des injecteurs voisins sont bien capturées par le numériquemontrant desmodes de propagation identiques à ceux obtenus expérimentalement (radial ou axial) et des temps d’allumage similaires. Pour finir, l’analyse détaillée de ces données numériques a permis d’identifier les mécanismes principaux qui sont à l’origine des différents modes de propagation. / For evident operational and safety reasons, ignition is a key feature of aeronautical gas turbine applications. In fact the design of a gas turbine combustion chamber imposes multiple contradicting objectives one of them being efficient ignition or re-ignition. Among all the parameters available to the engineers, the number of fuel injection systems and their spacing are crucial elements, that must be fixed early on in the design phase. Such choices however not only impact the manufacturing cost and size of the combustor but they also affect the operability of the engine as well as its ignition. To improve knowledge of the ignition process occurring in real engines, current research combines fundamental and increasingly complex experiments complemented by high fidelity numerical simulations. These actions focus on the one hand on the initial instants where the first flame kernel appears as well as the follow-on instants corresponding to the light-around phase or burner to burner flame propagation phase. Both phases are clearly important but are difficult to study simultaneously. The first purpose of this thesis aims at assessing LES models on a single experimental burner located at CORIA (France) to provide a reliable numerical methodology to achieve an ignition sequence in real engines. Indeed, various phenomena are involved in such numerical studies dedicated to real aeronautical combustion chambers and all need to be reproduced by numerics: swirling flows, ignition, quenching, flame propagation, flame/turbulence interactions. All of these processes interact and clearly raise the level of difficulty notably in terms of turbulent combustion modeling of an ignition transient. Having assessed the method on a single burner configuration, the work then investigates the second phase, using a multi-injector experiment simulated by LES to study the flame propagation during ignition. The comparison of numerical fully transient ignition sequences with experimental data shows that LES recovers features found in the experiment. Global events such as the propagation of the flame front to neighboring swirlers are well captured and correct propagation modes (radial or axial) as well as correct overall ignition time delay are obtained. Finally the detailed analysis of LES data allows to identify the driving mechanisms governing each of these propagation modes.

Continuous caster moulds are the last and most important stage in the steelmaking process, where inclusions can either be generated or removed. With increasing casting speed using conventional immersion nozzles critical problems, such as unstable bulk mould flow have been noticed. Mould flux entrapment due to vortex and shearing action from the oscillating surface waves have become of particular concern. It is therefore necessary to have a calm inlet flow at the entrance of the mould. Recently, it has been acknowledged that a swirl blade placed at the upstream of the immersion nozzle effectively resolves the problems arising from unstable bulk mould flow. Therefore, to increase the knowledge of effect of swirling flow on the flow pattern in the mould, fundamental mathematical models of a billet mould equipped with a swirl blade in the nozzle have been developed. The model was used to study the effect of divergent angle of the immersion nozzle and mould aspect ratio on the flow field and temperature distribution inside billets moulds. Data from water model experiments were used to verify the mathematical model predictions. A fairly good agreement was found between physical modeling data and predictions, which ensured that the numerical model is reliable. Thereafter, the differences between square and round billet moulds were studied. Next, the effect of changing aspect ratio of the rectangular mould on the fluid flow and heat transfer, while keeping mould surface area constant, was studied. Two types of immersion nozzles, bottomless and conventional, were also analyzed during the research. The model moulds were changed gradually from a square billet with an aspect ratio of 1x1 to a rectangular billet with an aspect ratio of 3x1. First, the temperature and velocity distributions were calculated. Later, unsteady calculations were done to determine velocity fluctuations on the meniscus level for two types of nozzles and several moulds geometries. / <p>QC 20101110</p>

No description available.

The complex multiscale physics of nanoparticle laden functional droplets in a reacting environment is of fundamental and applied significance for a wide variety of applications ranging from thermal sprays to pharmaceutics to modern day combustors using new brands of bio-fuels. Understanding the combustion characteristics of these novel fuels (laden with energetic nanoparticle NP) is pivotal for lowering ignition delay, reducing pollutant emissions and increasing the combustion efficiency in next generation combustors. On the way to understanding the complex dynamics of sprays is to first study the behaviour of an isolated droplet. A single droplet represents a sub-grid unit of spray. In vaporizing functional droplets under high heat flux conditions, the bubble formation inside the droplet represents an unstable system. This may be either through homogenous nucleation at the superheat limit or by dispersed nanoparticle acting as heterogeneous nucleation sites. First it is shown that such self-induced boiling in burning functional pendant droplets can induce severe volumetric shape oscillations in the droplet. Internal pressure build-up due to ebullition activity force ejects bubbles from the droplet domain causing undulations on the droplet surface and oscillations in bulk thereby leading to secondary break-up of the primary droplet. Through experiments, it is established that the degree of droplet deformation depends on the frequency and intensity of these bubble expulsion events. However, in a distinct regime of single isolated bubble growing inside the droplet, pre-ejection transient time is identified by Darrieus-Landau (DL) instability at the evaporative bubble-droplet interface. In this regime the bubble-droplet system behaves as a synchronized driver-driven system with bulk bubble-shape oscillations being imposed on the droplet. However, the agglomeration of suspended anaphase additives modulates the flow structures within the droplet and also influences the bubble inception and growth leading to distinct atomization characteristics. Secondly, the secondary atomization characteristics of burning bi-component (ethanol-water) droplets containing titania nanoparticle (NPs) at both dilute (0.5% and 1% by weight) and dense particle loading rates (PLR: 5% and 7.5 wt. %) are studied experimentally at atmospheric pressure under normal gravity. It is observed that both types of nanofuel droplets undergo distinct modes of secondary break-up that are primarily responsible for transporting particles from the droplet domain to the flame zone. For dilute nanosuspensions, disruptive response is characterized by low intensity atomization modes that cause small-scale localized flame distortion. In contrast, the disruption behavior at dense concentrations is governed by high intensity bubble ejections which result in severe disruption of the flame envelope. The atomization events occur locally at the droplet surface while their cumulative effect is observed globally at the droplet scale. Apart from this, a feedback coupling between two key interacting mechanisms, namely, atomization frequency and particle agglomeration also influence the droplet deformation characteristics by regulating the effective mass fraction of NPs within the droplet. Thus, third part of the study elucidates how the initial NP concentration modulates the relative dominance of these two mechanisms thereby leading to a master-slave configuration. Secondary atomization of novel nanofuels is a crucial process since it enables an effective transport of dispersed NPs to the flame (a pre-requisite condition for NPs to burn). Contrarily, NP agglomeration at the droplet surface leads to shell formation thereby retaining NPs inside the droplet. In particular, it is shown that at dense concentrations shell formation (master process) dominates over secondary atomization (slave) while at dilute particle loading it is the high frequency bubble ejections (master) that disrupt shell formation (slave) through its rupture and continuous out flux of NPs. These results in distinct combustion residues at dilute and dense concentrations, thus, providing a method of manufacturing flame synthesized microstructures with distinct morphologies. Next, it is shown that by using external stimuli (preferential acoustic excitation) the secondary atomization of the droplet can be suppressed i.e. the external flame-acoustic interaction with bubbles inside the droplet results in controlled droplet deformation. Particularly, by exciting the droplet flame in a critical, responsive frequency range i.e. 80 Hz ≤ fP ≤ 120 Hz, the droplet deformation cycle is altered through suppression of self-excited instabilities and intensity/frequency of bubble ejection events. The acoustic tuning also enables the control of bubble dynamics, bulk droplet-shape distortion and final precipitate morphology even in burning nanoparticle laden droplets. Droplets in a non-reacting environment (heated radioactively) are also subject to instabilities. One such instability observed in drying colloidal droplets is the buckling of thin viscoelastic shell formed through consolidation of NPs. In the final part of the thesis, buckling instability driven morphology transition (sphere to ring structure) in an acoustically levitated heated nanosilica dispersion droplet is elucidated using dynamic energy balance. Droplet deformation featuring formation of symmetric cavities is initiated by the capillary pressure that is two to three orders of magnitude greater than acoustic radiation pressure, thus indicating that the standing pressure field has no influence on the buckling front kinetics. With increase in heat flux, the growth rate of surface cavities and their post-buckled volume increases while the buckling time period reduces, thereby altering the buckling pathway and resulting in distinct precipitate structures. Thus, the cavity growth is primarily driven by evaporation. However, irrespective of the heating rate, volumetric droplet deformation exhibits linear time dependence and droplet vaporization is observed to deviate from the classical D2-law. Understanding such transients of buckling phenomenon in drying colloidal suspensions is pivotal for producing new functional microstructures with tenable morphology and is particularly critical for spray drying applications that produce powders through vaporization of colloidal droplets.

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.