bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of CuO Nanoparticle Concentration on R134a/Lubricant Pool-Boiling Heat Transfer
Authors: Kedzierski M. A.
Published: March 2009

This paper quantifies the influence of copper (II) oxide (CuO) nanoparticle concentration on the boiling performance of R134a/polyolester mixtures on a roughened horizontal flat surface. Nanofluids are liquids that contain dispersed nanosize particles. Two lubricant-based nanofluids (nanolubricants) were made with a synthetic polyolester and 30 nm diameter CuO particles to 1% and 0.5% volume fractions, respectively. As reported in a previous study for the 1% volume fraction nanolubricant, a 0.5% nanolubricant mass fraction with R134a resulted in a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) between 50% and 275%. The same study had shown that increasing the mass fraction of the 1% volume fraction nanolubricant resulted in smaller, but significant, boiling heat transfer enhancements. The present study shows that the use of a nanolubricant with half the concentration of CuO nanoparticles (0.5% by volume) resulted in either no improvement or boiling heat transfer degradations with respect to the R134a/polyolester mixtures without nanoparticles. Consequently, significant refrigerant/lubricant boiling heat transfer enhancements are possible with nanoparticles; however, the nanoparticle concentration is an important determining factor. Further research with nanolubricants and refrigerants is required to establish a fundamental understanding of the mechanisms that control nanofluid heat transfer.
Atomic-Scale Three-Dimensional Phononic Crystals With a Very Low Thermal Conductivity to Design Crystalline Thermoelectric Devices
Authors: Gillet Jean-Numa , Chalopin Yann , Volz Sebastian
Published: March 2009

Superlattices with thermal-insulating behaviors have been studied to design thermoelectric materials but affect heat transfer in only one main direction and often show many cracks and dislocations near their layer interfaces. Quantum-dot (QD) self-assembly is an emerging epitaxial technology to design ultradense arrays of germanium QDs in silicon for many promising electronic and photonic applications such as quantum computing, where accurate QD positioning is required. We theoretically demonstrate that high-density three-dimensional (3D) arrays of molecular-size self-assembled Ge QDs in Si can also show very low thermal conductivity in the three spatial directions. This physical property can be considered in designing new silicon-based crystalline thermoelectric devices, which are compatible with the complementary metal-oxide-semiconductor (CMOS) technologies. To obtain a computationally manageable model of these nanomaterials, we investigate their thermal-insulating behavior with atomic-scale 3D phononic crystals: A phononic-crystal period or supercell consists of diamond-cubic (DC) Si cells. At each supercell center, we substitute Si atoms by Ge atoms in a given number of DC unit cells to form a boxlike nanoparticle (i.e., QD). The nanomaterial thermal conductivity can be reduced by several orders of magnitude compared with bulk Si. A part of this reduction is due to the significant decrease in the phonon group velocities derived from the flat dispersion curves, which are computed with classical lattice dynamics. Moreover, according to the wave-particle duality at small scales, another reduction is obtained from multiple scattering of the particlelike phonons in nanoparticle clusters, which breaks their mean free paths (MFPs) in the 3D nanoparticle array. However, we use an incoherent analytical model of this particlelike scattering. This model leads to overestimations of the MFPs and thermal conductivity, which is nevertheless lower than the minimal Einstein limit of bulk Si and is reduced by a factor of at least 165 compared with bulk Si in an example nanomaterial. We expect an even larger decrease in the thermal conductivity than that predicted in this paper owing to multiple scattering, which can lead to a ZT much larger than unity.
The Leading Edge Cavitation Dynamics
Authors: Farhat Mohamed , Gennoun Faic¸al , Avellan Franc¸ois
Published: March 2009

In the present paper, we present an experimental investigation of the onset and detachment of a leading edge cavitation over a 2-D and 3-D hydrofoils. An insight of the flow field in the vicinity of the main cavity detachment is reached with the help of a miniature sensor fitted in a NACA009 hydrofoil suction side close to the leading edge stagnation point. Owing to theses experiments, we have demonstrated how the water may withstand negative pressure as low as −0.8 bar without any visible cavitation. As soon as the leading edge cavitation takes place, the pressure upstream to the cavity detachment rises significantly but remains negative while the measured pressure in the cavity is almost equal to the vapor pressure. This result confirms the assumption already stated by former studies according to which the liquid is in tension just upstream to the detachment point. Furthermore, flow visualization clearly shows that a well developed leading edge cavitation turns into bubble cavitation in a continuous way when the surrounding pressure is gradually increased. Owing to those results, we have introduced a physical model of the cavitation detachment in which, no laminar separation of the boundary layer is required to ensure its mechanical equilibrium as already stated by former studies. The nuclei in the vicinity of the blade surface explode as they cross the liquid-vapor interface, which is not a material surface. The main cavity is thus continuously fed with exploding bubbles at its detachment location. The negative pressure measured upstream to the cavity detachment may thus be explained by the dynamic delay of exploding nuclei due to inertia.
A Predictive Model for Cavitation Erosion Downstream Orifices
Authors: Archer Antoine
Published: March 2009

In order to improve pipes and valves maintenance in EDF power plants by the use of reliability analysis, some wear mechanisms have been listed and their wear kinetics have been investigated. One of these wear mechanisms is cavitation erosion. Cavitation may occur in numerous pipe components such as valves, orifices, elbows and tees, in any place where low pressure and/or high velocity is reached. The effects of cavitation developments are first noise and vibrations, then erosion of the downstream pipe if the material is not resistant enough and at least loss of performance of the component (choking). A predictive model to evaluate cavitation erosion in valves and pipes was proposed in 1998 by Lecoffre [4]. Experiments on a closed cavitation test loop for the simple geometry of two sharp-edge single hole orifices (opening factor β = d/D = 0.4 and β = d/D = 0.8) have been performed in order to validate this model. First, the cavitation developments downstream the orifice have been visualized through a transparent pipe section. Incipient and chocking pressure coefficient have been recorded and compared with data from Tullis [2]. Then the erosion has been quantified using pitting measurements on mirror polished 316L stainless steel samples, located flush on the inner pipe surface. Cavitation pits have been measured using a laser profilometer. The plastic deformation volume measurement allows to define a pitting velocity, which can be related to a « loss of thickness » velocity, taking into account previous erosion tests on the pipe material. These pitting tests were performed for five flow conditions, characterized by three flow rate values and three pressure coefficient values. In the predictive model, these flow conditions parameters are more precisely defined by the flow velocity in the contracted section, named V1 (m/s), and by the pressure coefficient calculated with the lowest pressure in the contracted section, named σ1 (-). At least, a predictive model was adjusted using these experimental results. We found that the cavitation erosion wear velocity of a 316L stainless steel straight pipe downstream the orifice (characterized by its opening factor β) for the flow condition (σ1, V1) with cavitation σ1<1 fits well to the following formula: Verosion=k.(1−σ1)6.8.(V1−V0)5 with : k = f(β), V0 = g(β) where f and g are functions of the opening factor β = d/D. Tests using the same methodology have since been performed on butterfly valves and on multiholes orifices.
Molecular Dynamics Study of Heterogeneous Bubble Nucleation in Liquid Oxygen Including Helium, Nitrogen, or Argon
Authors: Tsuda Shin-Ichi , Tokumasu Takashi , Kamijo Kenjiro , Matsumoto Yoichiro
Published: March 2009

Heterogeneous bubble nucleation in liquid oxygen including helium, nitrogen, or argon is simulated by using the molecular dynamics method. Molecular interaction is given as Lennard-Jones potential, and, basically, each potential parameter is determined so that a saturation curve obtained by MD data is consistent with an experimental value. In the case that helium is the impurity, a bubble is caused by density fluctuation at a lower concentration, while clusters of helium molecules become bubble nuclei at a higher concentration, and the point of bubble formation moves closer to the saturation point of pure oxygen when they form clusters. In the case that nitrogen or argon is the impurity, the above-mentioned clustering is not observed at a concentration where helium makes clusters, and these impurities have weaker action to make clusters compared with helium.
Nonlinear Analysis and Control of Detached Model Flows
Authors: Galletti Bernardo , Iollo Angelo , Zannetti Luca
Published: March 2009

In this paper we discuss some aspects related to feed-back control of detached flows. We use a very crude model, a single point vortex of variable intensity, to describe the massive separation region past a cusp followed by a smooth cavity. Then we investigate the controllability and the observabilty of the system by standard nonlinear control techniques. We concentrate on the possibility of controlling such a model by a feed-back controller and define the conditions under which the vortex can be asymptotically stabilized about unstable equilibria.
On the Coupling Between Adjacent Turbulent Jets
Authors: LePera S. D. , Vandsburger U.
Published: March 2009

A detailed examination of the coupling observed between adjacent turbulent jets, one of which was externally excited, is presented. The study is relevant to the active control of combustion and mixing devices; applications which demand maximum reliability and minimal power consumption by their active elements. To this end, it is desirable for each active component to affect several simple, non-active components in the system. An initial study of three collinear turbulent air jets was seen to enhance the mixing of adjacent, unforced jets with ambient fluid by 15–20%. The work presented here examines this phenomena using quantitative PLIF mixing measurements, three component and single-wire velocity measurements, qualitative flow visualization, and reduced singular value decomposition (SVD). This analysis of the data shows that the mixing improvement in the adjacent, unforced jets is primarily due to the low level perturbation at the exit of the unforced jets induced by the periodic forcing in the active center jet. This improved mixing is shown to be a result of amplification of the +/-1 spatial mode. Large scale structures in the central, forced jet are shown not to effect the behavior of the adjacent jets.
Performance Enhancement of Wet Gas Flow Meter
Authors: Park Kyung-Am , Choi Haeman , Oh Younkyun , Lee Dugki
Published: March 2009

Wet gas flow meters are used in the gas flow meter manufacturing industries as a reference meter. Also those meters are used for gas flow measurement in many areas. Oil is suggested for sealing fluid in a wet gas flow meter. But water is used because of cheap and easy handling fluid in the industries. In this case, the inlet gas flow rate is different with the outlet gas flow rate because the evaporation of water in the chamber. It is important to estimate the rate of water evaporation and effects on flow measurement error. The humidity of wet gas in the outlet of a flow meter was measured. Flow measurement error due to water evaporation is about 2%. Those results will be useful for flow measurement error reduction of wet gas flow meter using water as sealing fluid. The new developed wet gas meter measured pulse and frequency in encoder through magnetic coupling between the chamber and an encoder. The flow quantity was compensated with adjustment of flow rate error. So the rangeability and error of flow measurement were enhanced remarkably.
Numerical Methods for Tracking Interfaces With Surface Tension in 3-D Mold Filling Processes
Authors: Williams Matthew W. , Kothe Doug , Korzekwa Deniece , Tubesing Phil
Published: March 2009

Gravity-pour casting processes are simulated for both low and high Weber number flows. The validation problems examined are a symmetric side-fill problem and a more complex asymmetric top-fill problem with flow over and obstacle. A recently developed continuum surface force model was implemented within a transient three-dimensional software simulation tool and applied to the low Weber number problem. The resulting simulations are compared with experiments that were conducted in order to validate current and future gravity-pour casting simulations. The simulations are found to capture much of the qualitative behavior of the complex three-dimensional flows.
Surface Temperature Field Statistics at an Air/Water Interface for Grid-Generated Sub-Surface Turbulence
Authors: Flack Karen A. , Smith Geoffrey B.
Published: March 2009

Surface temperature fields and statistics are presented for the case of sub-surface grid-generated turbulence impacting an air/water interface. Temperature measurements are obtained with an infrared camera, sensitive in the 3–5 micron wavelength range. Results indicate that increased grid oscillation frequencies, and shallower grid depths, lead to increased surface mixing, yielding lower values of RMS temperature. Non-dimensionalization of the RMS temperatures using the difference in the average surface and the bulk fluid temperatures, collapses the data obtained for different grid depths and oscillation frequencies. This scaling is related to the thermal boundary layer thickness. The results are compared to the baseline case of turbulence due to evaporative convection without an oscillating grid.
Numerical Experiments on a Centrifugal Pump
Authors: Shukla S. N. , Kshirsagar J. T.
Published: March 2009

Advances in Computational Fluid Dynamics have made it possible to visualize some of features of flow through rotating machines, that are if not impossible, difficult to measure experimentally. Now Industry standard CFD codes are commercially available and are being used by leading industries as a design verification tool. Most of the commercial codes are general purpose, keeping in mind vast application needs. Various turbulence models, numerical schemes and grid types are given as an option for the end user. Appropriate choice of grid, numerical scheme, interfaces and turbulence models are the key parameters that influence the results from CFD software. A centrifugal pump was designed and developed using empirical and optimization tools developed in-house. The flow passages were checked for smooth flow pattern using CFD tools. The pump was manufactured and tested for its performance. The tested performance matched well with predictions within 3% at best efficiency point as regards to efficiency and head. This model was taken to carry out numerical experiments using CFD tools. The entire performance including flow-head-efficiency and cavitation performance in terms of NPSHr requirement was available. Numerical experiments were carried out to study impact of different parameters like numerical scheme, turbulence models, type of interface between stationary and rotating parts. The results from these numerical experiments are presented and discussed in the light of actual test results from laboratory testing.
3D Numerical Simulation of Pump Cavitating Behavior
Authors: Coutier-Delgosha O. , Fortes-Patella R. , Reboud J. L. , Pouffary B.
Published: March 2009

The quasi-steady cavitating behavior of three pumps was investigated by 3D unsteady viscous computations. The numerical model is based on the commercial code FINE/TURBO™, which was adapted to take into account the cavitation phenomenon. The resolution resorts to a time-marching algorithm initially devoted to compressible flows. A low-speed preconditioner is applied to treat low Mach number flows. The vaporization and condensation processes are controlled by a barotropic state law that links the void ratio evolution to the pressure variations. A radial pump, a centrifugal pump, and a turbopump inducer were calculated and the cavitating behaviors obtained by the computations were compared to experimental measurements and visualizations. A reliable agreement is obtained for the two pumps concerning both the head drop charts and the extension of the vapor structures. A qualitative good agreement with experiments is also observed in the case of the turbopump inducer. The accuracy of the numerical model is discussed for the three geometries. These simulations are a first attempt to simulate the complete 3D cavitating flows in turbomachinery. Results are promising, since the quasi-steady behaviors of the pumps in cavitating condition are found quantitatively close to the experimental ones. A continuing effort is pursued to improve the prediction accuracy, and to simulate unsteady effects observed in experiments, as, for example, rotating cavitation.
Suppression of Cavitation in a Francis Turbine Runner by Application of 3D Inverse Design Method
Authors: Okamoto Hidenobu , Goto Akira
Published: March 2009

This paper describes a new design method of blade geometry for a Francis turbine runner by using a three-dimensional inverse design method and the Computational Fluid Dynamics (CFD) technique. The design objectives are the suppression of cavitation by reducing the area in which static pressure is lower than the vapor pressure while keeping the efficiency high. In the inverse design method, it is possible to optimize the static pressure distribution in the runner by controlling blade loading parameters and/or stacking condition, which is related to a blade lean angle, for the same design specification. A Francis turbine runner was re-designed by the inverse design method for different blade loading and stacking conditions, and the flow fields were evaluated by applying CFD. It was confirmed that the present design method is very practical and effective to control low pressure region and achieve high efficiency for Francis turbine runners.
Effects of Blade Loading on Pump Inducer Performance and Flow Fields
Authors: Ashihara Kosuke , Goto Akira
Published: March 2009

Numerical and experimental investigations were performed to study the effects of blade loading on pump inducer performance and flow fields. To compare the performance of inducers with different blade loadings, a three-dimensional inverse design method was applied to control the blade loading distribution of inducers. Firstly, a conventional helical inducer was designed. The blade number is three and the blade angle at the tip was chosen by the conventional design method. Then, two inducers were designed using a three-dimensional inverse design method with different blade loading distributions. One inducer was designed with fore-loading and the other was designed with aft-loading, but both inducers were designed with no leading edge loading. These two inducers have the same design specification as the conventional helical inducer. The CFD (Computational Fluid Dynamics) analyses and water model tests were performed on these three inducers. Both results showed that the inlet backflow characteristics of the 3-D inverse design inducers are improved from those of the conventional inducer. It was also found that the inlet backflow characteristics of inducers that have no leading edge loading are almost same despite different blade loading distributions. The inducer designed with fore-loading showed almost the same suction performance as the conventional inducer. Cavitation visualization and FFT analysis of unstable phenomena were also performed in this study.
Simulation of Two-Dimensional Particle Flows
Authors: Prosperetti Andrea , Zhang Zhongzhen
Published: March 2009

This paper describes a new approach to the direct numerical simulation of particle flows and illustrates its performance on several examples. The method uses a fixed regular orthogonal grid and combines an analytic solution, valid in the neighborhood of each particle, with a finite-difference solution away from the particle. In effect, the analytic solution is used to ‘transfer’ the boundary conditions at the particle surface to the neighboring grid nodes, thus eliminating the complexity arising from the incompatibility of the particle geometry with that of the underlying fixed grid. Furthermore, the hydrodynamic force and couple on the particle are computed directly with no need for the error-prone extrapolation or interpolation procedures necessary with other methods.
An Approach to Parallel Computing in an Eulerian-Lagrangian Two-Phase Flow Model
Authors: Vance Marion W. , Squires Kyle D.
Published: March 2009

An approach to parallel solution of an Eulerian-Lagrangian model of dilute gas-solid flows is presented. Using Lagrangian treatments for the dispersed phase, one of the principal computational challenges arises in models in which inter-particle interactions are taken into account. Deterministic treatment of particle-particle collisions in the present work pose the most computationally intensive aspect of the simulation. Simple searches lead to algorithms whose cost is O(N2p) where Np is the particle population. The approach developed in the current effort is based on localizing collision detection neighborhoods using a cell-index method and spatially distributing those neighborhoods for parallel solution. The method is evaluated using simulations of the gas-solid turbulent flow in a vertical channel. The instantaneous position and the velocity of any particle is obtained by solving the equation of motion for a small rigid sphere assuming that the resulting force induced by the fluid reduces to the drag contribution. Binary particle collisions without energy dissipation or inter-particle friction are considered. The carrier flow is computed using Large Eddy Simulation of the incompressible Navier-Stokes equations. The entire dispersed-phase population is partitioned via static spatial decomposition of the domain to maximize parallel efficiency. Simulations on small numbers of distributed memory processors show linear speedup in processing of the collision detection step and nearly optimal reductions in simulation time for the entire solution.
3D PIV and LDV Measurements at the Outlet of a Francis Turbine Draft Tube
Authors: Iliescu Monica Sanda , Ciocan Gabriel Dan , Avellan Franc¸ois
Published: March 2009

For certain geometries of elbow draft tubes of a hydraulic turbine, a drop in the pressure recovery coefficient is observed for a small variation of the flow rate. In order to determine the possible causes of this characteristics shape, the flow field analysis for 4 nearby operating points have been investigated. For velocity and turbulence fields investigation in the outlet section of the studied draft tube, LDV measurements were performed in a transversal section and the 3D-PIV system was qualified for global velocity measurements in longitudinal sections, with an accuracy of less than 3%. By correlating the LDV and PIV results, the quantification of the flow rate through each channel, related to the operating points, and the description of the secondary flow in the outlet zone are possible.
Accuracy of the Operator Splitting Technique for Two-Phase Flow With Stiff Source Terms
Authors: Tiselj Iztok , Horvat Andrej
Published: March 2009

Code for analysis of the water hammer in thermal-hydraulic systems is being developed within the WAHALoads project founded by the European Commission [1]. Code will be specialized for the simulations of the two-phase water hammer phenomena with the two-fluid model of two-phase flow. The proposed numerical scheme is a two-step second-order accurate scheme with operator splitting; i.e. convection and sources are treated separately. Operator splitting technique is a very simple and “easy-to-use” tool, however, when the source terms are stiff, operator splitting method becomes a source of a specific non-accuracy, which behaves as a numerical diffusion. This type of error is analyzed in the present paper.
Statistical Investigation of Particle-Interaction in Multiphase Flows
Authors: Bru¨cker Ch.
Published: March 2009

This study was carried out to understand the particle-particle interaction in multiphase flows. We used light polstyrol particles (density 0.05 g/cm3) which were colored to detect the particles at higher void fractions. To investigate the influence of the shape on the particle interaction spherical as well as ellipsoidal particles were used. The polystyrol particles behave similar as air bubbles in water because of their relative low density. This was documented by several experiments of the motion and wake structure with single rising bubbles and particles [1]. The particles were studied in a water tank in counter-flow. A stereoscopic three-dimensional PTV systems was built up to record the particle motion. By using different concentration of water/glycerine mixture we are able to simulate different Reynolds-numbers. The results demonstrates that the shape of the particles plays a dominant role on the interaction, see the particle pair distribution shown in Fig. 1. In case of the spherical particles, most of the particle pairs are horizontally aligned which is a clear trend of bubble pairing. This agrees with the observations made by Fortes et al. (1987). In contrast, the prolate spheroids show a different distribution which can only be attributed to the different shape. It is likely, that the additional degree of freedom of the prolate spheroids to tilt or to align with the flow is the main reason which has also great influence on the drag and lift-forces.
Numerical Simulation of Coaxial Evaporating Spray in Nozzle Region of Circulating Fluidized Reactor
Authors: Zhu Chao , Wang Xiaohua , Liu Guangliang
Published: March 2009

Full field hydrodynamic mixing of a coaxial evaporating spray in the nozzle region of a circulating fluidized reactor was numerically investigated. An Eulerian-Lagrangian numerical code was developed for the field description of evaporating spray characteristics with strong phase interactions among evaporating droplets, solids and gas. The gas-solid flow is simulated using multi-fluid method coupled with kinetic theory modeling for inter-particle collisions while the spray is treated as the discrete droplets in a pseudo-continuum gas-solid flow. The Lagrangian simulation of the spray provides the needed coupling terms for the Eulerian simulation of gas-solid flows, such as droplet evaporation rate and interactions among phase of droplets, gas and solids. Phase distributions of temperature, velocity and concentration were achieved to explain the mixing process of evaporating spray in gas-solid flows. Effects of inlet solids loading and droplet size distribution on both spray structure and spray penetration depth were illustrated. An experimental system of liquid nitrogen spray into a circulating fluidized bed of fluid catalytic cracking particles is set up to provide experimental validation of our model. Good comparisons of the simulation and measurements are illustrated.
Numerical Simulation and Mixing Study of Non-Newtonian Fluids in an Industrial Helical Static Mixer
Authors: Rahmani Ramin K. , Keith Theo G. , Ayasoufi Anahita
Published: March 2009

Static mixers are increasingly being used to perform a variety of mixing tasks in industries, ranging from simple blending to complex multi-phase reaction systems. Use of static mixers to process non-Newtonian fluids is quite common. Data on the pressure drop of non-Newtonian fluids in static mixers and the degree of mixing of materials through the mixer are very useful in the design and engineering application of these tools. This paper extends a previous study by the authors on an industrial helical static mixer and illustrates how static mixing processes of single-phase viscous liquids can be simulated numerically. A further aim is to provide an improved understanding of the flow pattern of non-Newtonian single-phase liquids through the mixer. A three-dimensional finite volume simulation is used to study the performance of the mixer. The non-Newtonian fluid is modeled by the Carreau law model for the shear stress. The effects of the Reynolds number of the flow and also properties of non-Newtonian fluids on the static mixer performance have been studied. The flow velocities, pressure drops, etc. are calculated for various flow rates. The computed pressure drop is in good agreement with existing experimental data. A comparison of the mixer performance for both Newtonian and non-Newtonian fluids is presented. It is shown that for low Reynolds number flows, the fluid type is less effective on the degree of mixing, while as flow Reynolds number increases and the viscosity decreases, it manifests more influence on the downstream mixing. It is also shown that the fluid type has a major impact on the pressure drop across the mixer.
A Critical Examination of Correlation Methodology Widely Used in Heat Transfer and Fluid Flow
Authors: Adiutori Eugene F.
Published: March 2009

The correlation methodology widely used in heat transfer and fluid flow is based on fitting power laws to data. Because all power laws of positive exponent include the point (0,0), this methodology includes the tacit assumption that phenomena are best described by correlations that include the point (0,0). • If a phenomenon occurs near (0,0), the assumption is obviously valid. For example, laminar flow occurs near (0,0), and therefore the assumption is valid for laminar flow pressure drop correlations. • If a phenomenon does not occur near (0,0), the assumption is obviously invalid. For example, turbulent flow does not occur near (0,0)—it occurs only after a critical Reynolds number is reached. Therefore the assumption is invalid for turbulent flow pressure drop correlations. When the assumption is invalid, the correlation methodology widely used in heat transfer and fluid flow is lacking in rigor. The impact of the lack of rigor is evidenced by examples that demonstrate that, when this methodology is applied to phenomena that do not occur in the vicinity of (0,0), highly nonlinear power laws oftentimes result from data that exhibit highly linear behavior. Because the widely used methodology lacks rigor when applied to phenomena that do not occur near (0,0), power laws based on this methodology are suspect if they purport to describe phenomena that do not occur near (0,0). Data cited in support of such power laws should be recorrelated using rigorous correlation methodology. Rigorous correlation methodology is also used in heat transfer and fluid flow. It is described in the text, and should become the methodology in general use.
A Viscoelastic VOF-PROST Code for the Study of Drop Deformation
Authors: Renardy Y. , Renardy M. , Chinyoka T. , Khismatullin D. B. , Li J.
Published: March 2009

A volume of fluid method is developed with a parabolic representation of the interface for the surface tension force (VOF-PROST). This three-dimensional transient code is extended to treat viscoelastic liquids with the Oldroyd-B constitutive equation. Simulations of deformation for a Newtonian drop in a viscoelastic medium under shear are reported.
Turbulent Heat Transfer in Plane Couette Flow
Authors: Le Phuong M. , Papavassiliou Dimitrios V.
Published: March 2009

Direct numerical simulations of a turbulent plane Couette flow are combined with Lagrangian scalar tracking of thermal markers that are released in the flow field to determine the behavior of an instantaneous scalar line source located at the wall. The resulting probability density functions are used to calculate the behavior of instantaneous line sources of heat at the wall of the channel. The method is applied for fluids with a range of molecular Prandtl number, Pr, between 0.1 and 15,000, giving emphasis on the high Pr cases. The issues that are investigated are the effect of the Pr on turbulent dispersion, and the effect of the turbulence structure on turbulent heat transfer. The flow field for plane Couette flow is fundamentally different than that for channel flow, because the whole Couette flow domain is a constant stress region that forms an extensive logarithmic layer. For an instantaneous source at the wall, it is found that in both the channel flow and the Couette flow cases there are similar stages of development of the marker cloud that depend on the Prandtl number. This dependence becomes stronger as the Pr increases. However, this similarity is only qualitative.
Gas Holdup in a Cocurrent Air-Water-Fiber Bubble Column
Authors: Tang Chengzhi , Heindel Theodore J.
Published: March 2009

Effects of superficial liquid velocity (Ul), superficial gas velocity (Ug), and fiber mass fraction (C) on gas holdup (ε) and flow regime transition are studied experimentally in well-mixed water-cellulose fiber suspensions in a cocurrent bubble column. Experimental results show that the gas holdup decreases with increasing Ul when C and Ug are constant. The gas holdup is not significantly affected by C in the range of C < 0.4%, but decreases with increasing C in the range of 0.4% ≤ C ≤ 1.5%. When C > 1.5%, a significant amount of gas is trapped in the fiber network and recirculates with the water-fiber slurry in the system; as a result, the measured gas holdup is higher than that at C = 1.5%. The axial gas holdup distribution is shown to be a complex function of superficial gas and liquid velocities and fiber mass fraction. The drift-flux model is used to analyze the flow regime transitions at different conditions. Three distinct flow regimes are observed when C ≤ 0.4%, but only two are identified when 0.6% ≤ C ≤ 1.5%. The superficial gas velocities at which flow transition occurs from one regime to another are not significantly affected by Ul and slightly decrease with increasing C.
Comparison Between EVM and RSM Turbulence Models in Predicting Flow and Heat Transfer in Rib-Roughened Channels
Authors: Sleiti A. K. , Kapat J. S.
Published: March 2009

A 3-D analysis of two-equation eddy-viscosity (EVMs) and Reynolds stress (RSM) turbulence models and their application to solving flow and heat transfer in rotating rib-roughened internal cooling channels is the main focus of this study. The flow in theses channels is affected by ribs, rotation, buoyancy, bends and boundary conditions. The EVMs considered are: The standard k–ε Model: of Launder and Spalding Launder and Spalding [1], the Renormalization Group k-ε model: Yakhot and Orszag [2], the Realizable k-ε model Shur et al. [3], the standard k-ω Model, Wilcox Wilcox [4], and the Shear-Stress Transport (SST) k-ω Model, Menter [5]. The viscosity affected near wall region is resolved by enhanced near wall treatment using combined two-layer model with enhanced wall functions. The results for both stationary and rotating channels showed the advantages of Reynolds Stress Model (RSM), Gibson and Launder [6], Launder [7], Launder [8] in predicting the flow field and heat transfer compared to the isotropic EVMs that need corrections to account for streamline curvature, buoyancy and rotation.
Analytical Study of Natural Convection in a Cavity With Volumetric Heat Generation
Authors: Joshi Mandar V. , Gaitonde U. N. , Mitra Sushanta K.
Published: March 2009

A semi-analytical method for natural convection in a two dimensional rectangular enclosure, with uniform volumetric heat generation, having insulated horizontal boundaries, and isothermal vertical boundaries, has been studied here. In this method, the governing equations for natural convection, have been solved under the assumption that for a cavity with small aspect ratio, the flow in the central region of the cavity is only in the vertical direction. It is found that for the cavities with small aspect ratio, the temperature in central region of the cavity is nearly constant along the horizontal direction. However, there is a uniform temperature gradient in the vertical direction, which can be related to the maximum temperature in conduction. The velocity profiles and temperature profiles obtained in the present work, are compared with the numerical simulations by Fluent and a fair agreement is found between these results.
A Fast Time Convolution Algorithm for Unsteady Heat Diffusion Problems
Authors: Wang C. H. , Grigoriev M. M. , Dargush G. F.
Published: March 2009

A new algorithm is developed to evaluate the time convolution integrals that are associated with boundary element methods (BEM) for transient diffusion. This approach, which is based upon the multi-level multi-integration concepts of Brandt and Lubrecht, provides a fast, accurate and memory efficient time domain method for this entire class of problems. Conventional BEM approaches result in operation counts of order O(N2) for the discrete time convolution over N time steps. Here we focus on the formulation for linear problems of transient heat diffusion and demonstrate reduced computational complexity to order O(N3/2) for a couple of two-dimensional model problems using the multi-level convolution BEM. Memory requirements are also significantly reduced, while maintaining the same level of accuracy as the conventional time domain BEM approach.
Large Eddy Simulation of Fully Developed Turbulent Pipe Flow
Authors: Vijiapurapu Sowjanya , Cui Jie
Published: March 2009

Fully developed turbulent pipe flow is investigated by large eddy simulations (LES). The three-dimensional, unsteady, incompressible, filtered continuity and Navier-Stokes equations in cylindrical coordinates are discretized by a finite difference method. The spatial derivatives are approximated by second order conservative schemes. This scheme eliminates the numerical generation or dissipation of energy. The pressure Poisson equation is solved by FFT method and time is advanced through a third order Runge-Kutta method. The commonly used subgrid scale (SGS) models — the Smagorinsky model and the dynamic model are implemented and simulations are performed for fully developed turbulent pipe flow at two different Reynolds numbers. The flow features in terms of mean velocity as well as higher order turbulence intensities and correlations are presented and compared to experimental and DNS data available in literature. Extensive comparisons are made for cases using different grid resolution, different streamwise domain dimension, different sub-grid scale model, and, at two different Reynolds number. For two Reynolds numbers (5,000 and 30,000) tested in this study, the fine mesh (64 × 96 × 64, circumferential × radial × longitudinal) produces better results than the coarse mesh (32 × 48 × 32), indicating the significance of the grid resolution, especially near the pipe surface. On the fine mesh for the two Reynolds numbers, the results exhibit a slight Reynolds number effect, indicating the mesh needs to be further refined at higher Reynolds number. Simulations were performed for two domain sizes, namely 6D and 12D, where D is the pipe diameter. When the streamwise grid resolution remains unchanged, the two simulations show negligible difference. This ensures that a 6D domain is adequate to include the largest eddies in a fully developed turbulent pipe flow at the current Reynolds number. When the fine mesh is used, the subgrid scale models (Smagorinsky and Dynamic) provide limited contribution to the total turbulent kinetic energy. Although the current results agree quite well with other published LES simulations, when compared with the Law of the wall, benchmark experiments and DNS results, the simulated mean velocity in the log region is higher than the experimental and DNS data. Overall, it was observed that the numerical methods work satisfactorily well for turbulent pipe flows at low and high Reynolds numbers, and, the method has capability to be used in the simulation of flows with practical interest.
Prediction of Nanoscale Radiative Heat Transfer Between Silicon and Silicon or Another Material
Authors: Fu Ceji , Zhang Zhuomin M.
Published: March 2009

This work investigates the near-field radiative heat transfer between two semi-finite media separated by a vacuum gap. The fluctuational electrodynamics is used to calculate the net heat flux between a high-temperature medium, which is assumed to be silicon at 1000 K, and a room-temperature (300 K) medium, which is taken as either silicon or a different material, such as silicon carbide and aluminum. The dielectric function of silicon is modeled with a Drude model, considering the effects of temperature and doping level on the carrier concentrations and scattering rates. The calculated results show that the net radiative energy flux can be greatly enhanced in the near field. In the case of energy exchange between silicon and silicon, the net heat flux approaches to a constant value as the distance between the media is reduced to below 100 nm. Furthermore, increasing the doping level of the high-temperature medium causes a slight decrease in the near-field energy flux. On the contrary, in the case of energy exchange between silicon and a different material (silicon carbide or aluminum), the net heat flux continue to increase as the distance is reduced even below 1 nm. Increasing the doping level of silicon can significantly enhance the net energy flux, especially when the distance is shorter than 20 nm.
Solution of a Two-Dimensional Inverse Radiation Problem Using Evolutionary Algorithms
Authors: Suram Sunil , Bryden Kenneth M.
Published: March 2009

While solving inverse problems stability of the solutions is an issue. Stability ensures that solutions obtained are physically possible and not just mathematically feasible. Techniques like Truncated Singular Value Decomposition (TSVD), Tikhonov’s regularization have been used to stabilize solutions. These are called regularization techniques, which involve selection of regularizing parameters. The choice of these regularizing parameters dictates the accuracy of solutions obtained. In this paper an Evolutionary Algorithm (EA) based optimization procedure is used to solve an inverse radiation problem in a rectangular enclosure. The fitness function and the mutation operator have been designed to eliminate the need for regularization. The evolutionary technique developed in this paper ensures a better search on the fitness landscape, without the additional effort of choosing regularization parameters.
Non Conforming Meshes and RANS/LES Coupling: Two Challenging Aims for a CFD Code
Authors: Fournier Yvan , Benhamadouche Sofiane , Monfort David , Laurence Dominique
Published: March 2009

This paper focuses on the geometry/solver interface for the CFD Software Code_Saturne ® which has been developed by EDF R&D since 1997 to replace its FE and BFC solvers. The solver includes various RANS models, as well as LES features and is parallelized through a domain splitting technique. It is based on a cell centered unstructured finite volume scheme, and accepts cells of any shape. This opened the possibility of using non conforming meshes, making it easier to build meshes with well-controlled quality and refinement even for complex geometries. Adjacent boundary faces of non-conforming input meshes may be automatically split into their intersecting subsets so as to build a conforming mesh of polyhedra with an arbitrary number of faces per cell. This also extends to the handling of periodic boundary conditions as a geometrical condition. We will explain how this is handled and illustrate the algorithm’s behavior on different complex grid examples.
A Computational Study of Two Novel Non-Steady Pressure Exchange Ejectors With Environmental Benefits and Energy Efficiency
Authors: Hong Woo Jong , Zhang Hongfang , Garris Charles A. , Ababneh Amer K. , Al-Hussan Khaled
Published: March 2009

The latest progress on the design and performance of two different types of novel supersonic pressure exchange ejectors for thermally driven ejector refrigeration systems are presented. One is an axial rotor-vane pressure-exchange ejector whose main component parts are a supersonic nozzle, a rotor with some wedged vanes, and a shroud diffuser. The other is a radial rotor-nozzle pressure-exchange ejector that includes a rotor with some canted nozzles, and two disc-shaped diffusers. The energy exchange, which occurs between the fluids throughout the pressure exchange ejectors, is shown by computational simulation with two commercial CFD packages, Tascflow and Fluent. This paper shows how the supersonic aerodynamics is managed to provide the desirable flow induction characteristics through computational study for both radial-flow and axial-flow supersonic pressure exchange ejectors. For axial-flow supersonic rotor-vane pressure exchange ejectors, three different spin-angles, 0°, 10°, and 20° are used to better understand aerodynamic performance of three-dimensional, complex, supersonic pressure-exchange ejectors and to enhance a strong foundation for the problem at hand. Even though the larger spin-angle and the more number of rotor vane produce better flow induction and mixing between primary flow and secondary flow, the rotor rotating speed closer to free spinning one is more essential to produce better flow induction and mixing between primary flow and secondary flow. Meanwhile practically severe mechanical difficulties were observed with supersonic radial-flow pressure exchange ejectors whose design concept was turned out aerodynamically great by computational study that allowed one to make some idealized assumption on it. Because of its radial vaneless diffuser, a supersonic pressure-exchange ejector with a free-spinning rotor can be aerodynamically highly desirable. It is less likely to have strong shocks, which is a source for detrimental effects on the ejector. The radial configuration provides the opportunity for utilizing yet another mode of energy exchange “extended pressure exchange” that has been newly re-identified.
Simulation of Nano-Scale Multi-Fingered PD/SOI MOSFETs Using the Boltzmann Transport Equation
Authors: Pascual-Gutie´rrez Jose´ A. , Murthy Jayathi Y. , Viskanta Raymond , Joshi Rajiv V. , Chuang Ching-Te K.
Published: March 2009

This work uses a gray, a semi-gray, and a non-gray model based on the Boltzmann Transport Equation (BTE) in the relaxation time approximation to compute the temperature distribution in a nano-scale multi-finger, PD/SOI nMOSFET with copper interconnects. The BTE models were successfully incorporated in CFD software, Fluent 6.1. The BTE is used in the device layer, whereas in other regions of the device, such as the silicon substrate, buried oxide, gate oxide, poly-gate, and metal interconnects, the Fourier heat conduction equation is employed. The BTE is coupled with the heat conduction equation at the interfaces using the diffuse mismatch model (DMM). Heat dissipation in the channel region of the FET and in the metal lines of the device is specified based on circuit simulations for a clock buffer used in a microprocessor. The computed results for the temperature distribution in the multi-finger NFET using the different approaches are compared with simulations that employ the classical heat conduction equation in the entire domain. The comparisons demonstrate that the broad temperature fields in the transistor are primarily determined by the overall thermal resistances due to the various device structures; channel temperature, however, is determined in large part by sub-continuum effects. The need for direct measurements of channel temperature rather indirect gate temperature measurements is pointed out as well.