bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimization of a Small 2-Stage Centrifugal Blower by CFD Methods
Authors: Benra F.-K. , Dohmen H. J. , Kloda M.
Published: March 2009

Emissions legislation for internal combustion engines has been tightened yet further, demanding a reduction of more than 90% with respect to untreated emissions. In conjunction with measurements concerning the engine and the power system, λ = 1 regulation, and 3-way catalytic converter, it is possible to reduce the pollutants CO, HC and NOx by means of catalytic oxidation. Within the first few minutes after a cold start of the engine the temperature of the catalytic converter is below its specified “light-off” temperature. The injection of air into the exhaust manifold causes an after-burning of the flue gas. So this leads to a more quickly raising of the temperature of the catalytic converter and a better reduction of the pollutants CO and HC in the cold start phase of the combustion engine. The secondary air is compressed by a centrifugal blower which is directly driven by a 13-Volt DC motor. To reduce the power consumption for the electric drive and to minimize the construction volume of the machine, the blower has been redesigned by means of commercial CAx-methods [1]. First the single components of the turbomachine, the impeller, the return channel and the volute have been designed for the required operational characteristics. After an optimization process for these single components the complete machine consisting of a two stage rotor, a return channel between the impellers and a volute at the end of the machine has been investigated numerical by a commercial 3D Navier-Stokes solver. The differences between the computations of the single components compared to the according computations for the whole machine are shown as well. Beside the calculation of the flow field in the desired design point an investigation of the blower characteristic was accomplished. To validate the numerical results a prototype by the method of rapid prototyping was generated. The measured overall characteristic curves of the machine have been compared to the calculated ones. The differences are discussed in detail with the aid of pressure measurements at several locations in the machine.
Numerical Modeling of Strongly Heated Internal Gas Flows
Authors: Spall Robert E. , Richards Adam , McEligot Donald M.
Published: March 2009

The v2–f turbulence model was used to model an axisymmetric, strongly heated, low Mach number gas flowing upward within a vertical tube in which forced convection was dominant. The heating rates were sufficiently high so that fluid properties varied significantly in both axial and radial directions; consequently, fully developed mean flow profiles did not evolve. Results using both constant and variable turbulent Prandtl number approximations in the energy equation were obtained. Comparisons between computational results, and experimental results which exist in the literature, revealed that the v2–f model performed quite well in predicting axial wall temperatures, and mean velocity and temperature profiles. This result may be contrasted with most two-equation model results appearing in the literature for which the wall heat transfer rates are significantly over predicted.
Self-Preserving Properties of Steady Round Nonbuoyant Turbulent Jets in Uniform Crossflows
Authors: Diez F. J. , Bernal L. P. , Faeth G. M.
Published: March 2009

The properties of steady round nonbuoyant turbulent jets in uniform crossflows were studied, motivated by applications to the dispersion of heat and potentially harmful substances from steady exhaust flows. Emphasis was placed on self-preserving conditions far from the source where source disturbances have been lost and where jet properties are largely controlled by the conserved properties of the flow. The experiments involved steady round nonbuoyant fresh water jet sources injected into uniform and steady fresh water crossflows within the windowed test section of a water channel facility. Flow visualization was carried out by photographing dye-containing source jets. Mean and fluctuating concentrations of source fluid were measured over cross sections of the flow using Planar Laser-Induced Fluorescence (PLIF). The self-preserving properties of the flow were correlated successfully based on scaling analysis due to Fischer et al. (1979) which involves assumptions of no-slip convection in the cross stream direction (parallel to the crossflow) and a self-preserving nonbuoyant turbulent line puff having a conserved momentum force per unit length that moves in the streamwise direction (parallel to the initial source flow). The flow structure consisted of two counterrotating vortices, with their axes nearly aligned with the crossflow direction, that move away from the source in the streamwise direction due to the action of source momentum. Present measurements extended up to 160 source diameters from the source in the streamwise direction and yielded the following results: jet motion in the cross stream direction satisfied the no-slip convection approximation; geometrical features, such as the penetration of flow boundaries and the trajectories of the axes of the counter-rotating vortices, reached self-preserving behavior at streamwise distances greater than 40–50 source diameters from the source; parameters associated with the structure of the flow, e.g., contours and profiles of mean and fluctuating concentrations of source fluid, however, did not reach self-preserving behavior prior to reaching streamwise (vertical) distances greater than 70–80 source diameters from the source.
2-D CFD Computation on Effect of Flow Injection Into Base With Mixed Flows
Authors: Izadi Mohammad J.
Published: March 2009

Airplane engines play the key role on all aircraft. Propulsion engineers are all involved in making better engines by improving their performance. One of the usual problems on jet engines is the base drag. Base drag is maximum when an engine is having cold subsonic air flow around and high supersonic hot jet flows from the center of it which is more than 95% of the flight time. This drag on a 2D model was calculated by using Subsonic panel modeling as the outer body of the engine and the flow and also using supersonic paneling as the inner jet flow. At first an estimated model is given to the program and after some time a final model with some drag is obtained. Then by using this model some cold air (Same flow as free stream) at one time and hot air (Same flow as Jet) at another time is injected into the base between the solid body and the outer cold airflow and the inner hot jet flow. It was seen that this injection reduces the base area and also the length of the base wake consequently decreases the base drag. The injection of hot air reduces the drag more than when the injection is done by cold air.
Improvement of Firing Frequency Limits by Investigation of Ejection Failure Modes in Thermal Inkjet Print Heads
Authors: Kim Min Soo , Sohn Dong Kee , Shin Seung Joo , Kuk Keon , Oh Yong Soo
Published: March 2009

Ejection failure modes determining the firing frequency limit in thermal inkjet heads were investigated from performance experiments and numerical simulations. Dominant failure modes were affected by the flow resistance ratio in the ink flow passages. Optimal flow resistance ratios were obtained experimentally to provide maximum frequency limits for both mono and color inkjet heads. Numerical simulations were performed on the meniscus oscillation, the ejection behavior in consecutive firing, and the cross-talk induced flow. Numerical results supported the importance of flow resistance ratio in maximizing firing frequency limit in both unit nozzle and multi-nozzle firing. Our investigation will help to develop the inkjet print heads of more reliable high-speed performance.
DNS of Drag-Reducing Turbulent Channel Flow With Coexisting Newtonian and Non-Newtonian Fluid
Authors: Yu Bo , Kawaguchi Yasuo
Published: March 2009

In the present study, we numerically investigated drag-reducing turbulent channel flows by surfactant additives. Surfactant additives were assumed to be uniformly distributed in the entire flow region by turbulent convection and diffusion etc., but it was assumed that the shear-induced structure (SIS) (network of rod-like micelles) could form either in the region next to the walls or in the center region of the channel, making the fluid viscoelastic. In other regions surfactant additives were assumed to be incapable of building a network structure, and to exist in form of molecules or micelles which do not affect the Newtonian properties of the fluid. With these assumptions, we studied the drag-reducing phenomenon with coexisting Newtonian and non-Newtonian fluids. From the study we identified the effectiveness of the network structures at different flow regions, and showed that the phenomenon of drag-reduction (DR) by surfactant additives is not only closely associated with the reduction of Reynolds shear stress but also related to the induced viscoelastic shear stress.
Applications of the Lagrangian Dynamic Model in LES of Turbulent Flow Over Surfaces With Heterogeneous Roughness Distributions
Authors: Bou-Zeid Elie , Meneveau Charles , Parlange Marc B.
Published: March 2009

We study turbulent flow over surfaces with varying roughness scales, using large eddy simulation (LES). The goal is to use LES results to formulate effective boundary conditions in terms of effective roughness height and blending height, to be used for RANS. The LES are implemented with the dynamic Smagorinsky model based on the Germano identity. However, as is well-known, when this identity is applied locally, it yields a coefficient with unphysically strong fluctuations and averaging is needed for better realism and numerical stability. The traditional approach consists of averaging over homogeneous directions, for example horizontal planes in channel flow. This requirement for homogeneous directions in the flow field and the concomitant inability to handle complex geometries renders the use of this model questionable in studying the effect of surface heterogeneity. Instead, a new version of the Lagrangian dynamic subgrid-scale (SGS) model [1] is implemented. A systematic set of simulations of flow over patches of differing roughness is performed, covering a wide range of patch length scales and surface roughness values. The simulated mean velocity profiles are analyzed to identify the height of the blending layer and used to measure the effective roughness length. Extending ideas introduced by Miyake [2] and Claussen [3], we have proposed a simple expression for effective surface roughness and blending height knowing local surface patch roughness values and their lengths [4]. Results of the model agreed well with the LES results when the heterogeneous surface consisted of patches of equal sizes. The model is tested here for surfaces with patches of different sizes.
Multiple Length Scales for Maximal Heat Transfer Density in Forced and Natural Convection
Authors: da Silva Alexandre K. , Bejan Adrian , Fautrelle Yves
Published: March 2009

The present paper is a review of a new conceptual design for the maximization of heat transfer density (i.e., heat transfer rate per unit of volume) in channels installed in a fixed volume. The volume is filled optimally with parallel equidistant heated blades. The optimal spacing between two blades is such that the thermal boundary layers merge at the end of the channel. The blades can be cooled either by laminar natural or forced convection. Unheated volumes of fluid near the tips of the boundary layers are eliminated through the insertion of new generations of smaller blades. Based on the same principle, new generation of even smaller blades are added stepwise to the multi-scale structure. The optimal length of each family of new blades is determined based on the assumption that the flow downstream the smallest plates is not disturbed. This allows us to predict the exact height where the thermal boundary layer of two distinct generations will merge. As the number length of scales increase, the volume-averaged heat transfer density increases. The results show that the improvement associated with optimal insertion of the first and second generations of scales is significant. Diminishing returns are also observed as the complexity increases, meaning that the contribution of each smaller scale is less important than the contribution of the preceding scale. The theoretical results (i.e., the optimal spacings, optimal lengths, maximal heat transfer density and the cutoffs) are validated numerically.
Effects of Mesh-Induced Upstream Turbulence on Flip-Flop Flow Inside Diamond-Shaped Cylinder Bundles
Authors: Umeda Shinzaburo , Yang Wen-Jei
Published: March 2009

An experimental study is conducted to investigate effects of mesh-induced upstream turbulence on flip-flop flow inside diamond-shaped cylinder bundles. In order to quantitatively treat flip-flop flow induced by the self-excited oscillation of vortices, flow measurements by means of two-dimensional particle image velocimetry (PIV) and two-dimensional laser Doppler velocimetry (LDV) are employed. Flow cross-sectional area and flow rate are varied to change the Reynolds number in the bundles. A turbulence mesh is installed a very short distance upstream from the inlet of the test section. The LDV is employed to measure velocity changes in the flip-flop flow, while power spectra representing its oscillating characteristics are determined from LDV data. The dominant frequency is observed special features are disclosed in the relationship between the Reynolds number and the Strouhal number representing the dimensionless dominant frequency in all power spectra of the flip-flop flow. It is disclosed that both the flow cross section and the upstream turbulence are related to the generation of flip-flop flow in complex manner, and that the effects of the turbulence differ depending upon the flow cross section.
Hysteresis of Flow Around an Elliptic Cylinder in Critical Reynolds Number Regime
Authors: Ota Terukazu , Ueda Ken-ichiro , Yoshikawa Hiroyuki
Published: March 2009

Experimental investigations of the flow around an elliptic cylinder of axis ratio 1:2 were carried out for several angles of attack in a wide range of Reynolds number. The flow characteristics were studied by measuring the fluid forces and the surface pressure. In the critical Reynolds number regime, a discontinuous change of flow state was observed. This change was accompanied by the remarkable hysteresis phenomenon. The fluctuation of fluid forces and surface pressure, in general, markedly decreases in the critical flow state.
Particle/CIP Hybrid Method for Predicting Motions of Micro and Macro Free Surfaces
Authors: Ishii Eiji , Ishikawa Toru , Tanabe Yoshiyuki
Published: March 2009

To predict motions of micro and macro free surfaces simultaneously within gas-liquid flows, we have developed a particle/CIP (Cubic Interpolated Propagation) hybrid method. The micro free surfaces (smaller than grid sizes) were simulated by the particle method, and the macro free surfaces (larger than grid sizes) were simulated by the CIP method. And then the particles used in the particle method were assigned near the macro free surfaces by using volume fraction of liquid that was calculated by the CIP method. The developed method was used to predict the collapse of a liquid column. Namely, it predicted both the large deformation of the liquid column and the fragmentation of it simultaneously, and the predicted configurations of the liquid column agreed well with the experimentally measured ones. It was also used to predict breakup of liquid films in a fuel injector used for engines of automobiles, and the predicted profile of the liquid film was sharp in an air region where the thickness of the liquid film became thinner than the grid sizes.
Unsteady Cavitating Flow Around an Inclined Rectangular Cylinder
Authors: Ota Terukazu , Tsubura Isao , Yoshikawa Hiroyuki
Published: March 2009

Unsteady characteristics of cavitating flow around an inclined rectangular cylinder with a width to height ratio of 8.0 were experimentally investigated for various angles of attack and cavitation numbers. Measurements of fluid dynamic force and surface pressure were made and the cavity configuration was observed with a camera. Especially considered are the self-oscillating unstable flow characteristics along with the time variation of cavity configuration. It is found that a severe vibration occurs at some cavitation number, in which the attached cavity is formed in the separation bubble. As the cavitation number further decreases, the low frequency fluctuation of flow occurs.
Thermal Evaluations for Next Generation Nuclear Plant Fuel Testing
Authors: Ambrosek Richard G. , Utterbeck Debbie J. , Miller Brandon
Published: March 2009

The DOE Advanced Fuel Cycle Initiative and Generation IV reactor programs are developing new fuel types for use in the current Light Water Reactors and future advanced reactor concepts. The Advanced Gas Reactor program is planning to test fuel to be used in the Next Generation Nuclear Plant (NGNP) nuclear reactor. Preliminary information for assessing performance of the fuel will be obtained from irradiations performed in the Advanced Test Reactor large “B” experimental facility.
Enhancing the Teaching of Fluid Mechanics and Transport Phenomena via FlowLab: A Computational Fluid Dynamics Tool
Authors: Curtis Jennifer Sinclair , Henthorn Kimberly , Moeykens Shane , Krishnan Murali
Published: March 2009

Introducing Computational Fluid Dynamics (CFD) to engineering students at the undergraduate level has become more common in recent years, although there are significant barriers for doing so using a generalized CFD solver. A common constraint is the quantity of material to be covered in a fixed amount of time in a given course, which leaves little time left for learning the use of a generalized CFD package. With this consideration in mind, FlowLab (www.flowlab.fluent.com) was introduced by Fluent Inc. FlowLab may be described as a virtual fluids laboratory—a computer based analysis and visualization package. Using FlowLab, students solve predefined CFD exercises. These predefined exercises facilitate teaching and provide students with hands-on CFD experience. Through the design of each FlowLab exercise, students are introduced to engineering problems and concepts as well as CFD via a structured learning process. In the fall 2003 semester at Purdue University, FlowLab was used in CHE 540, a transport phenomena course offered within the School of Chemical Engineering. This course is open to advanced undergraduate engineering students and graduate students. Students were exposed to eight separate FlowLab exercises in this course. This paper gives a detailed summary of one of these specific exercises, developing flow in a pipe with and without heat transfer. The paper emphasizes how the use of CFD via FlowLab enhanced the teaching of specific concepts in transport phenomena as well as concepts in CFD such as creating a parametric geometry, discretizing the geometry, specifying boundary conditions, material properties and operating conditions, numerical solution techniques and post-processing. Experiences from this course are that FlowLab is a positive force for creating student interest and excitement in the area of fluid mechanics and transport phenomena. Using FlowLab’s post-processing capabilities, students were able to visualize complex flow fields and make direct comparison to analytical theory and experimental correlation. In addition, FlowLab provided a structured learning experience which reinforced proper pedagogy for applying CFD to engineering problems. Upon completion of the course, a student survey was performed in CHE 540 focusing on FlowLab integration and usage, and survey responses are summarized in this paper.
Numerical Validation of a New Methodology for the Generation of Realistic Turbulent Inflow Conditions for LES
Authors: Druault Ph. , Largeau J.-F. , Coiffet F. , Lardeau S. , Bonnet J.-P.
Published: March 2009

Due to the difficulty of specifying realistic inflow conditions for numerical simulations, a new method has been recently developed permitting to use time-dependent highly turbulent velocity field as inflow condition for spatially developing Large Eddy Simulation. This methodology is based on experimental time histories obtained at few selected reference locations with hot wires probes. From these measurements, the instantaneous velocity field is reconstructed on the inlet mesh of a numerical simulation and this reconstructed field is used as inflow condition for the simulation. The objective of this work is to validate the present methodology by using a purely numerical approach, based on Direct Numerical Simulation, in which experimental data are replaced by numerical data. Based on statistical analysis and instantaneous vorticity field visualizations, current numerical validation provides promising results for this new inflow condition generation method based on real experimental data.
Simulation of Acoustic Streaming in a Resonator
Authors: Farouk Bakhtier , Aktas Murat K.
Published: March 2009

Formation of vortical flow structures in a rectangular enclosure due to acoustic streaming is investigated numerically. The oscillatory flow field in the enclosure is created by the vibration of a vertical side wall of the enclosure. The frequency of the wall vibration is chosen such that a standing wave forms in the enclosure. The interaction of this standing wave with the horizontal solid walls leads to the production of Rayleigh type acoustic streaming flow patterns in the enclosure. All four walls of the enclosure considered are thermally insulated. The fully compressible form of the Navier-Stokes equations is considered and an explicit time-marching algorithm is used to explicitly track the acoustic waves. Numerical solutions are obtained by employing a highly accurate flux corrected transport (FCT) algorithm for the convection terms. A time-splitting technique is used to couple the viscous and diffusion terms of the full Navier-Stokes equations. Non-uniform grid structure is employed in the computations. The simulation of the primary oscillatory flow and the secondary (steady) streaming flows in the enclosure is performed. Streaming flow patterns are obtained by time averaging the primary oscillatory flow velocity distributions. The effect of the amount of wall displacement on the formation of the oscillatory flow field and the streaming structures are studied. Computations indicate that the nonlinearity of the acoustic field increases with increasing amount of the vibration amplitude. The form and the strength of the secondary flow associated with the oscillatory flow field and viscous effects are found to be strongly correlated to the maximum displacement of the vibrating wall. Total number of acoustic streaming cells per wavelength is also determined by the strength and the level of the nonlinearity of the sound field in the resonator.
CFD Analysis of Gas-Insulated Transformers
Authors: Lee D. J. , Seol S. S. , Park I. S. , Kim J. H. , Shin S. B. , Woo J. H.
Published: March 2009

A thermal design of transformers has been performed using an empirical formula. In order to reduce the developing cost and time, CFD analysis is used in thermal design process for gas-insulated transformers. We calculated the pressure loss of coolant and the temperature rise of winding with empirical formulas and CFD analysis. Also, we constructed some real machines and compared the analytic results with the experimental data. The comparison shows a good agreement between the CFD calculations and experimental results.
Investigation of a Two-Phase Siphon: Pressure Drop Characteristics, Flow Prediction, and Flow Regime Change Prediction
Authors: Arthur J. Howard , Morgan Charles D. , Engelhard Cory D. , Austin Berton
Published: March 2009

In some nuclear power plants, a passive siphon breaking system is used to prevent the spent fuel tank from draining in the event of a break in the vertical leg of the heat exchanger piping. A hole is drilled in the horizontal leg of the piping. When the water level in the tank drops below the pipe level air is sucked into the system. When sufficient air is entrained in the pipe the siphon will break. A model to predict the flow rate in a vertical siphon was developed in reference 1 using the homogeneous flow model. The predicted flow rates were greater than measured flow rates. In order to improve the predictive capability, pressure drop measurements were obtained from ten foot vertical test sections with nominal diameters of 0.5, 0.75, 1.0, 1.25, 1.5, and 2.0 inches. Values of the distribution parameter, Co, for the drift flux model were determined from the pressure drop data. When the model of reference 1 is changed from homogeneous flow to drift flux model with the distribution parameter determined from the pressure drop data, good agreement with measured liquid flow rates is obtained. The improved model, along with the correlation for the siphon break condition obtained provides a good method for determining the hole size required to break the siphon. There is a paucity of data for two-phase flow regime transition where the flow is in the downward direction that is typical in a siphon. Flow regime transition data were obtained using the test sections listed above. The flow map of Oshinowo2 et al. gave a reasonable prediction of the transition from bubbly to slug flow. None of the references investigated gave an adequate prediction of the point where the siphon would break. A correlation for the siphon break point was developed.
Fluid Flow Applications of Solid Freeform Fabrication: Valves in Single Build
Authors: Kumpaty Subha K. , Garten Dawn , Laursen Russell , Kamara Sheku
Published: March 2009

There is a constant need for custom designed and manufactured valves that are capable of precision flow control. The applications vary, such as medically related fluid situations; a check valve for a feeding tube for example. The fluids under consideration are diverse: air, water and blood, to name a few. Solid freeform fabrication (SFF) has immense potential for creating intricate valve structures without the limitations and numerous machining processes involved in conventional methods of valve assembly. This research demonstrated the usefulness of SFF in novel fluid mechanics applications. The primary goal was the design and fabrication of a functional valve in one build using a stereolithography apparatus. A valve with linear motion was designed that was rapid prototyping compatible and was fabricated using stereolithography under the NSF sponsored REU program in the summer of 2002. The valve was fully functional as a sealed system. Its performance was evaluated and compared to that of a traditional gate valve. Precision flow control and improved sealing were two goals that this research program achieved in summer of 2003. Through proper design, an angle valve has been built that retains the flow and sealing characteristics of a traditional valve, but has the advantages associated with SFF. The highlight of this work was the fact that moving parts, integral O-rings, and threaded connections were all shown to be possible and the entire valve prototyped in a single build using SFF. In addition, a downsized valve, 0.4 inch inside diameter, has been built that will broaden the applications of this technology. The accomplishments of this research will allow functional prototypes of valves to be built and tested using solid freeform fabrication and influence the use of specialty valves in medical and engineering applications.
Simulation of Dense Solid-Liquid Two-Phase Flow in a Pump Impeller
Authors: Liu Shuhong , Tang Xuelin , Wu Yulin , Nishi Michihiro
Published: March 2009

In the present study, simulation with the second-order dynamic model for sub-grid stress is used to solve the governing equations of dense solid-liquid two-phase (DSLTP) flow numerically, and continuity is conserved by a mass-weighted method to solve the filtered governing equations. In the current second-order dynamic model with double dynamic coefficients, the sub-grid-scale stress is a function of both the strain-rate tensor and the rotation-rate tensor. This model considers the symmetry of the sub-grid-scale stresses, dimensional consistency with the Cauchy-Helmholtz theorem, the Smagorinsky model and the invariants of the strain-rate tensor and the rotation-rate tensor in large eddy simulation (LES). In the numerical calculation, the SIMPLEC algorithm and a staggered grid system were applied for the solution of the discretized particle-liquid turbulent flow equations, and Body-fitted Coordinates were used to simulate the flow over the complex geometry field. In this paper, the second-order dynamic model for turbulent dense solid-liquid two-phase flows was applied to a centrifugal pump impeller.
Mass Transfer Enhancement for Improving the Performance of Polymer Electrolyte Fuel Cells
Authors: Li P. W. , Uysal U. , Chyu M. K.
Published: March 2009

The significance of mass transfer enhancement in polymer electrolyte fuel cells (PEFC) is presented and studied in this work based on experimental investigation. A novel structure of reactant gas distributors in PEFC is proposed for mass transfer enhancement purpose. For the PEFC with novel gas distributors, it is found that the large drop of the cell voltage, generally caused by a weak mass diffusion, is postponed to occur at relatively higher current density even though the same or less amount of air is fed when comparing to a PEFC with gas distributors in conventional structure. As a result, the maximum obtained electrical power in a PEFC and a PEFC stack both are dramatically improved under both free convective and forced convective airflow conditions.
Effect of Gas Distributor on Gas Holdup in Fiber Suspensions
Authors: Su Xuefeng , Heindel Theodore J.
Published: March 2009

Two different aeration plates are used to study their effect on gas holdup and flow regime transition in fiber suspensions. Two gas distributors with different open areas (A = 0.57% and 2.14%) and the same orifice diameter (do = 1 mm) are used, and experiments are performed using three different Rayon fiber lengths (L = 3, 6, and 12 mm) over a range of superficial gas velocities (Ug ≤ 18 cm/s) and a range of fiber mass fractions (0 ≤ C ≤ 1.8%) in a 15.24 cm diameter semi-batch bubble column. Experimental results show that the distributor with A = 2.14% tends to produce lower gas holdup than the one with A = 0.57% for both air-water systems and fiber slurries. However, the effect of distributor open area on gas holdup diminishes at high fiber mass fractions (C ≤ 1.2%). Both distributors generate homogeneous, transitional, and heterogeneous flow regimes over the range of superficial gas velocities for air-water and low fiber mass fraction suspensions. However, the distributor with A = 2.14% enhances the flow regime transition, i.e., the superficial gas velocity at which the transitional flow regime appears is lower. Additionally, the fiber mass fraction at which purely heterogeneous flow is observed is lower when A = 2.14%.
Unsteady Fluid Dynamics in a Semi-Closed Cavity
Authors: Filippone Antonio , Michelsen Jess A.
Published: March 2009

Results are presented for unsteady, time-true simulations of a shear driven cavity started impulsively from rest. The cavity is partially occluded by slits on its upper side, thereby forming a Helmholtz resonator type of geometry. Simulations with Reynolds-averaged Navier-Stokes solvers at the laminar Reynolds number Re = 3,000, for a spanwise aspect-ratio equal to 3, showed that the flow is unsteady and a-periodic over a normalised time of one hundred units. Taylor-Go¨rtler spanwise vortices appear early in the simulation (flow time 3 or 4 units); these vortices have a relatively short time span, and completely disappear at a flow time equal to 90. This study also reports about the findings on the secondary eddies (upstream and wall vortices), on the vortex filaments that are drawn from the cavity floor, and on the mass transport properties of the flow.
Laser Heating of Stationary Fused Silica Rods and Fibers: Effects of Heat Source and Radiation Modeling
Authors: Tian Weixue , Chiu Wilson K. S.
Published: March 2009

This paper investigates the effects of surface and volumetric absorption of laser energy on the temperature prediction of CO2 laser heating of silica rods and fibers. Radiative transfer within the fused silica is modeled using the diffusion approximation and the zonal method. Around 25 K in temperature difference is observed when the diffusion approximation is used in place of the zonal method to model radiative transfer in fused silica. The spectral dependence of the fused silica absorption coefficient in semitransparent wavelengths is approximated by a two-band model. The Weighted-Sum-of-Gray-Gas (WSGG) method is used to calculate the radiative source term for the zonal method. The importance of using the zonal method for radiative heat transfer in fused silica is also discussed.
A Reduced Dimension Finite Volume Method for Radiative Heat Transfer in Axisymmetrical Cylinders
Authors: Tian Weixue , Chiu Wilson K. S.
Published: March 2009

This paper presents a modified scheme to analyze the radiative heat transfer in axisymmetrical enclosures using the finite volume method. The modified scheme is derived from the conservation of radiative energy in an infinitely thin slice of an axisymmetrical cylinder. Therefore, the final discretized equations are based on a two-dimensional mesh in the spatial domain, and similar to meshes used for convective and conductive heat transfer analysis. The control angle overlap problem caused by misalignment of solid angles with control volume faces in the angular direction is eliminated. Error caused by the control volume face curvature is also eliminated. Comparison of results for several demonstration cases with literature yields satisfactory results.
An Investigation of Droplet Diffusion in Isotropic Turbulence With Digital Holographic PIV
Authors: Malkiel Edwin , Sheng Jian , Garber David , Katz Joseph
Published: March 2009

In-line digital holography is utilized to measure the Lagrangian trajectory of droplets in locally isotropic turbulence. The objective of these measurements is to determine the diffusion rate of these droplets as a function of density ratio between the continuous and dispersed phases, Stokes number and turbulence level relative to the quiescent settling/rise velocity of the droplets. The present experiments are conducted using diesel fuel with diameters of 0.5–2 mm, specific gravity of 0.85 and Stokes number in the 0.2 to 5 range. The droplets are injected into a 50 × 50 × 50 mm sample volume located in the center of a 160 1 tank. The turbulence is generated by four spinning grids, located symmetrically in the corners of the tank. Planar PIV has been used to characterize the turbulence prior to the experiments. A time series of in-line digital holograms is recorded at 2000 frames per second using a 1000×1000 pixel digital camera by back illuminating the sample volume with a collimated laser beam. Numerical reconstruction generates a time series of high-resolution images of the droplets and tracer particles throughout the sample volume. Subsequent analysis is used to obtain the velocity along the droplet trajectory. Lagrangian correlations can then be used for calculating the diffusion rate of these droplets. In a smaller sample volume, with a 15×15 mm cross section, and by using localized seeding, we can also simultaneously measure the droplet velocity along with the velocity of the fluid in the vicinity of this droplet. The results provide statistics on the correlations between the droplet and fluid velocities.
Analysis of Capillary Flow in Rounded Corners
Authors: Chen Yongkang , Weislogel Mark M.
Published: March 2009

The problem of capillary flow in interior corners that are rounded is re-visited analytically in this work. By the appropriate geometric scaling, and through the introduction of a new parameter that features the roundedness of the corner, the Navier-Stokes equation is reduced to a convenient form for both numerical and analytical solution. The scaling and analysis of the problem is expected to significantly reduce the reliance on numerical data for such problems, and the design process can be both shortened and improved as a result. For capillary flows of perfect wetting fluids in the rounded corner with an advancing tip, a finite interfacial curvature related to the corner roundedness results at the tip. Accordingly, an outer and inner region of the flow is suggested based on the impact of the corner roundedness on the flow. In this study, asymptotic solutions of the geometrical ‘cross-flow’ problem for the outer region are sought under several constraints and are expected to narrowly bracket parallel numerical solutions. A complete understanding of the flow will be obtained only after the cross-flow problem for the inner region is solved. However, for the flow in the outer region a similarity solution is obtained and presented that reveals how roundedness retards the flow.
Development of a Smart Material Integrated Sensor Thermal Switch for Thermal Management
Authors: Mensah Patrick F. , Abdi Solomon , Jana Amitava , Ibekwe Samuel , Crosby Karen , Henry Laurence , Notardonato Bill
Published: March 2009

In this work, advanced materials technology is employed to design a thermal switch that has integrated temperature sensing and the actuation of the switching device in tandem. The design involves the coupling of a variable area aluminum thermal sink with a Ni-Ti shape memory alloy (SMA) sensor to form an Autonomous Shape Memory Alloy Thermal Switch (ASMATS). The fabricated ASMATS is discussed in this article along with experiments conducted to ascertain its performance. In the experiments performed, the ASMATS is mounted in the common bulkhead of two fluid reservoirs maintained at different temperatures. The temperature in the lower reservoir was increased with a heater while that of the upper tank was maintained at a constant temperature. The rate of cooling from the lower reservoir was measured with and without the fin and a conclusive result was obtained on the effectiveness of the designed ASMATS. In addition, the transient natural convection heat transfer problem that results in the lower reservoir with the upper reservoir serving as a constant temperature heat sink is modeled and solved numerically. The simulation results are compared with experimental performance of the ASMATS.
Experimental Study on Fluidization Characteristics of Nanoparticles
Authors: Yu Qun , Zhu Chao , Pfeffer Robert , Dave Rajesh N.
Published: March 2009

Recent studies show that nano-sized particles can be fluidized in the form of nanoparticle agglomerates. However, fluidization behavior such as the minimum fluidization velocity and fluidization regime differ significantly for different nanoparticles. Hence this paper is aimed to experimentally investigate the general fluidization characteristics of different nanoparticles. It is interesting to note that a fluidized bed of nanoparticle agglomerates is optically semi-transparent due to the extremely high porosity (typically over 99%) of the bed with respect to the primary materials of nanoparticles. Taking advantage of this unique feature, traditional optical measurement techniques are applied to visualize the flow structure as well as to measure the size of the fluidizing nanoparticle agglomerates. Based on measurements of four different nanoparticle materials, two types of fluidization behavior have been identified, which closely resemble those of classical Geldart Group A and Group B particles, respectively. It shows, however, that the bed of “Group A” nanoparticles expands as long as there is a flow through the bed, which is different from the classical fluidization of Geldart Group A particles where there is no bed expansion until reaching the minimum fluidization velocity. It is also noted that, based on the apparent density and size, the fluidization behavior of nanoparticle agglomerates do not precisely follow the Geldart classification. To differentiate these particles with very similar fluidization characteristics, terms the APF and ABF are introduced for the fluidization classification of nanoparticle agglomerates. Typical fluidization characteristics including bed expansion, bed pressure drop and hysteresis effects of both APF and ABF nanoparticles. The sizes of nanoparticle agglomerates also have been measured using an in-situ optical measurement system.
Mass Transfer During Fluid Sphere Dissolution in an Alternating Electric Field
Authors: Elperin Tov , Fominykh Andrew , Orenbakh Zakhar
Published: March 2009

In this study we considered mass transfer in a binary system comprising a stationary fluid dielectric sphere embedded into an immiscible dielectric liquid under the influence of an alternating electric field. Fluid sphere is assumed to be solvent-saturated so that an internal resistance to mass transfer can be neglected. Mass flux is directed from a fluid sphere to a host medium, and the applied electric field causes a creeping flow around the sphere. Droplet deformation under the influence of the electric field is neglected. The problem is solved in the approximations of a thin concentration boundary layer and finite dilution of a solute in the solvent. The thermodynamic parameters of a system are assumed constant. The nonlinear partial parabolic differential equation of convective diffusion is solved by means of a generalized similarity transformation, and the solution is obtained in a closed analytical form for all frequencies of the applied electric field. The rates of mass transfer are calculated for both directions of fluid motion — from the poles to equator and from the equator to the poles. Numerical calculations show essential (by a factor of 2–3) enhancement of the rate of mass transfer in water droplet–benzonitrile and droplet of carbontetrachloride–glycerol systems under the influence of electric field for a stagnant droplet. The asymptotics of the obtained solutions are discussed.
High Heat Flux Cooling With Water Jet Impingement
Authors: Silverman I. , Nagler A.
Published: March 2009

Accelerator targets, both for radioisotope production and for neutron sources generate thermal energy at very high density due to the absorption of the particles beam in the target material. Total power is in the order of 10–50 kW. Average heat fluxes are about 1 kW/cm2 and maximum values can exceed 5 kW/cm2. The design of these targets requires efficient heat removal techniques in order to preserve the integrity of the target. Experimental cooling loops based on water and liquid gallium jet impingement have been designed in order to evaluate the actual potential of jet impingement for high heat flux cooling. The water cooling system is already operational and initial results have demonstrated a cooling capacity of 5 kW with average heat flux of 0.5 kW/cm2 and a maximum of about 1 kW/cm2 with a total target area of 10 cm2. In order to test the system at higher power level we build an electron gun heat source that is designed to provide up to 20 kW heating power.
Computational and Experimental Study of Effects of Guide Vanes and Tip Clearances on Performances of Axial Flow Fans
Authors: Lee Sungsu , Kim Hak-Sun , Nam Kwang-Hyun , Hong Jae Ik , Chun Seung Hyun
Published: March 2009

The effects of guide vanes and tip clearances on the characteristics of axial flow fans are investigated both computationally and experimentally. Performance test of fans carried out in full scale shows considerable effects of tip clearance between rotor tip and duct on the characteristics of fans. The tested results are compared with the computation based on the finite volume method to solve the Navier-Stoke equations with k–ε turbulence model. The comparison shows good agreements between experimental and computational results. In addition, the effects of shape of guide vanes are numerically studied. The results shows that increased volume of separated region around the guide vane reduces the recovery of tangential component of kinetic energy in the wake, resulting in loss of efficiency.
Simulation of Air Entrapment by a Plunging Liquid Jet of Finite Length
Authors: Krueger Paul S. , Bidoae Razvan , Raad Peter E.
Published: March 2009

The impingement of a finite length round water jet on a large pool of water was simulated numerically using a 3D Eulerian-Lagrangian Marker and Micro-Cell (ELMMC) method. The method allowed simulation of the initial impact of the jet on the pool surface, the deformation of the pool surface by the falling jet, and, under certain conditions, the entrapment of an air bubble as the pool closes in on the jet. The conditions considered were for ratios of jet length to radius (h/r) in the range of 4 to 25 and jet Froude number in the range of 16 to 74. The results agreed with previous experimental observations by Oguz et al. (J. Fluid Mech., 294, 1995) in terms of entrapped air volume and the possible geometries of entrapped bubbles (viz., toroidal or spheroidal). The simulation results also allowed for a detailed study of effects difficult to discern experimentally, such as vorticity generation and differences in entrapped air volume between toroidal and spheroidal bubbles.
Utilizing the Integral Technique for Obtaining a New Dimensionless Group for Prandtl Number Dependence for Natural Convection Heat Transfer From an Isothermal Vertical Flat Plate
Authors: Kobus Chris J. , Raikar Vinay , Guessous Laila
Published: March 2009

Research is presented in this paper where the integral technique is utilized to find a primary dimensionless group for natural convection heat transfer from an isothermal vertical flat plate. The discovery of this group from fundamental principles demonstrates the value of the chronologically mature integral technique even in this advanced era where numerical techniques are much more prevalent. The integral solution in the current research compares very favorably with experimental data for a wide range of Prandtl numbers, and also with several numerical models in the archival literature as well as empirical correlations. The good agreement, coupled with the closed-form nature of the solutions, should once again affirm the usefulness of the integral technique as a research as well as an educational tool. The result of the research in this paper is the discovery of a fundamental parameter for Prandtl number dependence in the case of natural convection from an isothermal flat plate. With this fundamental parameter, the Prandtl number dependence is effectively linearized over the entire range of data available, and a simple empirical correlation derived, whose general form may be used for other geometries.
Mutation Growth of Ice Crystal During Frost Formation
Authors: Li Z. M. , Peng X. F.
Published: March 2009

Frost formation on flat cold surfaces was experimentally investigated, particularly the dynamic process was visually observed. During test runs, a plastic film was used to separate the cold surface from moist air, and formation of ice crystals was observed using microscope visualization technique as the film was removed and the cold surface reached to a specified temperature. In crystal growth stage of frost formation, a new phenomenon was found during ice crystal growth process. A layer of irregular crystal embryos was formed at the earlier stage, and these crystal embryos vanished when ice crystals with regular form grew up along nicks on the plate. And then, ice crystals on the plate kept growing slowly, while the area without ice crystals kept clean. This process is divided into three stages: formation of crystal embryos, mutation of ice crystals, and growth of ice crystal. Duration times of the first two stages seemed to be constant for different cases.