Identification and Development of a Reliable Framework to Predict Passive Scalar Transport for Turbulent Bounded Shear Flows

Dissertation advisors: Amirfarhang Mehdizadeh and Majid Bani-YaghoubVitaIncludes bibliographical references (page 159-175)Thesis (Ph.D.)--School of Computing and Engineering, Department of Mathematics and Statistics. University of Missouri--Kansas City, 2020Title from PDF of title page viewed January 31, 2022Heat transfer modeling plays an integral role in optimization and development of highly efficient modern thermal-fluid systems. However, currently available heat flux models suffer from fundamental shortcomings. For example, their development is based on the general notion that an accurate prediction of the flow field will guarantee an appropriate prediction of the thermal field, as the Reynolds Analogy does. Furthermore, literature about advanced models that aim to overcome this notion, does not provide reliable information about prediction capabilities. These advanced models can be separated into two distinct heat flux model categories, namely the implicit and explicit models. Both model categories differ fundamentally in their mathematical and physical formulation. Hence, this dissertation presents a comprehensive assessment of the Reynolds Analogy regarding steady and unsteady calculations. It further analyses the entropy generation capability in detail and evaluates the prediction accuracy of implicit and explicit models when applied to turbulent shear flows of fluids with different Prandtl numbers. Moreover, the implicit and explicit models are modified such that important thermal second order statistics are included. This enables deeper insight into the mechanics of thermal dissipation and delivers a better understanding towards the sensitivity and reliability of predictions using heat flux models. Finally, to overcome the shortcomings of the Reynolds Analogy in unsteady calculations, an anisotropic extension is proposed. This dissertation shows that even for first order statistics within steady state calculations, the Reynolds Analogy is only appropriate for fluids with Prandtl numbers around unity. For second order statistics within unsteady simulations, the Reynolds Analogy could provide acceptable results only if an appropriate grid design/resolution is provided that allows resolving essential dynamics of the thermal field. Concerning entropy generation, the Reynolds Analogy provides acceptable results only for mean entropy generation, while it fails to predict entropy generation at small/sub-grid scales. The anisotropic extension of the Reynolds Analogy is a promising approach to overcome these shortcomings. Furthermore and concerning the implicit and explicit heat flux models, this work shows that only the explicit framework is potentially capable of dealing with complex turbulent thermal fields and to address longstanding shortcomings of currently available models, if the flow field is predicted accurately. Moreover, it has been shown that thermal time scale plays an integral role to predict thermal phenomena, particularly those of fluids with low/high Pr numbers.Introduction -- A comprehensive Assessment of the Reynolds Analogy in Predicting Heat Transfer in Turbulent Wall-Bounded Shear Flows -- Entropy Generation Assessment for Wall-Bounded Turbulent Shear Flows Based on Reynolds Analogy Assumptions -- An Assessment pf the Reynolds Analogy in Predicting Heat Transfer in Turbulent Flows of Low Prandtl Numbers -- A Wall-Adapted Anisotropic Heat Flux Model for Large Eddy Simulations of Complex Turbulent Thermal Flows -- Towards Identification and Development of a Reliable Framework to Predict the Thermal Field in Turbulent Wall-Bounded Shear Flow -- Conclusion and Outloo

Entropy generation assessment for wall-bounded turbulent shear flows based on the Reynolds Analogy assumptions

Heat transfer modeling plays a major role in design and optimization of modern and efficient thermal-fluid systems. Further, turbulent flows are thermodynamic processes, and thus, the second law of thermodynamics can be used for critical evaluations of such heat transfer models. However, currently available heat transfer models suffer from a fundamental shortcoming: their development is based on the general notion that accurate prediction of the flow field will guarantee an appropriate prediction of the thermal field, known as the . In this work, an assessment of the capability of the in predicting turbulent heat transfer when applied to shear flows of fluids of different Prandtl numbers will be given. Towards this, a detailed analysis of the predictive capabilities of the concerning entropy generation is presented for steady and unsteady state simulations. It turns out that the provides acceptable results only for mean entropy generation, while fails to predict entropy generation at small/sub-grid scales

A study of the internal flow of dense vapours used in Organic Rankine Cycle (ORC) turbines

An Organic Rankine Cycle (ORC) is a thermodynamic cycle utilizing a heat source at low-temperature. It can be used for the waste heat recovery of vehicle engines, industrial processes, solar thermal power plants, and geothermal power plants, leading to reduction of CO2 emissions. The turbine expander is a key component of this cycle, and its efficiency is critical to the system performance. To improve the design of an ORC turbine, the internal flow of the turbine should be studied. Unlike the fluids in conventional turbines, the fluids used in ORC turbines are dense vapours. These vapours have complex molecules and relatively large molecular weights, and they operate at states close to thermodynamic critical points or saturation line. Therefore, the thermodynamic behaviours of dense vapours are far from those of ideal air. However, the influence of these effects on the internal flows is not well understood. The fundamental flow behaviours of dense vapours, including gasdynamic behaviours in blade-shaped nozzle flows and turbulent behaviours in wall-bounded flows, are the focus of this work. A supersonic cascade using R1233zd(E) as working fluids is designed by the method of characteristics. The designed geometry is able to achieve a nearly uniform outlet flow at about $Ma=2$, which is checked in an RANS simulation. A blade-shaped nozzle is designed using the blade shape of this cascade, and the preliminary test results of this nozzle is presented. This nozzle is tested with both nitrogen and R1233zd(E) as working fluids. In the test with nitrogen, an adverse pressure gradient is measured on both nozzle surfaces downstream of the nozzle throat, and a shock train is observed at the corresponding position. Similar to the nitrogen test, the adverse pressure gradient is also found in the tests with R1233zd(E), but the Schlieren images cannot clearly show the shock train due to the disturbance of two-phase flows. A Direct Numerical Simulation (DNS) method for dense vapours is developed to obtain detailed information on turbulence. A modified Steger-Warming splitting is proposed to consider the strong non-ideal effects of gases, and the Span-Wagner EoS \cite{span2003equations} is used for studied dense vapours. The first studied benchmark case of wall-bounded flow is the supersonic fully-developed channel flow. Both the mean flow fields and the turbulent fluctuation fields are analysed. The mean profile and the fluctuation of thermodynamic properties are significantly affected by both molecular-complexity effects and non-ideal effects, and the sound-wave mode can be the dominant mode for generating fluctuations of thermodynamic properties ($T'$, $\rho'$) in dense vapours. Important modelling issues for dense vapours are also discussed, including the Strong Reynolds Analogy (SRA) and assumptions required for the $k-\varepsilon$ RANS method. The second studied benchmark case of wall-bounded flow is the bypass laminar-turbulent transition over a flat plate under a supersonic incoming flow. At the same incoming non-dimensional numbers ($Ma_\infty$ and $Re_x$), the breakdown of laminar flow starts earlier in dense vapours than in air. The mechanism of breakdown for both dense vapours and air is due to the interaction of two streamwise vortices in opposite rotational directions, leading to Kelvin-Helmholtz (KH) instability in a high shear layer. Proper Orthogonal Decomposition (POD) is also used to support the findings in vortex structure analysis.Open Acces

Intermittency and Self-Organisation in Turbulence and Statistical Mechanics

There is overwhelming evidence, from laboratory experiments, observations, and computational studies, that coherent structures can cause intermittent transport, dramatically enhancing transport. A proper description of this intermittent phenomenon, however, is extremely difficult, requiring a new non-perturbative theory, such as statistical description. Furthermore, multi-scale interactions are responsible for inevitably complex dynamics in strongly non-equilibrium systems, a proper understanding of which remains a main challenge in classical physics. As a remarkable consequence of multi-scale interaction, a quasi-equilibrium state (the so-called self-organisation) can however be maintained. This special issue aims to present different theories of statistical mechanics to understand this challenging multiscale problem in turbulence. The 14 contributions to this Special issue focus on the various aspects of intermittency, coherent structures, self-organisation, bifurcation and nonlocality. Given the ubiquity of turbulence, the contributions cover a broad range of systems covering laboratory fluids (channel flow, the Von Kármán flow), plasmas (magnetic fusion), laser cavity, wind turbine, air flow around a high-speed train, solar wind and industrial application

The Effect of Heat Transfer on Turbine Performance

Despite the blades downstream of the combustor being cooled in most large gas turbines, the effect this cooling has on their performance is still unclear. Fundamental questions, such as how turbine efficiency should be defined and which flow features reduce performance, remain unanswered. This thesis answers these questions and addresses the problem of how heat transfer affects turbine performance. Currently, there is a direct contradiction between industrial experience and the academic methods used to evaluate turbine performance. In a cooled turbine, these entropy-centric (exergy) methods predict that the irreversible heat transfer due to cooling would cause an extremely large drop in turbine efficiency (around 4-6%). In practice, this drop is not observed. By applying a new method called euergy, this thesis demonstrates that the performance of a cooled turbine can be defined in a way that agrees with industrial experience. This method shares the view of the practical device that the ideal work is defined by a reversible adiabatic turbine. A key consequence of this method is the value placed on heat, relative to work, becomes set by the Joule (Brayton) cycle efficiency. This means whenever heat is transferred, or when viscous reheat occurs, the value of this heat should be set by the Joule cycle efficiency. This new understanding finally allows both the efficiency to be defined and the flow features that change it to be identified. The method also provides cooling designers with a new way of raising turbine efficiency, a form of thermal recuperation in the flow. This mechanism offers the exciting potential that future cooling systems, when added to a blade profile, could reduce profile loss by up to 9%. Furthermore, the generality of the new method allows all cooled components, however complex, to be systematically analysed for the first time. This is demonstrated using a computational approach. The large-scale effect of heat transfer on performance is captured in newly developed loss models which are then compared with conjugate heat transfer simulations. The small-scale effect of heat transfer on performance is investigated by examining the near-wall region of cooled boundary layers using direct numerical simulation (DNS). This thesis establishes a new framework and illustrates how heat transfer affects turbine performance. This new understanding allows performance to be communicated across all levels of turbine design. The new method offers the prospect of future innovative cooling schemes which, in addition to cooling the blade, act to raise turbine efficiency

Free and Confined Buoyant Flows

Flows driven by density differences, whether natural or induced by man, surround us on a wide range of scales. As a density difference is generated by commonly occuring temperature or salinity differences, these flows are ubiquitous. On the largest scales, they transport and mix the water in our oceans and air in our atmosphere. At an intermediate scale, frequently referred to as the mesoscale, the cold outflows which flow downwards from thunderstorms and impact the ground are a common aviation hazard. On a smaller scale, the toxins emitted by fire plumes or dense gas releases are a threat to health. This work focusses on a continuous, localised release of buoyant fluid from a horizontal source whose dimension is significantly smaller than the horizontal and vertical scales of the quiescent, uniform environment into which the flow propagates. We consider both passive releases, which are only driven by the density difference between the source fluid and the denser ambient, and forced releases, where the fluid has source momentum in addition to its buoyancy. In general, these releases give rise to turbulent plumes, a familiar example being the cloud of smoke and water vapour seen rising from a chimney stack on a cold, still morning. The first part of the research presented in this thesis focusses on the freely-propagating plume. Velocity and temperature measurements are presented which contribute considerably to the existing experimental data available in the literature. This data-set is used to validate classical plume theory and make a check of the experimental set-up so that the subsequent results can be presented with confidence. It is also possible that this dataset will be used by other researchers to validate numerical simulations of buoyant flows. The effect of varying the source balance of buoyancy and momentum upon the plume dynamics is investigated. Measurements also reveal the development region or ‘zone of flow establishment’. Frequently, plumes are restricted by some form of confinement, either vertically, horizontally or both, for example the plumes rising from the occupants of a room. Whether this restriction takes the form of a solid wall, free surface or density discontinuity, the disturbance to the flow is typically significant. The simplest confining boundary is arguably a horizontal surface located some distance H from the source of buoyant fluid. The horizontal boundary forces the vertical flow to change direction and propagate radially outwards. This type of semi-confined flow can be frequently observed in the natural world with examples including the impingement of a fire plume against a ceiling and a plume of volcanic ash with the tropopause. An investigation into this type of flow, which we refer to as the ‘impinging buoyant plume’, constitutes the second part of the research. Plume impingement has not been studied as extensively as jet impingement and several key questions remain unanswered. For example, how much energy is lost as the vertical flow is forced to turn and propagate horizontally? What effect does buoyancy have on the horizontal flow? How does the flow evolve with increasing radial distance and what is the effect of changing the source-boundary separation? These are just some of the questions addressed in this thesis guided by the novel application of highly-resolved Particle Image Velocimetry measurements to this relatively low-speed, buoyant, turbulent flow. The free and impinging plume studies both employed similar experimental techniques and analysis methods. Statistics of the steady flow were determined from a highly-resolved data-set. The third part of the research concerns a time-dependent flow and is of a more qualitative nature. The complexity of the impinging plume increases considerably when a radial confinement is added to the geometry. This restricts the radial propagation of the flow produced by the impinging plume. The plume is now effectively enclosed and buoyant fluid begins to accumulate within and thereby fill the enclosure, a configuration known as the ‘filling-box’. While previous work, which we shall go on to review in detail, has contributed analytical solutions for the density profiles in the enclosure after a certain time-scale has elapsed, in many applications, such as the spread of smoke carried by a fire plume in a room, what happens in the early moments of a confined release following impingement with the horizontal and then vertical boundaries, may be critical. This has been overlooked in earlier studies, yet is crucial as it is during these early transients that the fire is best tackled by fire-fighters. Visualisations and velocity measurements of these early filling-box transients are reported. This work provides the first detailed measurements of the velocity field induced in the filling-box by the turbulent plume during the early transients and resolves the turbulent structures that comprise the plume outflow. The experiments which investigated the impinging jet were conducted on thermal air plumes in facilities at the Laboratoire de Mécanique des fluides et de l’Acoustique (LMFA) of École Centrale de Lyon (ECL). Filling-box experiments were performed on brine plumes in fresh water in visualisation tanks in the Department of Civil & Environmental Engineering at Imperial College London (ICL). The set of experiments at ECL used a combination of Particle Image Velocimetry (PIV) and thermocouples to measure flow velocities and temperatures. At ICL, Light-Induced Fluorescence (LIF) enabled visualisation of a plane through the centre of the axisymmetric flow to complement the PIV work. These experiments enabled effective use of the equipment, techniques and expertise available at both institutions. The principal objective of this research was to use experimental measurements to answer questions of importance regarding these impinging flows which remain unresolved in the literature. Using experimental techniques unavailable to earlier researchers, the work presented herein makes a substantial contribution to the existing knowledge of these flows. Free and impinging plumes and the dynamics of the filling-box flow are studied in detail. Notably, the data gathered are of very high spatial resolution and provide a resource for those interested by not only the plume dynamics, but also radial gravity currents and the filling-box

Prédiction de la génération des pertes des écoulements compressibles anisothermes appliquée aux distributeurs hautes pressions de turbine avec les simulations aux grandes échelles

Afin d'améliorer l'efficacité des moteurs aéronautiques, une des solutions envisagées par les industriels est d'augmenter la température d'entrée de la turbine. Cependant, ces hautes températures induisent de fortes contraintes thermiques sur les pales de turbine ce qui réduit leur durée de vie. Pour surmonter ces problèmes thermiques, des systèmes de refroidissement efficaces sont nécessaires. Afin d'évaluer la performance de ces systèmes, une prédiction précise de la température de paroi des pales de turbine et des pertes générées par ces systèmes est requise. Profitant de l'opportunité de récents développements d'outils de prédiction haute-fidélité, cette thèse financée par Safran Helicopter Engines à travers le projet FUI CASCADE, a pour but de valider la prédiction de la température de paroi des pales de turbine refroidie et des pertes générées par ces systèmes avec la Simulation aux Grandes Echelles (SGE). Pour atteindre ces objectifs, différentes configurations académiques et industrielles refroidies par film de refroidissement ont été simulées et étudiées. Les résultats obtenus dans cette thèse montrent que la SGE est capable de prédire l'aérodynamique et l'environnement thermique pour de tels systèmes. Pour faciliter l'utilisation de la SGE dans l’industrie et limiter le coût CPU lié à la résolution de l'écoulement dans le système de refroidissement des pales, un modèle de jets de refroidissement a été proposé et évalué dans ce travail. Les résultats montrent que ce modèle permet de reproduire l'aérodynamique des jets de refroidissement et la température de paroi des pales sans mailler le système de refroidissement. Pour évaluer les pertes dans ce contexte, l’approche Second Law Analysis (SLA) est adoptée. Contrairement aux bilans de température et pression totales, cette approche donne directement accès aux champs de perte 3D qui sont construits à partir des termes sources de l’entropie résolus sur le maillage. Ainsi, le mécanisme de génération de perte peut être localement étudié et ne requière pas de procédure de moyenne contrairement aux modèles de perte 1D. Ces champs de perte sont décomposés en deux contributions : une contribution aérodynamique et une contribution thermique liée au mélange chaud-froid. L'étude de ces champs montre que les pertes aérodynamiques sont principalement générées dans les régions de fort cisaillement (couche limite et de mélange) alors que les pertes de mélange sont générées dans les films de refroidissement et dans le sillage des pales. Des analyses avancées des champs de perte mettent en évidence que les fluctuations turbulentes dominent la génération des pertes pour ces systèmes. Ce dernier résultat met en évidence les bénéfices de l'approche Second Law Analysis pour prédire les pertes à partir des champs obtenus avec la SGE. En effet et contrairement aux approches RANS, les contributions turbulentes des pertes sont directement résolues sur le maillage avec la SGE et ne requiert aucune stratégie de modélisation. La principale conclusion de cette thèse est que l'approche Second Law Analysis couplée avec la SGE est une méthodologie très prometteuse et pertinente pour la prédiction des écoulements et des pertes pour les futurs designs de pale de turbine industriel

Annual research briefs, 1993

The 1993 annual progress reports of the Research Fellow and students of the Center for Turbulence Research are included. The first group of reports are directed towards the theory and application of active control in turbulent flows including the development of a systematic mathematical procedure based on the Navier Stokes equations for flow control. The second group of reports are concerned with the prediction of turbulent flows. The remaining articles are devoted to turbulent reacting flows, turbulence physics, experiments, and simulations

Institute for Computational Mechanics in Propulsion (ICOMP)

The Institute for Computational Mechanics in Propulsion (ICOMP) is operated by the Ohio Aerospace Institute (OAI) and the NASA Lewis Research Center in Cleveland, Ohio. The purpose of ICOMP is to develop techniques to improve problem-solving capabilities in all aspects of computational mechanics related to propulsion. This report describes the accomplishments and activities at ICOMP during 1993