bluuebunny/crossref_metadata_2025 huggingface dataset:
Similitude Analysis for Gas-Liquid-Fiber Flows in Cocurrent Bubble Columns
Authors: Tang Chengzhi , Heindel Theodore J.
Published: September 2008

Gas-liquid-fiber systems are different from conventional gas-liquid-solid systems in that the solid material (i.e., fiber) is flexible, has a large aspect ratio, and forms flocs or networks when its mass fraction is above a critical value. With its wide application to the pulp and paper industry, it is important to investigate the hydrodynamics of gas-liquid-fiber systems. In this paper, 19 parameters that influence gas holdup in gas-liquid-fiber bubble columns are critically examined and then a dimensional analysis based on the Buckingham Pi Theorem is used to derive the dimensionless parameters governing gas-liquid-fiber bubble column hydrodynamics. Seven dimensionless parameters that are related to the fiber effects on gas holdup are further analyzed, and a single dimensionless parameter combining these dimensionless parameters is derived based on a force analysis and experimental results. This dimensionless parameter is shown to be sufficient to quantify the influence of fiber on gas holdup in gas-liquid-fiber cocurrent bubble columns. It also reduces the number of parameters needed in correlating experimental gas holdup data in gas-liquid-fiber bubble columns.
Design and Analysis of a Turbopump for a Conceptual Expander Cycle Upper-Stage Engine
Authors: Dorney Daniel J. , Rothermel Jeffry , Griffin Lisa W. , Thornton Randall J. , Forbes John C. , Skelley Stephen E. , Huber Frank W.
Published: September 2008

As part of the development of technologies for rocket engines that will power spacecraft to the Moon and Mars, a program was initiated to develop a conceptual upper stage engine with wide flow range capability. The resulting expander cycle engine design employs a radial turbine to allow higher pump speeds and efficiencies. In this paper, the design and analysis of the pump section of the engine are discussed. One-dimensional meanline analyses and three-dimensional unsteady computational fluid dynamics simulations were performed for the pump stage. Configurations with both vaneless and vaned diffusers were investigated. Both the meanline analysis and computational predictions show that the pump will meet the performance objectives. Additional details describing the development of a water flow facility test are also presented.
Direct Numerical Simulations of Bubbles in Vertical Channels
Authors: Tryggvason G. , Lu J. , Biswas S.
Published: September 2008

Recent DNS studies of bubbly flows in channels are discussed. Simulations of nearly spherical bubbly flows in vertical channels show that the bubbles move towards the wall for upflow and away from the wall for downflow in such a way that the core is in hydrostatic equilibrium. For down flow the wall layer is free of bubbles but for upflow there is an excess of bubbles in the wall layer. The liquid velocity in the core is uniform. For laminar downflow the velocity in the wall layer can be computed analytically but for upflow the velocity is strongly influenced by the presence of the bubbles. Results for turbulent flow show similar behavior and for downflow the velocity is given (almost) by the law of the wall. Several simulations are used to examine the effect of void fraction and bubble size for turbulent downflow.
Three-Dimensional Direct Numerical Simulations of Autoignition in Turbulent Non-Premixed Flows With Simple and Complex Chemistry
Authors: Lo̸va˚s T. , Lowe A. , Cant R. S. , Mastorakos E.
Published: September 2008

3D Direct Numerical Simulations (DNS) of autoignition in turbulent non-premixed flows between fuel and hotter air have been carried out using both 1-step and complex chemistry consisting of a 22 species n-heptane mechanism to investigate spontaneous ignition timing and location. The simple chemistry results showed that the previous findings from 2D DNS that ignition occurred at the most reactive mixture fraction (ξMR) and at small values of the conditional scalar dissipation rate (N|ξMR) are valid also for 3D turbulent mixing fields. Performing the same simulation many times with different realizations of the initial velocity field resulted in a very narrow statistical distribution of ignition delay time, consistent with a previous conjecture that the first appearance of ignition is correlated with the low-N content of the conditional probability density function of N. The simulations with complex chemistry for conditions outside the Negative Temperature Coefficient (NTC) regime show behaviour similar to the single-step chemistry simulations. However, in the NTC regime, the most reactive mixture fraction is very rich and ignition seems to occur at high values of scalar dissipation.
Development of Euler-Euler LES Approach for Gas-Particle Turbulent Jet Flow
Authors: Riber Eleonore , Moreau Mathieu , Simonin Olivier , Cuenot Be´ne´dicte
Published: September 2008

A Large Eddy Simulation approach for Eulerian-Eulerian dispersed two-phase flow is presented. It is shown that not only the Random Uncorrelated Motion but also Sub-Grid Scales term modeling the unresolved field in the particle mesoscopic momentum transport equation and the particle Random Uncorrelated Energy need to be accounted for. Simulations of a non-homogeneous particle laden turbulent gas flow allow to compare dispersed phase quantities such as number density, time-averaged and rms mesoscopic velocity, fluid-particle correlations with experimental results.
Improvements to the Dynamic Range of Velocity Measurements Using DPIV
Authors: Charonko John J. , Vlachos Pavlos P.
Published: September 2008

The accurate measurement of velocity fields containing large dynamic ranges is important because many problems of interest feature regions of both very high and low velocity. Examples include mixing tanks, jets injected into quiescent chambers, and stagnation regions behind bodies in high speed flow. However, accurate calculation of the velocity is currently limited to displacements greater than the total error of the scheme used (typically in the range of 0.01 to 0.05 pixels) and less than one-quarter of the window size. To counteract this difficulty, two new methods for improving the dynamic range of DPIV calculations have been developed. The first can be used with any double-pulsed time-resolved DPIV system where closely spaced frame pairs are captured at kilohertz rates. Displacement measurements can be made both within each frame pair (resolving high velocity regions), and between successive pairs (resolving low velocity regions). The two displacement fields are then reconciled, resulting in a single flow field measurement. The second method uses multiple laser pulses per camera exposure. Four laser pulses are required per measurement (two per camera frame), however, unlike the first method, kilohertz repetitions rates are not required. By carefully selecting the intervals between pulses, it is possible to associate the each cross-correlation peak with the correct delay time, and thereby simultaneously obtain velocity measurements over a much wider dynamic range. These two methods have been applied to synthetic and experimental data and their performance has been characterized through error analysis. Results indicate that both methods can increase the dynamic range by one to two orders of magnitude as compared to traditional techniques, while retaining similar total error and spatial resolution characteristics.
Study of Flow Structures and Heat Transfer in Parallel Fins With Dimples and Protrusions Using Large Eddy Simulation
Authors: Elyyan M. , Rozati A. , Tafti D. K.
Published: September 2008

Flow field and heat transfer for parallel fins with dimples and protrusions are predicted with large-eddy simulations at a nominal Reynolds number based on fin pitch of 15,000. Dimple and protrusion depth and imprint diameter to channel height ratio are 0.4 and 2.0, respectively. The results show that on the dimple side, the flow and heat transfer is dominated by unsteady vorticity generated and ejected out by the separated shear layer in the dimple. The high turbulent energy which results from the unsteady dynamics is mostly responsible for heat transfer augmentation on the dimple side. A maximum augmentation of about 4 occurs in the reattachment zone of the dimple and immediately downstream of it. On the protrusion side, however, the augmentation in heat transfer is dominated by flow impingement at the front of the protrusion, which results in a maximum augmentation of 5.2. The overall heat transfer and friction coefficient augmentations of 2.34 and 6.35 are calculated for this configuration. Pressure drag from the dimple cavity and protrusion contribute 82% of the total pressure drop.
Spatio-Temporal Development of Supercavitation Over an Impulsively Launched Projectile
Authors: Weiland C. J. , Vlachos P. P.
Published: September 2008

Supercavitation inception and formation was studied over blunt projectiles. The projectiles were fired using a gas gun method. In this method, projectiles are launched under the action of expanding detonation gases. Both qualitative and quantitative optical flow diagnostics using high speed digital imaging were used to analyze the spatio-temporal development of the supercavitating flow. For the first time, quantification of the supercavitation was achieved using Time Resolved Digital Particle Image Velocimetry (TRDPIV) detailing the two phase flow field surrounding the translating projectiles and the gas vapor bubble. Experimental results indicate that the supercavity forms at the aft end of the projectile and travels forward along the direction of projectile travel. The impulsive start of the projectile generates two asymmetric vortices which are shed from the blunt nose of the projectile. The vortices interact with the moving cavity and subsequently deform. This interaction is believed to directly contribute to the instabilities in the flight path.
Transient Growth of Three-Dimensional Vortex Perturbations During Near-Parallel Collision With a Circular Cylinder
Authors: Liu X. , Marshall J. S.
Published: September 2008

A computational study is reported that examines the transient growth of three-dimensional flow features for nominally parallel vortex-cylinder interaction problems. We consider a helical vortex with small-amplitude perturbations that is advected onto a circular cylinder whose axis is parallel to the nominal vortex axis. The study assesses the applicability of the two-dimensional flow assumption for parallel vortex-body interaction problems in which the body impinges on the vortex core. The computations are performed using an unstructured finite-volume method for an incompressible flow, with periodic boundary conditions along the cylinder axis. Growth of three-dimensional flow features is quantified by use of a proper-orthogonal decomposition of the Fourier-transformed velocity and vorticity fields in the cylinder azimuthal and axial directions. The interaction is examined for different axial wavelengths and amplitudes of the initial helical waves on the vortex core, and the results for cylinder force are compared to the two-dimensional results. The degree of perturbation amplification as the vortex approaches the cylinder is quantified and shown to be mostly dependent on the dominant axial wavenumber of the perturbation. The perturbation amplification is observed to be greatest for perturbations with axial wavelength of about 1.5 times the cylinder diameter.
Application of CFD in the Design of Contra-Rotating Kaplan Turbine
Authors: Yang Wei , Liu Shuhong , Wu Yulin
Published: September 2008

A new type turbine which can be called contra-rotating Kaplan turbine has been designed in this paper. This new turbine, which can make power on both directions of the water flow, is mainly used in tidal power station. In this paper we discuss the feasibility of the new turbine by analyzing the velocity triangle of the inlet and outlet of the runner. According to this, we advance that two group of “S” type blades must be used to constitute the contra-rotating runner. Method of singular point distribution is used to design front-runner and rear runner based on the triangle velocity. Then the CFD simulation is used to validate and optimize the new turbine in different axial distances and different ratios of revolution speed between the front-runner and the rear runner. We simulate the internal flow field and the interaction between the front and rear runners using a three-dimensional steady turbulent flow with k-ε model and SIMPLEC method. Standard wall function is used in the turbulent calculation. From the simulation the hydraulic performance of the turbine is discussed.
Fluid Forces on a Circular Cylinder Subjected to a Transient Motion at Low Amplitude: Infinite Medium and Cylindrical Confinement
Authors: Melot Vincent , Leblond Ce´dric , Sigrist Jean-Franc¸ois , Laine Christian , Auvity Bruno , Peerhossaini Hassan
Published: September 2008

The present paper deals with the fluid forces induced by a rapidly moving rigid circular cylinder in an incompressible fluid initially at rest. The cylinder is subjected to an impulsive motion which corresponds to unique sinusoidal period of acceleration and is then stopped. Two fluid domains are considered: infinite and cylindrically confined. This study focuses on small displacements of the cylinder in regards to its radius, i.e. for low Keulegan-Carpenter numbers. In a first part, the flow is assumed potential. Only the inner cylinder is displaced and the outer one, in the confined case, is at rest. The problem, formulated as a two-dimensional boundary-perturbation problem, is solved thanks to a regular expansion. A non-linear analytical formulation of the fluid forces experienced by the moving cylinder is then proposed. Its range of validity is discussed with regards to the inner cylinder displacement. The results are confronted to numerical simulations with a CFD code based on a finite volume discretization on a moving mesh. In a second part, a two-dimensional viscous flow is considered. Analytical formulations of the fluid forces experienced by the cylinder subjected to arbitrary motions are proposed. The starting point of the analytical approach is the fluid forces expressions obtained with harmonic motion. These expressions come from the Rosenhead model for the infinite fluid domain. A Fourier transform is applied on the harmonic solutions to capture the wide frequency spectrum composing the transient motion. An inverse Fourier transform is then applied on the resulting expressions to derive the solutions in the temporal space. The analytical solutions are discussed and compared to numerical simulation results obtained in an infinite domain for various Stokes numbers. The competition between the viscous diffusion time and the wave duration time is studied which allows to underline history effects on the force.
Direct Coupling Between Eulerian and Lagrangian Approaches in Turbulent Gas-Solid Flows
Authors: Pialat Xavier , Simonin Olivier , Villedieu Philippe
Published: September 2008

The purpose of this paper is both to present and validate the methodology of a hybrid method coupling a Eulerian and a Lagrangian approaches in turbulent gas-particle flows. The knowledge of the dispersed phase is displayed in terms of a joint fluid-particle probability density function (pdf) which obeys a Boltzmann-like equation. We chose two different ways of resolution of this equation, depending on the required level of description. The first one is a stochastic Lagrangian approach which embeds a Langevin equation for the fluid velocity seen along the particle path. The second one is a Eulerian second-order momentum approach derived in the same frame as the preceding one. These two approaches are then coupled through half-fluxes. This procedures allows well-posed boundary conditions stemmed from previous time-step statistics for the two approaches. The aim is to provide a methodology able to take into account physical phenomena such as particle bouncing on rough walls or deposition in inhomogeneous flows with a reasonable numerical cost. The paper present the methodology and validations in the case of inert monodispersed particle in a turbulent shear flow without two-way coupling. Comparisons of the results of the hybrid method with each approach and LES/DPS results indicate that the hybrid method could become a powerful simulation tool for gas-particle flows.
The Hybrid Particle-Level-Set Method Applied to Two-Phase Flows
Authors: Gaudlitz Daniel , Adams Nikolaus A.
Published: September 2008

The hybrid particle-level-set method was proposed by Enright et al. (2002) for improving the efficiency and accuracy of the original level-set method. In this paper, we examine the discretization of the surface-tension term and of the reinitialization procedure with respect to their effect on the mass-conservation properties of the overall method. We apply the hybrid particle-level-set method for the computation of two-phase flows and assess its performance with different parameter choices in comparison with the original level-set method.
Development of a Two-Dye LIF Technique for Measuring Fluid Temperature Fields in Microfluidic Devices
Authors: Yamaguchi Eiichiro , Natrajan Vinay K. , Christensen Kenneth T.
Published: September 2008

The feasibility of implementing a two-color laser-induced fluorescence (LIF) technique to study thermal transport at the microscale is investigated. A temperature-sensitive fluorescent dye (Rhodamine B) and a temperature-insensitive dye (Nile Red) are used in tandem to determine fluid temperature with high accuracy and low noise using a pulsed Nd: YAG laser as an illumination source. While the fluorescence intensity of the temperature-sensitive dye is proportional to temperature, it can be biased by variations in the illuminating intensity. Therefore, a second, temperature-insensitive dye is required to account for noise associated with variations in illumination intensity. Calibration of the system reveals that the two-color dye mixture in ethanol yields 1.0%/K of temperature sensitivity with volume illumination from the Nd:YAG laser. The feasibility of this methodology is explored by measuring steady-state conduction in a heated micro-reservoir.
A Semi-Empirical One-Dimensional Model for Flow in Ejectors Used for Gas Evacuation
Authors: Al-Ansary Hany A.
Published: September 2008

Ejectors are considered an integral part of numerous engineering applications such as chemical processes, desalination, and refrigeration. The majority of past research in this field was aimed at developing simplified one-dimensional models for the ejector flow problem. The main advantage of those models is that results can be obtained almost instantly as opposed to experimental or CFD work and they are also flexible in that they allow the user to change various operating and geometric parameters. Despite the success of some of those models, they are inherently unable to incorporate common multi-dimensional phenomena such as oblique shocks and shock-induced boundary layer separation. This work relies on experimental and CFD results obtained earlier by the author for a certain ejector geometry to develop a semi-empirical one-dimensional model where empirical formulation is used wherever multi-dimensional effects need to be taken into account. One of the features of this model is that it can be used at both design and off-design conditions but is currently limited to low exit pressure conditions, making it suitable for modeling ejectors used in applications such as gas evacuation where exit pressure is nearly ambient. The paper presents the results of the semi-empirical one-dimensional model for three geometries. The validity of the results is tested by comparing it with reliable numerical data.
Bubble Behaviour in Viscous Flows: Experimental and Numerical Determination of 3D-Trajectories
Authors: Chaminade Sylvain , Toutain Jean , Dilhaire Stefan , Arquis Eric , Marraud Christine , Marchetto Virginie
Published: September 2008

We studied the air bubble behavhiour in viscous flows. A one-way coupling model was developed, based on lagrangian method. In order to validate this model, experiments were carried out. Our experimental installation was based on shadow visualization method. Bubbles were introduced in a transparent Plexiglas tank filling with viscous fluid (e.g. silicon oil) and were illuminated by diffused lights. In order to reconstruct 3D-trajectories, two video cameras, orthogonally placed, took pictures of shape bubbles and a post treatment of images acquired allowed us to determine the position of the center and the radius of each bubble. Thus, from this information of each plane, we could determine the 3D parameters of bubbles. Superluminescent LEDs with different wave-lengths were used as light sources. The choice of the wavelengths for each plane illumination and the selection of appropriated RGB filter of CCD sensor allowed us to prevent dissymmetry of bubble shape which was due to parasitic orthogonal illumination. Numerical results agree with experimental data.
Influence of Surfactants on the Dynamics of Fluid-Liquid Interfaces
Authors: Bothe Dieter , Alke Andreas
Published: September 2008

In many two-phase fluid-liquid systems at least one phase contains surface active agents (surfactants for short) which are adsorbed preferentially at the interface Γ(t) due to minimization of free surface energy. Important examples are emulsification processes and bubbles rising in a bubble column through water containing a surfactant - unmeant as a contamination or by determined addition in order to increase the efficiency of the column. The adsorption of a surfactant at a fluid-liquid interface causes a decrease of the surface tension, depending on the area specific concentration cΓ of the adsorbed surfactant, i.e. σ=f(cΓ)(1) with a decreasing function f. The adsorbed surfactant is distributed on the interface due to convective and diffusive interfacial fluxes. The resulting spatial inhomogeneity leads to surface gradients of the surface tension, ∇Γσ(cΓ), which effect the hydrodynamics via the interfacial momentum jump condition [pI−S]nΓ=σκnΓ+∇Γσ(cΓ).(2) These additional so-called Marangoni stresses often result in a pronounced change of the dynamical behavior.
Dual-Energy X-Ray Tomographic Measurement of Local Phase Fractions in 3-Phase Bubble Columns
Authors: Behling Martin , Mewes Dieter
Published: September 2008

For measuring local phase fractions in 2- and 3-phase bubble columns, an X-ray tomographic measurement system is applied. This measurement technique, also referred to as Computer Tomography or CT, is based on the attenuation of X-rays along their path through the bubble column. An X-ray source and an X-ray detector are mounted on opposite sides of the bubble column. The bubble column is irradiated by an X-ray fan beam perpendicular to the bubble column axis. The X-ray intensity measured by each detector pixel is an integral measure for the penetrated material along the path of the X-rays. By rotating the X-ray source and X-ray detector around the bubble column axis, multiple projection measurements of the measurement plane are collected. In a second step, the phase distribution in the measurement plane (the so-called CT-slice) is calculated from the projection measurements by applying mathematical reconstruction algorithms. The reconstructed phase fractions are time-averaged over the measurement interval of 200 seconds for the measurements presented in this work. In order to distinguish all 3 phases, a special dual-energy technique is used. In this technique, 2 separate CT measurements are conducted successively, applying 2 different X-ray wavelengths. By combining the information gained from these 2 measurements, all 3 phase fractions are determined for every image pixel. The local phase fractions of all 3 phases are measured simultaneously for the whole cross-section. The measurement technique is fully non-intrusive. It is not restricted to limited ranges of phase fractions, solid loadings or flow rates of any of the phases. A 244 mm diameter, 7 m high bubble column is examined. It can be operated either with only 2 phases (liquid and gas) or with additional solid particles. Measurements are conducted with air as gas, water as liquid and PVC particles as solid phase. In this paper, the measurement principle of the tomographic technique and the dual-energy algorithm are explained. The experimental setup is described and the results of the measurements are presented.
A New Design of Mini Heat Exchanger for PEMFC Bipolar Plates
Authors: Lasbet Yahia , Auvity Bruno , Castelain Cathy , Peerhossaini Hassan
Published: September 2008

Fuel cell is a power generator which directly converts chemical energy to electric energy through an electrochemical reaction instead of thermal combustion. The electrochemical reaction produces as much heat as electric energy. In transport applications where the specific power density of the fuel cell is relatively high a large amount of heat should be evacuated from the system. Therefore, heat exchangers integrated in the bipolar plates play an important role in the operation and life time of fuel cells. Conventional designs of the cooling system of the current bipolar plates consist of a net work of parallel straight channels of half of the bipolar plate’s thickness. These designs reach rapidly their limit of efficacy and new designs are needed. Reynolds number in the bipolar channel flow is around 200; therefore the flow regime is laminar which is known for its weak heat transfer efficiency. In this work we present a new geometry of the cooling channels of bipolar plates in which heat transfer efficiency in laminar regime is enhanced by generating chaotic trajectories. Here we characterize the heat transfer characteristics of a single channel. Firstly, hydrodynamic and heat transfer characteristics of several channel geometries are characterized by using the CFD code Fluent. Thermo-physical properties of the working fluid are those of water and the velocity profile at the channel entrance is that of a fully developed Poiseuile flow. Secondly, fluid mixing along the channels is evaluated using two different criteria. For these calculations, thermal boundary conditions on the channel walls are adiabatic and the entrance of the channel is divided in two (horizontal or vertical) parts. In one part water flows at 300K while in the other part water is at 320K. The first criteria is the ratio of (Tmin/Tmax-Ro)/(1-Ro) over cross-section surfaces calculated for Reynolds numbers 100 and 200 and for both horizontal and vertical positions of the dividing surface at the entrance.
RANS and LES of Particle Dispersion in Turbulent Pipe Flow: Simulations Versus Experimental Results
Authors: Berrouk Abdallah S. , Douce Alexandre , Laurence Dominique , Riley James J. , Stock David E.
Published: September 2008

Turbulent transport and dispersion of inertial particles in fully-developed turbulent vertical pipe flow has been investigated (Reτ = 2,200, based on the friction velocity and the pipe diameter) using two approaches: large-eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) both employing Lagrangian tracking of a dilute suspension of particles (glass beads in air with different Stokes’ numbers, namely 0.022 and 2.8). Detailed numerical simulations are performed in order to: (a) assess the capabilities of these two approaches to match the experimental measurements of Arnason and Stock [1, 2]; and (b) validate the extension of the stochastic approach based on Langevin modeling used in a RANS framework to the generation of sub grid-scale fluctuating velocities as seen by solid particles in LES. Results for the particle dispersion coefficient and preferential distribution of particles in different sections of the vertical pipe as well as streamwise and radial particle velocities, are computed and compared to the results of the experimental measurements. The following conclusions are drawn. (a) Both RANS and LES, using stochastic modeling for the fluid velocity, are seen to predict reasonably well the dispersion of solid particles with different Stokes’ numbers in a high Reynolds number, nonhomogeneous and anisotropic turbulent flow. (b) The extension of stochastic modeling based on the Langevin equation to the construction of the subgrid-scale fluctuating velocity field as seen by the particles is successful; it contributes to the better results obtained, compared to RANS results, especially for those predicted for the small particles. (c) As shown in experimental results [1, 2] and demonstrated by theoretical studies [3, 4], the numerical predictions supported the conclusions that large inertia particles can disperse faster than small inertia particles, depending on the combined effects of inertia and drift parameters.
Study on the Reversible Pump-Turbine Closing Law and Field Test
Authors: Zhang Jian , Hu Jianyong , Hu Ming , Fang Jie , Chen Ning
Published: September 2008

In the operation of pumped storage plant, the sudden change of load in the large range often happen, the design of reversible pump-turbine emergency closing law should satisfy limit of rotation speed-rise and pressure-rise. The broken-line closing law of reversible pump-turbine usually used in pumped storage plants is discussed and based on full hydraulic characteristic of reversible pump-turbine, the broken-line closing law of three stages with delayed segment is designed, virtues and defects of the pump-turbine emergency closing law are also analyzed. Numerical calculation and field test indicate that water hammer in pumped storage plant water conveyance system caused by wicket gate closing together with rotation speed-rise, is effectively reduced by the closing laws presented above, especially to the latter, its turning point position can change following the different operating case, the detection of poor robustness is overcome. A new idea for the design of the reversible pump-turbine emergency closing law is proposed with good prospects.
A Hybrid Method for Simulating Flows Including Fluid Particles
Authors: Maeda Akiyoshi , Sou Akira , Tomiyama Akio
Published: September 2008

A hybrid CMFD (computational multi-fluid dynamics) method is proposed for the prediction of multiphase flows including large-scale interface, poly-dispersed bubbles and/or drops. The method is the hybrid integration of an interface tracking method (ITM), three kinds of particle tracking methods (PTM) and an averaging method based on a multi-fluid model (MFM). The integration enables us (1) to cover a wide range of d* = d/Δx, where d is the particle diameter and Δx the grid size, and (2) to perform various kinds of multiphase CFD such as standard interface tracking, particle tracking and multi-fluid simulations, and hybrid simulations using an arbitrary combination of ITM, PTM and MFM. The field and constitutive equations of the proposed method are described in detail. A poly-dispersed air-water bubbly flow and several bubble plumes in a small open vessel are simulated using the proposed hybrid method to demonstrate its potential.
Small Turbine Unit Replacing Reducing Valve in Water Supply Systems of Hydropower Plants
Authors: Zheng Yuan , Zhang Fei , Liu Dakai
Published: September 2008

This paper is on the selection of the type of a small turbine unit, which can replace the reducer and stabilize the operation in water supply systems of hydropower plants. In high-head large-sized plants, a small turbine unit can be adopted as pressure relief installation, converting the energy of water into electric power. At the same time, a stable water pressure can be achieved for the water system at the exit. In this paper, a hydropower plant is taken as an example. This scheme is suitable for the main water supply system and its operation is simpler than the pump system. The present study is of practical value to many high-head plants.
Experimental Study on Energy Characteristics of Axial-Flow Pumps
Authors: Zheng Yuan , Hu Ming , Xu Feng , Zhang Jingxuan
Published: September 2008

An experimental study has been performed on energy characteristics of axial-flow pumps designed for one pumping station in Jiangsu province, China. The energy characteristic tests for two different model runners were carried out. The result shows that the airfoil shape of the blade profile has significant effect on the performances of the pumps. Efficiency of the pumps can be improved by using smaller camber and larger blade density.
Numerical Jet Atomization: Part I — DNS Simulation of Primary Break Up
Authors: Me´nard T. , Beau P. A. , Tanguy S. , Demoulin F. X. , Berlemont A.
Published: September 2008

DNS simulations are carried out to obtain information in the dense zone of a spray where nearly no experimental data are available. Interface tracking is ensured by Level Set method, Ghost Fluid Method (GFM) is used to capture accurately sharp discontinuities for pressure, density and viscosity. Coupling between Level Set and VOF method is used for mass conservation. Fluid motion is predicted with a projection method for incompressible flows. The numerical methods are described and validations are presented. First results are then presented for 3D simulation of the primary break-up of a liquid jet with the Level Set-VOF-Ghost Fluid method and the results can help to determine closure relation in the ELSA model [Part II of the paper] which is based on a single-phase Eulerian model and on the transport equation for the mean liquid/gas interface density in turbulent flows.
Numerical Jet Atomization: Part II — Modeling Information and Comparison With DNS Results
Authors: Beau P. A. , Me´nard T. , Lebas R. , Berlemont A. , Tanguy S. , Demoulin F. X.
Published: September 2008

The main objective of our work is to develop direct numerical simulation tools for the primary break up of a jet. Results can help to determine closure relation in the ELSA model [1] which is based on a single-phase Eulerian model and on the transport equation for the mean liquid/gas interface density in turbulent flows. DNS simulations are carried out to obtain statistical information in the dense zone of the spray where nearly no experimental data are available. The numerical method should describe the interface motion precisely, handle jump conditions at the interface without artificial smoothing, and respect mass conservation. We develop a 3D code [2], where interface tracking is ensured by Level Set method, Ghost Fluid Method [3] is used to capture accurately sharp discontinuities, and coupling between Level Set and VOF methods is used for mass conservation [4]. Turbulent inflow boundary conditions are generated through correlated random velocities with a prescribed length scale. Specific care has been devoted to improve computing time with MPI parallelization. The numerical methods have been applied to investigate physical processes that are involved in the primary break up of an atomizing jet. The chosen configuration is close as possible of Diesel injection (Diameter D = 0.1 mm, Velocity = 100m/s, Liquid density = 696kg/m3, Gas density = 25kg/m3). Typical results will be presented. From the injector nozzle, the turbulence initiates some perturbations on the liquid surface, that are enhanced by the mean shear between the liquid jet and the surrounding air. The interface becomes very wrinkled and some break-up is initiated. The induced liquid parcels show a wide range of shapes. Statistics are carried out and results will be provided for liquid volume fraction, liquid/gas interface density, and turbulent correlations.
DNS of Particle Dispersion and Material Erosion in Gas-Solid Two-Phase Circular Cylinder Wakes
Authors: Luo K. , Fan J. R. , Cen K. F.
Published: September 2008

Particle dispersion and the resulting material erosion in a three-dimensional wake of circular cylinder were investigated by using direct numerical simulation (DNS). The domain decomposition method with patched grid and the high-order finite difference schemes were used to solve the flow. A new Lagrangian tracking solver was developed to trace the trajectories of particles in the non-uniform and unstructured grid. It is observed that particles at the smaller Stokes numbers can follow the vortex motion and have a relatively uniform distribution in the flow field. They usually collide with the downstream surface of the cylinder. The particles at the larger Stokes numbers tend to maintain their own motion and the particle-cylinder collision occurs in the upstream surface of the cylinder. But particles at the intermediate Stokes number of 1 are observed to assemble in the outer boundaries of the vortex structures and the thin band regions which connect two adjacent eddies. Due to the particular response characteristics, neither can these particles collide with the upstream surface, nor can they collide with the downstream surface of the cylinder. The increase of particle diameter leads to an exponential increase of the collision frequency and the material erosion. The local collision frequency is higher for each kind of particles when the inclination angle approximates zero. But the maximum local erosion happens when the inclination angle is between 30 degree and 60 degree. The larger the particle diameter is, the larger the inclination angle where the maximum erosion happens is. In addition, it is confirmed that the highest erosion ratio occurs when the collision angle between particles and cylinder is around 25 degree for the plastic material.
Numerical Study of Two-Dimensional Turbulent Jets
Authors: Abdel-Salam Tarek , Micklow Gerald , Williamson Keith
Published: September 2008

The current study reports numerical analysis of turbulent jets. Effects of various parameters on the characteristics of two-dimensional turbulent plane parallel and offset jets are investigated. The emphasis is put on the effect of the wall angle and nozzle width on the location merging and the combining points. The flowfield under consideration are two-parallel plane jets and offset jets issued from plane wall. Four angles and three values of the nozzle width are used. Also, different values of Reynolds number between 9000 and 39000 have been examined. It is noted that the wall angle and the nozzle width linearly affect the location of the merging and the combining points, while Reynolds number plays no role in their location. The effect of the wall angle on the reattachment point is found to be non linear.
Three-Dimensional Unsteady Turbulent Simulation and Analysis of Pressure Fluctuation in the Prototype Kaplan Turbine
Authors: Wu Shangfeng , Wu Yulin , Liu Shuhong
Published: September 2008

In recent years, stability research of the hydraulic turbine has been one of the most important problems of analysis of the large turbines. In this paper, the simulation model is based on the large prototype Kaplan turbine of the Shuikou hydroelectric station in Fujian Province, China. The three-dimensional unsteady turbulent flow is simulated through the whole flow passage of the turbine mainly by FLUENT, and pressure fluctuation of several points in the passage are measured including the front of the stay vane, runner and the front of the draft tube, meanwhile, the frequency and amplitude of the pressure fluctuation are gained by FFT transform, through that we can find the source of the pressure fluctuation, therefore we will discuss the occurrence reason and spread elements of the pressure fluctuation in the flow passage of the turbine.
An Experimental Investigation to Improve the Performance of Centrifugal Compressors Operating at Off-Design Conditions
Authors: Ahmed Saad A. , Mohammed Dhafar S.
Published: September 2008

Experiments to investigate the feasibility of controlling (delaying/or suppressing) the stall in the radial diffuser of a low speed centrifugal blower were carried out. The mass flow rate of air through the blower could be gradually reduced until stall inception is accomplished. The static and dynamic pressures in the diffuser were measured along a radial or a circumferential path using static and dynamic pressure transducers. The technique was very simple and involved changing the flow area at the diffuser exit with the use of throttle rings to either diffuser shroud, or the diffuser hub. Simultaneous attachment of the throttle rings to both the diffuser shroud and the hub was also made. The diffuser exit blockage ratio varied between 0% and 75% when throttle rings were attached to the diffuser walls to reduce its exit flow area. The results showed that the flow instability in the diffuser (stall) was delayed to a lower flow coefficient (the critical mass flow rate could be reduced to 55% of its value without the blockage). The experimental data also confirmed that the throttle rings could be an effective way to control the stall in the vaneless radial diffuser.
The Interaction of Downwind Sails
Authors: Lasher William C.
Published: September 2008

When a sailboat is sailing in the general direction of the wind, it is known as downwind sailing. Under these conditions boats generally carry two sails — a mainsail and a spinnaker. The flow over these sails is complicated and poorly understood. An issue of importance to sailors is how the adjustment of one sail affects the aerodynamics of the other sail and the total driving force. In the present work a CFD analysis was performed on two different sailboat rigs, with an emphasis on the interactions between the main and spinnaker. The results show that the optimum angle of attack of the mainsail is generally 60° to 70°, depending on the direction of the wind relative to the boat. The correct setting for a mainsail has been a point of debate amongst sailors, and the present analysis supports the theory of easing the mainsail until it luffs (or flaps), then pulling it in. A surprising result is that when the wind is directly behind the boat, the mainsail should not be set to maximize projected area, but eased out past 90°. This is difficult to accomplish in practice due to restrictions of the rig, but raise some interesting questions.
Numerical Simulation of Density Current Using Two Phase Flow
Authors: Afshin H. , Firoozabadi B.
Published: September 2008

Due to shear layer at the interface of density current and ambient fluid, density current disturbs and entrains the surrounding fluid. Most existing analytical and numerical models for density current flows are based on the equations for single-phase flows. In this research, the density current has been modeled with two-phase flow model. The governing equations are continuity, x- momentum, and y- momentum equations for every fluid. The volume-of-fluid (VOF) interface tracking technique which uses a piecewise-linear interface calculation (PLIC) in each cell is used to determine the deformation of free surface in density current, numerically. Surface tension is implemented by the continuous surface stress (CSS), and continuous surface force (CSF) methods. Finally, results of these two models are compared. The model has been verified with the experimental data sets. The Results show that the model can be applied to experimental cases and has acceptable level of accuracy.
Effects of Particle-Particle Collisions on Particle Concentration in a Turbulent Channel Flow
Authors: Nasr H. , Ahmadi G. , McLaughlin J. B.
Published: September 2008

This study is concerned with the effect of inter particle collisions on the particle concentration in turbulent duct flows. The time history of the instantaneous turbulent velocity vector was generated by the two-way coupled direct numerical simulation (DNS) of the Navier-Stokes equation via a pseudospectral method. The particle equation of motion included the Stokes drag, the Saffman lift, and the gravitational forces. The effect of particles on the flow is included in the analysis via a feedback force on the grid points. Several simulations for three classes of particles (28 μm Lycopodia, 50μm glass and 70μm copper) and different mass loadings were performed, and the effect of inter particle collisions on the particle concentration was evaluated and discussed. It was found that the particle-particle collisions reduce the tendency of particles to accumulate near the wall. This might be because collisions decorrelate particles with coherent eddies which are responsible for accumulation of particles near the wall. The spatial distribution of particles at the channel centerplane was compared with the experimental results of Fessler et al. (1994). The simulation results showed that the copper and glass particles had a random distribution while Lycopodium particles showed a non-random distribution with bands of particles that were preferentially concentrated.
Numerical Investigation and Optimization of a Stator for a Radial Submersible Pump Stage With Minimum Stage Diameter
Authors: Roclawski H. , Weiten A. , Hellmann D.-H.
Published: September 2008

For radial multistage pumps with stationary vaned diffusers the diameter of the pump casing is much larger than the diameter of the impeller. In the case of submersible pumps, where just a limited space for installing the pump is available, smaller stator diameters need to be realized resulting in a loss in efficiency. Improving the design of these pumps and in order to save space and to decrease costs, alternative stators are being developed at the Institute of Turbomachinery and Fluid mechanics at University of Kaiserslautern. With respect to space, the optimum is a pump stage with a stage diameter equal to the outer impeller diameter. For realizing this concept, it is necessary for the flow to exit the radial impeller in axial direction before entering the stator. The advantage of this design is that when keeping the stage diameter constant, a much larger impeller diameter and thus a much higher hydraulic head can be realized. On the other hand, when keeping the hydraulic head or specific speed constant, the diameter of the pump casing can be much smaller. In this paper, the numerical calculation of the flow through the pump stage with minimum stage diameter is presented. Fully transient calculations with the sliding mesh method are performed. The numerical model consisting of the complete impeller with side gaps and stator is validated by measurements of characteristic curves and hot-wire data. After discussing the deficit of the initial stator layout, an optimized stator with a pump efficiency comparable to a conventional stator is presented. All results are compared to characteristic curves of a pump stage with conventional stator.
Effects of Geometrical Parameters on Improving the Transversal Mass Transport in PEM Fuel Cells
Authors: Zhao Yanxia , Mei Renwei , Klausner James F.
Published: September 2008

A computational model using Lattice Boltzmann Equation (LBE) method is employed to investigate the fluid transport on the anode side of Polymer Electrolyte Membrane (PEM) fuel cells, with an emphasis on mass transfer enhancement. A 3-dimensional LBE code is developed to solve the flows in the channel and the porous media in the gas diffusion layer (GDL) simultaneously. Multiple flow enhancers (obstructions in the flow channel) are placed in the channel to enhance the transversal flow across the GDL. The mass flow rate, the velocity field and the pressure distribution are analyzed. The effects of flow enhancers are assessed. The results show that the transversal flow across the GDL is enhanced by placing flow enhancers in the channel. Increasing flow enhancer size can significantly increase the transversal flow rate, with high pressure-loss through the flow channel. The results also demonstrate that the location of flow enhancers in the flow channel have a remarkable impact on the transversal flow rate. The transversal flow rate increases as the GDL porosity increases.