Search

The purpose of this paper is to understand the capability and consistency of large eddy simulation (LES) in Eulerian–Lagrangian studies aimed at predicting inertial particle dispersion in turbulent wall-bounded flows, in the absence of ad hoc closure models in the Lagrangian equations of particle motion. The degree of improvement granted by LES models is object of debate, in terms of both accurate prediction of particle accumulation and local particle segregation; therefore, we assessed the accuracy in the prediction of the particle velocity statistics by comparison against direct numerical simulation (DNS) of a finer computational mesh, under both one-way and two-way coupling regimes. We performed DNS and LES at friction Reynolds number Re τ= 180 in smooth and rough channels, tracking particles with different inertia, with the aim to conduct a parametric study that examines the accuracy of particle statistics obtained from LES computations. The issue has been widely analysed in turbulent flow bounded by smooth walls, whereas the effect of rough boundaries on momentum coupled two-phase flows has been much less investigated until now. The action of the roughness of the wall is studied in terms of both turbulence modification and particle interaction with the wall surface due to particle rebounding off the solid boundary, without the introduction of a virtual rebound model. Results show that resolved LES adequately predicts particle-induced changes in both fluid and particle statistics in rough channels, at least for the range of parameters considered here. © 2020, Springer-Verlag GmbH Austria, part of Springer Nature.

When converting a baffled stirred reactor to work with a different fluid, usually the original impeller must be replaced with a customized one. If the original impeller was designed for mixing liquids, its performance for liquid–solid suspensions may not be satisfactory. A case study is presented, where a two-blade original impeller is replaced with a new three-blade design. The new impeller shows clear improvements in mixing a liquid–solid suspension, while keeping the shaft power practically at the same level. As a result, a practically homogenous liquid–solid mixture is obtained, thus ensuring the required quality of the final product. The present numerical investigations employ the Eulerian multiphase model with renormalization (RNG) k–ε turbulence model to simulate the three-dimensional unsteady free-surface liquid–solid flow in a stirred tank. A sliding mesh approach was used to account for the impeller rotation within the expert code, FLUENT 16. The comparative quantitative analysis of the solid phase distribution and the relevant velocity profiles show that the new design of three-blade-impeller is significantly increasing the sedimentation time of the solid phase beyond the chemical reaction specific time. The necessary power to drive the new impeller has a slightly higher value than for the original impeller but it can be sustained by the existing driving system.

The paper explores a tandem configuration of three-blade impellers in a stirred reactor. The working fluid is a liquid-solid mixture and the stirring mechanism fitted with the two impellers must prevent the sedimentation of solid particles while homogenously dispersing them in the bulk liquid. The present numerical investigation, performed with the expert software Ansys® Fluent, Release 16, employs the Eulerian multiphase model along with the RNG k–ε turbulence model to simulate the free-surface liquid–solid flows in the baffled stirred reactor. A sliding mesh approach is used to model the impellers rotation. The tandem configuration is clearly superior to a single impeller, while the existing electrical motor that drives the stirring mechanism still provides the necessary power.

Performance characteristics of a novel cyclone with tangential inlet were presented in axial and reverse flow operation modes. 3-D and unsteady governing equations were used for the numerical solution of the two-phase turbulent flow in the cyclone separator. The Eulerian approach was used to solve the flow field, and the Reynolds Stress Model (RSM) with the scalable wall function was employed for the numerical study. The Lagrangian approach with the Discrete Phase Model was used to calculate the discrete phase by releasing particles from the inlet surface. CFD calculations were run for different geometric configurations to analyze the performance of the cyclones regarding pressure drop, cut-off diameter, and fractional efficiency. Axial and tangential velocity profiles are presented at the defined sections. The computational results of pressure drop, velocity field, and separation efficiency were also compared for the axial and reverse flow cyclones at the same flow rate. The results show that pressure drop and collection efficiency in reverse flow mode are higher than that of the axial flow operation. However, axial flow cyclones seem to be more efficient for small particles comparing to reverse flow cyclones.

In the present paper a set of closure relations for interphase momentum exchange and for bubble-induced turbulence within the Euler-Euler framework is presented and validated against a set of tests performed at the HZDR-facility MT-Loop. The facility was equipped with wire-mesh sensors that allow cross sectional distributions of gas fraction, gas velocity, and bubble sizes to be measured at different distances from the gas injection. The here applied models were derived for the simulation of fully developed turbulent vertical upward bubbly flow in a pipe. The radial gas fraction profile for this kind of flow is the result of the ratio of the radial force components of the so-called non-drag forces and can be used for model validation. However, only the ratio, not the absolute value of the bubble forces can be tested in this way. In the present paper more detailed information from the experiment is exploited by consideration of the evolution of gas fraction distribution particularly after the gas injection region. The change of cross sectional gas volume fraction distribution is the result of the action of non-drag forces. In addition to vertical pipe flow tests further insight is obtained from the investigation of the effect of a slight tube inclination which shifts the gas distribution. Here the disturbance of the cylindrical symmetry of a vertical pipe gives hints on the absolute value of the non-drag force components. The results show that the presented model framework at least is able to describe the phenomena qualitatively. Possible reasons for quantitative deviations are discussed and require further investigations.

Dam is considered as a strategic structure whose collapse and destruction is a catastrophic event and could bring about significant life threatening and financial losses. Also its destruction may cause environmental damages due to uncontrollable exit of large amounts of water and sediment from the reservoir which results into propagation of devastating flood at downstream. Presence of barriers and buildings changes the flow patterns downstream of a dam. Regarding the importance of this issue, in this research modeling of this phenomenon was performed in the presence of a trapezoidal barrier using the finite volume method and OpenFOAM software. Modeling is in 2D form and, for validation of the results, use has been made of the numerical and experimental research conducted by other researchers. The results show that this model has a good performance in simulation of these problems and has been able to simulate the results with a good accuracy, compared to the experimental results. For simulation of other phenomena similar to the dam break, the present model could be developed.

Direct numerical simulation is used to assess the importance of compressibility in turbulent channel flow of a mixture of air and water vapor with dispersed water droplets. The dispersed phase is allowed to undergo phase transition, which leads to heat and mass transfer between the phases. We compare simulation results obtained with an incompressible formulation with those obtained for compressible flow at various low values of Mach number. We discuss differences in fluid flow, heat- and mass transfer and dispersed droplet properties. Results for flow properties such as mean velocity obtained with the compressible model converge quickly to the incompressible results in case the Mach number is reduced. In contrast, thermal properties such as the heat transfer, characterized by the Nusselt number, display a systematic difference between the two formulations on the order of 15%, even in the low-Mach limit. This shows the necessity of the use of a compressible formulation for accurate prediction of heat transfer, even in case of an initial relative humidity of 100%. Mass transfer properties display a difference between the models on the order of 5%, for example in the prediction of the droplet mean diameter near the walls.

This paper discusses the interaction between droplets and entrained turbulent air flow in the far-downstream locations of a confined polydispersed isothermal spray. Simultaneous and planar measurements of droplet and gas velocities in the spray along with droplet size are obtained with the application of a novel experimental technique, developed by Hardalupaset al. (Exp. Fluids, vol. 49, 2010, pp. 417–434), which combines interferometric laser imaging for droplet sizing (ILIDS) with particle image velocimetry (PIV). These measurements quantified the spatial correlation coefficients of droplet–gas velocity fluctuations ($R_{dg}$) and droplet–droplet velocity fluctuations ($R_{dd}$) conditional on droplet size classes, for various separation distances, and for axial and cross-stream velocity components. At the measurement location close to the spray edge, with increasing droplet size,$R_{dg}$was found to increase in axial direction and decrease in cross-stream direction. This suggests that as the gas-phase turbulence becomes more anisotropic away from the spray axis, the gravitational influence on droplet–gas correlated motion tends to increase. The effective length scales of the correlated droplet–gas motion were evaluated and compared with that for gas and droplet motion. The role of different turbulent eddies of the gas flow on the droplet–gas interaction was examined. The flow structures were extracted using proper orthogonal decomposition (POD) of the instantaneous gas velocity data, and their contribution on the spatial droplet–gas velocity correlation was evaluated, which quantified the momentum transfer between the two phases at different length scales of the gas flow. The droplets were observed to augment turbulence for the first three POD modes (larger scales) and attenuate it for the rest of the modes (smaller scales). It has been realized that apart from droplet Stokes number and mass loading, the dynamic range of length scales of the gas flow and the relative turbulent kinetic energy content of the flow structures (POD modes) must be considered in order to conclude if the droplets enhance or reduce the carrier-phase turbulence especially at the lower wavenumbers.

The NUPEC “PWR Subchannel and Bundle Tests” (PSBT) were designed to investigate critical heat flux phenomena in rod bundles and yield data to validate hot channel codes. Some of the tests are suitable for validating a CFD wall boiling model. The paper presents CFD calculations of the void distribution tests of the PSBT benchmark using ANSYS CFX. First, relevant aspects of the implemented wall boiling model are reviewed highlighting the uncertainties in several model parameters. It is then shown that the measured cross sectionally averaged values can be reproduced well with a single set of calibrated model parameters for different tests cases. For the reproduction of patterns of void distribution cross sections attention has to be focussed on the modelling of turbulence in the narrow channel. Only a turbulence model with the capability to resolve turbulent secondary flows is able to reproduce at least qualitatively the observed void distribution patterns. Furthermore the influence of bubble forces on the calculated void distribution is investigated.

In this work, an experimental and numerical investigation of hydrodynamics in a liquid induced by a bubble plume is carried out. The gas is introduced through a needle in the center of the tank containing water. The gas-liquid flow in such systems is inherently unsteady. Particle image velocimetry (PIV) was used to experimentally determine the transient velocity fields in the system. For this gas-liquid flow system, both the fluctuating and mean liquid velocities were determined experimentally by 2D and 3D PIV. The system was investigated for a liquid phase Reynolds number in the range of 3.7 � 104 to 1.8 � 105 and bubble phase Reynolds number in the range from 2350 to 11 773. The behavior of the system was simulated in FLUENT 6.3.26 using a two fluid Euler-Lagrangian (EL) model with a constant bubble size of 5 mm. Here, water is treated as the continuous phase, and gas bubbles are treated as the dispersed phase. Motion of the bubbles renders the flow turbulent, and this effect is captured by the standard k-? turbulence model. The temporal prediction of the flow field is compared with experimental results obtained from 2D and 3D measurements. The predictions from the 3D simulations capture the oscillating behavior found using 3D PIV. The 2D simulations predict a significantly higher value of turbulent viscosity. This is hypothesized as the reason as to why these simulations do not capture the oscillating behavior. � 2011 American Chemical Society.

Spray cooling of the superheated steam occurs with the interaction of many complex physical processes, such as initial droplet formation, collision, coalescence, secondary break up, evaporation, turbulence generation, and modulation, as well as turbulent mixing, heat, mass and momentum transfer in a highly non-uniform two-phase environment. While it is extremely difficult to systematically study particular effects in this complex interaction in a well defined physical experiment, the interaction is well suited for numerical studies based on advanced detailed models of all the processes involved. This paper presents results of such a numerical experiment. Cooling of the superheated steam can be applied in order to decrease the temperature of superheated steam in power plants. By spraying the cooling water into the superheated steam, the temperature of the superheated steam can be controlled. In this work, water spray cooling was modeled to investigate the influences of the droplet size, injected velocity, the pressure and velocity of the superheated steam on the evaporation of the cooling water. The results show that by increasing the diameter of the droplets, the pressure and velocity of the superheated steam, the amount of evaporation of cooling water increases. .

This work presents a computational fluid dynamics (CFD) calculation to investigate the flow field and the heat transfer characteristics in a tangential inlet cyclone which is mainly used for the separation of the dens phase of a two phase flow. Governing equations for the steady turbulent 3D flow were solved numerically under certain boundary conditions covering an inlet velocity range of 3 to 30 m/s. Finite volume based Fluent software was used and the RNG k − ɛ turbulence model was adopted for the modeling highly swirling turbulent flow. Good agreement was found between computed pressure drop and experimental data available in the literature. The structure of the vortices and variation of local heat transfer were studied under the effects of inlet velocity.

cited By 6; International audience; The bifurcation of slugs to bubbles within two-phase flow patterns in a minichannel is analyzed. The two-phase flow (water-air) occurring in a circular horizontal minichannel with a diameter of 1 mm is examined. The sequences of light transmission time series recorded by laser-phototransistor sensor is analyzed using recurrence plots and recurrence quantification analysis. Recurrence parameters allow the two-phase flow patterns to be found. On changing the water flow rate we identified partitioning of slugs or aggregation of bubbles. © 2015, The Author(s).

V nalogi je narejena primerjava med eksperimentalnim in numeričnim modeliranjem uparjanja vode v vertikalnem kanalu. Poudarek je na obravnavi dvofaznega toka tekočine z uporabo programskega paketa ANSYS CFX. Ugotovili smo, da lahko s pomočjo tega programa precej natančno opišemo tako uparjanje vode kot tudi tok brez uparjanja. This diploma thesis represents comparison between experimental and numerical modelling of water boiling in a vertical pipe. Emphasis is on modelling two phase flows using ANSYS CFX software package. We found that ANSYS CFX is very suitable for calculations of characteristics of either water boiling or water flow without boiling.

The presentation shows CFD calculations of the void distribution tests of the PSBT benchmark using ANSYS CFX-12.1. First, relevant aspects of the implemented wall boiling model are reviewed highlighting the uncertainties in several model parameters. It is then shown that the measured cross sectionally averaged values can be reproduced well with a single set of calibrated model parameters for different tests cases. For the reproduction of patterns of void distribution cross sections attention has to be focussed on the modelling of turbulence in the narrow channel. Only a turbulence model with the capability to resolve turbulent secondary flows is able to reproduce at least qualitatively the observed void distribution patterns. Furthermore the influence of bubble forces on the void distribution cross sections was shown.

The presentation shows CFD calculations of the void distribution tests of the PSBT benchmark using ANSYS CFX-12.1. First, relevant aspects of the implemented wall boiling model are reviewed highlighting the uncertainties in several model parameters. It is then shown that the measured cross sectionally averaged values can be reproduced well with a single set of calibrated model parameters for different tests cases. For the reproduction of patterns of void distribution cross sections attention has to be focussed on the modelling of turbulence in the narrow channel. Only a turbulence model with the capability to resolve turbulent secondary flows is able to reproduce at least qualitatively the observed void distribution patterns.

The paper presents CFD calculations using CFX-12.1 on the void distribution tests of the PSBT benchmark. First, relevant aspects of the implemented wall boiling model are reviewed highlighting the uncertainties in several model parameters. It is then shown that the measured cross-sectionally averaged values can be reproduced well with a single set of calibrated model parameters for different tests cases. For the reproduction of void distribution cross-sections attention has to be focussed on the modelling of turbulence in the narrow channel. Only a turbulence model which is able to resolve the secondary flows is able to reproduce at least qualitatively the void distribution images.

In the Euler/Eulerian approach simulating bubbly flow, the influence of the bubbles on the turbulence of the liquid has to be modelled. Vice versa, the structures of the turbulent liquid flow influence the gas void fraction distribution, which is expressed as a turbulent dispersion force. Reliable models for turbulence are an urgent precondition for the improvement of models describing bubble coalescence and bubble breakup in any population balance model. In the present work the different approaches simulating the influence of bubbles on the turbulence are revised and compared to measurements using ANSYS-CFX. Models for the turbulent dispersion force are validated.

Numerical modeling of a particle separation process is carried out to understand the gas-particle two-phase flow field inside a cyclone prolonged with a dipleg and results of the numerical simulations are compared with experimental data to validate the numerical results. The flow inside the cyclone separator is modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow is predicted by solving Navier–Stokes equations using the differential RSM turbulence model with nonequilibrium wall functions. The second phase is modeled based on a Lagrangian approach. Analysis of computed results shows that the length of the dipleg considerably influences the cyclone separation efficiency rather than the cyclone pressure drop, especially for lower inlet velocities in relatively short cyclones, by providing more separation space.

The mixing in two-phase gas-liquid and three-phase gas-liquid-solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed. � 2005 Elsevier Ltd. All rights reserved.

A finite volume method-based CFD model has been developed to simulate steady, turbulent, two-dimensional annular gas-liquid flow in a duct. The gas flow is treated as being equivalent to flow through a rough-walled duct. The effect of the liquid film on the gas phase is included in the form of modified wall functions which incorporate the well-known triangular relationship (Annular Two-Phase Flow, Pergamon Press, Oxford, 1970) that exists among wall shear stress, film flow rate and film thickness in annular flow. The presence of droplets is accounted for by solving an additional scalar transport equation for the mass fraction of the droplets. Entrainment and deposition of droplets are included as source term and boundary condition, respectively, in the mass fraction equation. It is shown that the resulting model, while retaining simplicity of formulation, gives good predictions of the literature data of annular flow parameters under equilibrium and non-equilibrium conditions. ? 2004 Elsevier Ltd. All rights reserved.

PARTIKÜLLERIN BOYUTSAL SINIFLANDIRILMASINDA GAZ-KATI PARTİKÜL İKİ FAZLI AKIŞIN SAYISAL İNCELENMESİ (Doktora Tezi) Aydın KARABEY GAZİ ÜNİVERSİTESİ FEN BİLİMLERİ ENSTİTÜSÜ Şubat, 2001 ÖZET Bu çalışmada, toz metalürji yöntemleriyle üretilen değişik büyüklüklerdeki metal tozlarann boyutsal olarak smıflandırılmasmda görülen gaz-katı partikül iki fazlı akışm sayısal incelenmesi yapılmıştır. Buradaki temel amaç, sürekli fazı oluşturan hava ile dağınık fazı oluşturan partiküllerin akış alam içerisindeki hareketlerinin analiz edilmesi ve sonuç olarak partiküllerin boyutsal sınıflandırılması için gerekli olan çalışma şartlarının belirlenmesidir. Gaz-katı partikül iki fazlı akış Eulerian-Lagrangian metodu ile formüle edilerek, PSI-CELL yöntemi ile sayısal olarak çözülmüştür. Sürekli faz içerisindeki türbülansı modellemek için standart k-e modeli kullanılmıştır. Sayısal çözüm için geliştirilen bilgisayar programı literatürdeki çalışmaların simülasyonlan için kullanılarak test edilmiş ve programın bu tipte akışları simüle edebileceği doğrulanmıştır. Yapılan çalışmada sözü edilen iki fazlı akışı simüle etmek amacıyla; Partikül giriş noktasının yükseldiğinin annulus hidrolik yarıçapına oram H/L, partikül çapının annulus hidrolik yarıçapına oranı, d/L, akışkan yoğunluğunun partikül yoğunluğuna oranı, p /pp, hava giriş hızının partikül giriş hızına oram, Vf/Upo, akışm Reynolds sayısı, Re ve partikül kütlesel debisinin akışkanın kütlesel debisine oram (yükleme oram), m, boyutsuz parametrelerinin değişimlerinin akışa etkileri incelenmiş ve sınıflandırma sisteminin verimli çalışması için uygun değerler belirlenmiştir.Yapılan simülasyonlar neticesinde: Büyük boyutlu partiküllerin küçük boyutlu partiküllere oranla aynı akış şartlarında daha uzak mesafelere gittiği tespit edilmiştir. Küçük boyutlu partiküllerin sürükleme ivmesi, büyük partiküllerin sürükleme ivmesine göre daha fazla olduğundan küçük boyutlu partiküllerin radyal yöndeki hızları daha çabuk azalarak bu yönde daha az yol almaktadırlar. Reynolds sayışırım artmasıyla, havanın hızı ve partiküllere etkiyen sürükleme kuvveti artmakta, böylece partiküllerin radyal yöndeki hızları daha çabuk azalmaktadır. Ayrıca, hava hızının artmasıyla partiküller daha hızlı bir şekilde akış alanının dışına taşındıklarından, partiküller daha yakın mesafelere düşmektedir. Partikül yoğunluğunun sürekli faz yoğunluğuna oram (p /pp) gaz-katı partikül iki fazlı akışlarda önemli bir parametredir. Yoğunluğu fazla olan partiküller daha uzak mesafelere düşmektedir. Partikül yoğunluğunun artması, partikül üzerindeki birim kütleye düşen sürükleme kuvvetini azaltmaktadır. Böylece, akış alanına aynı hızla giren partiküllerden daha ağır olanlar daha geç yavaşlayarak radyal yönde daha fazla yol almaktadırlar. Bu çalışmada ele alman hız oranının (Vh/Upo) artmasıyla partikül giriş hızı azalmaktadır. Partikül giriş hızı azaldıkça, partiküller akış alanında daha az yol alarak daha yakın mesafelere düşmektedir. Yükleme oranının artmasıyla akış alanına giren partikül miktarı artmakta ve havanın her bir partikül üzerindeki etkisi azalmaktadır. Böylece, yükleme oranının artmasıyla partiküller daha uzak mesafelere düşmektedir. Partikül giriş yükseldiğinin değişimi belirli aralıklarda partiküllerin düşme mesafelerini değiştirmemekle birlikte, büyük giriş yüksekliklerinde partiküller geriye dönüş hareketi yapmakta, düşük giriş yüksekliklerinde ise partiküller radyal yöndeki hızlan sıfır olmadan akış alanım terk etmektedir. Bilim Kodu : 625.04.03 Anahtar Kelimeler : İki-fazlı akış, gaz-katı partikül akışı, türbülans, k-e modeli, SAD simülasyonu, Eulerian-Lagrangian formülasyonu Sayfa Adedi : 208 Tez Yöneticisi : Prof. Dr. Haşmet TÜRKOĞLU m NUMERICAL ANALYSIS OF GAS-SOLID PARTICULE TWO PHASE FLOW FOR SIZE CLASSIFICATION OF PARTICULES (Ph. D. Thesis) Aydm KARABEY GAZİ UNIVERSITY INSTITUTE OF SCIENCE AND TECHNOLOGY February, 2001 ABSTRACT In this study, gas-solid particule two-phase flow in the systems used for classification of different size particulles for powder metallurgy applications was analyzed numerically. The main goal of this work is to analyze the motions of both continuous and dispersed phases and consequently, determine the optimum values of system parameters and operating conditions for particule classification systems. Eulerian-Lagrangian approach was used to model the flow. PSI-CELL method was employed for numerical solutions. The turbulence in continuous phase was modeled by using standart k-e model together with appropriate wall functions. A computer program was developed for simulation of gas-solid particle two- phase flows. This computer program and model used were tested by comparing the results with works found in literature. It was confirmed that both the model and the program could simulate such flows with reasonable accuracy. In this study, to simulate the considered flow, the effect of changes on the predetermined dimensionless parameters such as; the ratio of the height of particule entrance port to the annulus hyraulic radius, H/L, the ratio of partikül diameter to the annulus hyraulic radius, d/L, the ratio of the air density to the particule density, p fpp, the ratio of inlet velocity of air to the inlet velocity of particule, Vf/Upo, flow Reynolds number, Re and the ratio of mass flow rate of gas to the mass flow rate of particules (loading ratio), m, on the particle motion were examined and optimum operating conditions of the classification system were determined.IV By the simulations made, followings were concluded: For the same flow conditions, larger particles travel further, compared with the smaller ones. Since drag acceleration of the smaller particules are greater than that of larger particules, radial velocities of smaller particules decrease faster than larger ones and hence, smaller particules travel less distances in the radial direction. As Reynolds number increases, the velocity of air increases, so does the drag acceleration of particules. Thus, the radial velocity of particules decrease faster. Also, as air velocity increases particules are carried to outlet of flow domain faster and particules fall closer to the entrance port. The ratio of gas density to the particule density (p/pp) is an important parameter in gas-solid two phase flows. Particules with a greater density, travel further. As particule density increases, the drag force per unit mass of the particule decreases. Thus, the heavier particules decelarate slower and travel further in radial direction. As the velocity ratio (fVUpo) increases, particule inlet velocity decreases. As particule inlet velocity decreases, the distance taken by the particules in radial direction decreases too. The number of particules entering to the flow field increases as loading ratio increases. This lessens the affect of air flow on individual particle. Thus, particules travel further away from the entrance point in radial direction. The change in height of particule entrance point doesn't change the distance, traveled by the particules in radial direction, much. But, after a specific value, inreasing the height causes particules to move backward. Decreasing the height of the particle inlet port under a certain value causes the particules to leave the flow field before particules' radial velocity components reduce to zero. Science Code : 625.04.03 Key Words : Two-phase flow, gas-solid particulate flow, turbulence, k-E modeling, CFD simulation, Lagrangian-Eulerian approach Page Number : 208 Adviser : Prof. Dr. Haşmet TÜRKO?LU 208

Hidroelektrik santrallerin şaft tipi dolu savaklarında, termik ve nükleer santrallerin soğutma suyu sistemlerinde, binaların kullanılmış ve yağmur suyu tesisatlarında, serbest yüzeyli akışa sahip bir akımın düşey bir boru boyunca serbest düşüşü, bugüne kadar çoğunlukla amprik karakterde izah edilmeye çalışılmıştır. Bu akım yapısı incelendiğinde, akışın serbest düşüşü esnasında içerdiği hava ile birlikte çift fazlı bir karakter taşıdığı görülmektedir, Kullanılmış suların akışında ise bu akım yapısı içereceği katılarla üç fazlı bir karaktere sahip olacaktır. Düşey sistemlerdeki bu kısmi serbest yüzeyli akım, yaratabileceği kavitasyon ve yapı ile etkileşime girerek oluşturacağı vibrasyon, bu tip sistemlerde önemli problemlere neden olmaktadır. Düşey boru akışları, slug akımı ile kesikli bir yapı gösterebilmektedir. Slug akımı eksenel simetriye sahip büyük mermi şeklinde gaz kabarcıkları veya Taylor kabarcıkları ile karakterize edilen kesikli akımların bütünü olarak tanımlanmaktadır. Taylor kabarcıkları borunun hemen hemen tamamını kaplayarak üniform bir yapıda hareket etmektedirler. Taylor kabarcığı ile boru cidarı arasında ince bir film şeklinde akış söz konusu olmaktadır. Ancak burada dikkate alınacak atam alanı iki ucu atmosfere açık düşey bir boruya, yanal bir borudan akışın olması halidir. Bu durumda oluşan kısmi dolu akış, kabarcıklı bir akım yapısına sahip olduğu gibi, borunun kalan kısmında hava akışı meydana gelecektir. Akım yerçekimi etkisindedir ve borunun belirli bir yüksekliğine kadar hızlanacak ancak hava ve boru cidarıyla olan sürtünme direnci nedeniyle, sabit bir değere ulaşacaktır. Hızdaki bu değişim borunun tam dolu olması haline göre belirgin olarak farklılık gösterecektir. Kısmi dolulukta düşen akışın, düşey boru boyunca ve boru enkesitlerinde hava-su oranları değişmektedir. Borunun büyük bir kısmında slug atanıma benzer kesikli akışlar söz konusu olabilecektir, böylece üniform olmayan bir düşey atam alanı meydana gelecektir. Bu çalışmada dairesel kesitli bir akış yolunda serbest yüzeyli atam düşüşü sırasında akış yolu boyunca hız ve basınç alanındaki değişim, hava-su fazlan birlikte düşünülerek modellenecektir. Atam tamamiyle türbülanslı karakter taşımaktadır, bu nedenle çok iyi bilinen RNG k-8 modeli, problemin çözümünde ele alınacaktır ve sonlu hacimler yaklaşımı kullanılarak problem modellenecektir. Ancak model, problemin fiziğinin sahip olduğu geometri, hesap alanında dikdörtgen koordinat sistemiyle dönüştürülerek çalıştırılacaktır. Elde edilen sonuçlar konu ile ilgili deneysel verilerle karşılaştırılarak yorumlanmaya çalışılacaktır. Elde edilen veriler yardımıyla, düşey akış sırasında gerek hava, gerekse su akımının hız dağılımı ile basınç dağılımlarının her kesit içinde ve düşey boyunca değişimleri belirlenecektir. Ayrıca akış hızının hangi düşey mesafeden sonra sabit değer ulaştığı ve yine sistemin ne kadar hava çektiği belirlenecektir. Böylece düşey boyunca herbir kesitteki hava- su karışım yüzdeleri de bulunabilecektir. vı Gas-liquid flow in vertical tubes is a flow pattern commonly found in many practical applications. It occurs in civil engineering applications and process equipment, e.g. in two- phase oil and gas pipelines, particularly in risers from subsea wells to platforms, as well as in oil, gas and geothermal wells, drain pipes for buildings. When gas-liquid mixtures flow in a tube, two phases distribute in a number of ways which characterize the spatial distribution of liquid and gas within the conduit. These are called flow patterns. Four dominant patterns in vertical pipes are suggested; bubbly, slug, churn and annular flows for upward flows. However there is only emprical knowledge existing in the literature for partical free surface downward flows. The objective of this work was to develop a physically-based hydrodynamic model for downward flows in vertical pipes and obtain experimental data by which the model could be evaluated. The results should permit the evaluation of the pressure and velocity distribution and volume fraction for gas-liquid flow through vertical pipes. 306

We propose a theory to calculate the pressure gradient and the liquid fraction in two-phase gas-liquid fully developed stratified flow in a cylindrical duct of arbitrary cross-section and slope and its application to near horizontal ducts. Its validity is limited to Reynolds numbers high enough to ensure turbulent flow in both phases. Droplet entrainment or bubble inclusion have not been taken into account. The comparison between calculations and experimental values obtained with air and water in circular and rectangular ducts is very satisfactory.

The response of turbulent pipe flow to sudden changes in wall roughness and flow cross-sectional area has been studied experimentally and numerically. Changes typical of those encountered by the gas phase in annular gas-liquid flow have been considered. The results show that the flow field and the pressure field can be significantly distorted at these transitions. Good agreement has been obtained between the measured results and those calculated using the Harwell-FLOW3D computational fluid dynamics (CFD) code.