bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effects of Casing Geometry and Flow Instability on Rotordynamic Fluid Forces on a Closed Type Centrifugal Impeller in Whirling Motion
Authors: Hata Shinichiro , Richert Julien , Nishiyama Yumeto , Horiguchi Hironori , Tujimoto Yoshinobu
Published: June 2012

The rotordynamic fluid forces on a closed type centrifugal impeller in whirling motion were measured with a vaneless diffuser at various flow rates for three types of casing, with the casing/impeller shroud clearance of 2[mm], 5[mm], and 5[mm] with seven radial grooves to break the flow swirl. The rotordynamic fluid forces are destabilizing within small positive whirl speed ratio Ω/ω, where Ω and ω are a whirling speed and a rotational speed of the shaft, respectively. The radial grooves were effective to reduce the fluid forces and the destabilizing region due to the reduction of the circumferential velocity of the clearance flow. At low flow rates, the fluid forces increased at Ω/ω, ≈ 0.7. By the measurement of unsteady pressure on the casing wall and the CFD analysis of flow field in the pump, it was found that the large fluid forces at Ω/ω ≈ 0.7 were caused by the rotating stall of the impeller. The rotating stall was caused by the backflow along the suction surface of the main blade due to the separation vortices near the middle of the chord.
Numerical Prediction of Pump Performance Drop and Erosion Area due to Cavitation in a Double-Suction Centrifugal Feedpump
Authors: Miyabe Masahiro , Maeda Hideaki
Published: June 2012

This paper presents the numerical cavitatoin prediction methods of pump performance drop and erosion area by using a commercial CFD code. A cavitation model which is so called full cavitation model was used to predict pump performance drop. Furthermore, some indexes which are associated with void fraction and static pressure rising were applied to estimate erosion area on the impeller blades. A centrifugal boiler feedpump with a double-suction was tested and the cavitation characteristics were visualized and compared with the calculated results at three pump operating points of 0.63Qopt, Qopt and 1.50Qopt, respectively. As a result, the calculated pump performance drop and cavitation behavior agree well qualitatively with the experimental ones. Based on the calculation results of partial flow rate, it was found that pump performance drop is caused when the cavity reaches the blade-to-blade throat at the hub-side and the decrease of blade loading at the impeller outlet. In addition, simple cavitatoin erosion indexes were applied to esitimate the erosion area. The paint erosion test was conducted and compared with the calculated results at three pump operating points. One of them is in good agreement with the experiment. It is concluded that the present numerical cavitation prediction method is available at a design process of the pump.
Influence of Axial Vane on Inner Flow and Performance of a Molten-Salt Pump
Authors: Kang Can , Zhou Liang , Wang Weifeng , Yang Minguan
Published: June 2012

A vertically installed two-stage pump transporting molten salt was studied. Axial vane diffuser was designed to relieve the effect of non-uniformity of circumferential pressure distribution near the impeller outlet. Performance test was carried out to evaluate the working ability and acquire the operation performance parameters of the designed pump. Numerical simulation was conducted to capture the variation of main flow parameters with time and space. Both steady and unsteady computations were performed and mediums of water and molten salt were defined in computation for comparative analysis. The results indicate that circumferential pressure distributions at different axial positions exhibit apparently different tendencies. Under the designed flow rate, energy loss in the rotor-stator interaction zone is relatively small, while under the condition of small flow rate, the loss can reach as high as 3.5% of the pump head. With impeller’s rotation, the flow pattern near blade outlet experiences drastic change. Dimension, shape and location of the typical flow structure change accordingly due to the interplay between rotating impeller and stationary axial vane.
Three-Dimensional Structure and Turbulence Within the Tip Leakage Vortex of an Axial Waterjet Pump
Authors: Miorini Rinaldo L. , Wu Huixuan , Tan David , Katz Joseph
Published: June 2012

The flow structure and dynamics of turbulence are investigated by means of three-dimensional stereo particle image velocimetry (Stereo-PIV) measurements within the tip leakage vortex (TLV) of an axial waterjet pump rotor. Both the blades and casing of the pump are transparent and their optical refractive indices are matched with that of the pumped fluid, providing unobstructed optical access to the sample area without image distortion. Data are acquired on selected meridional planes in the rotor passage as well as in three-dimensional domains obtained by stacking closely-spaced planes situated within the rotor passage. Presented data have been sampled in one of these 3D regions, at 67% of the blade tip chordlength. All components of velocity and vorticity are calculated, together with the whole strain-rate and Reynolds stress tensors. The entire set of contributors to the turbulence production-rate is also available. The TLV and associated flow structures are completely 3D and change significantly along the blade tip chordwise direction. The vortex originates from the rollup of a multi-layered tip leakage flow, and propagates within the rotor passage towards the neighboring blade. Because of layered backflow rollup, vorticity entrained in the TLV is convected along different paths and re-oriented several times within the vortex. As a result, the TLV consists of a core surrounded by a tube of three-dimensional vorticity that wraps around it helically. Propagation of tip leakage backflow into the passage and subsequent TLV rollup also cause flow separation at the casing endwall with ejection of boundary layer vorticity that is finally entrained into the outer perimeter of the TLV. This complex TLV flow dominates the tip region of the rotor and involves non-uniform distributions of strain-rate and Reynolds stresses resulting in well-defined peaks of turbulence production-rate. For instance, turbulence is produced locally both at the flow contraction point near the region of aforementioned endwall separation and in the shear layer that connects the vortex with the suction side corner of the blade tip. The spatial inhomogeneity of turbulent kinetic energy (TKE) distribution within the TLV, and the mismatch between locations of TKE and production-rate peaks can be explained by analyzing the 3D mean flow advection of turbulence, for example from the region of endwall boundary layer separation towards the outer region of the TLV. In addition to being spatially non-uniform, turbulence is also anisotropic in both the shear layer and periphery of the TLV. Conversely, turbulence is intense and relatively isotropic near the TLV core, as well as monotonically increasing along the vortex centerline. This trend cannot be described solely in terms of local production of turbulence; it must also involve slow turbulence dissipation associated with the meandering of relatively large-size, interlaced vortex filaments in the TLV core region.
Computational Fluid Dynamics Analyses of a Micro Pediatric Ventricular Assist Blood Pump
Authors: Yang Xiao-chen , Zhang Yan , Gui Xing-min , Hu Sheng-shou
Published: June 2012

The heart failure patients supported by the mechanical rotary blood pumps have been validated and investigated in recent decades. A series of adult blood pumps have been investigated in our research group in the last several years and one of them is currently under clinical trials. This present paper aimed at analyzing a micro pediatric blood pump (MPBP) with Computational fluid dynamics (CFD) tool. MPBP is developed to assist the ventricular according to the practice of pediatric heart failure in Fuwai Hospital of Chinese Academy of Medical Sciences. The blade tip diameter of the MPBP is 10 mm. Some advanced structures proposed in our adult blood pumps were further improved in the MPBP and a cantilevered stator applied in the blood pump is a novel try. The results of the numerical simulation show that the MPBP can generate the flow rates of 0.74–3.21 lpm at the rotational speeds of 9,000–11,000 rpm, producing the pressure rises of 36.9–89.7 mmHg. The structural advantage, hydraulic performance and hemolytic property of the MPBP were analyzed in detail. Overall, the attempt of the cantilevered stator blade improved the performance of the blood pump effectively and the MPBP deserves a promising prospect.
An Optimization Design of the Auxiliary Impeller of Centrifugal Pumps Based on Orthogonal Methods
Authors: Yuan Jianping , Jin Rong , Li Shujuan , Wang Longyan , Ge Aixiang
Published: June 2012

In order to research the influence laws of the main geometrical parameters of auxiliary impeller and different operation conditions on the centrifugal pump with an auxiliary impeller, which aimed to act as dynamic seal, the orthogonal experiment was designed with four factors and three values. The factors respectively are auxiliary impeller axial clearance, blade width, outlet diameter and blade number. With simulation by Fluent, major and minor factors were investigated which influence the performance of the centrifugal pump with an auxiliary impeller. The cases with optimization sealing pressure value and optimization efficiency were obtained and it was proved by the experimental results. Then, two optimization cases and the original case were simulated and analyzed. The research results show that the major factor of auxiliary impellers for the pump efficiency is the outlet diameter. For sealing pressure head of auxiliary impellers, the major factor is the outlet diameter of auxiliary impeller and the axial clearance and blade number of the auxiliary impeller are secondary important factors. For the optimization of centrifugal pumps with an auxiliary impeller, numerical orthogonal tests can replace actual orthogonal tests.
Analysis of Static Characteristics of Pressure Seal in Actual High Pressure Pump
Authors: Hagiya Isao , Kobayashi Katsutoshi , Chiba Yoshimasa , Yoshida Tetsuya , Arai Akira
Published: June 2012

We predicted the leakage flow rates of a pressure seal in an actual high-pressure multistage pump. Since the pressure of the actual pump is higher than that of a model pump, accurate prediction of leakage flow rate and rotor dynamic forces for an actual pump is more difficult than that for a model pump. A non-contacting seal is used as a pressure seal to suppress leakage flow for high-pressure multistage pumps. When such pumps are operated at high speed, the fluid force acting on an eccentric rotor may cause vibration instability. For vibration stability analysis, we need to estimate static and dynamic characteristics of the pressure seals, i.e., leakage flow rate and rotor dynamic coefficients. We calculated the characteristics of the pressure seal based on Iwatsubo group’s method. The pressure seal we developed has labyrinth geometry consisting of grooves with different sizes. This method numerically calculates the characteristics of the grooved seal by using a three-control-volume model and a perturbation method. We compared the calculated and measured leakage flow rates. We found that the calculated results quantitatively agreed with the measured one in the actual pump and the characteristics of pressure and velocity for the seal with non-uniform-sized grooves were clarified.
Identification of Francis Turbine Helical Vortex Rope Excitation by CFD and Resonance Simulation With the Hydraulic System
Authors: Alligne´ Se´bastien , Nicolet Christophe , Avellan Franc¸ois
Published: June 2012

Due to the growing development of new renewable energies, which production is difficult to foreseen, power grid is subjected to disturbances. Hydropower plants are one of the solution to restore the grid stability by allowing hydraulic machines, especially Francis turbines, to change quickly of operating points in a very large range of heads and power in order to cover the variation of the electrical demand. In part load conditions, the cavitating vortex rope is an excitation source for the whole hydraulic circuit. The frequency of the excitation may matches with one of the eigenfrequency of the system leading to resonance phenomena. The aim of this paper is to simulate this hydroacoustic resonance by identifying the excitation source with CFD numerical simulations of the cavitating vortex rope and simulating the response of the hydraulic system with a one dimensional compressible model. A one dimensional draft tube model including three key parameters is used: the excitation momentum source corresponding to the force induced by the vortex rope acting on the wall, the excitation mass source induced by the cavitation volume fluctuations and the thermodynamic damping modeling energy dissipation during the phase change between cavitation and liquid. These parameters are computed for the FLINDT reduced scale model with the help of unsteady CFD simulations considering both one phase and two phase simulations. Finally these parameters are injected in the one dimensional hydroacoustic model to simulate the resonance phenomenon. In out of resonance conditions, maximum of pressure fluctuations are found in the draft tube cone with an amplitude of 1% of the turbine head. However, when resonance occurs, maximum amplitude of pressure fluctuations reaches up to 6.8%.
Hydrodynamics of a Pump-Turbine at Off-Design Operating Conditions: Numerical Simulation
Authors: Hasmatuchi Vlad , Roth Steven , Botero Francisco , Farhat Mohamed , Avellan Franc¸ois
Published: June 2012

Flow numerical simulations in a low specific speed radial pump-turbine scale model are performed to investigate off-design operating conditions in generating mode. The Best Efficiency Point (BEP) and the runaway operating conditions at 10° guide vanes opening are addressed. The computational domain includes the full reduced scale model water passage from the spiral casing inlet to the draft tube outlet. The numerical simulation is performed using the Ansys CFX code, solving the incompressible unsteady Reynolds-Averaged Navier-Stokes equations. Wall pressure measurements in the stator are used to validate the numerical results. Then, detailed analysis is focused on the onset of the flow instabilities when the machine is brought from BEP to runaway. In these severe operating conditions, one single stall cell is found to rotate with the impeller at subsynchronous speed in the vaneless gap between the impeller and the guide vanes. It is found to be the effect of flow separation developed at the inlet of several consecutive impeller channels which lead to their blockage.
Reduction of Cavitation Damage in a High-Energy Water Injection Pump
Authors: Cooper Paul , Ungewitter Ron , Farooqi Rehan , McKenzie James , Schiavello Bruno , Robinson Wayne
Published: June 2012

The conventionally-designed first-stage impeller of a high-energy, two-stage 19MW seawater injection pump, running at 4950 rpm and generating 1500m of head at a flow rate of 1.05 m3/s was seriously damaged by cavitation erosion in the first two months of operation. The impeller was redesigned by reshaping the blades in the region near the leading edges so as to reduce the inception cavitation number. This impeller has been running for more than a year, and the cavitation erosion rate is predicted to be low enough for it to last 40,000 hours. However, a prominent tone at blade passing frequency appeared with the new impeller, which interacts more effectively with the distorted inflow from the side-suction approach passage. Acoustic analysis of both single- and two-phase unsteady RANS CFD solutions corroborate the presence of this tone, which had not been observed when the pump operated with the original, conventional impeller.
A New Methodology for Suppressing Pressure Pulsation in a Draft Tube by Grooved Runner Cone
Authors: Sano Takeshi , Ookawa Masayuki , Watanabe Hiromi , Okamoto Nobuaki , Yano Hiroshi , Fukuda Nobuhide , Maekawa Masatake , Miyagawa Kazuyoshi
Published: June 2012

High amplitude of pressure fluctuation is observed in a draft tube, for the case of partial load operation. Several methods had been reported to decrease the amplitude so far, such as, air or water injection to the draft tube, fins on the draft tube surface, or runner replacement with optimized velocity profile at runner exit. However, several problems for each method can be considered, such as, negative influence on efficiency, high cost, technical difficulties for installation, and so on. To solve these problems and satisfy the demand for decreasing the amplitude of pressure fluctuation simultaneously, a new runner cone with grooves on the surface was developed. As for the case with reversible pump turbine, grooved runner cone was developed with unsteady draft tube calculation based on Design of Experiment (DOE) method, and confirmed by model tests. Finally, developed runner cone was installed to the prototype pump turbine, and predicted performance was confirmed by on-site tests. Using this result, development of a new runner cone for hydraulic turbine had been started. For this case, meridian shape of the runner cone was also selected as a design parameter. Finally, we obtained the optimized shape, “diverged runner cone with spiral groove”, and the performance is confirmed by the model test. In this paper, details of development, especially for the case of hydraulic turbine and considerations of the mechanism are treated.
Experimental Study on a Twin Unidirectional Impulse Turbine for Wave Energy Conversion
Authors: Takao Manabu , Alam Md. Mahbubu , Setoguchi Toshiaki , Jayashankar V.
Published: June 2012

A twin unidirectional impulse turbine has been proposed in order to enhance the performance of wave energy plant. This turbine system uses two unidirectional impulse turbines and their flow direction is different each other. However, the turbine characteristics have not been clarified to date. The performances of a unidirectional impulse turbine under steady flow conditions were investigated experimentally by using a wind tunnel with large piston/cylinder in this study.
The Growth of Vorticity in the Vicinity of a Wall of an Elastic Moving Airfoil and Their Developing Processes
Authors: Kurinami Tomoki , Fuchiwaki Masaki , Tanaka Kazuhiro , Nagata Tetsushi
Published: June 2012

The behavior of a vortex generated by a moving object subjected to vibration and fluidic noise has been investigated both experimentally and numerically. Animals fly and swim by skillfully controlling vorticies generated around their bodies. Moreover, an elastic airfoil that enables elastic deformation in order to fly and swim with low energy has recently attracted a great deal of attention, and industrial applications of this elastic airfoil are expected. The authors performed the PIV measurement for vortex flow in the vicinity of a wall of the elastic NACA0010 and clarified quantitatively the change in the developing process of vortices generated in the vicinity of a wall with elastic deformation. The vorticity generated in the vicinity of the wall near the elastic NACA0010 continues to increase until the acceleration becomes maximum, because the vorticity develops for main flow direction (x-direction). Therefore, the growth and development of vortices generated in the vicinity of the wall near the trailing edge of a heaving airfoil depend strongly on the variation of accelerations of the trailing edge.
Finite Element Simulation of Blood Flow in a Flexible Carotid Artery Bifurcation
Authors: Lee Sang Hoon , Choi Hyoung Gwon , Yoo Jung Yul
Published: June 2012

To investigate the effect of the flexible artery wall on the flow field and to determine the wall shear stresses in the carotid artery wall, numerical simulations for the blood flow are carried out. For solving the equation of motion for the structure in typical fluid-structure interaction (FSI) problems, it is necessary to calculate the fluid force on the surface of the structure explicitly. To avoid the complexity due to the necessity of additional mechanical constraints, we use the combined formulation which includes both the fluid and structure equations of motion into single coupled variational equation. The Navier-Stokes equations for fluid flow are solved using a P2P1 Galerkin finite element method (FEM) and mesh movement is achieved using arbitrary Lagrangian-Eulerian (ALE) formulation. The Newmark method is employed for solving the dynamic equilibrium equations for linear elastic solid mechanics. The time-dependent, three-dimensional, incompressible flows of Newtonian fluids constrained in the flexible wall are analyzed. The study shows strongly skewed axial velocity and flow separation in the internal carotid artery (ICA). Flow separation results in locally low wall shear stress. Further, strong secondary motion in the ICA is observed.
Critical Liquid Velocities for Low Concentration Sand Transport
Authors: Hill Alan L. , Arevalo Balbino , Almutahar Faisal M. , McLaury Brenton S.
Published: June 2012

The transport of solid particles is a concern for several industries and has been the source of extensive research for many years. One topic that has been studied is the critical velocity required to transport particles, but the transport behavior used to determine the critical velocity can be different. For example, one commonly defined critical velocity is the minimum velocity necessary to keep the particles fully suspended. However, this study examined the sand behavior with two different definitions for critical velocity. The first definition simply requires the sand to move enough to prevent a permanent bed from forming. The sand behavior when this velocity requirement is met can be described as intermittent scouring and stopping. All particles do not need to move continuously. The second definition of critical liquid velocity requires the sand particles to continuously move along the bottom of the pipe. The sand behavior when this velocity requirement is met is described as continuous scouring and not fully suspended. Numerous past experiments have amassed a significant amount of slurry data, but the majority of the data is for particle volume concentrations greater than 1%. However for many cases such as in oil and gas production, particle concentrations much below 1% are present. So to examine transport for lower concentrations, experiments were conducted for volume concentrations of 0.01%, 0.1%, and 1%. Three different sand diameters and two different liquid viscosities were also used to observe their effects on the critical liquid velocities. All tests were conducted in transparent, horizontal pipe to observe particle behavior. Results have shown that as particle size, concentration, and liquid viscosity increase, the required critical liquid velocity also increases. Experimental results are compared to existing data and models to examine the effect of concentration, focusing on volume concentrations below 1%.
Numerical Study on Bubble Departing Behavior From Heated Surface in Subcooled Pool Boiling
Authors: Ose Yasuo , Kawara Zensaku , Kunugi Tomoaki
Published: June 2012

In this study, in order to clarify the heat transfer characteristics of the subcooled pool boiling and to discuss its mechanism, the boiling and condensation model for numerical simulation on subcooled boiling phenomena has been developed. In this paper, the three dimensional numerical simulations based on the MARS (Multi-interface Advection and Reconstruction Solver) with the boiling and condensation model which consisted of the improved phase-change model and the relaxation time based on the quasi-thermal equilibrium hypothesis have been conducted for the subcooled pool boiling phenomena especially regarding to the bubble departure behavior from the heated surface. The results of the numerical simulations were compared with the experimental data obtained by the high-speed camera (Phantom 7.1) with the Cassegrain optical system, and then the influence of the degree of subcooling for the bubble departing behaviors including their shape changes from the heated surface were numerically predicted. As the results, the numerical results of the bubble departing behavior from the heated surface showed in good agreement with the experimental observations quantitatively.
Correlation Between a Local Void Fraction and Dissolved Gas Concentration in a Bubble Column by Using a Newly Developed Photoelectric Optical Fiber Probe
Authors: Yamada Masahiro , Saito Takayuki
Published: June 2012

A bubble column is frequently used in various industrial plants for the purpose of dissolving gases into liquid phases. Clarifying a relationship between the mass transfer mechanism and the structure of the bubbly flow is important in order to improve the efficiency of the gas absorption. In the present study, we discuss a correlation between the concentration transportation and the large-scale structure of the bubbly flow, by using a newly developed photoelectric optical fiber probe (POFP). The POFP has two functions of electrical concentration measurement and optical bubble measurement; i.e. measurement of the time-spatial correlation of the bubble swarms and the concentration transportation is achieved simultaneously by using a pair of the POFPs. A bubble column of 380 mm in inner diameter and 1500 mm in height was employed. We pumped a mixture gas of CO2 and pure air into the bubble column through a perforated plate. Based on the fluctuation characteristics of the void fractions, the flow field in the column was divided into three zones: bottom, middle and upper zones. A long-period fluctuation of the void fractions fades out toward the upper zone of the bubble column. Simultaneously, we measured local CO2 concentrations. In the bottom zone, the increase rate of the concentration significantly differed by position. In contrast, in the upper zone, little difference in the increase rate was observed by position. The differences in the increase rate of the CO2 concentration faded out toward the upper zone. Furthermore, the fluctuations of the CO2 concentration showed a very close correlation with the fluctuations of the void fraction. We will discuss correlations between the mass transportation characteristics and the large-scale structure of the bubbly flows (i.e. the gas-phase and liquid-phase large-scale structure).
CFD Analysis to Study Evaporation of a Single Ethanol/Iso-Octane Binary Mixture Droplet
Authors: Banerjee R. , Gopinath R.
Published: June 2012

Evaporation of a single droplet of a binary mixture of ethanol/iso-octane was numerically studied. Liquid/gas interface was tracked using VOF multiphase model. As ethanol and iso-octane form a highly non-ideal mixture, vapour-liquid-equilibrium (VLE) calculations were made using UNIFAC group contribution method. Source terms arising due to interfacial mass transfer are implemented in the continuity, momentum, energy and species equations. In this study, commercial CFD solver, Ansys Fluent 12.1 was used. VLE calculations and the source terms were implemented using user defined functions. A non-isothermal flow condition with droplet and surrounding medium at different temperatures was studied. Time history of temperature, evaporation rate, droplet size and shape were studied and reported. A parametric study of droplet composition and Reynolds number has been performed and reported in this study.
Thermo-Fluid Dynamic Behavior of Water Droplets Injected Into Air Stream
Authors: Suryan Abhilash , Kim Heuy Dong , Lee Jung Keun , Kim Chang Ki , Kim Dong Sun
Published: June 2012

Behavior of water droplets in air is of interest in many engineering applications. In evaporative cooling, the evaporation of liquid droplets sprayed into air streams in measured quantities cools the air. In the current study, experiments and numerical simulations are carried out to investigate the thermo-fluid dynamic behavior of water droplets of different diameters in long air ducts. Droplet size measurements at different locations were obtained with global sizing velocimetry technique. Effect of initial droplet size on temperature and relative humidity distribution of air stream in the duct is investigated. Droplet spray patterns are also analyzed to identify suitable locations for spray nozzles within the duct. Cooling effectiveness is calculated for different conditions. Experiments were conducted to validate the numerical results. Results obtained provide a clear understanding of the thermo-fluid dynamic behavior of water droplets within the duct.
Visualization Study on Gas Bubble Break-Up and Transport Phenomena in Microchannels
Authors: Yim Dongwook , Sung Jaeyong , Yoo Jung Yul
Published: June 2012

In the present research, a visualization study is carried out to understand the flow patterns and break-up formation in flow-focusing microfludic device, which usually involve gas-liquid two-phase flow patterns such as bubbly flow, slug flow, and annular flow. High-speed camera and fluorescence microscopy are used to the characterize microscale gas-liquid two-phase flows in 191.64 μm × 200 μm rectangular PDMS microchannel which are fabricated by using soft lithography technique. N2 and Silicone oil serve as the gas and liquid phases, respectively. The influence of gas and liquid flow rates and the viscosity of the liquid phase on the bubble size, flow patterns, and generating frequency are experimental studied. This study is focused on viscosity of liquid phase. In order to influence of viscosity of liquid phase, various viscosity silicone oils are used in this study. The breakup mechanism, bubble generating rate, and gas bubble making time are investigated at the cross junction. The gas bubble size and space of between bubbles are dependent on the flow rates of the liquid and gas, and viscosity of liquids. The slug length L is also dependent on the flow rates of the liquid and gas in such a manner that the ratio of slug length to the channel width is a function of the flow rate ratio of gas to liquid.
Inhomogeneous Structure in High-Speed and High-Number-Density Sprays
Authors: Kawaharada Noritsune , Sakaguchi Daisaku , Komada Keisuke , Ueki Hironobu , Ishida Masahiro
Published: June 2012

A L2F (Laser 2-Focus velocimeter) was applied for the measurements of the velocity and size of droplets in diesel fuel sprays. The micro-scale probe of the L2F has an advantage in avoiding the multiple scattering from droplets in a dense region of fuel sprays. A data sampling rate of 15MHz has been achieved in the L2F system for detecting almost all of the droplets which passed through the measurement probe. Diesel fuel was injected into the atmosphere by using a common rail injector. Measurement positions were located in the planes 15, 20, and 25 mm apart from the injector nozzle exit. Measurement result showed that the integral time scale of turbulence in size was nearly the same as the one in frequency. And the integral time scale of turbulence in velocity was about two times larger than the time scale of size and frequency.
Flow of a Viscoelastic Fluid Around a Falling Sphere
Authors: Shiratori Takahisa , Tasaka Yuji , Murai Yuichi , Oyama Kazuya , Kumagai Ichiro , Takeda Yasushi
Published: June 2012

Velocity vector fields around a falling sphere in a 1.0 wt % polyacrylamide (PAA) solution are obtained on a vertical cross section by particle image velocimetry (PIV). PAA solution is known as non-Newtonian fluid, which has shear thinning and viscoelastic property. Strain rate tensor fields and deformation fields are calculated from the velocity vector fields in order to visualize the dynamic behavior of the fluid quantitatively. In velocity vector field, two typical flow regions are observed in the wake of the sphere: approaching flow to the sphere, rising flow called “negative wake” [1]. Results show that the strain rate tensor field gives fluid strain at the approaching flow region and the edge of the negative wake. Furthermore deformation history of one portion of the fluid shows that fluid is strained in the approaching flow region, and the strain rate at the edge of the negative wake represents their recovery to the original status of the fluid in the moving frame.
A Relationship Between the Motion of a Zigzagging Bubble and its Surrounding Liquid Motion
Authors: Tachibana Rintarou , Saito Takayuki
Published: June 2012

In the present study, a mechanism of zigzagging bubble motion was experimentally and quantitatively investigated. We focused on a relationship between the bubble motion (gravity-center motion and surface motion) and its surrounding liquid motion. We visualized the bubble motion and its surrounding liquid motion simultaneously using IST (Infrared Shadow Technique) and SPIV (Stereo Particle Image Velocimetry). The interrogation areas were located just above a needle (just after bubble launch) to the first inversion of the zigzagging trajectory (transition from the linear to the zigzag). First, the bubble trajectory, the surface motion and the bubble inclination angle of major axis were obtained from the IST images. Second, the pseudo-3D motions (2D-3C velocity fields) of the surrounding liquid were calculated from the SPIV results. Based on the velocity data, we calculated the surrounding pressure field around the bubble by solving simplified Navier-Stokes Equation. Finally, from these results, we quantitatively discuss the close relationship between the bubble motion and its surrounding liquid motion. In particular, we discuss the mechanism of the zigzagging motion. At the inversion point, the shape deformation of the bubble and the fluctuation of the surrounding liquid motion show distinctive behavior. We tentatively consider that the zigzagging motion is caused by the periodical fluctuation of the bubble deformation (the surface motion and the inclination angle of major axis) and the surrounding liquid motion; i.e. the asymmetry of the surrounding liquid (velocity and pressure) induced by the bubble surface oscillation characterizes the zigzag motion.
Liquid Entrainment by Round Turbulent Gas Jets Submerged in Water
Authors: Drew Brady , Charonko John , Vlachos Pavlos
Published: June 2012

Entrainment characteristics of two-phase flow (liquid-gas) buoyant jets differ significantly from their single-phase flow counterparts. Past studies have not adequately described the mechanisms that cause the gas jet to entrain liquid from its surroundings and expand. In this work, Particle Image Velocimetry (PIV) and shadowgraph flow visualization experiments have been conducted on submerged round gas jets of varying speeds and nozzle diameters with the goal of improving our understanding of the processes of entrainment and expansion in a two-phase jet. We hypothesize that liquid is entrained into the gas column through (1) shear entrainment due to instabilities at the interface between the fast-moving gas jet and stagnant liquid, and (2) convective entrainment that occurs when the jet begins to pinch off and transform into a bubbly plume. The total entrainment estimated using the PIV measurements is higher than the respective values that single-phase buoyant jet theory suggests, especially at low jet speeds. This may be an effect of increased convective entrainment as the jet slows down. The shadowgraph flow visualization experiments provide valuable information pertaining to the structure of the jet and the interfacial dynamics.
Visualization of a Single Boiling Bubble in a DC Electric Field
Authors: Chen Feng , Liu Dong , Song Yaozu , Peng Yao
Published: June 2012

The application of electric field has been demonstrated as an effective way to enhance pool boiling heat transfer. In past studies, adiabatic experiments were often conducted to simulate the dynamics of nucleate bubbles in the presence of an electric field, where gas bubbles were injected from an orifice, to avoid complexities involved in the nucleate boiling experiments. While adiabatic studies yield useful information of the bubble dynamics, further studies about bubble dynamics during nucleate boiling heat transfer are still necessary for a full understanding of the effects of applied electric field on the liquid-vapor phase change heat transfer. In this paper, the dynamics of a single boiling bubble in a direct current (DC) electric field was studied experimentally employing R113 as the working fluid. The life cycle of the boiling bubble was visualized using high-speed photography and was compared with that of an injected nitrogen bubble. Under the same electric field, a more appreciable elongation along the field direction was observed for the boiling bubble. A modified relationship between the bubble deformation and the electrical Weber number was proposed for the boiling bubble. As the electric field strength increases, it was found that, although the growth time of the boiling bubble increases, the waiting period decreases. However, it was also found that, the change of the whole life cycle with electric field strength increasing is relevant to the wall temperature. In this work, the wall temperature measured in the vicinity of the nucleation site upon the bubble departure decreases when the electric field is applied.
Study on the New Fine Particle Size Measurement Technique With Laser Interference Fringes
Authors: Sakaki Hiroto , Kaneko Akiko , Abe Yutaka , Ike Masatoshi
Published: June 2012

We propose one fine partice size measurement technique with laser interference fringes which is expected to achieve both non-contact and in-line measurement of the diameter of several hundred nano meter particles. However, the principle of this measurement technique with laser interference fringes is not clear and applicability of fine particle size measurement is unclear. The objective of this study is to examine the principle of this measurement technique with laser interference fringes and evaluate its applicability. We measure the flow structure in a flow cell which is the test section with Particle Image Velocimetry (PIV) to optimize the setting of the flow cell in the measurement system. From the velocity profile along the flow cell, 20 mm downward from the inlet is set as the measurement point. Next, we visualize the laser interference fringes formed with a high-speed video camera to identify the appearance and the behavior of the laser interference fringes through fine particles in the flow cell. Finally, we measure both the time variation of the intensity of the laser interference fringes and the intensity of the scattering light with photodiodes. Passage time of a particle in the laser beam and the voltage intensity have correlation each other. The effect of particle sizes on the laser signal is discussed.
On the Application of Active Flow Control to Wind Turbines
Authors: Seifert Avraham , Stalnov Oksana , Troshin Victor , Avnaim Maor Hai
Published: June 2012

This paper reviews recent studies conducted at Tal-Aviv University that were aimed at applying active flow control technology to large and small scale wind turbines with the purpose of improved performance and reduced noise emission. Large wind turbines suffer from degradation of surface quality over the 20 years life span. Premature transition results and promotes early turbulent separation. Energizing the boundary layer with vortex generators (VG’s) and active flow control is studied. While both methods are capable of recovering lost performance, VG’s have an off-design drag penalty, while AFC can be turned off when not needed. It is demonstrated that AFC is highly energy efficient. Small wind turbines suffer from low Reynolds number effects resulting from low wind speed, small chord and low tip speed ratio. Passive and semi-active flow control methods are applied to a small vertical axis wind turbine (VAWT) with the purpose of delaying boundary layer separation via forced transition and transient semi-active fluidic jets. Power extraction at low wind speeds is demonstrated to be greatly improved.
Wake Structure Behind Circular Cylinder by Plasma Actuators
Authors: Yamada Shunsuke , Shibata Koui , Yanagihara Hikaru , Doi Takahiro , Ishikawa Hitoshi , Segawa Takehiko
Published: June 2012

The separation flow causes the decrease of the driving efficiency of the fluid machines. It is important to control the separation on the bluff body, boundary layer and so on. The purpose of the present study is to control the separation on a circular cylinder and investigate the wake structure using the induced by dielectric barrier discharge (DBD) plasma. The electrode of the DBD plasma was mounted. In previous study, it is reported that the three dimensional wake structures are effective for the drag reduction. We investigate the three dimensional structure of the wake due to three dimensional jets of plasma actuators. The plasma actuators have pulse driving frequency of 0, 0.22, 1.0 and 2.0. A voltage of 4 kVpp and a frequency of 10 kHz are applied to the electrode. The velocity profiles behind the circular cylinder were measured by X type hot-wire anemometer at a Reynolds number of 1.0 × 103. The wake structure changes because the roll up of vortex is suppressed using pulse driving of the plasma actuators. The effect of plasma actuators is evaluated based on the half width and Reynolds stress in the wake.
Generation of Velocity Fluctuations in a Boundary Layer by a Piezoelectric Actuator
Authors: Okawa Hajime , Kondo Shigenori , Shigeta Masaya , Izawa Seiichiro , Fukunishi Yu
Published: June 2012

A feedforward control system for suppressing the instability waves in a boundary layer has been examined and improved. An array of piezoelectric actuators attached on a flat-plate surface was used as an active control device of the feedforward control system. Previous works have demonstrated that an array of piezoelectric actuators was able to generate controlling waves that were 180 degrees out-of-phase with the target instability waves, but finding the optimum timing and amplitude for driving the actuators depended on the experience and skill of the experimenters. To resolve this problem, we focused our attention on the process of generating these artificial waves. First, the flow field near an actuator was measured in detail in order to clarify the wave-generating process. It was found that the velocity fluctuation pattern started to appear from the actuator’s surface, not from its trailing edge. Next, the effect of the driving signal waveform on the control was examined by comparing two different waveforms, sinusoidal and rectangular. A difference was found only in the third harmonic component of the generated velocity fluctuations, where the peak in the rectangular case was higher than that in the sinusoidal case. This suggested that it was not necessary to operate the actuators by a sinusoidal signal. Finally, a new semi-automatic algorithm for determining the control parameters was proposed and applied in an active feedforward control experiment. Our new algorithm demonstrated its ability to reduce the amplitude of the incoming oblique waves.
Analysis of a Subsonic Expansion Jet Under the Influence of a Finite Wall at the Exit
Authors: Shukla Vaibhav , Kushari Abhijit
Published: June 2012

With the advent of short takeoff and landing, thrust vectoring has become an integral part of aircraft systems. Various schemes are employed to achieve jet offset, one of which is by fluidic control using mechanisms such as perturbing boundary layer or providing a control bleed. The work presented in this paper provides an unconventional method of offsetting the main jet by inducing virtual Coanda effect by introducing a finite wall. It also lays down the importance of aspect ratio which determines whether the flow will bend or go straight. The proposed scheme requires neither moving parts nor control bleed making this a cutting edge discovery. The case presented here addresses the influence of finiteness of domain following expansion on the flow characteristics. It comprises a small inlet pipe to ensure full development of flow before expansion; and a main domain with a finite wall below, but is open above. So, not just the front but the top face too, is open to atmosphere. For various lengths, domain depth is increased till the flow switches from touching the wall to a straight jet. This is repeated for multiple inlet velocities and turbulent intensities. The expansion ratio is taken to be quite large so that it does not influence the results. It is observed that for a given velocity and length, there exists a critical depth before which flow sticks to the wall permanently. If the depth of wall is more than this critical depth then, the flow completely ignores the presence of wall and goes straight. It is also observed that this critical ratio is independent of the velocity and domain size to a large extent.
Influence of Shear Rate History on Generation of Shear-Induced Structure of Wormlike Micelles Solutions
Authors: Chiba Shogo , Ito Masatoshi , Fujii Shuji , Takahashi Tsutomu
Published: June 2012

The influence of the history of the shear rate on generation of shear hardening phenomenon of wormlike micelles aqueous solutions was investigated. An accelerated shear flow was applied to change the shear rate history and the influence of an acceleration rate on the time evolution behavior of the shear stress was observed. The relationship between the strain and the shear stress is given by one curve that is independent of a history of shear rate if the shear rate or the accelerated shear coefficient is higher than a certain critical value. A very high shear rate flow can be caused without the shear hardening by using the slowly accelerated shear flow but the SIS is generated if the shear rate in the moment exceeds the critical shear rate of the SIS formation.
Numerical Study on the Thermal Characteristics of Large Sized Silicon Polycrystal Growth by DS Method
Authors: Seo Hyeon-Seok , Kim Youn-Jea
Published: June 2012

The development of the manufacturing processes of solar silicon ingot is one of the more important issues to guarantee the growth of the photovoltaic industry, because of the saving of manufacturing costs of ingots, wafers, solar cells and of solar modules. Several solidification processes have been developed by industries, including casting, heat exchanger method and electromagnetic casting. However, the market growth using mono- and polycrystalline Si wafers might be saturated due to the shortage of Si feedstock. One of the methods to solve this problem is to make higher quality polycrystalline Si wafers which are capable of producing higher efficiency solar cells. In this paper, the effects of changing several geometrical configurations of cooling path were evaluated to improve the directional solidification (DS) method and to achieve the main issues. The developed DS method has the advantages of the small heat loss, short cycle time and efficient directional solidification. Based on the CFD (computational fluid dynamics) model, the simulation was performed on the thermal characteristics during the DS process. Using a commercial CFD code, Fluent, the thermal characteristics in the DS system are calculated, and the results are graphically depicted.