bluuebunny/crossref_metadata_2025 huggingface dataset:
Natural Convection With Porous Medium in a Narrow Horizontal Cylindrical Annulus With Constant Heat Generation
Authors: Mansour Kamyar
Published: March 2011

We consider the two-dimensional problem of steady natural convection in a narrow horizontal cylindrical annulus filled with porous medium due to a constant temperature variation on the outer and adiabatic conditions at the inner boundaries with constant volumetric heat flux. The solution is expanded in powers of a single combined similarity parameter, which is the product of the gap ratio to the power of two, and Rayleigh number. The series is extended by means of symbolic calculation up to 28 terms. Analysis of these expansions allows the exact computation for arbitrarily accuracy up to 50000 figures. Although the range of the radius of convergence is small, but Pade approximation leads our results to be good even for much higher value of the similarity parameter.
Prediction of Droplet Tear-Off and Meniscus Formation in the Top-Spot Experiment Using TransAT
Authors: Narayanan Chidambaram , Lakehal Djamel
Published: March 2011

For the design and development of new microfluidic devices reliable modeling and simulation tools must be made available. Many extensions to conventional computational fluid dynamics are required, especially multiphase fluid dynamics simulation capability. A new dynamic contact angle model is presented here, which does not require the specification of a contact angle or contact-line velocity. The level-set method is used for interface capture. The model is tested for unit problems such as relaxation to equilibrium of a contact line. It is then applied to the problem of fluid filling in a prototypical microdevice to show its utility as a design tool.
Performance Test of an Innovative Vertical-Axis, Twin-Rotor Wind Turbine System
Authors: Lee S. , Kim H.-R. , Lee J.-K. , Sabourin Tom , Cho H.-J. , Choi G.-B.
Published: March 2011

The patented vertical-axis wind turbines for distributed applications have been developed. The wind turbine system is very efficient in terms of the power-to-swept area and works very quiet even for the installation near houses and buildings and in other applications where traditional, three-blade, horizontal axis turbines are not suitable. The technology behind the new VAWT having twin rotors lies in its guided turbine design which combines inlet-guide vanes, a top-guide vane, and an impulse-type rotor. Its functional assembly makes it unique from all the Savonius rotors or their derivatives. To minimize the inertia effect of yawing system for frequent wind direction change, the twin-rotor turbine is designed to align with the wind direction by a stabilizing tail. The objective of this study is to test a 5 kW, twin-rotor VAWT installed at the Kansas site in terms of the performance, noise level, safety, and durability. The twin-rotor, WindJet turbine of the rated capacity of 5 kW resulted in an annual capacity of 26% or more at average wind speed of 6m/s, and a turbine efficiency of 50% or more.
Large-Eddy Simulation of Urban Flows: Porous-Media Analogy
Authors: Wyszogrodzki Andrzej A. , Smolarkiewicz Piotr K.
Published: March 2011

In analogy to theoretical formulation for the microscopic fluid flows in porous media [1] we perform series of numerical simulations of urban boundary layer flows through realistic street-level building structures of the Oklahoma City downtown area. The corresponding numerical solutions relate the morphology of the real building structures to topological and geometrical properties of the physical media. We analyze the results in statistical terms of random porous media, seeking the relation between the momentum flux and the macroscopic pressure gradient; i.e., a high-Reynolds-number analogy of the Darcy’s law. We relate urban “pore-space” spatial properties (e.g., homogeneity and anisotropy) to the estimates of permeability and tortuosity. We evaluate further the utility of a porous-media analogy for parameterizing urban effects in the mesoscale weather and air-quality prediction models.
Moderate Reynolds Number Mixing in a T-Channel
Authors: Thomas Susan , Ameel Tim
Published: March 2011

An experimental investigation of water flow in a T-shaped channel with rectangular cross section (20 × 20 mm inlet ID and 20 × 40 mm outlet ID) has been conducted for a Reynolds number Re range of 56 to 422, based on inlet diameter. Dynamical conditions and the T-channel geometry of the current study are applicable to the microscale. This study supports a large body of numerical work, and resolution and the interrogation region are extended beyond previous experimental studies. Laser induced fluorescence (LIF) and particle imaging velocimetry (PIV) are used to characterize flow behaviors over the broad range of Re where realistic T-channels operate. Scalar structures previously unresolved in the literature are presented. Special attention is paid to the unsteady flow regimes that develop at moderate Re, which significantly impact mixing but are not yet well characterized or understood. An unsteady symmetric topology, which develops at higher Re and negatively impacts mixing, is presented, and mechanisms behind the wide range of mixing qualities predicted for this regime are explained. An optimal Re operating range is identified based on multiple experimental trials.
Injection, Transport and Removal of Gas Bubbles in Microchannels for Heat Transfer Enhancement
Authors: Attinger Daniel
Published: March 2011

Liquid cooling is an efficient way to remove heat fluxes with magnitudes up to 10,000 W/cm2. One limitation of single-phase microchannel heat transfer is the relatively low Nusselt number, due to laminar flow. Several methods have been used to improve the Nusselt number such as geometric obtrusions, pins and fins and nanofluids. In this talk, we experimentally investigate the heat transfer enhancement of a heat sink where air bubbles are periodically injected. The segmented flow pattern generates recirculation loops that enhance transport phenomena. We show that segmented flow can enhance the Nusselt number of laminar flows in short channels by a factor two. Also, we demonstrate a simple and high-throughput method for removing bubbles from microchannels, using a hydrophobic porous membrane. The role of the thin liquid film coating the bubbles in the heat transfer and the bubble removal is investigated.
Viscous Core-Annular Flows in Microfluidic Chambers
Authors: Darvishi Samira , Cubaud Thomas
Published: March 2011

The manipulation of highly viscous materials at the microscale is a key challenge for implementing lab on chips with the ability to manage a variety of complex and reactive fluids. We describe methods for producing and controlling high-viscosity fluid threads flowing in sheath of less viscous fluids, i.e., viscous core-annular flows, in microchannels. The self-lubrication property of multi-fluid flows having large viscosity contrasts offers a promising means for manipulating interfaces between “thick” and “thin” fluids and for reducing the hydraulic resistance in micro- and nanofluidic devices. In particular, we focus on the flow behavior of threads as they traverse diverging-converging slit microfluidic chambers. The alteration of convective time-scales using extensional microgeometries permits the manipulation of complex phenomena such as viscous buckling, wetting, and coalescence. We examine the interrelation between these phenomena that are useful for passively enhancing mixing between miscible fluids and for initiating continuous emulsification processes between immiscible fluids having widely disparate viscosities.
Nanoparticle-Coated Microtubes for the Manipulation of Cancer Cells
Authors: Zhou Robin , Hughes Andrew , Liesveld Jane L. , King Michael R.
Published: March 2011

The development of novel methods for the isolation of primary stem and progenitor cells is important for the treatment of blood cancers, tissue engineering, and basic research in the biomedical sciences. Our lab has previously shown that microtubes coated with P-selectin protein can be used to capture and enrich hematopoietic stem and progenitor cells from a mixture of cells perfused through the tube at physiologically-relevant shear stresses[1][2], and that using a surface coating of colloidal silica nanoparticles (12 nm diameter, 30% by weight SiO2) increased cell capture and decreased rolling velocity[3]. Here we show that 50 nm colloidal silica nanoparticle coatings may similarly increase cell capture, and that these protocols are effective for enrichment of human adult CD34-positive HSCs from primary apheresis and bone marrow aspirate samples. Future research may include long-term colony-forming assays to confirm stem cell activity of enriched cells, and transplantation in immune-deficient mice.
Photothermal Energy Conversion in Liquid Nanoparticle Suspensions
Authors: Phelan P. E. , Taylor R. , Adrian R. , Prasher R. S. , Otanicar T.
Published: March 2011

Liquid nanoparticle suspensions, popularly termed “nanofluids,” have been the subject of numerous investigations because of their interesting thermal transport properties. Their propensity to scatter and absorb electromagnetic radiation enables other applications that can take advantage of both their radiative and thermal transport properties. In particular, we are working to develop direct-absorption solar thermal collectors in which nanofluids serve to absorb incident sunlight, thus heating the fluid directly and more efficiently than conventional solar collectors. Our experimental results, in which we irradiate nanofluids with a continuous-wave laser, demonstrate that boiling can be induced at lower incident light fluxes compared to a thin layer of pure water in front of a black absorptive backing. These findings suggest that improved solar energy conversion systems can be developed, including solar-driven direct-steam generators.
Nanofluidics Meets Plasmonics: Flow-Through Surface-Based Sensing
Authors: Escobedo C. , Brolo A. G. , Gordon R. , Sinton D.
Published: March 2011

Nanostructures exhibit both nanofluidic and nanophotonic phenomena that can be exploited in sensing applications. In the case of nanohole arrays, the role of surface plasmons on resonant transmission motivates their application as surface-based biosensors. Research to date, however, has focused on dead-ended (or ‘blind’) holes, and therefore failed to harness the benefits of nanoconfined transport combined with plasmonic sensing. A flow-through nanohole array format presented here enables biomarker sieving and rapid transport of reactants to the sensing surface. Proof of concept operation is demonstrated and compared with previous methods. The various transport mechanisms are characterized with the aim to utilize the metallic plasmonic nanostructure as an active element in concentrating as well as detecting analytes. The invited presentation will provide an overview of all our experimental, computational and analytical work in this area. This paper is focused on the analysis and evaluation of flow-through nanohole arrays for analyte sensing.
Neutrophil Motion, Adhesion and Activation in an In Vitro Micropipette Model of a Lung Capillary
Authors: Tees David F. J. , Sundd Prithu , Choi Young Eun , Goetz Douglas J. , Pai Anand , Rogers Steven
Published: March 2011

White blood cell (WBC) sequestration in lung capillaries is a key step in the inflammatory response to lung infection. P-selectin and ICAM-1 have well-defined roles in WBC adhesion in venules but their role in pulmonary capillaries is still unclear. Here, a novel in vitro Micropipette Cell Adhesion Assay used P-selectin, ICAM-1 or BSA-coated capillary-sized glass micropipettes as an in vitro model of a lung capillary. WBC were aspirated into adhesion molecule-coated vessels of varying diameters. Cell velocities and activation times were determined under pressures representative of lung capillaries. WBC velocities in this assay were significantly lower on P-selectin than BSA and decreased with increasing P-selectin concentration. These results demonstrate that P-selectin at low density mediates WBC adhesion in the pulmonary capillary geometry. WBC can also become activated upon aspiration into micropipettes and under some circumstances can be seen to exhibit a cyclic migratory behavior. This work was supported by grant BES-0547165 from the National Science Foundation and by an award from the American Heart Association.
Simulation of Gaseous Microscale Transport Phenomena via Kinetic Theory
Authors: Pantazis Sarantis , Valougeorgis Dimitris
Published: March 2011

Kinetic theory of gases, as described by the Boltzmann or model kinetic equations, provides a solid theoretical approach for solving microscale transport phenomena in gases. Due to significant advancement in computational kinetic theory and due to the availability of high speed parallel computers, kinetic equations may be solved numerically with modest computational effort. In this framework, recently developed upgraded discrete velocity algorithms for solving linear and nonlinear kinetic equations are presented. In addition, their applicability in simulating efficiently and accurately multidimensional micro flow and heat transfer problems is demonstrated. Analysis and results are valid in the whole range of the Knudsen number.
A Visualization Study During Flow Boiling of R134a in a Horizontal Microchannel
Authors: Ali Rashid , Palm Bjo¨rn , Maqbool Mohammad H.
Published: March 2011

In this paper, the experimental flow boiling visualization results of a microchannel are presented and discussed. A series of visualization experiments have been conducted in a horizontal, circular, uniformly heated microchannel, to record the two-phase flow patterns evolved during the boiling process and to study the ebullition process. A high speed camera (REDLAKE HG50LE) with a maximum of 100000 fps together with tungsten lights was used to capture the images along the test section. Microchannel was made of circular fused silica tube having an internal diameter of 0.781 mm and a uniformly heated length of 191 mm. Outside of the test tube was coated with a thin, electrically conductive layer of Indium Tin Oxide (ITO) for direct heating of the test section. Refrigerant R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 25 °C and 30 °C. Mass flux was varied from 100 kg/m2 s to 400 kg/m2 s and heat flux ranged from 5 kW/m2 to 45 kW/m2. Visualization results show that the bubble growth is restricted by the tube diameter which results in very short existence of isolated bubbly flow regime except essentially restricted to a very short length of test tube. Flow patterns observed along the length were: Isolated bubble, elongated bubble, slug flow, semi annular and annular flow. Rigorous boiling and increased coalescence rates were observed with increase in heat flux. Bubble frequency was observed to increase with both heat and mass flux. A comparison with our previous flow boiling visualization studies, carried out for a test tube of 1.33 mm internal diameter, shows that the number of active nucleation sites is less while the bubble frequency is higher for the current study. Mean bubble length and bubble velocity during elongated bubble flow pattern have also been calculated from the images obtained during the tests.
Critical Mass Flow Rate Through Capillary Tubes
Authors: Nouri-Borujerdi A. , Javidmand P.
Published: March 2011

This paper presented a numerical study that predicts critical mass flow rate, pressure, vapor quality, and void fraction along a very long tube with small diameter or capillary tub under critical condition by the drift flux model. Capillary tubes are simple expansion devices and are necessary to design and optimization of refrigeration systems. Using dimensional analysis by Buckingham’s π theory, some generalized correlations are proposed for prediction of flow parameters as functions of flow properties and tube sizes under various critical conditions. This study is performed under the inlet pressure in the range of 0.8 ≤ pin ≤ 1.5Mpa, subcooling temperature between 0 ≤ ΔTsub ≤ 10 °C. The tube diameter is in the range of 0.5 ≤ D ≤ 1.5mm and tube length between 1 ≤ L ≤ 2m for water, ammonia, refrigerants R-12, R-22 and R-134 as working fluids. Comparison between the results of the present work and some experimental data indicates a good agreement. Cluster of data close to the fitted curves also shows satisfactory results.
Industrial Compressed Air Use: Two Case Studies
Authors: Meskell Craig , O’Donnell Garret , Eret Petr , Harris Colin , De Lasa Tom , Whelan Tom
Published: March 2011

While there are several best practice standards available for minimizing the energy requirement for compressed air use in an industrial context, moving to best practice often requires investment and operational change. In production facilities, there is often a reluctance to commit to this type of change without a clear view of the benefit. Furthermore, there is very little detailed information available in the open literature that allows even a qualitative assessment of priorities. In order to address this shortcoming, analyses of two industrial compressed air systems which are already installed in manufacturing plants have been conducted in the context of energy usage. The installations are quite different in compressed air needs: one is focused on actuation and drying; while the other uses compressed air primarily for material handling. In both sites, the energy of the compressed air is evaluated at each key element of the system and the typical end use application profile is assessed. Simple models of the consumption rates are used to relate duty cycle and device count with actual total consumption. A new way of assessing the leak rate from the entire system has been developed, based on the pressure decay time, and has been implemented at one site. In this way, the energy balance of the system entire has been analyzed quantitatively, with the effect of distribution leaks accounted for directly. It is found that in both sites, open blowing operations (e.g. drying) are the largest, consumers which are amenable to optimization. It is also found that the measured leak rate at one site represented 23% of the compressed air generated, with an energy input of 455kWh per day. It is concluded that this approach can help to identify priorities for optimizing CA use at an industrial site.
Simulation of Particle-Wall Collisions in a Viscous Fluid Using a Resolved Discrete Particle Method
Authors: Feng Zhi-Gang , Michaelides Efstathis E. , Mao Shaolin
Published: March 2011

The process of particle-wall collisions is very important in understanding and determining the fluid-particle behavior, especially near walls. Detailed information on particle-wall collisions can provide insight on the formulation of appropriate boundary conditions of the particulate phases in two-fluid models. We have developed a three-dimensional Resolved Discrete Particle Method (RDPM) that is capable of meaningfully handling particle-wall collisions in a viscous fluid. This numerical method makes use of a Finite-Difference method in combination with the Immersed Boundary (IB) method for treating the particulate phase. A regular Eulerian grid is used to solve the modified momentum equations in the entire flow region. In the region that is occupied by the solid particles, a second particle-based Lagrangian grid is used, and a force density function is introduced to represent the momentum interactions between particle and fluid. We have used this numerical method to study both the central and oblique impact of a spherical particle with a wall in a viscous fluid. The particles are allowed to move in the fluid until they collide with the solid wall. The collision force on the particle is modeled by a soft-sphere collision scheme with a linear spring-dashpot system. The hydrodynamic force on the particle is solved directly from the RDPM. By following the trajectories of a particle, we investigate the effect of the collision model parameters to the dynamics of a particle close to the wall. We report in this paper the rebound velocity of the particle, the coefficient of restitution, and the particle slip velocity at the wall when a variety of different soft-sphere collision parameters are used.
High Resolution Measurement of Wall Temperature Distribution During Forced Convective Boiling in a Single Minichannel
Authors: Schweizer Nils , Freystein Martin , Stephan Peter
Published: March 2011

Cooling systems incorporating convective boiling in mini- and microchannels achieve very high thermal performance. Although many investigations related to the subject have already been conducted, the basic phenomena of the heat transfer mechanisms are not yet fully understood. The development of empirical correlations based only on flow pattern maps does not lead to a deeper knowledge of the mechanisms. In this study a comprehensive measurement technique that was successfully adapted in pool boiling experiments [8,9] was used for the investigation of forced convective boiling of FC-72 in a single rectangular minichannel. This technique allows the measurement of the local temperature with very high spatial and temporal resolution. High speed video recording was used to observe the flow inside the minichannel. The inlet Reynolds number was kept constant for the first measurements to Re = 200 corresponding to a hydraulic diameter of the minichannel of 800 μm. The Bond number for the proposed setup is about Bo ≈ 1.2. Several flow pattern regimes such as bubbly flow, slug flow and partially dryout were observed for heat fluxes up to 25 kW / m2. From an energy balance at each pixel element of the thermographic recordings the local transient heat flux could be calculated and compared to the flow pattern video recordings. The results of the first experiments already give an indication about the heat transfer mechanisms at different flow regimes.
Formation Mechanism and Characteristics of Liquid Microlayer in Mini-Gap Boiling System
Authors: Zhang Yaohua , Utaka Yoshio
Published: March 2011

Experiments were performed using the laser extinction method to directly measure the thickness of the liquid film between the bubbles and heating plates in a mini-gap formed by two parallel vertical quartz glass plates for the dielectric fluid HFE-7200 at gap sizes of 0.15, 0.3 and 0.5mm. High-speed movies were also taken to measure the velocity of bubble forefront simultaneously. It was confirmed that the microlayer thickness was determined by the gap size and velocity of bubble forefront. At the region of small Weber number, microlayer thickness of HFE-7200 was obviously thicker than that of water, toluene and ethanol at the same velocity and gap size for its small surface tension. Furthermore, by nondimensional analyzing of experimental data, the empirical correlation proposed in previous study which was based on water, toluene and ethanol is still reliable for HFE-7200.
Pore Scale Simulation of Two-Phase Fluid Flow in Berea Sandstone Core Using Lattice Boltzmann Method
Authors: Gunde Akshay C. , Mitra Sushanta K. , Babadagli Tayfun
Published: March 2011

Study of flow through porous media has been an area of major interest due to its application in diverse areas like Enhanced Oil Recovery. In order to gain a better understanding of the physical processes taking place inside a porous structure, a large number of attempts have been made to computationally simulate multiphase fluid flow at pore-scale. Recently, application of Lattice Boltzmann Method has gained popularity for this very purpose, considering its relative superiority in dealing with complex boundaries and multiphase flow. However, in order that such a numerical analysis is successful, a proper understanding of the geometry of the pore structure at the microscale is required. This paper uses a Micro-CT scan image of a Berea Sandstone core, which displays a two dimensional representation of pore network inside the scanned sample. The processed image has been imported and simulation of an immiscible two-phase flow has been carried out by using a Lattice Boltzmann program. The resident fluid (oil) has been displaced by the invading fluid (water) due to application of a pressure gradient. The pore surfaces have been treated as solid boundaries and bounce back scheme has been implemented on them to account for the no-slip condition. The ability of the code to import an arbitrary porous geometry and perform numerical analysis of fluid flow has been demonstrated.
Shear-Improved Smagorinsky Modeling of Turbulent Channel Flow Using Generalized Lattice Boltzmann Equation
Authors: Jafari Saeed , Rahnama Mohammad
Published: March 2011

Generalized Lattice Boltzmann Equation (GLBE) was used for computation of turbulent channel flow for which Large Eddy Simulation (LES) was employed as a turbulence model. The subgrid-Scale turbulence effects were simulated through a Shear-Improved Smagorinsky Model (SISM) which is capable of predicting turbulent near wall region accurately without any wall function. Computations were done for a relatively coarse grid with shear Reynolds number of 180 in a parallelized code. Good numerical stability was observed for this computational framework. Results of mean velocity distribution across the channel showed good correspondence with Direct Numerical Simulation (DNS) data. Negligible discrepancies were observed for computed turbulent statistics between present computations and those reported from DNS. Three-dimensional instantaneous vorticity contours showed complex vortical structures appeared in such flow geometries. It is concluded that such framework is capable of predicting accurate results for turbulent channel flow without adding significant complication and computational cost to the standard Smagorinsky model.
Research on Improving Cavitation Performance of High Temperature Vacuum Tower Bottom Pump
Authors: Ye Xiao-yan , Ding Ya-na , Zhang Jun-hui , Huang Ming-ke , Pan Bing-hui
Published: March 2011

In petrochemical industry the working environment of vacuum tower bottom pump is harsh and its cavitation requirement is high. In view of this problem, we optimally designed two kinds of impeller. With well designed suction chamber and volute, two groups of hydraulic model were assembled. The 3d models were made in PROE; the flow field was meshed in the pre-processing module ICEM-CFD of CFX; then CFX was used to calculate flows under three discharges in two phase motion patterns of the two hydraulic models, and also cavitation was predicted. The simulation results show that the first hydraulic model is better than the second, so the first hydraulic model was made into full mold and also test was done. The simulation results and test results are contrasted: at Q = 406m3/h, the predicted critical NPSH is 3.74m, while the test critical NPSH is 4.02m, which meet the design commands and the deviation is not large. This shows that CFD technology is helpful on performance prediction in engineering. This research provides a theoretical guidance for study on improving cavitation performance of vacuum tower bottom pump.
Extended Self-Similarity in Drag-Reduced Turbulent Flow of Viscoelastic Fluid
Authors: Li Feng-Chen , Zhang Hong-Na , Cai Wei-Hua , Yang Juan-Cheng
Published: March 2011

Direct numerical simulations (DNS) have been performed for drag-reduced turbulent channel flow with surfactant additives and forced homogeneous isotropic turbulence with polymer additives. Giesekus constitutive equation and finite extensible nonlinear elastic model with Peterlin closure were used to describe the elastic stress tensor for both cases, respectively. For comparison, DNS of water flows for both cases were also performed. Based on the DNS data, the extended self-similarity (ESS) of turbulence scaling law is investigated for water and viscoelastic fluids in turbulent channel flow and forced homogeneous isotropic turbulence. It is obtained that ESS still holds for drag-reduced turbulent flows of viscoelastic fluids. In viscoelastic fluid flows, the regions at which δu(r)∝r and Sp(r)∝S3(r)ζ(p) with ζ(p) = p/3, where r is the scale length, δu(r) is the longitudinal velocity difference along r and Sp(r) is the pth-order moment of velocity increments, in the K41 (Kolmogorov theory)-fashioned plots and ESS-fashioned plots, respectively, are all broadened to larger scale for all the investigated cases.
Hydrodynamic Considerations in Bioreactor Selection and Design
Authors: Kadic Enes , Heindel Theodore J.
Published: March 2011

The biological production of renewable fuels and chemicals, medicines, and proteins is not possible without a properly functioning bioreactor. Bioreactors are expected to meet several basic requirements and create conditions favorable to the biological material such that the desired production is maximized. The basic requirements, which are strongly influenced by fluid mechanic principles, may include minimum damage to the biological material, maximum reactor volume utilization, optimized gas-liquid mass transfer, and/or enhanced mass transfer from the liquid to the biological species. Each of these goals may be achieved within any of the major bioreactor designs, which generally fall under the categories of stirred tank, bubble column, or airlift bioreactor. Yet, each of the bioreactor designs has strengths and weaknesses. This paper provides an overview of bioreactor hydrodynamic developments and the fluid mechanic issues that should to be considered when selecting a bioreactor for experimental and production purposes.
A Method to Quantify Mixing in a Two Component Fluidized Bed
Authors: Keller Norman K. , Heindel Theodore J.
Published: March 2011

Fluidized bed technology can be used for pyrolysis and gasification of solid fuel particles such as biomass, which is important to industry because of its potential as an alternative for petroleum-based fuels. To efficiently utilize a fluidized bed reactor it is necessary, among other factors, to investigate the mixing and segregation behavior of the fuel particles with the bed material. In order to characterize the material distribution, a technique to visualize the biomass inside a fluidized bed reactor has been developed using X-ray computed tomography (CT) scans. This paper presents an image analysis procedure that can be used to quantify and characterize the local mixing and segregation in a 3D fluidized bed.
Combined Experimental/Numerical Development of Propulsor Evaluation Capability
Authors: Dropkin Amanda M. , Huyer Stephen A. , Henoch Charles
Published: March 2011

Propulsor design methods utilize Computational Fluid Dynamics (CFD) to develop initial propulsor configurations and predict the full-scale in-water performance of these optimal designs. However, like all numerical models, these CFD models need experimental validation to provide a sufficient level of confidence in the design. The actual data needed to validate CFD models include propulsor inflow velocities and thrust and are impractical to collect for full-scale vehicles. As a result, the in-water propulsor performance can be significantly different than CFD predictions. Another approach in the propulsor design process is to experimentally test a subscale version of the vehicle and appropriately scale results. This scaling is often unreliable due to differences between open water conditions and the flow in the laboratory facility. This paper presents a method to combine CFD modeling with subscale experiments to improve full-scale propulsor performance prediction. Laboratory experiments were conducted on subscale generic torpedo models in the 12″ × 12″ water tunnel located at the Naval Undersea Warfare Center in Newport, Rhode Island. This model included an operational ducted post-swirl propulsor. Laser Doppler Velocimetry was used to measure several velocity profiles along the torpedo hull. The experimental data were used in this project to validate the CFD models constructed using the commercial CFD software, Fluent®. Initially, axisymmetric two-dimensional simulations investigated the bare body, hull only case, and a shrouded body without the propulsor. These models were selected to understand the axisymmetric flow development and investigate methods to best match the propulsor inflow. A variety of turbulence models including the realizable k-epsilon model and the Spallart-Almaras model were investigated and ultimately the numerical and experimental velocity profiles were found to match within 3%. Based on these water tunnel simulations, differences between the flow in the facility and open water could then be characterized. These differences quantified both the effect of Reynolds number as well as local flow acceleration due to tunnel blockage effects. Full 3-D flow simulations were then conducted with an operating propulsor and compared with the corresponding subscale experimental data. Finally, simulations were conducted for full-scale tests and compared with actual in-water data. While the in-water data was limited to propulsor rpm and vehicle velocity, the operating advance ratio could be determined as well as the estimated vehicle thrust. This provided a method to utilize CFD/experiments to bridge the gap between subscale and full-scale tests. The predicted in-water advance ratio of 1.87 was very close to the measured value of 1.75.
Experimental and Numerical Investigations of a Transonic, High Turning Turbine Cascade With a Divergent Endwall
Authors: Abraham Santosh , Panchal Kapil , Xue Song , Ekkad Srinath V. , Ng Wing , Brown Barry J. , Malandra Anthony
Published: March 2011

The paper presents detailed measurements of midspan total pressure loss, secondary flow field, static pressure measurements on airfoil surface at midspan, near hub and near the end walls in a transonic turbine airfoil cascade. Numerous low-speed experimental studies have been carried out to investigate the performance of turbine cascades. Profile and secondary loss correlations have been developed and improved over the years to include the induced incidence and leading edge geometry and to reflect recent trends in turbine design. All of the above investigations have resulted in better understanding of flow field in turbine passages. However, there is still insufficient data on the performance of turbine blades with high turning angles operating at varying incidences angles at transonic Mach numbers. In the present study, measurements were made at +10, 0 and −10 degree incidence angles for a high turning turbine airfoil with 127 degree turning. The exit Mach numbers were varied within a range from 0.6 to 1.1. Additionally, the exit span is increased relative to the inlet span resulting in one end wall diverging from inlet to exit at 13 degree angle. This was done in order to obtain a ratio of inlet Mach number to exit Mach number which is representative to that encountered in real engine and simulates the blade and near end wall loading that is seen in an engine. 3D viscous compressible CFD analysis was carried out in order to compare the results with experimentally obtained values and to further investigate the design and off-design flow characteristics of the airfoil under study. All aerodynamic measurements were compared with CFD analysis and a reasonably good match was observed.
The Influence of Channel Orientation and Flow Rates on the Bubble Formation in a Liquid Cross-Flow
Authors: Siddiqui Kamran , Chishty Wajid A.
Published: March 2011

For gas turbines burning liquid fuels, improving fuel spray and combustion characteristics are of paramount importance to reduce emission of pollutants, improve combustor efficiency and adapt to a range of alternative fuels. Effervescent atomization technique, which involves the bubbling of an atomizing gas through aerator holes into the liquid fuel stream, has the potential to give the required spray quality for gas turbine combustion. Bubbling of the liquid stream is presently used in a wide range of other applications as well such as spray drying, waste-water treatment, chemical plants, food processing and bio- and nuclear-reactors. In order to optimize control of the required aeration quality and thus the resulting spray quality over a wide range of operating conditions, it is important that the dynamics of bubble formation, detachment and downstream transport are well understood under these circumstances. The paper reports on an experimental study conducted to investigate the dynamics of gas bubbles in terms of bubble detachment frequency when injected from an orifice that is subjected to a liquid cross-flow. The experiments were conducted over a range of gas and liquid flow rates and at various orientations of the liquid channel. Analyses presented here are based on shadowgraph images of two-phase flow, acquired using a high speed camera and a low intensity light source. An image processing algorithm was developed for the detection and characterization of the bubble dynamics. Results show that bubble detachment frequency is a function of both liquid cross-flow rate and the gas-to-liquid flow rate ratio.
Numeric Analysis on Internal Flow of High-Speed Pump of Different Structural Style
Authors: Shi Hai-xia , He Yu-jie , Wu Jian-bo , Li Qiang
Published: March 2011

In order to study the effect of the structural style of high-speed pump on internal flow, 4 types high-speed pump of different structural style are selected for comprehensive analysis, S-A turbulence model and SIMPLIEC algorithm are adopted for numerical simulation of internal flow. By comprehensively comparing the static pressure, total pressure, sectional velocity vector and flow path line, two types of high-speed pumps with cylindrical-blade impeller matching nozzle type pump casing have relatively ideal pressure field and flow condition, can reduce hydraulic loss. The comprehensive performance of 4 types of high-speed pump is predicted through computational simulation, the high-speed pump with open cylindrical-blade impeller matching nozzle type pump casing has most excellent performance, indicating the structural style of high-speed pump has effect on its internal flow and performance.
Optothermal Control of Local Fluid Temperature in Microfluidics
Authors: Akbari M. , Bahrami M. , Sinton D.
Published: March 2011

This paper outlines an optothermal approach to manipulate local fluid temperatures in microfluidic and lab-on-chip systems. The system has the ability to control the size, location and the source heat flux using an external computer and is not constrained by predefined geometries, complex fabrication or control system. The thermal performance is evaluated using a temperature-dependent fluorescent dye. Experiments demonstrate localized heating up to 35°C over ambient temperature for a heat source spanning a downstream length of 1.5 mm. Experimental results are compared with a 1D thermal analysis of the system based on the Taylor-Aris dispersion.
Identification of Flow Induced Vibration in Steam Exhaust Hood-Duct System
Authors: Motriuk R. Wojciech Voytek
Published: March 2011

Vibration and noise of concern were observed at the hood-duct of the exhaust steam turbine at a power generating facility. The field personnel noticed this problem after a startup of the unit which followed the maintenance replacement of the expansion joints. The author was contacted by the field personnel and asked to formally evaluate this problem and advise on the steps needed to eliminate it. Several similar units were scheduled for the same maintenance work. The analysis carried out was based on the acoustic and structural dynamic modeling of the critical parts of the hood-duct system in question. Both the flow induced excitations and mechanical structural responses were considered. The results of the analysis combined with field measurements indicate a vibration generation mechanism which is flow induced in nature and is coupled with the acoustic field and enhanced by the high stream-wise flow velocities. Consequently, there are two categories of solutions identified to the observed problem. The first considers addressing the problem at the source and the second suggests path treatment. Addressing the problem at the source by means of aeroacoustic optimization of the flow path is recommended. Detuning the mechanical response from the flow excitation(s), i.e. the vibration path treatment, involves an aspect of greater or lesser trial and error since the responses of the studied structure depend on many variables; therefore, even the most accurate calculations might not be sufficient to give the expected results. Both recommendations are discussed.
Axial Evolution of Asymmetric Vortical Flow Around Slender Body of Revolution at High Incidence
Authors: Guan Xiaorong , Fu Song , Xu Cheng
Published: March 2011

For studying the axial evolution of the flow around slender body of revolution at high incidence under different conditions, numerical simulations are performed. Based on the computational results, several conclusions and deductions are obtained. When the flow is asymmetric and whether the asymmetry is remarkable or not, downstream axially it always presents itself in the structure of leeside vortices forming, rising and shedding alternately from opposite sides of the body and induces the sectional side force of waving sinusoidally. Based on the idea of vortex dividing, a forming mode of shed and new leeside vortices is put forward, which is composed of two idiographic manners. The axial evolutions on the forming manner can be reduced to three idiographic laws. The global asymmetry degree of the flow lies on both the axial evolution law on the forming manner and the intensity of leeside vortex. The influences of incidence, freestream Mach number and nose-perturbation location on the axial evolution of the asymmetric vortical flow are achieved as well.
Numerical Analysis of Unsteady Flow in Centrifugal Pumps With Impellers Based on Different Hydraulic Design Principles
Authors: Yuan Jianping , He Hongqin , He Zhixia , Yuan Shouqi
Published: March 2011

The centrifugal pump is one of the most widely used pumps. Lower efficiency, worse cavitation performance and stronger unsteady flow are three main problems for the low-specific-speed centrifugal pumps. Due to unsteady flow within pumps, pressure is fluctuant, which further induces vibration and noise. For a centrifugal pump with different impellers, its characteristics of unsteady flow are also different. In this paper, five different impellers were designed adopting low-specific-speed design method, splitter-blades design method and velocity-coefficient method with a set of performance parameters (Q = 25m3/h, H = 10m, n = 1450r/min). 3D unsteady turbulent flow field within the centrifugal pump was simulated. The periodic fluctuation phenomenon and the unsteady flow characteristics were investigated. The static pressure fluctuation in the volute and at the volute outlet and the instantaneous head changes were showed for the pumps with five different impellers operated at design and off-design conditions. The pressure fluctuation is the strongest near the tongue and is slighter at the volute outlet section. The transient head fluctuation increases with the flow rate. The transient head fluctuation of the pump with the low-specific-speed designed impeller is the biggest, while that of the pump with the normal-designed impeller is the smallest. Among the three splitter-blade impellers, the transient head fluctuation of the impeller with splitter blades leaned to the corresponding suction side of the long blades is the smallest. Eventually, the different design schemes were evaluated using the unsteady flow analysis. These conclusions from this paper can supply some references for the design of low-specific-speed centrifugal pumps considering its pressure fluctuation and flow-induced vibration and noise.
Optimal Pump Station Operation Based on the Single Variable-Frequency Speed Regulation
Authors: Tang Yue , Zhao Kun , Yuan Shouqi , Zhang Xinpeng , Xu Yanfei
Published: March 2011

Aiming at the situations that pumps of the same model are used in most pump stations and the price of frequency converter is too high, the reasons are analyzed for the low efficiency of pump assembly operating under the single frequency converter configuration condition. In multipled pump assembly equipped with pumps of the same model, adding an extra pump operating as a transitional pump whose maximum flow rate is half of the previous ones’ will reduce variable-speed pump’s speed regulation ratio. According to this, a model for optimising operation of pump station is constructed with the minimum shaft power being objective function and solved by simulated annealing genetic algorithm. And then a comparison analysis about energy consumption and economical efficiency is made toward these three speed regulation strategies. Energy consumption analysis results indicate that single speed regulation with transitional pump consumes a little more energy than that of all speed regulation, while economical efficiency analysis results show that it cost only one twelfth of all-speed-regulation in extra investment and its payback period is one eighth of the latter’s. The effect of energy saving of this strategy of speed regulation is obvious and solves well the problem that pump station equipped with the same model of pump operates at low efficiency when equipped with a single frequency converter.
Detached-Eddy Simulations of the Effects of Different Wind Gust Models on the Unsteady Aerodynamics of Road Vehicles
Authors: Favre Tristan , Efraimsson Gunilla
Published: March 2011

The relative flexibility of nowadays simulation techniques offers an alternative way to experiments in order to investigate unsteady crosswind aerodynamics in an industrial framework. In this study, time-accurate simulations, Detached-Eddy Simulations (DES), are used to simulate the flow around a simple vehicle shape, the so-called Windsor model. The ReL of the corresponding flow case is 2.0 · 106. The influence of different deterministic models of wind gusts on the aerodynamic loads and moments are studied. The wind gusts are varied in the stream-wise and the vertical direction. The magnitude of the gusts models corresponds to a yaw angle of 20°. The aerodynamic loads calculated show a large excess of drag coupled with a reduction of the pitch moment. In addition, although the side force has a smooth variation in the gust, overshoots up to 18% higher than the steady value of yaw moment are also observed.
Stability of a Shear-Thinning Film on a Porous Substrate
Authors: Usha R. , Millet S. , BenHadid H. , Rousset F.
Published: March 2011

A significant feature of gravity-driven film flows of Newtonian and rheologically complex fluids down an inclined/vertical substrate is the instability of the free surface which manifests as surface waves having wavelengths much larger than the film thickness. There are a number of applications which can be modeled as thin film flow systems on porous substrates. Pascal [1] investigated the stability of a falling power-law film on an inclined porous substrate. This model for the fluid predicts a viscosity that goes to infinity as the shear rate approaches zero. There is a need to employ a more appropriate model to examine the effects of non-Newtonian rheology on the dynamics and stability of thin film free surface flows down inclined or vertical rigid/porous substrates. The four-parameter Carreau model predicts a viscosity that remains finite as the shear rate approaches zero and is given by η−η∞η0−η∞=[1+(δγ)˙2]n−12.(1) Weinstein [2] and Rousset et al. [3] have considered the Carreau model and have examined the temporal stability of a film flow down an impermeable rigid inclined substrate. The authors show that a shear-thinning Carreau fluid film on a rigid impermeable substrate is more unstable than a Newtonian film. This calls for an analysis that includes both the effects of Carreau non-Newtonian rheology and bottom permeability and the present study reports such an investigation of a Carreau non-Newtonian film on a porous inclined substrate.