359 research outputs found
Study of Liquid Flow with Bubbles in Pipes
In oil and gas industry, fluid flow in pipe is a common occurrence and creates several critical issues. Produced wellhead fluids are often mixtures of different compounds of carbon, all with different densities, vapor pressures, and other characteristics. The change of pressure and temperature results in the evolving of gas as the production is lifted up to higher elevation. This multiphase flow complicates the flow metering. Cavitation, which involves the evolving and collapsing of bubbles in liquid, is another problem related to fluid flow. The existence of bubbles in liquid flow causes unfavorable issues for example pressure drop through pipeline, vibration, cavitation and multiphase metering. Thus, the objective of this project is to study the behavior of bubbles in flowing liquid and understand how it affects the fluid flow
Towards large eddy simulation of dispersed gas -liquid two-phase turbulent flows
This study presents a detailed investigation of all essential components of computational and modeling issues necessary for a successful large-eddy simulation (LES) of dispersed two-phase turbulent flows. In particular, a two-layer concept is proposed to enable the LES capability in two-phase flows involving dispersed bubbles that are relatively large compared to the mesh size. The work comprises three major parts.;Part I focuses on the development and verification of a transient, three-dimensional, finite-volume-method (FVM) based accurate Navier-Stokes solver, named DREAM II (second generation of the DREAM code). Several high-order schemes are implemented for both the spatial and temporal discretization. Solution of the coupled partial differential equations is attacked with a fractional step (projection) method. The developed solver is verified against various benchmarks including Taylor\u27s vortex, free-shear layer, backward-facing step flow and square cavity. A second-order overall accuracy is achieved in both space and time.;Part II concerns the modeling and LES of single-phase turbulent flows. A review of the LES theory and subgrid-scale (SGS) models is presented. Three SGS models, namely, Smagorinsky model, dynamic model and implicit model, are implemented and investigated. Then turbulent channel flow, plane mixing layer, and flow past a square cylinder are simulated, and comparisons of the first-, second-order statistics, and characteristic flow structures are made with direct numerical simulation (DNS) and/or benchmark experiments. The test results show superior quality of the present LES.;Part III delves into the theory, modeling and simulation of dispersed two-phase flow systems. A conceptual review of the characteristics and description of such system is made, considering both Eulerian-Eulerian (E-E) and Eulerian-Lagrangian (E-L) approaches, but with an emphasis on the latter. Various hydrodynamic forces acting on particles or bubbles are summarized and interpreted. Formulations regarding interphase coupling is discussed in depth. Typical computational treatments of modeled two-way couplings in an E-L DNS/LES are reviewed. Issues related to the interpolation are addressed. A general Lagrangian particle-tracking (LPT) program, named PART, is developed and verified using analytical solutions. (Abstract shortened by UMI.)
Computational fluid dynamics modelling of multi-phase flow transition in presence of solid particles.
Multi-phase flow is the type of flow common in the oil and gas industry, as oil reservoirs contain mixtures of oil, gas and water with sand particles from sandstone reservoirs. Accurate design of oil and gas production equipment greatly depends on detailed understanding of this flow phenomena. Previously, multi-phase studies relied upon empirical correlation and mechanistic equations developed from experimental data, but these approaches have limitations because of limited experimental data and underlying simplified assumptions. Thus, these methods cannot be used for complex flow situations often encountered in mature oil and gas fields. Hence, the lack of scalability of the existing classical empirical correlations and mechanistic models has called for a high-fidelity modelling method. In this research, a computational fluid dynamics (CFD) method is used to investigate gas-liquid and gas-liquid-solid multi-phase flow in a vertical pipe. A hybrid model, the multi-fluid Euler-Euler and Euler-Euler-Euler model with interfacial area transport equation (IATE), were used to simulate the flow regime spectrum in a large diameter vertical pipe. The hybrid model could simulate the mean gas volume fractions and bubble size changes as a function of fluid rate. The predicted gas volume fractions were benchmarked against experimental data and were in agreement. Changes in the gas flow rates were seen to generate flow transitions from bubble to annular flow, which compared favourably with appropriate literature across the vertical flow regime spectrum. However, sand particle inclusion in the flow scheme were seen to change the flow dynamics, which were found to be greatly dependent on the particle concentration. Solid particle concentrations were seen as the major deposition influencer. The results of this research elucidate the regime transition in a three-phase gas-liquid-solid flow scheme of a typical production well, and is viable for well-production optimisation and completion design in large-diameter vertical pipes
Numerical Investigations of Bubble Column Equipped with Vertical Internals in Different Arrangements
Bubble columns are multiphase contactors with wide applications in industrial processes. Often they are equipped with longitudinal tube bundles to facilitate heat exchange. Studying effects of these internals on column hydrodynamics is vital for the design of these internals. Computational Fluid Dynamic (CFD) simulations provide an understanding of the complex two-phase flow enabling the study of the effects of the internals on the column hydrodynamics. In the present work, an Eulerian-Eulerian based two-fluid model (TFM) coupled with a population balance model (PBM) is used to simulate the gas-liquid two-phase flows in bubble columns. The models studied were validated using experimental data from the literature. The selected model was used to simulate the effects of the tube-to-tube distance and height of the internals on the hydrodynamics in the column. It was found that the tube-to-tube distance has a significant impact on the liquid axial velocity distribution and flow recirculation. Decreasing the tube-to-tube space reduces the axial liquid flow and the height of internals affects the liquid recirculation only in homogeneous flow regime
Development and Application of a Model for the Cross-Flow Induced by Mixing Vane Spacers in Fuel Assemblies
CFD investigations of the flow in a 5x5 rod bundle with mixing vane spacer grid were performed with single-phase and two-phase flow conditions. A non-linear k-epsilon model was used to simulate secondary flow and non-isotropic turbulence. A detailed analysis of the results showed that mixing vane induced swirling flow strongly affects the cross-flow. A model was developed to predict the forced cross-flow between the sub-channels and implemented into the sub-channel analysis code COBRA-FLX
Large eddy simulations of ventilated micro-hydrokinetic turbine and pump-turbines
Large eddy simulations of ventilated hydrokinetic turbine and pump-turbine are conducted. The mathematical modeling of oxygen dissolution and the flow model employed were validated by comparing predicted dissolved oxygen concentration against reported experimental measurements. A parametric study is performed to investigate the influence of interfacial forces, surface tension and bubble breakage and coalescence terms. It is demonstrated that aeration via hydrokinetic turbines can be used to improve the dissolved oxygen level in rivers for better water quality. It is also shown that aeration can effectively be achieved via the pump-turbine system to provide the desired dissolved oxygen level for the microorganisms’ growth during the wastewater treatment process. Air injection is applied to the wake region of each unit. The influence of aeration on the turbine performance, flow induced vibration and oxygen dissolution characteristics are investigated. The numerical predictions reveal that the aeration can be utilized in both hydro systems without experiencing a significant penalty in power generations. Aeration significantly reduces the flow induced vibration in the pump turbine system. The pressure pulsation on the draft tube surface of the pump-turbine is reduced significantly with both central and peripheral aeration. In hydrokinetic turbine, the variation in the standard deviation of power, which is related to the vibration of the turbine unit, is strongly dependent on the turbine operating conditions. Draft tube aeration provided 30% greater amount of dissolved oxygen and 3.2 times higher dissolution efficiency inside the draft tube as compared to the central aeration. The mathematical approaches and the numerical methods employed here can be used to design and optimize the aeration process in these systems
A hybrid lagrangian-eulerian approach for simulation of bubble dynamics
A mutiscale numerical approach is developed for the investigation of bubbly flows in turbulent environments. This consists of two different numerical approaches capable of capturing the bubble dynamics at different scales depending upon the relative size of the bubbles compared to the grid resolution: (i) fully resolved simulations (FRS) wherein the bubble dynamics and deformation are completely resolved, and (ii) subgrid, discrete bubble model where the bubbles are not resolved by the computational grid. For fully resolved simulations, a novel approach combining a particle-based, mesh-free technique with a finite-volume flow solver, is developed. The approach uses marker points around the interface and advects the signed distance to the interface in a Lagrangian frame. Interpolation kernel based derivative calculations typical of particle methods are used to extract the interface normal and curvature from unordered marker points. Unlike front-tracking methods, connectivity between the marker points is not necessary. For underresolved bubbles, a mixture-theory based Eulerian-Lagrangian approach accounting for volumetric displacements due to bubble motion and size variations is developed. The bubble dynamics is modeled by Rayleigh-Plesset equations using an adaptive timestepping scheme. A detailed verification and validation study of both approaches is performed to test the accuracy of the method on a variety of single and multiple bubble problems to show good predictive capability. Interaction of bubbles with a traveling vortex tube is simulated and compared with experimental data of Sridhar and Katz [1] to show good agreement.http://deepblue.lib.umich.edu/bitstream/2027.42/84270/1/CAV2009-final74.pd
Experimental Analysis and Improved Modelling of Disperse Two-Phase Flows in Complex Geometries
Gas-liquid two-phase flows are encountered in different industrial applications such as, chemical reactors, wastewater treatment, oil and gas exploration and nuclear reactors. In nuclear reactors, boiling two-phase flows occur under both normal and accident conditions. For the design and safety operation of nuclear reactors, Computational Fluid Dynamics (CFD) based on the Euler−Euler framework has become a popular tool. However, accurate CFD prediction for a fuel assembly geometry is still a challenge. The reason is that the accuracy of two-phase flow simulations is highly dependent on adequate modelling of phase interactions including interfacial forces (i.e. drag, lift, wall lubrication, turbulent dispersion and virtual mass), bubble-induced turbulence (BIT) and bubble breakup/coalescence.
Through the Euler−Euler framework, modelling of these phase interactions is provided by different approaches. These approaches include closure equations, most of which have been determined empirically. These closures are important for the accurate prediction of mean flow profiles, including void fraction and phase velocity distributions. A variety of closure models has been proposed by different researchers. However, it is difficult to differentiate them and make an appropriate choice for a particular problem without knowing their predictive properties in detail. While an extensive number of models have been developed and have meanwhile been well validated for simple pipe and column geometries, there is yet limited analysis and qualification for more complex three-dimensional flow domains. One reason for this is the lack of suitable experimental validation data. In addition, it is important to mention that most of the available models were generally obtained considering laminar or low turbulence conditions.
Therefore, it is necessary to further investigate the modelling capabilities for two-phase flows with flow complexity/high turbulence as they occur in nuclear reactors. For this purpose, additional validations are required in the CFD modelling of two-phase flows. However, studies on the capabilities of two-phase flow models directly for rod bundles are very complicated and time-consuming.
Hence, a capability analysis of the models for the three main phenomena, i.e. breakup/coalescence, drag and turbulence, was first carried out for the case of a semi-obstructed pipe under adiabatic flow conditions. The results were validated using the experimental data obtained by Neumann-Kipping (2022) on the void fraction, mean bubble diameter, bubble size distribution, liquid velocity and gas velocity for two different turbulence conditions.
Subsequently, experiments were conducted in a 3 x 3 rod bundle with a spacer and vanes using X-ray computed tomography (CT), which provides high quality void data without disturbing the flow. The effects of different mass and heat fluxes on the void fraction and its distribution downstream of the spacer were analyzed. In addition, the effects of different vane angles on the distribution of the void fraction were discussed. Furthermore, an experimental database was obtained in a rod bundle with a spacer under different flow conditions to validate the numerical modelling.
Finally, the improved CFD model obtained from the semi-obstructed pipe geometry was applied to the 3 x 3 rod bundle geometry under two different turbulence conditions. The numerical results were compared with the X-ray CT data on the void fraction.Gas-Flüssig-Zweiphasenströmungen kommen in verschiedenen industriellen Anwendungen wie Blasensäulen, Rührkesseln und Kernreaktoren vor. In Kernreaktoren treten siedende Zweiphasenströmungen sowohl unter Normal- als auch unter Störfallbedingungen auf. Für die Auslegung und den sicheren Betrieb von Kernreaktoren ist die numerische Strömungsmechanik (engl. Computational Fluid Dynamics, CFD) auf der Grundlage des Euler−Euler-Konzepts zu einem wichtigen Instrument geworden. Eine genaue CFD-Vorhersage für eine Brennelementgeometrie ist jedoch nach wie vor eine Herausforderung. Der Grund dafür ist, dass die Genauigkeit von Zweiphasenströmungssimulationen in hohem Maße von einer genauen Modellierung der Phasenwechselwirkungen abhängt, einschließlich der Grenzflächenkräfte (d. h. Widerstand, Lift, Wand, turbulente Dispersion und virtuelle Masse), der blaseninduzierten Turbulenz (BIT) und des Aufbrechens/Koaleszierens von Blasen.
Durch den Euler−Euler-Rahmen wird die Modellierung dieser Phasenwechselwirkungen durch verschiedene Ansätze ermöglicht. Zu diesen Ansätzen gehören Schließungsgleichungen, von denen die meisten empirisch ermittelt wurden. Diese Schließungsgleichungen sind wichtig für die genaue Vorhersage von mittleren Strömungsprofilen, einschließlich Gasgehalt und Phasengeschwindigkeitsverteilungen. Es gibt eine Vielzahl von Schließungsmodellen, die von verschiedenen Forschern innerhalb ihrer experimentellen Bereiche vorgeschlagen wurden. Es ist jedoch schwierig, sie zu unterscheiden und eine geeignete Wahl für ein bestimmtes Problem zu treffen, ohne ihre Vorhersageeigenschaften im Detail zu kennen. Während für einfache Rohr- und Säulengeometrien eine große Anzahl von Modellen entwickelt und inzwischen gut validiert wurde, gibt es für komplexere dreidimensionale Strömungsgebiete noch wenig Analyse und Qualifizierung. Ein Grund dafür ist der Mangel an geeigneten experimentellen Validierungsdaten. Darüber hinaus ist es wichtig zu erwähnen, dass die meisten der verfügbaren Modelle im Allgemeinen unter laminaren oder geringen Turbulenzbedingungen erstellt wurden.
Daher ist es notwendig, die Modellierungsmöglichkeiten für Zweiphasenströmungen mit komplexer Strömung/hoher Turbulenz, wie sie in Kernreaktoren auftreten, weiter zu untersuchen. Zu diesem Zweck sind zusätzliche Validierungen bei der CFD-Modellierung von Zweiphasenströmungen erforderlich. Untersuchungen zur Leistungsfähigkeit von Zweiphasenströmungsmodellen direkt für Stabbündel sind jedoch sehr kompliziert und zeitaufwändig. Daher wurde zunächst eine Fähigkeitsanalyse der Modelle für die drei Hauptphänomene, d. h. Aufbrechen/Koaleszenz, Widerstand und Turbulenz, für den Fall eines halbgeschlossenen Rohrs unter adiabatischen Strömungsbedingungen durchgeführt. Die Ergebnisse wurden anhand der von Neumann-Kipping (2022) gewonnenen experimentellen Daten über den Gasgehalt, den mittleren Blasendurchmesser, die Blasengrößenverteilung, die Flüssigkeitsgeschwindigkeit und die Gasgeschwindigkeit für zwei verschiedene Turbulenzbedingungen validiert.
Anschließend wurden Experimente in einem 3 x 3-Stabbündel mit einem Abstandshalter und Fahnen unter Verwendung der Röntgen-Computertomographie (CT) durchgeführt, die qualitativ hochwertige Gasgehaltdaten liefert, ohne die Strömung zu stören. Die Auswirkungen unterschiedlicher Massen- und Wärmeströme auf den Gasgehalt und seine Verteilung stromabwärts des Abstandshalters wurden analysiert.
Außerdem wurden die Auswirkungen verschiedener Fahnenwinkel auf die Verteilung des Gasgehaltes diskutiert. Darüber hinaus wurde eine experimentelle Datenbasis in einem Stabbündel mit einem Abstandshalter unter verschiedenen Strömungsbedingungen gewonnen, um die numerische Modellierung zu validieren.
Schließlich wurde das verbesserte CFD-Modell, das aus der halbgeschlossenen Rohrgeometrie gewonnen wurde, auf die 3 x 3 Stabbündelgeometrie bei zwei verschiedenen Turbulenzbedingungen angewendet. Die numerischen Ergebnisse wurden mit den Röntgen-CT-Daten für den Gasgehalt verglichen
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