bluuebunny/crossref_metadata_2025 huggingface dataset:
Particulate Aggregate Formation and Wall Adhesion in Microchannel Flows
Authors: Marshall J. S.
Published: September 2008

A multiple-time step discrete-element approach is presented for efficient computational modeling of the transport, collision and adhesion of small particles in microchannel flows. Adhesive particulates have been identified as a leading cause of failure in many different microfluidic devices, including those currently being developed by different research groups for rapid biological and chemical contaminant sensing, fluid drag reduction, etc. As these microfluidic devices enter into the marketplace and become more extensively used in field conditions, the importance of particle adhesion and clogging will increasingly limit the reliability of such systems. At a larger scale, clogging of vehicle radiators by small adhesive particles is currently a major problem for construction vehicles operating in certain environmental conditions and certain soil types. Cooling system fouling leads to the need for frequent maintenance and machine down time. Dust fouling of equipment is also of concern for potential human occupation on dusty planets, such as Mars. The discrete-element method presented in this paper is developed to enable efficient prediction of aggregate structure and breakup, for prediction of the effect of aggregate formation on the bulk fluid flow, and for prediction of the effects of small-scale flow features (e.g., due to surface roughness or lithographic patterning) on the aggregate formation and particle deposition. We present an overview of the computational structure and modeling assumptions, including models for various forces and torques present during particle-particle collisions. We then utilize the computational method to examine the physical processes involved in aggregate formation and capture of particulate aggregates by walls in microchannel flows.
Active Laminar Mixing Induced by Surface Disturbance
Authors: Williams Alicia , Akle Barbar , Vlachos Pavlos
Published: September 2008

Traditionally, passive mixers and chaotic advection have played a key role in the field of microfluidics by stretching and folding the fluid. Some examples of these passive mixing techniques include channels with riblets or grooves on the walls, or serpentine curved channels. In this work, we explore an actuated wall for use as a closed-chamber micromixer. Although active mixers for low Reynolds number flows have been previously studied, a significant portion of that work has involved piezoelectrics. This work presents a novel polymer that offers many advantages over conventional piezoelectric stacks, including very small thickness and inexpensive construction cost. Moreover, these actuators are water-based, eliminating the need for coating or electrical isolation. The polymer used to create this moving boundary or “active skin” is a flexible surface that deforms mechanically in response to an electrical signal. This was accomplished with a polymer that was approximately 560 mm2 divided into nine sections. Every other section of the polymer was wired to actuate simultaneously under the influence of an applied voltage cycling between 1 and 70 hertz with a 2 volt amplitude. The flow over the active skin was measured via TRDPIV (Time Resolved Digital Particle Image Velocimetry) using an Nd:YAG laser. The result of this research indicates that flow stirring is generated by the use of an active skin. Moreover, even though the deflection of the polymer is on the order of microns, its influence extends to the entire area of the channel studied in this work (approximately 13 mm height and 43 mm length).
Magnetic Targeting of Particle Transport Under Pulsatile Flow
Authors: Williams Alicia , Sinha Ashok , Vlachos Pavlos , Puri Ishwar K.
Published: September 2008

Magnetic Drug Targeting (MDT) has been shown to be a promising technique to effectively deliver medicinal drugs via functionalized [1] magnetic particles to target sites during the treatment of cancer and other diseases [2,3,4]. In this paper, we investigate the interaction of steady and pulsatile flows injected with a ferrofluid, which is a colloidal suspension of superparamagnetic nanoparticles in a glass tube under the influence of a magnetic field. Ferrofluids are colloidal suspensions of single domain magnetic nanoparticles that are of the order of 10 nm in diameter. In this experiment, the ferrofluid particles were directed to a particular region of interest within a 10 mm diameter glass vessel by means of an applied localized magnetic field that originated outside of the vessel. The magnetic field was generated using a rare earth sintered permanent magnet which produced the magnetic field gradient required for inducing a body force on the volume of the ferrofluid. The experimental results reveal flows with rich dynamical phenomena. The aggregation of the ferrofluid produces a self-assembled hemispherical structure which dynamically interacts with the host flow. The aggregation generates an occlusion creating a flow field that is similar to that past an obstruction. However, since the structure itself is of a fluidic nature, it is subject to shear forces caused by the host fluid. In addition, the wake of the flow behind the aggregation creates vortices which are critical to study the stability of the ferrofluid aggregate. This paper presents a detailed investigation of the dynamics of the flow using Time-Resolved Digital Particle Image Velocimetry. To the best of the authors’ knowledge, these are the first quantitative, spatiotemporally resolved measurements documenting the interaction of a host fluid with a ferrofluid aggregate under steady or pulsatile flow conditions.
Variations in the Flow Field of a Pulse Detonation Engine for Different Operating Conditions
Authors: Raghupathy Arun Prakash , Ghia Urmila , Ghia Karman
Published: September 2008

Pulse Detonation Engine (PDE) is considered to be the propulsion system of future air and space vehicles because of its low cost, light weight, and high performance. Hybrid PDE is a relatively new concept where a turbine is integrated with a PDE. This hybrid system is expected to operate under fuel-rich conditions during take-off (stoichiometric), and fuel-lean (φ = 0.44) conditions during cruise. Hence, the objective of the present study is to simulate the external flow field of a stand alone PDE system and study its variation during the above mentioned operating conditions. In order to study Hybrid PDE systems, the underlying concept of the working of a stand alone PDE, namely, detonation, has to be simulated first. For this purpose, the one-dimensional reactive Euler equations are solved. Since a propagating detonation wave is the result of chemical reactions in a very small region, flow adaptive grids are used for the one dimensional detonation simulations. The global chemical mechanisms employed predicted all the detonation quantities for both stoichiometric and lean mixture of hydrogen-air with the least error. The results from the global chemical mechanism for both mixtures are used in the two-dimensional PDE simulations. Analyses of the axial pressure and temperature distribution in the external flow field show the nature of the blowdown process and its variation for different operating conditions. Flow exergy analysis shows that there is 25% loss in available work when a turbine is placed at one tube length away from the exit of the PDE. One of the important outcomes of this study is the information that can guide in the placement of the turbine downstream of the PDE to achieve lower blowdown time.
Analysis of the Performance of Linear and Non-Linear k-ε-ζ-f Turbulence Model on Various Applications
Authors: Basara Branislav
Published: September 2008

The present paper proposes a new ‘default’ turbulence model for general purpose Computational Fluid Dynamics (CFD). The four equations k-ε-ζ-f turbulence model of Hanjalic et al. (2004) is assessed on various test cases. There is no significant penalty on computing time and robustness when compared with the present standard industrial model, namely the standard k-ε model. The k-ε-ζ-f model is applied in conjunction with the universal wall treatment which combines the integration up to the wall with wall functions. Such a solution is the optimum for any computational mesh. The original model uses the linear stress-strain relationship but here, the assessment is also made for the non-linear k-ε-ζ-f model. The key application in this paper is the tangential dual-intake port and for that case, the calculations are also performed with the full Reynolds-stress model in order to provide an objective view on the k-ε-ζ-f model’s performances.
Technique for Designing Forward Curved Bladed Fans Using Computational Fluid Dynamics and Numerical Optimization
Authors: Iwase Taku , Sugimura Kazuyuki , Shimada Ryuuichi
Published: September 2008

We developed a technique for designing forward curved bladed fans using computational fluid dynamics (CFD) and numerical optimization. The target is a forward curved bladed fan including an impeller with blades and a volute casing. In our research, we developed a two-step calculation method with a blade-to-blade grid model and a full grid model. Using these models individually according to purpose, we reduced the design time by one quarter. An automatic grid-generation program that was developed in-house generated the grids for the CFD calculation. The fan performance was calculated using commercial CFD software based on an incompressible Reynolds-averaged Navier-Stokes (RANS) solver. For numerical optimization, we used a simulated annealing algorithm (SA) to determine the optimized design variables. Using the developed technique, we attempted to minimize the total pressure loss of an impeller and a suction cone. We could obtain the optimized design variables: the gap between the impeller and suction cone, the inside diameter of the shroud rim cover and the number of blades. Our results demonstrated that the optimized fan design had smaller shaft power than the initial design, especially at the low flow rate. Clearly therefore, our technique is capable of designing an energy saving fan in a short time. Moreover, it was found that that the leak flow between the impeller and suction cone of the optimized fan was suppressed. The change in these design variables contributed to this suppression.
Design, Fabrication and Characterization of a Fully Integrated Micro Vapor-Jet Vacuum Pump
Authors: Doms Marco , Mu¨ller Jo¨rg
Published: September 2008

A MEMS vapor-jet pump for vacuum generation in miniaturized analytical systems, e.g. micro mass-spectrometers [1, 2], is presented. A high velocity gas- or vapor-jet is used for vacuum generation. Starting from atmospheric pressure, a high throughput of more than 23 ml/min and an absolute pressure of 495 mbar were obtained with this new type of micropump. An approach for the full integration of all components of the pump is presented, validated by experimental results. The pump is fabricated from silicon and glass substrates using standard MEMS fabrication techniques including DRIE, trichlorosilane MVD and metal-assisted chemical etching for porous silicon fabrication. Micromachined pressure sensors based on the Pirani principle have been developed and integrated into the pump for monitoring.
Mixing in a T-Shaped Microreactor: Mechanisms and Scales
Authors: Bothe Dieter , Stemich Carsten , Warnecke Hans-Joachim
Published: September 2008

The large area-to-volume ratio of microreactors gives prospect of better yield and selectivity than for conventional designs, since diffusive fluxes of mass and heat in micro-devices scale with area, while the rate of changes corresponding to sources and sinks are proportional to volume. Indeed, theoretical considerations of the scaling behavior [1] support the fact that micro-reactors allow for faster chemical reactions and provide better thermal control. Moreover, specific applications prove that these advantages of micro-reactors can be realized in order to perform fast exothermic reactions [2–4] and to enhance selectivity [5]. For such applications, the mixing of chemical species is of special interest, since it is an essential condition for chemical reactions. To obtain efficient mixing for the short residence times in micro-systems, the contact area between regions of higher and lower species concentration has to be increased significantly. To avoid large pressure drops, secondary flows instead of turbulent flow fields are preferred. In case of a T-shaped micro-mixer, the secondary flow acts mainly in cross directions, i.e. perpendicular to the axial direction, and can be used to mix the two feed streams. To assess and optimize the mixing process, this qualitative picture has to be understood more thoroughly and significant quantitative information has to be added. In particular, the interplay of convective and diffusive transport to bridge the gap between the reactor and the molecular scale, have to be further investigated, since even if no new physico-chemical phenomena occur, new aspects enter the picture in case of micro-systems; [6] and the references given there. The present contribution employs CFD-simulations to obtain first steps in this direction.
Effects of Roughness on Liquid Flow Behavior in Ducts
Authors: Zou Jiang , Peng Xiao-Feng
Published: September 2008

In this paper, liquid laminar flow friction in micro/mini ducts is considered for investigating the effects of roughness. The available research in this field is briefly reviewed, and the results are comprehensively discussed. The flow behavior is theoretically analyzed in the region adjacent to rough wall, and one of roughness effects is considered equivalent to the reduction of flow cross area, which is referred to the constricted flow model. A modified formula and coefficient η are introduced to correct the reduction value of hydraulic diameter. Two important factors, the space of two neighboring roughness elements and the reattachment length, are involved in accounting for the flow cross-section reduction. Eventually, an expression of flow resistance calculation is reduced in terms of relative roughness ε/d, parameter A and Reynolds number Re. Based on the ultimate friction factor formula, the influences of relative roughness ε/d, parameter A and Reynolds number Re on friction coefficient f are discussed by figuring and comparing f variation with those parameters.
A Chemical Probe for Characterising Turbulent Micromixing
Authors: Durandal C. , Lemenand T. , Della Valle D. , Peerhossaini H.
Published: September 2008

Micromixing strongly affects the molecular-scale contact of chemical species in numerous chemical reactions. Understanding the mechanisms governing micromixing lets us explain the relationships between the turbulence structure and the selectivity of the reaction. The micromixing models in the literature for large Schmidt numbers (Sc∼1000) are of two sorts: stochastic models (principally the Curl model) and deterministic models (such as IEM and the “engulfment” model). These models are essentially based on phenomenological considerations and on the properties of small-scale eddies related to the micromixing time. Micromixing can be quantified locally by analysing the products of a local chemical reaction if this reaction is “fast” compared to the micromixing time. This so-called chemical probe method is used in the present work, following the Villermaux method [1] on the [iodide/iodate] chemical reaction. A numerical study is carried out here to compare the different micromixing models with the experimental results of Fournier [2] in a stirred tank. In order to compute the concentrations, mass transfer equations including a reactive term are solved by a pdf model using a Monte-Carlo method. A benchmark proposed here for the micromixing models shows that the engulfment model is the more efficient in evaluating the local power dissipation means of the chemical probe.
Discrete Bubble Modeling of Unsteady Cavitating Flow
Authors: Xu Zhiliang , Samulyak Roman , Glimm James , Li Xiaolin
Published: September 2008

A discrete vapor bubble model is developed to simulate the unsteady cavitating flows. The mixed vapor-liquid mixture is modeled as a system of pure phase domains (vapor and liquid) separated by free interfaces. On the phase boundary, a numerical solution for the phase transition is developed for compressible flows.
Rapid Computations to Match Impellers With Inducers
Authors: Kaupert Kevin
Published: September 2008

Simplifying assumption are examined to quicken the fluid dynamic design of centrifugal pump impellers when an upstream inducer is applied. First a complete inducer with a complete impeller is computed. Second a more rapid calculation method is investigated which utilizes only one inducer passage and one impeller passage. Pitchwise averaging is performed at their interface. A third and even more rapid approach replaces the entire inducer with simulated velocity and pressure boundary conditions upstream of a single impeller passage. In all 3 computed cases a vaned channel diffuser is included at the impeller outlet. The 3 computed cases are compared to the measured pump pressure and efficiency characteristic. In terms of computational intensity the complete calculation presented here took 5 days to generate a 9 point characteristic curve. For the same number of points the averaged case took 2.5 days while the simulated case took 1.5 days to compute. In terms of the results the 3rd simulated method is reasonable until the final centrifugal impeller design phase is reached, when more accurate calculations are required.
Effects of Slip on the Flow Characteristics of Laminar Flat Plate Boundary-Layer
Authors: Vedantam Nanda K. , Parthasarathy Ramkumar N.
Published: September 2008

A uniform stream of viscous fluid flowing past a flat plate with slip at the fluid-plate interface is considered. The aim of this investigation is to examine the effects of slip at the wall on the laminar boundary-layer flow characteristics. For this purpose, an analytical and a computational study were conducted. The analytical study is based on the Blasius similarity solution for laminar boundary-layer flow past a flat plate. In the computational study, the flow over a flat plate is modeled and solved (using FLUENT) by dividing the computational domain into small control volumes and discretizing and solving the governing equations around these control volumes. The slip at the fluid-solid interface is accounted for by the Navier boundary condition (NBC). Three cases are considered, wherein the slip boundary condition is incorporated in three different ways, i.e., by specifying the slip length, or the slip velocity, or by assuming the slip length to be a function of shear rate. The flow characteristics are evaluated for different amounts of slip. The wall shear stress, the skin-friction coefficient, and the drag coefficient decrease by about 38% when the nondimensional slip length is increased from zero (noslip) to 2. The boundary-layer thickness, the displacement thickness, and the momentum thickness also decrease with increase in slip. The reduction in displacement thickness is much greater (about 68%) than the boundary-layer thickness or the momentum thickness. An excellent agreement between the analytical and computational results is noticed.
Measurement and Analysis on the Velocity Field of Tubular Pump Unit Flow Passage
Authors: Zhou Daqing , Liu Shuhong , Wu Yulin , Zheng Yuan , Jiang Xiaoxin
Published: September 2008

Being required to have the functions of both pumping and drainage, some pumping stations are suggested to adopt two-way standard tube tubular pump unit, which also meets the needs of low heads and large discharge condition. In the paper, the velocity field of flow passage among the passage of tubular pump unit is measured by micro 5-hole probes system, in order to provide experimental evidences for improving the pump unit performance further. Experimental results show that axial velocity become larger from the wall to the center of inlet passage; however, axial velocity in the outlet passage center is less than that near the wall. At the same time, the uniformity of axial velocity in the outlet passage also turns badly. It is also found that transverse velocity of inlet passage is small and it becomes larger in the outlet passage because of the outlet circulation. The axial velocity of curved pipe becomes less from the inside to the outside, which is caused by centrifugal effort. The above data can make for both reference to design perfect tubular pump unit and comparison with CFD results.
Quantification of Particle and Fluid Scales in Particle-Laden Turbulent Channel Flow
Authors: Marchioli Cristian , Picciotto Maurizio , Soldati Alfredo
Published: September 2008

In this work, we study the dispersion of inertial particles in fully-developed turbulent channel flow to evaluate the relationship between particle and fluid time scales, and to identify suitable scales for parametrization of near-wall particle behavior. Direct Numerical Simulation (DNS) and Lagrangian particle tracking are used to build a complete and homogeneous dataset which covers a large target parameter space and includes statistics of particle velocity and particle concentration at steady state. Our results show that the Lagrangian integral time scale of the fluid is adequate to characterize particle wall deposition and that such fluid time scale will be different when sampled at the position of either fluid particles or inertial particles. Differences become particularly evident in the range 5 < St < 25. These observations can be crucial to improve the accuracy of engineering models for particle deposition.
ELSA Model for Atomization: To Benefit of the Eulerian and Lagrangian Descriptions of the Liquid Phase
Authors: Lebas Romain , Beau Pierre-Arnaud , Blokkeel Gre´gory , Demoulin Franc¸ois-Xavier
Published: September 2008

In the context of the Euler-Lagrange Spray Atomization (ELSA) model [1,2,3] for two-phase flows, a Eulerian approach is used to describe the dense part of the spray and it is completed elsewhere by a standard Lagrangian approach. A recurrent issue of the Lagrangian approach is the difficulty to reach a number of stochastic samples large enough, in each mesh cell, to achieve statistical convergence. It is necessary to propose methods that can benefit of the Lagrangian formulation to describe the statistical dispersion but that are converged for certain key quantities like the mass of liquid. Such ideas are already developed under the so-called Direct Quadrature Method Of Moments (DQMOM) [4]. This complex method is not directly used here, a more practical approach that used the usual formalism of Lagrangian methods for spray is proposed. The method will take into account the originally unused Eulerian equation in the diluted part of the spray to transport the key mean quantities of the spray. In each mesh cell that contains Lagrangian stochastic particles the liquid mass fraction can be obtained from the Eulerian equation. Equivalently, in the mesh cell considered, a Lagrangian liquid mass fraction is defined. It is clear that big statistical fluctuations of this variable can be expected if the number of particles in the cell is not high enough. The least noisy Eulerian variable will be used to correct the Lagrangian one. There are numerous possible ways to correct the Lagrangian variable, one method will be presented. In the same manner, the Eulerian equation for the density of liquid surface is used to correct the Sauter mean diameter that can be obtained from the Lagrangian description. Hence, the Lagrangian phase is linked to the Eulerian equations. But the Eulerian equations have to be linked also to the Lagrangian phase, this is achieved through the liquid turbulent diffusion flux closure and through the source terms of surface density transport equation, i.e. those modeling break-up and coalescence effects. By this way, the Lagrangian phase, whenever it is available, enables to take into account the joint probability density functions of fluctuating variables such as droplet diameter or droplet velocities.
A Novel Computational Approach for Modeling Stent Reconstructing of an Aortic Bifurcation
Authors: Miller Ross , Battaglia Francine
Published: September 2008

Stent reconstruction of the aorta-iliac bifurcation was studied using computational fluid dynamics. Stents were modeled using a novel method, whereby the stents were represented as porous media. Three-dimensional numerical simulations using FLUENT were performed to determine how stent orientation affects the fluid dynamics. Three cases were studied and compared for both iliacs unstented, one iliac stented, and both iliacs stented. The stents lowered wall shear stress along the stented artery walls as compared to the unstented aorta-iliac model. However, the stent presence elevated vorticity, both in magnitude and size of the region. Additional studies were conducted to determine the effects for stent mis-alignment, where one stent protruded more into the aorta. It was found that when the stents were misaligned, wall shear stress increased near the stent inlet for the stent further inserted into the iliac. The resulting flow phenomena were consistent with other numerical models and medical investigations of stent reconstruction.
Drop Motion Simulation on a Surface Due to a Wettability Gradient
Authors: Nasr H. , Ahmadi G. , McLaughlin J. B. , Jia X.
Published: September 2008

This study is concerned with the numerical simulation of a drop moving on a surface due to a wettability gradient. The wettability gradient is introduced by varying the static contact angle along the surface. The unsteady laminar Navier-Stokes equation is solved in each phase using a fixed Eulerian unstructured grid. The Volume of Fluid Model (VOF) is used to account for tracking the gas/liquid interface. A water drop was placed on a solid surface with a linear contact angle distribution, and the simulation results for the drop velocity were compared with the analytical results of Brochard (1989).
Numerical Simulation of Heterogeneous Bubbly Flow in a Bubble Column
Authors: Maekawa Munenori , Shimada Naoki , Kinoshita Kouji , Sou Akira , Tomiyama Akio
Published: September 2008

Numerical methods for predicting heterogeneous bubbly flows are indispensable for the design of a Fisher-Tropsh reactor for GTL (Gas To Liquid). It is necessary to take into account bubble size distribution determined by bubble coalescence and breakup for the accurate prediction of heterogeneous bubbly flows. Hence we implemented several bubble coalescence and breakup models into the (N+2) field model, which is a hybrid combination of an interface tracking method and a multi-fluid model. Void and bubble size distributions in an open rectangular bubble column were measured and compared with predicted ones. As a result, the following conclusions were obtained: (1) Void and bubble size distributions were not affected by inlet bubble sizes because the bubble size distribution reaches an equilibrium state at which the birth rate is equal to the death rate, and (2) the combination of Luo’s bubble breakup model and a coalescence model consisting of Prince & Blanch’s model and Wang’s wake entrainment model gave good predictions.
Velocity and Pressure Measurements Along a Row of Confined Cylinders
Authors: Smith Barton L. , Stepan Jack J. , McEligot Donald M.
Published: September 2008

The results of flow experiments performed in a cylinder array designed to mimic a VHTR Nuclear Plant lower plenum design are presented. Pressure drop and velocity field measurements were made. Based on these measurements, five regimes of behavior are identified that are found to depend on Reynolds number. It is found that the recirculation region behind the cylinders is shorter than that of half cylinders placed on the wall representing the symmetry plane. Unlike a single cylinder, the separation point is found to always be on the rear of the cylinders, even at very low Reynolds number. Boundary layer transition is found to occur at much lower Reynolds numbers than previously reported.
Fluorescent Particle Injection Technique for Two-Phase Flow Measurement Using Particle Image Velocimetry
Authors: O’Halloran Steven P. , Beck B. Terry , Hosni Mohammad H. , Eckels Steven J.
Published: September 2008

Particle image velocimetry (PIV) is a well established measurement technique to measure velocity in a variety of different fluids. Using PIV to measure single-phase flow is well established, but recently PIV has been used to measure two-phase flows as well. Most two-phase PIV measurements have been for dispersed or bubbly flows, often utilizing the bubbles or droplets as PIV seed particles. However, there are other types of two-phase flow situations, such as stratified or slug flow, in which PIV measurement techniques are not yet well established. Situations such as these require both liquid and gas phases to be seeded separately with particles that can distinguish each phase. A particle injection method is presented for the air phase of a two-phase system using fluorescent tracer particles. Information about the system, including details of the fluorescent particles and injection device are given. The device injects micron sized fluorescent particles at a uniform rate into the flow of interest. A cut-off lens filter on the PIV camera is used to distinguish the fluorescent particles used for the air phase from non-fluorescent particles used in the liquid phase. Results using the technique with a two-phase air/water system in a thin rectangular channel for stratified/wavy flow are given. The channel is enclosed in a clear acrylic plastic tank and the dimensions of the channel are 600 mm long, 40 mm high, and 15 mm wide. The results demonstrate the ability to use PIV to measure the gas phase of a two-phase system for stratified/wavy flow and the method could be extended to other two-phase flow regimes as well.
Research on the Field Test of Desilting and Bank-Strengthening for the Yellow River
Authors: Chen Yanqiang , Luo Xianwu , Xu Hongyuan , Shi Wei
Published: September 2008

Due to the large amount of sand in the Yellow River water, the sediment accumulation of the sand in the lower reaches of the river has been increasing for years. In order to control the Yellow River flood, the height of the stream channel of this river should be decreased, and the banks have to be widened and its height should be increased. Desilting and bank-strengthening is an effective path to reinforce the banks of the Yellow River by taking advantages of the silt in the river appropriately. In the Desilting and bank-strengthening project, owing to the long distance of transportation of the sand, three parallel submersible liquid-solid pumps are operated together with one or two slurry pumps in series. This paper treats the field test of the Yellow River sand transportation system. Based on the on-site experiment, the relationship between the transportation efficiency and sand properties, the resistance of pipeline, etc has been investigated. It is noted that the amount of sand increased the resistance of pipeline, and sand concentration had obvious effect on transportation efficiency. With the same flow-rate, the larger sand concentration is, the less time is needed to transport the same amount of sand, and the less the cost of both power consumption and the consumption water would be; With the same sand concentration, the larger the flow-rate is, the larger the production output of sand transportation and economic profits would be achieved during the same operation period. Thus, the transportation system should be recommended to run in the condition of as large flow-rate and as high concentration as possible so that the working time and the cost of production would be reduced, and better economic performance was available.
Parameter Selection of Fluid Dynamic Sealing for a Submersible Slurry Pump
Authors: Ran Hongjuan , Luo Xianwu , Xu Hongyuan , Luo Wen , Peng Junqi
Published: September 2008

In the case of slurry pumps, mechanical seal is usually used. It is well known that the pump is very dangerous if any solid particle enters the seal. Nowadays, people prefer to install a fluid dynamic sealing including 1 or 2 sets of minor vanes and an auxiliary impeller rather than using high-pressure cleaning water in a slurry pump. However, if a fluid dynamic sealing is not designed properly, the mechanical seal is apt to be destroyed by the invading solid particle, and the slurry pump axes is worn seriously and rapidly. Though there are few empirical methods for designing a fluid dynamic sealing, the definite design concept is much deficiency. In the present work, the geometry selection of the fluid dynamic sealing for a submersible slurry pump is carried out so as to establish the basic principle for a fluid dynamic sealing design. The model pump is of centrifugal type, and has the specific speed of 136 m·m3/min·rpm. In order to validate the design concept, three dimensional turbulent flow in the pump was simulated. The total pump channel including inlet pipe, impeller with attached minor vane set, volute casing, and auxiliary impeller is treated as the calculation domain. The mesh grid is more than 2,000,000 nodes. The numerical simulation has been conducted by using a commercial code Fluent v6.1.2. To see the effect of minor vanes and auxiliary impeller geometrical parameters on the sealing performance, we change vane number, vane height, and radius of the minor vanes and auxiliary impeller, flow-rate of sealing water, etc. Based on the numerical results, a set of favorable geometrical parameters of the fluid dynamic sealing was selected for a submersible slurry pump. It is noted that the pump with the newly designed fluid dynamic sealing had been successfully applied for actual operation in the Yellow River sand transportation system in Shandong province, China.
Unsteady Flow Analysis of the Vertical Axis Cross-Flow Wind Turbine
Authors: Ejiri E. , Yabe S. , Hase S. , Ogiwara M.
Published: September 2008

Flow through the vertical axis cross-flow wind turbine was analyzed using computational fluid dynamics (CFD) to clarify current aerodynamic issues and to propose an improved design configuration for achieving better performance. The computed torque coefficients and power coefficients of a reference cross-flow wind turbine runner were compared with the experimental results. Flow around each blade of the turbine runner was then investigated based on the computed flow results. As a countermeasure to the issues found, a new wind turbine design was devised which has two guide vanes point-symmetrically arranged outside the turbine runner. It was experimentally shown that this improved design with the guide vanes increased turbine efficiency. However, performance predictions by CFD lack sufficient accuracy in the case of the turbine runner with the guide vanes, where complexity and unsteadiness prevail over the entire flow fields.
Different Measurement Techniques to Determine Hole to Hole Dispersion in a Real Diesel Injector
Authors: Desantes Jose Mari´a , Payri Rau´l , Salvador Francisco Javier , Gimeno Jaime
Published: September 2008

In the present paper four experimental techniques are used to determine hole to hole dispersion in Diesel injector nozzles. The four methodologies used in the present work are the silicone technique to determine the real internal geometry, mass flow rate measurements of each orifice against ambient pressure, spray momentum flux measured at a constant distance from the nozzle orifice and Spray visualization in high pressure nitrogen ambient conditions. Generally, all these techniques can be used in order to determine the hole to hole dispersion due to different angle inclination of the orifices, different internal geometry, fouling, nozzle needle off-center, etc. The objective of this article is to apply the four methodologies to a real VCO nozzle used for a two valve engine equipped with a common rail fuel injection system.
Aerothermal Characteristics of a Mixing Layer on a Multiperforated Plate
Authors: Messaadi A. , Dorignac E. , Fe´not M. , Vullierme J. J.
Published: September 2008

In the aeronautics field, engine manufacturers are continually confronted with the cooling problems associated with walls in contact with overheating gases. In such situations, full coverage film cooling techniques are commonly used to protect aeronautic engine walls. For example, the first stage of gas turbine rotor or stator as well as the combustion wall chamber need to be protected from the ever-increasing temperature of combustion products. The present experimental work is an attempt to both better understand the behaviour of the flow obtained on several rows of 30° inclined staggered jets and to control heat transfers. When the ambient temperature injected flow meets the hot main flow (its temperature ranges from ambient temperature to 60°C), it should protect the wall from the main flow. The wall is heated by electric dissipated power and maintained at constant heat flux. The number of injection rows may vary from 0 to 9. So, we can observe cold layer formation in function of the successive opening rows. Experiments are carried out for different injection rates (M = 3 to 5) as well. Injection flow velocity is maintained constant and equal to 7.5m/s. Association of temperature profile measurement by thermocouples and wall temperature measurement with infrared thermography allows us to describe the behaviour of the flows and to propose the injection leading to the most satisfactory cooling of the plate. Using wall temperature measurement with infrared thermography, we write the heat balance at the wall and present the results in form of heat transfer coefficient h variations and film cooling effectiveness η variations. The analysis of these results suggests an optimal number of injection rows and may enable researchers to determine the topology of the mixing layer and to thereby more effectively control their use of wall cooling techniques.
The Study on the Flow in a Horizontal Pump by 3D-PIV Experiments
Authors: Li Yong , Pan Hailin , Wei Yanming , Liu Shuhong , Wu Yulin
Published: September 2008

In order to investigate the flow conditions in a horizontal open pump sump, and to validate chest wall with different angles on preventing the vortex, and to find the key factor to the entrained vortex generating from the free surface, 3D PIV experiments were carried out in the pump sump. In the research, 3D PIV system was used to measure the flow near the free surface in the sump with different chest wall with angle of 0° and 30°. In the research, the refractivity-revised method was applied to get clear images in different media. In the method, an accessory device was used to eliminate the refractive effect between air and water. In the experiments, the multi-step calibrated method was applied to improve the measurement precision of the W vector. For each kind of chest wall, the experiments were performed under three operation conditions with different flow rates. From the experiments, the 3D flow conditions on the measuring plane can be obtained, such as velocity, streamline, and W velocity component. Analyzing those results, it can be concluded that the chest wall with angle of 30° can improve the flow conditions obviously and weaken the entrained vortex flow in the sump. Analyzing the experimental results, it can be found that the entrained vortex are more depended on ratio of the W velocity to the total velocity than other factors.
Dispersion and Deposition of Particles in Rayleigh-Be´nard Turbulent Flows
Authors: Oresta Paolo , Lippolis Antonio , Verzicco Roberto , Soldati Alfredo
Published: September 2008

In this paper, Direct Numerical Simulation (DNS) and Lagrangian Particle Tracking are used to investigate dispersion and deposition of particles swarms in convective flow confined in a cylindrical domain with aspect ratio (diameter over height) 0.5. The numerical simulations are carried out with Prandtl and Rayleigh numbers respectively equal to Pr = 0.7 and Ra = 2108. For these values of aspect ratio, Rayleigh and Prandtl numbers the flow is turbulent and time-dependent. In such flow, three sets of particles with Stokes numbers, based on the large eddy time scale, equal to Stf = 0.01, Stf = 0.005 and Stf = 0.001 are randomly dispersed. Particles distribution in turbulent convective flow is highly inhomogeneous and shows the clustering correlated with the vortical structures. The level of clustering is computed with the deviation of particles probability density function (PDF) from Poisson distribution. With this technique is available the size of the cluster but their geometry is unknown. The organisation along lines, planes and surfaces was investigated using the fractal dimension of the cluster. Finally, the flow topology is studied to relate the particles dispersion to coherent flow structures.
Strategies for Dispersion Control by Pulsed Injection: Particle Dispersion by Jet in Crossflow
Authors: Campolo Marina , Cremese Andrea , Soldati Alfredo
Published: September 2008

The dispersion produced by a jet injecting microparticles in a cross stream is controlled by the interaction between dispersed species and large scale time-dependent flow structures populating the transverse jet. These structures span over a wide range of spatial and temporal scales and are not equally effective in advecting and dispersing species. In many environmental and industrial applications, the species advected by the jet stream are expected to undergo rapid and homogeneous dilution away from the injection point. Preferential accumulation of particles into specific flow regions is to be avoided since this may have consequences on the overall industrial process. For instance, non uniform distribution of droplets of ammonia solution can severely downgrade the efficiency of post-combustion control devices. In a previous work (Campolo et al., 2005), we addressed the problem of identifying which of the flow structures in a jet in crossflow control the dispersion mechanisms of inertial particles, focusing specifically on the issue of their preferential distribution. Based on these results, in this work we try to identify a strategy for particle pulsed injection which can be used to optimize their dispersion. The flow field produced by the transverse jet is calculated using a finite volume solver of Navier-Stokes equations; the dispersion of injected particles is computed using a Lagrangian approach. Particle dispersion and segregation are evaluated considering the effect of the synchronicity between the particle injection time and the evolution dynamics of the mixing structures. Numerical results show that (i) transport of species is dominated by specific flow structures, (ii) particle dispersion is not uniform and (iii) the synchronicity between species injection and evolution dynamics of flow structures influences the dispersion process. These results indicate that pulsed injection may be used to control effectively particle dispersion.
Characterization of Superheated Liquid Jet Atomisation With Phase Doppler Anemometer (PDA) and High-Speed Imaging
Authors: Yildiz Dilek , Rambaud Patrick , van Beeck Jeroen , Buchlin Jean-Marie
Published: September 2008

A flashing phenomenon is often met in liquid propulsion of safety fields in industrial environments. This violent evaporation occurs when a liquid finds itself suddenly in a thermodynamic non-equilibrium and becomes superheated. To investigate theoretically the source processes and validate models for design and safety assessments, knowledge of accurate and reliable data such as distribution of droplet size, velocity and temperature in the closest field of flashing occurrence is mandatory. In this present work, an experimental study is undertaken in order to characterize the two-phase jet after a sudden accidental release and aims to quantify the effects of initial conditions such as initial storage pressure, temperature, geometrical effects of the release points etc on the spray characteristics. To fulfil this goal, a laser-based optical technique like Phase Doppler Anemometry (PDA) is used to obtain information for particle diameter and velocity evolution in this harsh environment. Cases for different initial pressures, temperatures and orifice diameters are studied and the droplet size and velocity evolution are presented in function of initial parameters.
Unsteady 3D Simulation of a Jet Fan With Symmetric Blades
Authors: Ferna´ndez Oro J. M. , Argu¨elles Di´az K. M. , Santolaria Morros C.
Published: September 2008

This work develops the numerical modeling of a monoplane axial jet fan with symmetric blades. The goal of the study is the simulation of the flow inside a rotor with elliptic airfoils, where the Kutta condition cannot be satisfied. The unsteady 3D model includes tip clearance gridding and a sliding mesh technique to simulate transient effects. The flow patterns inside the blade passage and the wake-core structure will be studied at design operating conditions. Also, the interaction of the tip leakage flow with time-averaged structures will be analyzed in detail. Therefore, the impact of the tip vortex in the mean time performance of the jet fan will be introduced. The investigation shows how the tip leakage vortex modifies the blade loading on the suction surface. The leakage flow rolls-up in a vortical structure at the suction side, establishing a mixing mechanism that produces a low axial velocity region. As a result, the adverse pressure gradient is enhanced and a major flow separation overcomes. This feature is especially critical in case of a rotor with symmetric blades, where the flow is always detached at the trailing edge. The simulation is carried out using a commercial code, FLUENT, which resolves the Navier-Stokes set of equations. An extremely high dense mesh is introduced in the model, so tip leakage is expected to be well-captured. In addition, fully-developed detachment of the boundary layer requires superior discretizations and high quality meshes, so restrictive y+ criteria have been employed for both endwall boundaries and blade surfaces. Turbulence modeling is closed using URANS models. The Reynolds Stress Model (RSM) has been employed because of its suitable predictions for rotating flow passages. In addition, this model considers anisotropic turbulence, and effects of curvature and rotation are directly addressed in the transport equations. Therefore, swirl effects of the tip vortex are expected to be well-captured. The numerical results are compared with previous experimental data of velocity fields to validate the simulation. Axial and tangential velocity profiles were obtained using a five-hole probe. Complementary, the instantaneous wake flow structure was measured with a dual hot wire anemometer.
Suppression of Self-Excited Flow Instability in a Radial Diffuser Using Rough Surfaces
Authors: Ahemd Saad A. , Salem Hayder
Published: September 2008

Flow instabilities in a compression system at low flow rates set the flow limit of the stable operating range. Experiments to investigate the feasibility of controlling the stall in the radial diffuser of a low speed centrifugal compressor were carried out. The technique was very simple and involved using rough surfaces (i.e., sand papers) attached to the diffuser shroud. The results showed that the flow instability in the diffuser (stall) was delayed to a lower flow coefficient (the mass flow rate could be reduced to 70% of its value with the smooth surface) when the rough surfaces were positioned on the diffuser shroud.