bluuebunny/crossref_metadata_2025 huggingface dataset:
A Generalized Gas-Liquid-Solid Three-Phase Flow Analysis for Airlift Pump Design
Authors: Margaris D. P. , Papanikas D. G.
Published: February 2008

The object of the present study is to access the performance of an airlift pump under predetermined operating conditions. The gas-liquid-solid three phase flow in an airlift pump is described by a system of differential equations, which derives from the fundamental conservation equations of continuity and momentum. This approach leads to a more general mathematical model which is applicable to a wide range of installations, from small airlift pumps to very large systems, suitable for deep-sea mining. For the frictional pressure drop calculation a new correlation, based on a pseudoliquid model, has been proposed. In addition, parameters such as the drag coefficient of both solid and gas phase, the shape of particles and the compressibility factor, which is very important for deep-sea mining, have been incorporated in the governing equations. The application of the computational algorithm to different geometry and flow conditions of an airlift pump leads to the optimization of the system. The numerical simulation results clearly show a very good agreement with experimental and computational data of other researchers. The analysis methods have been combined in an easily used computer code which is a very useful tool for the optimum design of airlift pump systems.
Effects of Particulate Diffusion on the Compressible Boundary-Layer Flow of a Two-Phase Suspension Over a Horizontal Surface
Authors: Chamkha Ali J.
Published: February 2008

The problem of steady, laminar, compressible flow and heat transfer of a particulate suspension past a semi-infinite horizontal flat surface is formulated and solved numerically using an implicit finite-difference scheme. The mathematical formulation of the governing equations is based on the Eulerian description familiar from fluid mechanics where both phases are treated as two separate interacting continua. These equations account for Brownian diffusion which is important when the particle phase consists of very tiny particles and allow for a general power-law fluid-phase viscosity-temperature and particle-phase diffusion-temperature relations. Obtained flow and heat transfer results are illustrated graphically to show interesting features of this type of flow.
Time Resolved Torque of Three-Dimensional Rotating Bluff Bodies in a Cylindrical Tank
Authors: Maynes D. , Klewicki J. , McMurtry P.
Published: February 2008

Torque measurements have been made on rotating three-dimensional bluff bodies in a cylindrical container from start-up to mean flow steady state. The flow is observed to pass through three distinct temporal regimes in the transient process. These regimes include a build-up period where the torque remains approximately constant, a decay period where the torque decreases, and a mean steady state where the mean torque remains at a constant level. Effects of body geometry, rotation rate, acceleration rate, and fluid height in the tank are quantified. The torque coefficient during the build-up and mean steady-state regimes is shown to be a function of Reynolds number and body geometry. A time scale marking the beginning of the decay period is presented in terms of the problem parameters. Over the range of parameters studied, the build-up, decay, and mean steady-state regimes are completely specified by the Reynolds number, geometric parameters, and decay time scale.
A Procedure for Using DNS Databases
Authors: Parneix S. , Laurence D. , Durbin P. A.
Published: February 2008

A second moment closure (SMC) computation is compared in detail with the direct numerical simulation (DNS) data of Le et al. (1997) for the backstep flow at Re = 5100 in an attempt to understand why the intensity of the backflow and, consequently, the friction coefficient in the recirculation bubble are under-estimated. The data show that this recirculation bubble is far from being laminar except in the very near wall layer. A novel “differential a priori” procedure was used, in which the full transport equation for one isolated component of the Reynolds stress tensor was solved using DNS data as input. Conclusions are then different from what would have been deduced by comparing a full simulation to a DNS. In particular, the ε-equation, usually blamed for faults in model predictions, has been found to give excellent results in this case. In fact, the main problem comes from the uv-equation which predicts a too high turbulent force. A modification, by including the gradients of mean flow in the transport model, has then been attempted and has cured 50 percent of the backflow discrepancy.
The Steady Horizontal Flow of a Wall Jet Into a Large-Width Cavity
Authors: Homan K. O. , Soo S. L.
Published: February 2008

This paper treats the steady flow of a wall jet into a large-width cavity for which the primary axis is normal to the direction of the jet inflow. Numerical solutions of the two-dimensional Navier-Stokes equations are computed for inlet Reynolds numbers of 10 to 50 and tank width to inlet height ratios of 16 to 128. The length and velocity scales of the wall jet boundary layer exhibit close agreement with the classic wall jet similarity solution and published experimental data but the width of the region for which the comparison proves to be favorable has a limited extent. This departure from a self-similar evolution of the wall jet is shown to result from the finite domain width and its influence on the large recirculation cell located immediately above the wall jet boundary layer.
Angular Range and Uncertainty Analysis of Nonorthogonal Crossed Hot Wire Probes
Authors: Blanco-Marigorta Eduardo , Ballesteros-Tajadura Rafael , Santolaria Carlos
Published: February 2008

Crossed hot wire probes are widely used for measuring two-dimensional flows. Many applications in fluid mechanics like turbomachinery require an angular range as large as possible and a reasonable uncertainty in both direction and velocity values. The classic design places the wires orthogonally, while an angle between the probe wires allows a wider angular range to be obtained. The paper shows an experimental and a theoretical study of the response of this kind of probes, by analyzing the dependence of the calibration procedure and uncertainties of flow angle and velocity with the angle between the wires.
Anisotropy Invariants of Reynolds Stress Tensor in a Duct Flow and Turbulent Boundary Layer
Authors: Mazouz A. , Labraga L. , Tournier C.
Published: February 2008

The present study shows that the Reynolds stress anisotropy tensor for turbulent flow depends both on the nature of the surface and the boundary conditions of the flow. Contrary to the case of turbulent boundary layers with k-type surface roughness, the measured anisotropy invariants of the Reynolds stress tensor over a series of spanwise square bars separated by rectangular cavities (k-type) in duct flows show that roughness increases the anisotropy. There is a similarity between the effect of roughness on channel flow turbulence and that on pipe flow turbulence. The present data show that the effect of introducing a surface roughness significantly perturbs the entire thickness of the turbulent flow.
Perspective: Flow at High Reynolds Number and Over Rough Surfaces—Achilles Heel of CFD
Authors: Patel V. C.
Published: February 2008

The law of the wall and related correlations underpin much of current computational fluid dynamics (CFD) software, either directly through use of so-called wall functions or indirectly in near-wall turbulence models. The correlations for near-wall flow become crucial in solution of two problems of great practical importance, namely, in prediction of flow at high Reynolds numbers and in modeling the effects of surface roughness. Although the two problems may appear vastly different from a physical point of view, they share common numerical features. Some results from the ’superpipe’ experiment at Princeton University are analyzed along with those of previous experiments on the boundary layer on an axisymmetric body to identify features of near-wall flow at high Reynolds numbers that are useful in modeling. The study is complemented by a review of some computations in simple and complex flows to reveal the strengths and weaknesses of turbulence models used in modern CFD methods. Similarly, principal results of classical experiments on the effects of sand-grain roughness are reviewed, along with various models proposed to account for these effects in numerical solutions. Models that claim to resolve the near-wall flow are applied to the flow in rough-wall pipes and channels to illustrate their power and limitations. The need for further laboratory and numerical experiments is clarified as a result of this study.
Unbalanced Hydraulic Forces Caused by Geometrical Manufacturing Deviations of Centrifugal Impellers
Authors: Yoshida Yoshiki , Tsujimoto Yoshinobu , Kawakami Takashi , Sakatani Toru
Published: February 2008

When an impeller has geometrical manufacturing deviations, synchronous vibration occurs from an unbalanced hydraulic force rotating with the impeller. This phenomenon is termed “Hydraulic unbalance.” In this study, experiments and analyses were carried out to study the hydraulic unbalance. We made several model impellers with intentional “manufacturing deviations”: in vane angle, vane pitch, and with eccentricity. Hydraulic forces were estimated from the rotor vibrations. The results of the experiments showed that the magnitude of the hydraulic force increased as the increase of the extent of manufacturing deviations. It was also shown that the amplitude and the phase of the vibration due to the unbalanced hydraulic force depend on the flow rate. This character of the unbalanced hydraulic force is quite different from that of the mass unbalance force, which is independent of the flow rate. A two-dimensional inviscid flow analysis was carried out to clarify the characteristic of the unbalanced hydraulic force. It was found that the hydraulic force can be divided into three components which are constant, linear and parabolic function of the flow rate. Applications of the present results show that the “hydraulic unbalance” of a high-speed pump impeller manufactured within a geometrical tolerance recommended by a typical standard can be larger than the ordinary “mechanical unbalance.” It was thus shown that the hydraulic unbalance is one of the important factors in the balancing of high-speed pump rotors.
Investigation and Control of Backflow Ahead of a Shrouded Inducer
Authors: Offtinger Brissaud C. , Henry C. , Morel R. , Spettel F.
Published: February 2008

Incorporating a backflow recirculator is a way to reduce part load instabilities of inducers. Experimental results demonstrate that two devices with simple geometry considerably soften pressure pulsations in suction and discharge pipes associated with part load instabilities, even at low NPSH conditions and low flowrates. This is accomplished with negligible adverse effects on pump performance.
A Numerical Methodology to Predict Exhaust Plumes of Propulsion Nozzles
Authors: Nasuti F. , Niccoli R. , Onofri M.
Published: February 2008

A simple methodology to simulate the mixing layers that occur between nozzle exhaust jet and the external air is proposed. The method is based on a simplified model of the plume, that replaces the mixing layer with a contact discontinuity surface, thus avoiding the cumbersome calculation of the turbulent mixing of two flows with different chemical composition. The contact discontinuity is numerically treated by an advanced fitting technique, capable of tracking the discontinuity by points floating over the computational grid. The numerical method is discussed and its capability is demonstrated with validation tests, as well as with a discussion of some practical applications for underexpanded nozzle flows.
Nonlinear Rupture Theory of a Thin Liquid Film With Insoluble Surfactant
Authors: Hwang Chi-Chuan , Lin Chaur-Kie , Hou Da-Chih , Uen Wu-Yih , Lin Jenn-Sen
Published: February 2008

Effects of insoluble surfactant on the dynamic rupture of a thin liquid film coated on a flat plate are studied. The strong nonlinear evolution equations derived by the integral method are solved by numerical method. The results show that enhancing (weakening) the Marangoni effect (the surface diffusion effect) will delay the rupture process. Furthermore, the rupture time predicted by the integral theory is shorter than that predicted by the long-wave expansion method. In addition, the quantitative difference in the rupture time predicted by two models enlarges with the increase of Marangoni effect, however, without obvious change as the diffusion effect increases.
Nonlinear Spin-Up of a Rotating Stratified Fluid: Theory
Authors: Hewitt Richard E. , Duck Peter W. , Foster Michael R. , Davies Peter A.
Published: February 2008

We consider the boundary layer that forms on the wall of a rotating container of stratified fluid when altered from an initial state of rigid body rotation. The container is taken to have a simple axisymmetric form with sloping walls. The introduction of a non-normal component of buoyancy into the velocity boundary-layer is shown to have a considerable effect for certain geometries. We introduce a similarity-type solution and solve the resulting unsteady boundary-layer equations numerically for three distinct classes of container geometry. Computational and asymptotic results are presented for a number of parameter values. By mapping the parameter space we show that the system may evolve to either a steady state, a double-structured growing boundary-layer, or a finite-time breakdown depending on the container type, rotation change and stratification. In addition to extending the results of Duck et al. (1997) to a more general container shape, we present evidence of a new finite-time breakdown associated with higher Schmidt numbers.
Thermoelectrically Driven Melt Motion During Floating Zone Crystal Growth With an Axial Magnetic Field
Authors: Khine Y. Y. , Walker J. S.
Published: February 2008

During semiconductor crystal growth with an externally applied magnetic field, thermoelectric currents may drive a melt circulation which affects the properties of the crystal. This paper treats a model problem for a floating zone process with a uniform axial magnetic field, with planar solid-liquid interfaces, with a cylindrical free surface, with a parabolic temperature variation along the crystal-melt interface, and with an isothermal feed rod-melt interface. The ratio of the electrical conductivities of the liquid and solid is a key parameter. The azimuthal velocity is much larger than the radial or axial velocity. There is radially outward flow near the crystal-melt interface which should be beneficial for the mass transport of dopants and species.
Particle Arrestance Modeling Within Fibrous Porous Media
Authors: Giuliani James , Vafai Kambiz
Published: February 2008

In the present study, particle growth on individual fibers within a fibrous medium is examined as flow conditions transition beyond the Stokes flow regime. Employing a numerical model that solves the viscous, incompressible Navier-Stokes equations, the Stokes flow approximation used in past research to describe the velocity field through the fibrous medium is eliminated. Fibers are modeled in a staggered array to eliminate assumptions regarding the effects of neighboring fibers. Results from the numerical model are compared to the limiting theoretical results obtained for individual cylinders and arrays of cylinders. Particle growth is presented as a function of time, angular position around the fiber, and flow Reynolds number. From the range of conditions examined, particles agglomerate into taller and narrower dendrites as Reynolds number is increased, which increases the probability that they will break off as larger agglomerations and, subsequently, substantially reduce the hydraulic conductivity of the porous medium.
Analysis on Liquid-Vapor Bubbly-Flow Systems in Reciprocating Motion
Authors: Iyer Claudia O. , Yang Wen-Jei
Published: February 2008

An analytical study is performed on the dynamics and hydrodynamic stability of liquid-vapor mixtures in the bubbly-flow range in reciprocating motion through a horizontal channel. The perturbation technique is applied on the one-dimensional conservation equations for laminar flow and on the thermodynamic equation of state. The Laplace transform is operated on the linearized equations from which a transfer function is derived, relating the flow rate change due to a change in pressure drop along the channel. The resulting characteristic equation is analyzed to determine the dynamic behavior of the two-phase flow in reciprocating motion and the conditions for neutral stability under which self-induced oscillations occur. The natural frequency of the physical system is derived, which can be used to predict the resonance that will occur in forced vibrations. Results can be applied to systems such as car suspensions (shock absorbers) in which oil is susceptible to cavitation, resulting in bubbly flow due to vibrations. Conditions under which resonance occurs in the two-phase system are determined. Resonance leads to severe oscillations and noise generation, as experienced in shock absorbers in car suspensions.
Propeller Sheet Cavitation Predictions Using a Panel Method
Authors: Mueller A. C. , Kinnas S. A.
Published: February 2008

A boundary element method is used to predict the time-dependent cavitation on a propeller subject to nonaxisymmetric inflow. The convergence of the method is studied. The predicted cavities agree well with those observed in CAPREX, an experiment performed at MIT’s variable pressure water tunnel. The method is modified so that prediction of cavities detaching at mid-chord regions is possible. An algorithm for predicting the cavity detachment location on the blade is described and applied on a blade geometry which exhibits mid-chord cavitation.
Monotonic Convergence Property of Turbulent Flow Solution With Central Difference and QUICK Schemes
Authors: Hayase Toshiyuki
Published: February 2008

Monotonic convergence of numerical solutions with the computational grid refinement is an essential requirement in estimating the grid-dependent uncertainty of computational fluid dynamics. If the convergence is not monotonic, the solution could be erroneously regarded as convergent at the local extremum with respect to some measure of the error. On the other hand, if the convergence is exactly monotonic, estimation methods such as Richardson extrapolation properly evaluate the uncertainty of numerical solutions. This paper deals with the characterization of numerical schemes based on the property of the monotonic convergence of numerical solutions. Two typical discretization schemes of convective terms were considered; the second-order central difference scheme and the third-order Leonard’s QUICK scheme. A fully developed turbulent flow through a square duct was calculated via a SIMPLER based finite volume method without a turbulence model. The convergence of the numerical solution with the grid refinement was investigated for the mean flow property as well as fluctuations. The comparison of convergence process between the discretization schemes has revealed that the QUICK scheme results in preferable monotonic convergence, while the second-order central difference scheme undergoes non-monotonic convergence. The latter possibly misleads the determination of convergence with the grid refinement, or causes trouble in applying the Richardson extrapolation procedure to estimate the numerical uncertainty.
The Effects of Secondary Flow and Passive Injection on the Motion of Solid Particles Entrained in Flow Through a Curved Converging Channel
Authors: Ventresca James J. , Rouleau Wilfred T.
Published: February 2008

The three-dimensional effects of secondary flow, passive injection, and particle size on the motion of solid particles entrained in a laminar, incompressible flow through a curved, converging, rectangular passage were numerically investigated. Emphasis was placed on observing the physical mechanisms that cause particles 5 μm and smaller in diameter to deposit on passage surfaces and to concentrate near the endwalls and mid-span at the passage exit. Particle trajectories were calculated for 5, 30, and 300 μm diameter solid particles. It was observed that the paths of 5 μm particles were similar to the streamlines of the three-dimensional flow in the channel until the particles encountered the boundary layers on the blade surfaces and endwalls, where they would graze the surfaces (contributing to particle deposition) and concentrate at the exit of the channel. Particles of 30 μm diameter, however, were only slightly affected by secondary flows, but were affected enough to be made to concentrate at the exit near the endwall and mid-span surfaces. Particles of 300 μm diameter were not affected by secondary flows at all. The particle trajectories showed that the passage secondary flow convected particles across endwalls toward the pressure and suction surface boundary layers of the blades. It was observed that small particles were made to decelerate and/or concentrate in the boundary layers near the passage exit. It was found that this concentration of particles along the suction surface and endwalls could be significantly reduced by means of passive injection. (Passive injection is a method of inducing the flow of jets in the curved portion of an airfoil shaped surface due to the pressure difference on opposing sides. This is accomplished by means of holes or slots that have been drilled through the surface at strategic locations.)
Jets in a Crossflow: Effects of Geometry and Blowing Ratio
Authors: Findlay M. J. , Salcudean M. , Gartshore I. S.
Published: February 2008

The flow field characteristics of three different geometries of square jets in a crossflow at various blowing ratios are examined. The geometries considered are: perpendicular, streamwise-inclined, and spanwise-inclined jets. The inclined jets are at a 30 deg angle to the wind tunnel floor. Mean velocity and turbulence measurements along with film cooling effectiveness and scalar transport data were obtained. Jet-to-crossflow blowing ratios of 1.5, 1.0 and 0.5 are used with a density ratio of 1. It is shown that the flow field at the jet exit is strongly influenced by the crossflow as well as by the inlet conditions at the entrance to the jet orifice. The strong streamline curvature which is present in the perpendicular and spanwise injection cases appears to result in the greatest turbulence anisotropy. The film cooling effectiveness is best at the lowest blowing ratios as the jet is deflected strongly towards the floor of the wind tunnel, although the improvement is more significant for the streamwise injection case.
Experimental Investigations of Steady Flow in a Tube With Circumferential Wall Cavity
Authors: Cieslicki Krzysztof , Lasowska Anna
Published: February 2008

The problem of an axisymmetrical laminar flow past a square cavity was treated experimentally. The cavity flow patterns obtained in flow visualization were of a special interest. Some geometrical features of dividing streamline like coordinates of separate and reattachment points as well as the coordinates of the vortex center were measured for a range of Re numbers and compared with the numerical results obtained by Stevenson. The detected flow patterns were also related to the global pressure-flow characteristic. Two zones of laminar flow characterized by the different nonlinear corrections to Darcy’s law were distinguished.
A Macroscopic Turbulence Model for Flow in a Porous Medium
Authors: Nakayama A. , Kuwahara F.
Published: February 2008

A complete set of macroscopic two-equation turbulence model equations has been established for analyzing turbulent flow and heat transfer within porous media. The volume-averaged transport equations for the mass, momentum, energy, turbulence kinetic energy and its dissipation rate were derived by spatially averaging the Reynolds-averaged set of the governing equations. The additional terms representing production and dissipation of turbulence kinetic energy are modeled introducing two unknown model constants, which are determined from a numerical experiment using a spatially periodic array. In order to investigate the validity of the present macroscopic turbulence model, a macroscopically unidirectional turbulent flow through an infinite array of square rods is considered from both micro- and macroscopic-views. It has been found that the stream-wise variations of the turbulence kinetic energy and its dissipation rate predicted by the present macroscopic turbulence model agree well with those obtained from a large scale microscopic computation over an entire field of saturated porous medium.
Multistage Simulation by an Adaptive Finite Element Approach Using Structured Grids
Authors: Sleiman M. , Tam A. , Robichaud M. P. , Peeters M. F. , Habashi W. G.
Published: February 2008

This paper presents the application of a three-dimensional Navier-Stokes finite element code (NS3D) in the context of turbomachinery rotor-stator multistage interaction. A mixing-plane approach is used, in which boundary conditions at a common interface plane between adjacent blade rows are iteratively adjusted to yield a flow satisfying the continuity, momentum, and energy conservation equations, in an average sense. To further improve the solutions, a mesh adaptation technique then redistributes the mesh points of the structured grid within each component, according to an a posteriori edge-based error estimate based on the Hessian of the local flow solution. This matrix of second derivatives controls both the magnitude and direction of the required mesh movement at each node, is then implemented using an edge-based spring analogy. The methodology is demonstrated for two test cases with two types of data: a well-instrumented experimental large-scale rotating rig for a second stage compressor at UTRC and an actual engine. The latter, a two-stage compressor of a turboprop, has been only tested as a single-stage configuration, because of the quality of the experimental data available. All results compare well to the data and demonstrate the utility of the approach. In Particular, the mesh adaptation shows large improvements in agreement between the calculations and the experimental data.
Velocity Statistics of Round, Fully Developed, Buoyant Turbulent Plumes
Authors: Dai Z. , Tseng L. K. , Faeth G. M.
Published: February 2008

An experimental study of the structure of round buoyant turbulent plumes was carried out, limited to conditions within the fully developed (self-preserving) portion of the flow. Plume conditions were simulated using dense gas sources (carbon dioxide and sulfur hexafluoride) in a still air environment. Velocity statistics were measured using laser velocimetry in order to supplement earlier measurements of mixture fraction statistics using laser-induced iodine fluorescence. Similar to the earlier observations of mixture fraction statistics, self-preserving behavior was observed for velocity statistics over the present test range (87–151 source diameters and 12–43 Morton length scales from the source), which was farther from the source than most earlier measurements. Additionally, the new measurements indicated that self-preserving plumes are narrower, with larger mean streamwise velocities near the axis (when appropriately scaled) and with smaller entrainment rates, than previously thought. Velocity statistics reported include mean and fluctuating velocities, temporal power spectra, temporal and spatial integral scales, and Reynolds stresses.
Computational Evaluation of Approximate Rayleigh–Debye–Gans/Fractal-Aggregate Theory for the Absorption and Scattering Properties of Soot
Authors: Farias T. L. , Carvalho M. G. , Ko¨ylu¨ U¨. O¨. , Faeth G. M.
Published: February 2008

A computational evaluation of an approximate theory for the optical properties of soot is described, emphasizing the small-angle (Guinier) regime. The approximate theory (denoted RDG-FA theory) is based on the Rayleigh–Debye–Gans scattering approximation while treating soot as mass-fractal aggregates of spherical primary particles that have constant diameters and refractive indices. The approximate theory was evaluated by more exact predictions from the solution of the volume integral equation formulation of the governing equations, using the method of moments, and based on the ICP algorithm of Iskander et al. (1989). Numerical simulations were used to construct statistically significant populations of soot aggregates having appropriate fractal properties and prescribed numbers of primary particles per aggregate. Optical properties considered included absorption, differential scattering, and total scattering cross sections for conditions typical of soot within flame environments at wavelengths in the visible and the infrared. Specific ranges of aggregate properties were as follows: primary particle optical size parameters up to 0.4, numbers of primary particles per aggregate up to 512, mean fractal dimensions of 1.75, mean fractal prefactors of 8.0, and refractive indices typical of soot. Over the range of the evaluation, ICP and RDG-FA predictions generally agreed within numerical uncertainties (ca. 10 percent) within the Guinier regime, complementing similar performance of RDG-FA theory in the power-law regime based on recent experiments. Thus, the use of approximate RDG-FA theory to estimate the optical properties of soot appears to be acceptable—particularly in view of the significant uncertainties about soot optical properties due to current uncertainties about soot refractive indices.
Local Friction and Heat Transfer Behavior of Water in a Turbulent Pipe Flow With a Large Heat Flux at the Wall
Authors: Choi E. , Cho Y. I.
Published: February 2008

The present study investigated the behavior of friction and heat transfer coefficients of water flowing turbulently in a relatively long (i.e., 950 diameter long) circular pipe. When a large heat flux was applied at the wall, the viscosity of water significantly decreased along the axial direction due to the increasing temperature of water. A concept of a “redeveloping region” was introduced, where the local heat transfer coefficient increased while the local friction coefficient decreased due to the above-mentioned viscosity change. The present study proposed the use of local bulk-mean temperature to determine local Nusselt numbers by using local Reynolds (ReLB) and Prandtl numbers (PrLB), a method that automatically took into account the effect of axial viscosity change on the evaluation of local heat transfer coefficients. A new turbulent heat transfer correlation for the prediction of the local Nusselt number is given as Nux=0.00425ReLB0.979PrLB0.4(μw/μb)−0.11.
Augmented Heat Transfer in a Recovery Passage Downstream From a Grooved Section: An Example of Uncoupled Heat/Momentum Transport
Authors: Greiner M. , Chen R.-F. , Wirtz R. A.
Published: February 2008

Earlier experiments have shown that cutting transverse grooves into one surface of a rectangular cross-sectional passage stimulates flow instabilities that greatly enhance heat transfer/pumping power performance of air flows in the Reynolds number range 1000 < Re < 5000. In the current work, heat transfer, pressure, and velocity measurements in a flat passage downstream from a grooved region are used to study how the flow recovers once it is disturbed. The time-averaged and unsteady velocity profiles, as well as the heat transfer coefficient, are dramatically affected for up to 20 hydraulic diameters past the end of the grooved section. The recovery lengths for shear stress and pressure gradient are significantly shorter and decrease rapidly for Reynolds numbers greater than Re = 3000. As a result, a 5.4-hydraulic-diameter-long recovery region requires 44 percent less pumping power for a given heat transfer level than if grooving continued.
Nonisothermal Nongray Absorbing-Emitting-Scattering Suspension of Fe3O4 Particles Under Concentrated Solar Irradiation
Authors: Mischler D. , Steinfeld A.
Published: February 2008

Radiation transfer within a cloud of magnetite (Fe3O4) particles contained in an infinite slab is considered. The particulate cloud is modeled as a pseudo-continuous, nongray, nonisothermal, absorbing, emitting, and anisotropically scattering medium. The energy source is concentrated solar irradiation, which is assumed to be diffusely and uniformly distributed over a circular opening and has a 5780 K blackbody spectrum. Mie-scattering theory is applied to calculate the spectrally and directionally dependent optical properties of the particles. The Monte Carlo ray-tracing method is used to calculate the attenuation characteristics of the cloud and the temperature distribution under radiative equilibrium. The Monte Carlo simulation is optimized by incorporating the appropriate cumulative probability density functions via Bezier surfaces. The effect of spectral and directional dependency is investigated by comparing the results with those obtained for a gray and isotropic-scattering medium under diffuse or perpendicular incident radiation. It is found that a cloud of Mie-scattering Fe3O4 particles under perpendicular incident radiation requires approximately twice as much optical thickness to obtain the same attenuation (of radiation at all wavelengths and directions) as a cloud of isotropic-scattering particles under diffuse incident radiation. It is demonstrated that the gray approximation using Planck mean values can lead to considerable error in the temperature solution because the spectral absorption coefficient is higher in the region of longer wavelengths where the peak emission by Fe3O4 particles occurs.
Condensation of a Vapor Flowing Inside a Horizontal Rectangular Duct
Authors: Lu Q. , Suryanarayana N. V.
Published: February 2008

Condensation of a vapor flow inside a horizontal rectangular duct, using the bottom plate as the only condensing surface, was experimentally investigated. The experimental measurements included condensate film thickness and heat transfer coefficients with R-113 and FC-72. The condensate film thickness, measured with an ultrasonic transducer, was used to obtain the local heat transfer coefficient. The heat transfer coefficient increased with increasing inlet vapor velocity. The rate of increase was enhanced noticeably after the appearance of interfacial waves. Within the limited range of the experimental variables, a correlation between St and RegL was developed by a linear regression analysis. However, because of the effect of the interfacial waves, instead of a single correlation for the entire range of RegL, two separate equations (one for the wave-free regime and another for the regime with waves) were found. Analytical predictions of heat transfer rates in the annular condensation regime require the proper modeling of the interfacial shear stress. A properly validated interfacial shear stress model with condensation is not yet available. The measurement of condensate film thickness at several axial locations opens the door for determining the local interfacial stress and, hence, a model for the interfacial shear stress.
Non-Darcian Forced Convection in Porous Media With Anisotropic Dispersion
Authors: Adnani P. , Catton I. , Abdou M. A.
Published: February 2008

Convective heat transfer in a particle packed tube is modeled in this paper. Axial and radial dispersion are both included in the governing equations. Results are compared with experimental data, and with previously developed models that did not include axial dispersion. It is shown that heat transfer in the thermally developing region is affected by axial dispersion when Peclet number is smaller than 10. Graphic results are provided to show the importance of axial dispersion for various Peclet numbers.
Real-Time Physiconeural Solutions for MOCVD
Authors: Kelkar A. S. , Mahajan R. L. , Sani R. L.
Published: February 2008

This paper presents an integrated physiconeural network approach for the modeling and optimization of a vertical MOCVD reactor. The basic concept is to utilize the solutions obtained from a physical model to build an accurate neural network (NN) model The resulting model has the attractive features of self-adaptiveness and speed of prediction and is an ideal starting tool for process optimization and control. Following this approach, a first-principles physical model for the reactor was solved numerically using the Fluid Dynamics Analysis Package (FIDAP). This transient model included property variation and thermodiffusion effects. Using software developed in house, neural networks were then trained using FIDAP simulations for combinations of process parameters determined by the statistical Design of Experiments (DOE) methodology. The outputs were the average and local deposition rates. It is shown that the trained NN model predicts the behavior of the reactor accurately. Optimum process conditions to obtain a uniform thickness of the deposited film were determined and tested using the physical model. The results demonstrate the power and robustness of NNs for obtaining fast responses to changing input conditions. A procedure for developing equipment models based on physiconeural network models is also described.
Solution of the Inverse Heat Conduction Problem From Thermal Strain Measurements
Authors: Blanc G. , Raynaud M.
Published: February 2008

Another approach for the solution of the inverse heat conduction problem is presented. The unknown boundary conditions are recovered from thermal strain and temperature measurements instead of temperature measurements only. It is required to calculate both the temperature field and the strains induced by this field. The sensitivity coefficient analysis and the results of two benchmark test cases show that it is possible to recover higher temporal frequencies when the inversion is done from strains instead of temperatures. An experimental setup was specially designed to validate the numerical results. The numerical predictions are verified. Special attention is given to the strain gage measurements.
Observations of Spanwise Symmetry Breaking for Unsteady Mixed Convection in Horizontal Ducts
Authors: Spall R. E.
Published: February 2008

Three-dimensional mixed convection in a horizontal rectangular duct heated from below was studied numerically for a non-Boussinesq fluid. The duct was of aspect ratio four, with cold side-wall boundary conditions. Results were obtained at a reduced temperature of 2.33, Rayleigh number of 130,700, and Reynolds numbers of 5 and 20. Using the Re = 20 solution as initial conditions, the Re = 5 solution developed asymmetries about the duct centerline plane, with a streamwise periodic generation and dissolution of the inner pair of vortices. The consequence of these coherent vortical structures was a complex time-dependent distribution of the Nusselt number on the lower wall, with implications in the area of chemical vapor deposition. Specifically, when compared with the steady flow case occurring at Re = 20, an increased spatial uniformity of the time-averaged Nusselt number along the lower duct surface resulted.
Experimental Evidence of Hyperbolic Heat Conduction in Processed Meat
Authors: Mitra K. , Kumar S. , Vedevarz A. , Moallemi M. K.
Published: February 2008

The objective of this paper is to present experimental evidence of the wave nature of heat propagation in processed meat and to demonstrate that the hyperbolic heat conduction model is an accurate representation, on a macroscopic level, of the heat conduction process in such biological material. The value of the characteristic thermal time of a specific material, processed bologna meat, is determined experimentally. As a part of the work different thermophysical properties are also measured. The measured temperature distributions in the samples are compared with the Fourier results and significant deviation between the two is observed, especially during the initial stages of the transient conduction process. The measured values are found to match the theoretical non-Fourier hyperbolic predictions very well. The superposition of waves occurring inside the meat sample due to the hyperbolic nature of heat conduction is also proved experimentally.
Natural Convection Heat Transfer in a Rectangular Enclosure With a Transverse Magnetic Field
Authors: Alchaar S. , Vasseur P. , Bilgen E.
Published: February 2008

In this paper the effect of a transverse magnetic field on buoyancy-driven convection in a shallow rectangular cavity is numerically investigated (horizontal Bridgman configuration). The enclosure is insulated on the top and bottom walls while it is heated from one side and cooled from the other. Both cases of a cavity with all rigid boundaries and a cavity with a free upper surface are considered. The study covers the range of the Rayleigh number, Ra, from 102 to 105, the Hartmann number, Ha, from 0 to 102, the Prandtl number, Pr, from 0.005 to 1 and aspect ratio of the cavity, A, from 1 to 6. Comparison is made with an existing analytical solution (Garandet et al., 1992), based on a parallel flow approximation, and its range of validity is delineated. Results are presented for the velocity and temperature profiles and heat transfer in terms of Ha number. At high Hartmann numbers, both analytical and numerical analyses reveal that the velocity gradient in the core is constant outside the two Hartmann layers at the vicinity of the walls normal to the magnetic field.
The Effect of Turbulence on Solidification of a Binary Metal Alloy With Electromagnetic Stirring
Authors: Prescott P. J. , Incropera F. P.
Published: February 2008

Electromagnetic induction is considered as a means of altering convection during the solidification of a Pb–19 wt pct Sn alloy. Application of a time-varying magnetic field induces Lorentz forces, which augment thermal buoyancy forces in the melt and oppose solutal buoyancy forces in the mushy zone. A continuum model for binary solid–liquid phase change is extended to account for turbulence, and laminar and turbulent flow predictions are contrasted. Results indicate that turbulence decreases the propensity for channel development and macrosegregation by enhancing mixing and reducing the effective Lewis number from 8600 to near unity.
Early Initiation of Natural Convection in an Open Porous Layer Due to the Presence of Solid Conductive Inclusions
Authors: Delmas A. , Arquis E.
Published: February 2008

The effects of solid conductive blocks on the initiation of convection in a porous medium are reported in this paper. A two-dimensional convective code was used to determine the temperature field, the structure of the motion, and the global heat transfer through a composite medium consisting of permeable and impermeable areas. Influence of the size of impermeable regions on convection as well as the effect of the distance between these solid blocks was studied in terms of Nusselt number and maximum of the stream function. The predicted heat transfer in this type of composite medium, obtained with the code, was compared with experimental results where the porous medium is a low-density insulating material in which some wood joists are included. This configuration corresponds to the layer of insulation on the floor of a residential attic.
Prediction of Jet Impingement Heat Transfer Using a Hybrid Wall Treatment With Different Turbulent Prandtl Number Functions
Authors: Morris G. K. , Garimella S. V. , Amano R. S.
Published: February 2008

The local heat transfer coefficient distribution on a square heat source due to a normally impinging, axisymmetric, confined, and submerged liquid jet was computationally investigated. Numerical predictions were made for nozzle diameters of 3.18 and 6.35 mm at several nozzle-to-heat source spacings, with turbulent jet Reynolds numbers ranging from 8500 to 13,000. The commercial finite-volume code FLUENT was used to solve the thermal and flow fields using the standard high-Reynolds number k–ε turbulence model. The converged solution obtained from the code was refined using a post-processing program that incorporated several near-wall models. The role of four alternative turbulent Prandtl number functions on the predicted heat transfer coefficients was investigated. The predicted heat transfer coefficients were compared with previously obtained experimental measurements. The predicted stagnation and average heat transfer coefficients agree with experiments to within a maximum deviation of 16 and 20 percent, respectively. Reasons for the differences between the predicted and measured heat transfer coefficients are discussed.
The Intertube Falling Film: Part 1—Flow Characteristics, Mode Transitions, and Hysteresis
Authors: Hu X. , Jacobi A. M.
Published: February 2008

When a liquid film falls from one horizontal tube to another below it, the flow may take the form of discrete droplets, jets, or a continuous sheet; the mode plays an important role in the wetting and heat transfer characteristics of the film. Experiments are reported that explore viscous, surface tension, inertial, and gravitational effects on the falling-film mode transitions. New flow classifications, a novel flow regime map, and unambiguous transition criteria for each of the mode transitions are provided. This research is part of an overall study of horizontal-tube, falling-film flow and heat transfer, and the results may have important implications on the design and operation of falling-film heat exchangers.
Correlations for Film Regeneration and Air Dehumidification for a Falling Desiccant Film With Air in Crossflow
Authors: Park M. , Howell J. R. , Vliet G. C.
Published: February 2008

The coupled heat and mass transfer between a falling triethylene glycol (TEG) desiccant film and air in crossflow have previously been presented and solved numerically for the cases of regeneration and dehumidification. Here, correlations for the effects of independent variables on the rate of regeneration in the regenerator and on the rate of dehumidification and sensible cooling in the absorber are developed by statistical analysis of the numerical results. The functional correlations developed should be useful in the design of regenerators and absorbers having falling liquid desiccant films and air in crossflow.
Nucleate Boiling Heat Transfer in Spray Cooling
Authors: Yang J. , Chow L. C. , Pais M. R.
Published: February 2008

An experimental study to determine the effect of liquid and secondary gas flow in droplet impingement cooling is presented. The nucleate boiling regime in particular is analyzed. A correlation to predict the Nusselt number based on the liquid film thickness is derived and compared with the experimental data.
The Onset of Flow Instability for Downward Flow in Vertical Channels
Authors: Stelling R. , McAssey E. V. , Dougherty T. , Yang B. W.
Published: February 2008

A simple analytical model to predict the onset of Ledinegg instability in vertical channels under downflow conditions has been developed and evaluated. The model divides the flow field into three regions based upon the fluid temperature. The pressure drop is then found by solving an appropriate set of equations for each region. The theoretical results are compared to an extensive set of experimental data covering a range of channel diameters and operating conditions. Agreement is excellent, and the prediction of the velocity at which the minimum point in the demand curve occurs is within 12 percent over the range of experimental results. A parameter, the ratio between the surface heat flux and the heat flux required to achieve saturation at the channel exit for a given flow rate, is found to be a very accurate indicator of the minimum point velocity.