bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effect of Biofilms on Turbulent Boundary Layers
Authors: Schultz M. P. , Swain G. W.
Published: February 2008

Materials exposed in the marine environment, including those protected by antifouling paints, may rapidly become colonized by microfouling. This may affect frictional resistance and turbulent boundary layer structure. This study compares the mean and turbulent boundary layer velocity characteristics of surfaces covered with a marine biofilm with those of a smooth surface. Measurements were made in a nominally zero pressure gradient, boundary layer flow with a two-component laser Doppler velocimeter at momentum thickness Reynolds numbers of 5600 to 19,000 in a recirculating water tunnel. Profiles of the mean and turbulence velocity components, including the Reynolds shear stress, were measured. An average increase in the skin friction coefficient of 33 to 187 percent was measured on the fouled specimens. The skin friction coefficient was found to be dependent on both biofilm thickness and morphology. The biofilms tested showed varying effect on the Reynolds stresses when those quantities were normalized with the friction velocity.
On Pressure-Velocity Correlation of Steady and Oscillating Flows in Regenerators Made of Wire Screens
Authors: Hsu Chin-Tsau , Fu Huili , Cheng Ping
Published: February 2008

A facility capable of generating steady and oscillating flows was constructed and experiments were conducted to investigate the pressure-drop characteristics of regenerators packed with wire screens. Both the velocity and pressure-drop across the regenerator were measured. To accurately determine the correlation between pressure-drop and velocity, the experiments covered a wide range from very low to very high Reynolds numbers, Reh. The steady flow results reveal that a three-term correlation with a term proportional to Reh−1/2 in addition to the Darcy-Forchheimer two-term correlation will fit best to the data. This Reh−1/2 term accounts for the boundary layer effect at intermediate Reynolds number. The results also show that the correlation for oscillating flows coincides with that for steady flows in 1 < Reh < 2000. This suggests that the oscillating flows in the regenerators behave as quasi-steady at the frequency range of less than 4.0 Hz, which is the maximum operable oscillating flow frequency of the facility.
Modeling Turbulent Wall Flows Subjected to Strong Pressure Variations
Authors: Hanjalic´ K. , Hadzˇic´ I. , Jakirlic´ S.
Published: February 2008

Mean pressure gradient affects the turbulence mainly through the modulation of the mean rate of strain. Modification of the turbulence structure feeds, in turn, back into the mean flow. Particularly affected is the near wall region (including the viscous sublayer) where the pressure gradient invalidates the conventional boundary-layer “equilibrium” assumptions and inner-wall scaling. Accurate predictions of such flows require application of advanced turbulence closures, preferably at the differential second-moment level with integration up to the wall. This paper aims at demonstrating the potential usefulness of such a model to engineers by revisiting some of the recent experimental and DNS results and by presenting a series of computations relevant to low-speed external aerodynamics. Several attached and separated flows, subjected to strong adverse and favorable pressure gradient, as well as to periodic alternation of the pressure gradient sign, all computed with a low-Re-number second-moment closure, display good agreement with experimental and DNS data. It is argued that models of this kind (in full or a truncated form) may serve both for steady or transient Reynolds-Averaged Navier-Stokes (RANS, TRANS) computations of a variety of industrial and aeronautical flows, particularly if transition phenomena, wall friction, and heat transfer are in focus.
The Fluid Mechanics of Microdevices—The Freeman Scholar Lecture
Authors: Gad-el-Hak Mohamed
Published: February 2008

Manufacturing processes that can create extremely small machines have been developed in recent years. Microelectromechanical systems (MEMS) refer to devices that have characteristic length of less than 1 mm but more than 1 micron, that combine electrical and mechanical components and that are fabricated using integrated circuit batch-processing techniques. Electrostatic, magnetic, pneumatic and thermal actuators, motors, valves, gears, and tweezers of less than 100-μm size have been fabricated. These have been used as sensors for pressure, temperature, mass flow, velocity and sound, as actuators for linear and angular motions, and as simple components for complex systems such as micro-heat-engines and micro-heat-pumps. The technology is progressing at a rate that far exceeds that of our understanding of the unconventional physics involved in the operation as well as the manufacturing of those minute devices. The primary objective of this article is to critically review the status of our understanding of fluid flow phenomena particular to microdevices. In terms of applications, the paper emphasizes the use of MEMS as sensors and actuators for flow diagnosis and control.
Pump Research and Development: Past, Present, and Future—An American Perspective
Authors: Gopalakrishnan S.
Published: February 2008

Pump research and development efforts are primarily driven by the needs of the customer. Today, these needs are centered around cost and reliability issues with the understanding that certain threshold levels of performance are achieved. As centrifugal pumps have reached high levels of maturity in most industrial applications, we can anticipate, that in the future, customer expectations will change subtly but significantly. They will demand continuously reducing costs with the understanding that reliability and technology needs will be satisfied. This would lead to a strong emphasis on consistent predictability of performance in the field and to less of a focus on innovations in design. R&D efforts in the past were intended to stretch the envelope to produce better hydraulic performance, to improve mean-time-between-failures, and to operate at higher speeds. In contrast, R&D efforts in the future would be aimed towards cost reduction, accurate hydraulic, guarantees, and flawless performance in the field. In this paper, the R&D efforts of the past, present, and future are discussed in terms of three core competencies, which are essential for today’s pump manufacturer. These are hydraulics (with an emphasis on improving predictability of performance and improving impeller life), vibrations (with a view to providing cost effective problem solving/avoidance capability), and pump designs which capitalize on improved understanding of the underlying technologies.
The Rotordynamic Forces on an Open-Type Centrifugal Compressor Impeller in Whirling Motion
Authors: Yoshida Yoshiki , Tsujimoto Yoshinobu , Ishii Nobuhiro , Ohashi Hideo , Kano Fumitaka
Published: February 2008

In recent years, increasing interest has been given to the rotordynamic forces on impellers, from the view point of the shaft vibration analysis. Previous experimental and analytical results have shown that the fluid-induced forces on closed-type (with shroud) centrifugal impellers in whirling motion contribute substantially to the potential destabilization of subsynchronous shaft vibrations. However, to date nothing is known of the rotordynamic forces on open-type (without shroud) centrifugal impellers. This paper examines the rotordynamic fluid forces on an open-type centrifugal compressor impeller in whirling motion. For an open-type impeller, the variation of the tip clearance due to the whirling motion is the main contribute to the rotordynamic forces. Experiments were performed to investigate the rotordynamic forces by direct measurements using a force balance device, and indirectly from the unsteady pressure on the casing wall over a range of whirl speed ratio (Ω/ω) for several flow rates. In this paper, the following results were obtained: (1) Destabilizing forces occur at small positive whirl speed ratio (0 ≤ Ω/ω ≤ 0.3) throughout the flow range of normal operation; (2) At smaller flow rate with inlet backflow, the magnitude of the fluid force changes dramatically at a whirl speed ratio close to Ω/ω = 0.8, resulting in destabilizing rotordynamic forces. From the measurement of unsteady inlet pressure, it was shown that the drastic changes in the fluid force are related to the coupling of the whirling motion with a rotating flow instability, similar to “rotating stall”; (3) The forces estimated from the unsteady pressure distribution on the casing wall and those estimated from the pressure difference across the impeller blades were compared with the results from the direct fluid force measurements. The direct fluid forces correlate better with the forces due to the pressure distribution on the casing wall.
The Cavermod Device: Hydrodynamic Aspects and Erosion Tests
Authors: Filali E. G. , Michel J. M.
Published: February 2008

The Cavermod (CAVitation ERosion MODel) is an erosion test device first described by Dominguez-Cortazar et al. (1992, 1997). Recently, it was modified in two steps: first by increasing its maximum rotation rate (from 4500 to 8000 rpm) and second by shortening its vapor core (from 156 to 66 mm). This paper plans to present the main results which are obtained in both configurations (long and short vortex) and for “slow” or “rapid” regime of rotation. They mainly concern 1. the hydrodynamic aspects of the vapor core collapse, as deduced from observation of rapid films (evolution of the vortex length, collapse, velocity), 2. the erosion patterns produced on metallic targets such as pure aluminium and copper. A second companion paper will present the results of force measurements in both configuration and an attempt to estimate the local erosive pressures.
The Cavermod Device: Force Measurements
Authors: Filali E. G. , Michel J. M. , Hattori S. , Fujikawa S.
Published: February 2008

The Cavermod device, as described in the companion paper (Filali et al., 1999), allows us to produce the axial collapse of a cavitating vortex at high velocities. From a global point of view, we can consider that it produces a high momentum in the axial direction. Large forces, concentrated on a small area and able to produce erosion pits on hard materials, result from the sudden momentum stopping against a solid wall. In this paper, the results of the forces measurements are given. Four different measurements devices are used to analyze the Cavermod performance in both cases of long and short vortex: dislocations in MgO (Magnesium Oxide) single crystal, two special piezoelectric ceramic transducers and a PVDF film transducer. Special attention is given to the PVDF film response which is found twice the response of other devices. In addition, an attempt is made to interpret the temporal force signal given by a ceramic transducer in terms of local erosive pressure.
Predictable Model for Characteristics of One-Dimensional Solid-Gas-Liquid Three-Phase Mixtures Flow Along a Vertical Pipeline With an Abrupt Enlargement in Diameter
Authors: Hatta Natsuo , Omodaka Masaaki , Nakajima Fumitaka , Takatsu Takahiro , Fujimoto Hitoshi , Takuda Hirohiko
Published: February 2008

This paper treats the numerical analysis of the rising process of a solid-gas-liquid three-phase mixture along a vertical pipeline with an abrupt enlargement in diameter. The system of governing equations used is based upon the one-dimensional multifluid model and the transitions of gas flow pattern are taken into account in the system of governing equations. For the case of a sudden enlargement in diameter in a coaxial pipeline, the procedure of the numerical calculation to obtain the flow characteristics in the pipeline section after a sudden change in diameter has been established here. Furthermore, in order to confirm the validity of the present theoretical model by the comparison between the calculated and experimental values, the experiments have been made using four kinds of lifting pipes, including the straight one. Thereby, it has been found that the numerical model proposed here gives good fit to the prediction of the flow rates of lifted water and solid particles against that of air supplied for the case of a sudden change in diameter. In addition, the flowing process for each phase has been investigated from a photographic point of view. As a result, we found that the moving process of the solid particles depends strongly upon the volumetric flux of gas-phase as well as the submergence ratio.
Unstable Asymmetric Modes of a Liquid Jet
Authors: Shi S. X. , Xi D. G. , Qin J. R. , Liu N. , Shu G. C.
Published: February 2008

This paper reports the results of a linear instability analysis for a viscous liquid jet injecting into a quiescent inviscid gas medium with three-dimensional disturbances. A dispersion equation that accounts for the growth of asymmetric waves is derived, and the maximum rates of growth of various modes are calculated. The asymmetric breakup phenomenon of the jet and its structures at different modes is also studied by using a high-speed multi-frame holographic system. The theoretical predictions agree well with the experimental observations. The results of this study thus confirm the existence and even domination of unstable asymmetric modes under certain physical conditions in the breakup process.
Investigations of 3D Turbulent Flow Inside and Around a Water-Jet Intake Duct Under Different Operating Conditions
Authors: Hu Peixin , Zangeneh Mehrdad
Published: February 2008

In this paper the flow field in the intake duct of a model water-jet unit is studied by using a commercial 3D CFD code. In order to model the intake duct/hull interaction, the computational domain includes a large section of the hull in the vicinity of the intake duct opening. Appropriate boundary conditions are used on the far upstream and downstream of the duct inlet in order to minimize the effect of the domain boundaries on the flow field in the vicinity of the intake duct. Computations are performed for different boat speeds and flowrates. In addition, the effects of the impeller shaft, shaft rotation, boat trim as well as the traverse flow across the hull are investigated. The results of the computations are compared with some preliminary experimental results obtained from model self-propulsion tests in a towing tank. Good correlation is obtained between the predictions and the experimental results.
Flow Measurements in a Fishtail Diffuser With Strong Curvature
Authors: Yaras M. I.
Published: February 2008

The paper presents detailed measurements of the incompressible flow development in a large-scale 90 deg curved diffuser with strong curvature and significant streamwise variation in cross-sectional aspect ratio. The flow path approximates the so-called fishtail diffuser utilized on small gas turbine engines for the transition between the centrifugal impeller and the combustion chamber. Two variations of the inlet flow, differing in boundary layer thickness and turbulence intensity, are considered. Measurements consist of three components of velocity, static pressure and total pressure distributions at several cross-sectional planes throughout the diffusing bend. The development and mutual interaction of multiple pairs of streamwise vortices, redistribution of the streamwise flow under the influence of these vortices, the resultant streamwise variations in mass-averaged total-pressure and static pressure, and the effect of inlet conditions on these aspects of the flow are examined. The strengths of the vortical structures are found to be sensitive to the inlet flow conditions, with the inlet flow comprising a thinner boundary layer and lower turbulence intensity yielding stronger secondary flows. For both inlet conditions a pair of streamwise vortices develop rapidly within the bend, reaching their peak strength at about 30 deg into the bend. The development of a second pair of vortices commences downstream of this location and continues for the remainder of the bend. Little evidence of the first vortex pair remains at the exit of the diffusing bend. The mass-averaged total pressure loss is found to be insensitive to the range of inlet-flow variations considered herein. However, the rate of generation of this loss along the length of the diffusing bend differs between the two test cases. For the case with the thinner inlet boundary layer, stronger secondary flows result in larger distortion of the streamwise velocity field. Consequently, the static pressure recovery is somewhat lower for this test case. The difference between the streamwise distributions of measured and ideal static pressure is found to be primarily due to total pressure loss in the case of the thick inlet boundary layer. For the thin inlet boundary layer case, however, total pressure loss and flow distortion are observed to influence the pressure recovery by comparable amounts.
Fluid Motion in Ultrasonic Flowmeter Cavities
Authors: Lo̸land Tore , Sætran Lars R. , Olsen Robert , Gran Inge R. , Sakariassen Reidar
Published: February 2008

The ultrasonic flow meter is a newcomer among flow meters for measuring large quantities of natural gas. It has notable advantages compared to traditional meters. The ultrasonic flow meter is much more compact and has a wider dynamic range for flow measurements than the orifice plate meter. When manufactured, the ultrasonic sensors are often set back from the pipe wall in a cavity. When the fluid flows past the cavities, a secondary flow of vortices with characteristic size equal to the cavity width is established inside the cavities. The aim of this study has been to investigate the influence of this secondary flow on the accuracy of the ultrasonic flow meter. Both measurements and numerical simulations of the cavity flow have been conducted. It has been found from the present work, that the influence of the flow in the cavities on the measurements increases nonlinearly with the pipe flow rate.
Numerical Study of Vortex Shedding From a Circular Cylinder in Linear Shear Flow
Authors: Mukhopadhyay A. , Venugopal P. , Vanka S. P.
Published: February 2008

A three-dimensional numerical simulation of linearly sheared flow past a circular cylinder has been performed for a shear parameter β of 0.02 and a mean Reynolds number of 131.5. A cylinder of 24 diameters span is considered. A second-order accurate finite volume scheme is used to integrate the unsteady Navier-Stokes equations. Present computations confirm both qualitatively and quantitatively, the aspects of cellular shedding as reported by several investigators through experimental studies. Up to five constant frequency cells of obliquely shedding vortices are observed. The nondimensional frequencies of these cells are observed to be lower than those given by parallel shedding correlations at the equivalent Reynolds numbers. It is also observed that the cell boundaries continuously move in time. Detailed distributions of vorticity and velocity components are presented to describe the flow. The influence of end-wall boundary conditions is studied by computing two cases, one with free-slip condition, and the other with no-slip condition on disks of radius of five cylinder diameters.
Natural Convection in an Inclined Fluid Layer With a Transverse Magnetic Field: Analogy With a Porous Medium
Authors: Vasseur P. , Hasnaoui M. , Bilgen E. , Robillard L.
Published: February 2008

In this paper the effect of a transverse magnetic field on buoyancy-driven convection in an inclined two-dimensional cavity is studied analytically and numerically. A constant heat flux is applied for heating and cooling the two opposing walls while the other two walls are insulated. The governing equations are solved analytically, in the limit of a thin layer, using a parallel flow approximation and an integral form of the energy equation. Solutions for the flow fields, temperature distributions, and Nusselt numbers are obtained explicitly in terms of the Rayleigh and Hartmann numbers and the angle of inclination of the cavity. In the high Hartmann number limit it is demonstrated that the resulting solution is equivalent to that obtained for a porous layer on the basis of Darcy’s model. In the low Hartmann number limit the solution for a fluid layer in the absence of a magnetic force is recovered. In the case of a horizontal layer heated from below the critical Rayleigh number for the onset of convection is derived in term of the Hartmann number. A good agreement is found between the analytical predictions and the numerical simulation of the full governing equations.
Dependent Scattering Properties of Woven Fibrous Insulations for Normal Incidence
Authors: Kumar Sunil , White S. M.
Published: February 2008

The scattering properties of woven fibrous materials are examined in this paper and a simple model is presented to account for the interactions between the scattered radiation from different individual fibers. The case of a normally incident plane electromagnetic wave is considered. Fiber sizes in the Rayleigh regime are considered for developing closed-form solutions. Previous studies in the literature that have addressed the scattering properties of fibrous materials have mostly ignored the effect of constructive or destructive addition of scattered waves from individual fibers, the exception being the case of parallel fibers. The difference in the effects of interference on scattering properties of parallel fibers and of woven fabrics arises from the additional interaction of radiation scattered from mutually perpendicular fibers in the latter case, which further complicates the analysis.
Subcooled Flow Film Boiling Across a Horizontal Cylinder: Part I—Analytical Model
Authors: Chou X. S. , Witte L. C.
Published: February 2008

An analytical model of stable subcooled flow film boiling on the front of a horizontal cylinder and a model for the wake region downstream of the flow separation points were developed. The flow and temperature fields upstream of the separation points were represented by a “local-similarity” solution obtained through a rigorous mathematical transformation. The transformed governing equations were solved numerically using a finite-difference scheme. Numerical solutions for the vapor layer thickness, the velocity, and the temperature fields were obtained for both the liquid and vapor layers. The results showed that the liquid boundary layer was thicker than the vapor film. Increases in the liquid subcooling and in the free-stream velocity decreased the vapor layer thickness. The influence of convection in the vapor layer is small yielding a near-linear temperature distribution. A two-dimensional vapor wake model was developed based on mass and energy balances. Numerical solutions, including the vapor layer thickness and the temperature field of the front part and the wake part, were matched at the separation points. The results showed that increases in the liquid subcooling decreased the vapor layer thickness. Heat transfer in the wake region can amount up to 20 percent of the heat transfer in the forward region and should not be neglected especially at high subcooling.
Stratified Flow Film Boiling Inside Horizontal Tubes
Authors: Chan A. M. C.
Published: February 2008

Stratified flow film boiling inside a horizontal tube was investigated. A one-dimensional flow model was used to solve for the local vapor film thickness and vapor velocity in the vapor channel in the circumferential direction. Analytical solutions for the local vapor film thickness and heat transfer coefficient at the bottom of the tube were derived. The solutions were compared with expressions obtained for other external geometries. A simple experiment was also performed to provide local heat transfer data for direct comparison. It was found that the local heat transfer coefficient normalized with respect to ho can be represented by a single curve for different liquid inventory and initial wall temperatures. The curve was expressed in the form of a polynomial and it provides a simple way of calculating the local heat transfer coefficient in stratified flow film boiling in horizontal tubes.
Transient Temperature During Pulsed Excimer Laser Heating of Thin Polysilicon Films Obtained by Optical Reflectivity Measurement
Authors: Xu X. , Grigoropoulos C. P. , Russo R. E.
Published: February 2008

The transient reflectivity of a continuous wave (CW) HeNe laser was measured during the pulsed excimer laser heating of thin polysilicon films at the nanosecond time scale. Polysilicon films with thicknesses from 0.1 to 0.4 μm were deposited by Low-Pressure Chemical Vapor Deposition (LPCVD) on thermally oxidized crystalline silicon wafers. The complex refractive index of these films at the HeNe laser wavelength (λprobe = 0.6328 μm) was measured in the temperature range from 300 K to approximately 1400 K by combined ellipsometric and normal incidence reflectivity measurements. Numerical heat transfer and optical reflectivity analysis based on the measured optical properties of polysilicon films were conducted. The calculated reflectivity histories were compared with the experimental results to reveal the transient temperature field.
The Interfacial Thermodynamics of Micro Heat Pipes
Authors: Swanson L. W. , Peterson G. P.
Published: February 2008

Successful analysis and modeling of micro heat pipes requires a complete understanding of the vapor–liquid interface. A thermodynamic model of the vapor–liquid interface in micro heat pipes has been formulated that includes axial pressure and temperature differences, changes in local interfacial curvature, Marangoni effects, and the disjoining pressure. Relationships were developed for the interfacial mass flux in an extended meniscus, the heat transfer rate in the intrinsic meniscus, the “thermocapillary” heat-pipe limitation, as well as the nonevaporating superheated liquid film thickness that exists between adjacent menisci and occurs during liquid dry out in the evaporator. These relationships can be used to define quantitative restrictions and/or requirements necessary for proper operation of micro heat pipes. They also provide fundamental insight into the critical mechanisms required for proper heat pipe operation.
Holographic Interferometry Study of Spatially Periodic Heat Transfer in a Channel With Ribs Detached From One Wall
Authors: Liou Tong-Miin , Wang Wen-Bin , Chang Yuan-Jen
Published: February 2008

The effects of clearance ratio (C/H) and Reynolds number (Re) on the turbulent heat transfer and friction in a rectangular duct with ribs detached from one wall were characterized quantitatively using laser holographic interferometry and pressure measurements. The investigated flow was periodic in space both hydrodynamically and thermally. C/H and Re were varied from 0.25 to 1.5 and 5 × 103 to 5 × 104, respectively. The obtained interferograms, local (Nu) and average (Nu) Nusselt number, and thermal performance (Nup/Nus*) allowed the critical C/H characterizing different mechanisms of heat transfer augmentation to be identified and allowed a comparison of Nu, Nu, and Nup/Nus* among the detached ribbed duct, the attached ribbed duct, and the smooth duct to be made. It was found that the detached ribbed geometry has the advantage of eliminating the hot spots behind the attached ribs. Optimal clearance ratios for heat transfer enhancement between the present periodic detached ribs and previous single detached cylinder were also compared. Furthermore, compact heat transfer and friction correlations were developed for a detached ribbed duct for the first time.
A Correlation for Heat Transfer and Wake Effect in the Entrance Region of an In-Line Array of Rectangular Blocks Simulating Electronic Components
Authors: Molki M. , Faghri M. , Ozbay O.
Published: February 2008

An experimental investigation is carried out to study heat transfer in the entrance region of an array of rectangular heated blocks. The focus of the work is on the entrance heat transfer coefficients and the associated thermal wake effects. The experiments were performed with air as the working fluid. The geometric parameters of the array were varied in the range identified with B/L = 0.5, S/L = 0.128–0.33, and H/L = 0.128–1. The Reynolds number, based on the height above the blocks and the fluid mean velocity in the bypass channel, ranged from 3000 to 15,000. The adiabatic heat transfer coefficients and thermal wake effects are correlated for the entrance region. These correlations are incorporated into a user-friendly FORTRAN program, which can be used by the engineers to predict the working temperatures of the components of circuit boards with similar layout. A typical computer output indicated that the mean deviation between the measured and predicted temperatures is 11.0 percent.
Heat Transfer and Pressure Drop Characteristics of Spirally Fluted Annuli: Part II—Heat Transfer
Authors: Garimella S. , Christensen R. N.
Published: February 2008

This paper is the second of two papers that present the results of a comprehensive study of heat transfer and pressure drop in annuli with spirally fluted inner tubes for the laminar, transition, and turbulent flow regimes. Fourteen fluted tubes with varying geometries were studied, with up to three outer smooth tubes for each fluted tube. Flow patterns and transitions between flow regimes investigated through visualization tests, friction factor data (from Part I), and tube surface-temperature measurements were used to explain the enhancement phenomena. The fluted inner tubes induced a significant degree of swirl in the flow, and transition occurred in the 310 < Re < 1000 range. A Nusselt number correlation was developed in terms of the fluted annulus friction factor developed in Part I and geometric parameters. Nusselt numbers were between 4 and 20 times the smooth annulus values in the low Re range, while turbulent enhancements were between 1.1 and 4.0. These enhancement values can be used in conjunction with friction factor increase values reported in Part I to determine appropriate ranges of applicability for spirally enhanced annuli.
A Lumped-Parameter Model for Stagnant Thermal Conductivity of Spatially Periodic Porous Media
Authors: Hsu C. T. , Cheng P. , Wong K. W.
Published: February 2008

Based on a lumped-parameter method, algebraic expressions for the stagnant thermal conductivity of some two-dimensional and three-dimensional spatially periodic media are obtained. The geometries under consideration include arrays of touching and non-touching in-line square and circular cylinders (two-dimensional), as well as touching and nontouching in-line cubes (three-dimensional). A comparison of results based on these algebraic expressions with existing numerical solutions and experimental data shows that they are in excellent agreement.
Pressure Drop and Heat Transfer in Turbulent Duct Flow: A Two-Parameter Variational Method
Authors: Ghariban N. , Haji-Sheikh A. , You S. M.
Published: February 2008

A two-parameter variational method is introduced to calculate pressure drop and heat transfer for turbulent flow in ducts. The variational method leads to a Galerkin-type solution for the momentum and energy equations. The method uses the Prandtl mixing length theory to describe turbulent shear stress. The Van Driest model is compared with experimental data and incorporated in the numerical calculations. The computed velocity profiles, pressure drop, and heat transfer coefficient are compared with the experimental data of various investigators for fully developed turbulent flow in parallel plate ducts and pipes. This analysis leads to development of a Green’s function useful for solving a variety of conjugate heat transfer problems.
Heat Transfer and Flow Characteristics of an Oblique Turbulent Impinging Jet Within Confined Walls
Authors: Ichimiya K.
Published: February 2008

Experiments were conducted to determine the turbulent heat transfer and flow characteristics of an oblique impinging circular jet within closely confined walls using air as a working fluid. The local temperature distribution on the impingement surface was obtained in detail by a thermocamera using a liquid crystal sheet. A correction to the heat flux was evaluated by using the detailed temperature distribution and solving numerically the three-dimensional equation of heat conduction in the heated section. Two-dimensional profiles of the local Nusselt numbers and temperatures changed with jet angle and Reynolds number. These showed a peak shift toward the minor flow region and a plateau of the local heat transfer coefficients in the major flow region. The local velocity and turbulent intensity in the gap between the confined insulated wall and impingement surface were also obtained in detail by a thermal anemometer.
Stabilization of the No-Motion State of a Horizontal Fluid Layer Heated From Below With Joule Heating
Authors: Tang J. , Bau H. H.
Published: February 2008

Using linear stability theory and numerical simulations, we demonstrate that the critical Rayleigh number for bifurcation from the no-motion (conduction) state to the motion state in the Rayleigh–Be´nard problem of an infinite fluid layer heated from below with Joule heating and cooled from above can be significantly increased through the use of feedback control strategies effecting small perturbations in the boundary data. The bottom of the layer is heated by a network of heaters whose power supply is modulated in proportion to the deviations of the temperatures at various locations in the fluid from the conductive, no-motion temperatures. Similar control strategies can also be used to induce complicated, time-dependent flows at relatively low Rayleigh numbers.
Pigments to Reflect the Infrared Radiation From Fire
Authors: Berdahl P.
Published: February 2008

Conventional paints transmit or absorb most of the intense infrared radiation emitted by fire, a fact that contributes to the spread of fire. Techniques used for design of coatings with high visible reflectivity are here adapted to the design of coatings that have high reflectivity in the 1 to 8 μm wavelength range important in fires. Promising materials for pigments include titanium dioxide, iron oxide, chromium oxide, and silicon, with particle diameters in the 1 to 2 μm range. Aluminum flake pigments are also effective. Pigment synthesis by the deposition of coatings on mica flakes is briefly discussed.
A Painting Technique to Enhance Pool Boiling Heat Transfer in Saturated FC-72
Authors: O’Connor J. P. , You S. M.
Published: February 2008

A benign method of generating a surface microstructure that provides pool boiling heat transfer enhancement is introduced. Pool boiling heat transfer results from an enhanced, horizontally oriented, rectangular surface immersed in saturated FC-72, indicate up to an 85 percent decrease in incipient superheat, a 70 to 80 percent reduction in nucleate boiling superheats, and a ∼ 109 percent increase in the critical heat flux (CHF = 30 W/cm2), beyond that of the nonpainted reference surface. For higher heat flux conditions (19 to 30 W/cm2), localized dryout results in increased wall superheats (8 to 48°C). The enhanced surface heat transfer coefficients are four times higher than those from the reference surface and similar to those from the Union Carbide High Flux surface. Photographs that identify differences in bubble size and departure characteristics between the painted and reference surfaces are presented.
Shape of a Vapor Stem During Nucleate Boiling of Saturated Liquids
Authors: Lay J. H. , Dhir V. K.
Published: February 2008

The transport processes occurring in an evaporating two-dimensional vapor stem formed during saturated nucleate boiling on a heated surface are modeled and analyzed numerically. From the heater surface heat is conducted into the liquid macro/microthermal layer surrounding the vapor stems and is utilized in evaporation at the stationary liquid–vapor interface. A balance between forces due to curvature of the interface, disjoining pressure, hydrostatic head, and liquid drag determines the shape of the interface. The kinetic theory and the extended Clausius–Clapeyron equation are used to calculate the evaporative heat flux across the liquid–vapor interface. The vapor stem shape calculated by solving a fourth-order nonlinear ordinary differential equation resembles a cup with a flat bottom. For a given wall superheat, several metastable states of the vapor stem between a minimum and maximum diameter are found to be possible. The effect of wall superheat on the shape of the vapor stem is parametrically analyzed and compared with limited data reported in the literature.
Interfacial Heat Transfer Between Steam Bubbles and Subcooled Water in Vertical Upward Flow
Authors: Zeitoun O. , Shoukri M. , Chatoorgoon V.
Published: February 2008

In two-fluid modeling, accurate prediction of the interfacial transport of mass, momentum, and energy is required. Experiments were carried out to obtain a data base for the development of interfacial transport models, or correlations, for subcooled water–steam bubbly flow in vertical conduits. The experimental data of interest included the interfacial area concentration, interfacial condensation heat transfer, and bubble relative velocity. In the present investigation, bubble condensation in subcooled water–steam flow in a vertical annulus at low flow rate and low pressure is investigated experimentally. A high-speed video system (up to 1000 frame/s) was used to visualize two orthogonal views of the flow simultaneously. A digital image processing technique was used to track and measure the velocity and size of the collapsing bubbles. The axial void fraction distribution was also measured by a single beam gamma densitometer. The results were compared with existing correlations and a new correlation for bubble condensation Nusselt number was obtained based on the present data. In the proposed correlation, the bubble condensation Nusselt number is expressed in terms of the bubble Reynolds number, Jakob number, and void fraction. The latter term accounts for the multi-bubble effect. This correlation can predict the present data within ±20 percent with a correlation coefficient of 96 percent.
Exact Solution to the One-Dimensional Inverse-Stefan Problem in Nonideal Biological Tissues
Authors: Rabin Y. , Shitzer A.
Published: February 2008

A new analytic solution of the inverse-Stefan problem in biological tissues is presented. The solution, which is based on the enthalpy method, assumes that phase change occurs over a temperature range and includes the thermal effects of metabolic heat generation, blood perfusion, and density changes. As a first stage a quasi-steady-state solution is derived, defined by uniform velocities of the freezing fronts and thus by constant cooling rates at those interfaces. Next, the fixed boundary condition leading to the quasi-steady state is calculated. It is shown that the inverse-Stefan problem may not be solved exactly for a uniform initial condition, but rather for a very closely approximating exponential initial condition. Very good agreement is obtained between the new solution and an earlier one assuming biological tissues to behave as pure materials in which phase change occurs at a single temperature. A parametric study of the new solution is presented taking into account property values of biological tissues at low freezing rates typical of cryosurgical treatments.
Deterministic Chaotic Behavior of Heat Transfer in Gas Fluidized Beds
Authors: Pence D. V. , Beasley D. E. , Riester J. B.
Published: February 2008

Instantaneous heat transfer signals acquired in the bubble flow regime in a gas fluidized bed were examined for evidence of deterministic chaotic behavior. Signals were acquired using a heat flux sensor mounted on the surface of a horizontal, heated cylinder. Both standard statistical measures and nonlinear analysis techniques were employed to provide qualitative as well as quantitative information regarding dynamic behavior of heat transfer in such a facility. Power spectra provided preliminary evidence that heat transfer in fluidized beds may be chaotic in nature. Positive finite estimates of the Kolmogorov entropy provide further evidence of behavior consistent with chaos. Mutual information has a dual purpose in this work. It was used to evaluate a time delay in which to embed the time series heat transfer signal, and as a qualitative measure of the degree of chaos inherent in the signals. The quantitative information derived from mutual information allows the system dynamics to be represented as phase trajectories in a pseudo-phase space created by embedding a measured discrete signal in the space defined by time-delay coordinate axes. Embedding dimensions determined via the correlation integral suggest that four degrees of freedom quantify the system dynamics for the fluidizing conditions investigated in this work. Embedding dimensions were found to be independent of the fluidization velocity. This is a consequence of maintaining operating conditions within the bubble flow regime. The Kolmogorov entropy was shown to delineate fundamental changes in hydrodynamics conditions at the surface of the cylinder. Phase plots provide a means of analyzing the dynamics of heat transfer occurring at the surface of the cylinder. Two distinct characteristic emulsion phase contacts were identified using phase space trajectories. Phase space trajectories that result from particles remaining motionless at the heat transfer surface are distinctly different from those that result from particles actively mixing at the surface of the cylinder.
Predicting the Impact of Quenching on Mechanical Properties of Complex-Shaped Aluminum Alloy Parts
Authors: Hall D. D. , Mudawar I.
Published: February 2008

The mechanical properties of age-hardenable aluminum alloy extrusions are critically dependent on the rate at which the part is cooled (quenched) after the forming operation. The present study continues the development of an intelligent spray quenching system, which selects the optimal nozzle configuration based on part geometry and composition such that the magnitude and uniformity of hardness (or yield strength) is maximized while residual stresses are minimized. The quenching of a complex-shaped part with multiple, overlapping sprays was successfully modeled using spray heat transfer correlations as boundary conditions within a finite element program. The hardness distribution of the heat-treated part was accurately predicted using the quench factor technique; that is, the metallurgical transformations that occur within the part were linked to the cooling history predicted by the finite element program. This study represents the first successful attempt at systematically predicting the mechanical properties of a quenched metallic part from knowledge of only the spray boundary conditions.
Assessing Halon Alternatives for Aircraft Engine Nacelle Fire Suppression
Authors: Grosshandler W. , Presser C. , Lowe D. , Rinkinen W.
Published: February 2008

A coaxial turbulent spray burner was built to evaluate the relative effectiveness of different chemicals for suppressing fires in a jet engine nacelle. The fire suppressant of current choice, halon 1301 (CF3Br), must be replaced because of its detrimental effect on the ozone layer. The alternatives being considered lack the chemical activity of CF3Br, so that the ability of the agents to mix into the flame convectively and to absorb heat is critical to their success. An agent delivery system was designed to inject the desired amount of material into the air upstream of a fuel nozzle and to control the agent injection rate through variation of the storage pressure and the duration of time that a solenoid valve remains open. The influence of air velocity, fuel flow, and injection period on the amount of nitrogen required to extinguish a jet fuel spray flame is discussed. The effectiveness of eleven different fluorocarbons, hydrofluorocarbons, and hydrochlorofluorocarbons is compared to that of halon 1301. The alternatives required 1.7 to 2.3 times the amount (on a mass basis) of CF3Br to extinguish the spray flame, with HCFC-22 being the most efficient and FC-31-10 the least.
Fog Inerting Effects on Hydrogen Combustion in a PWR Ice Condenser Containment
Authors: Luangdilok W. , Bennett R. B.
Published: February 2008

A mechanistic fog inerting model has been developed to account for the effects of fog on the upward lean flammability limits of a combustible mixture based on the thermal theory of flame propagation. Benchmarking of this model with test data shows reasonably good agreement between the theory and the experiment. Applications of the model and available fog data to determine the upward lean flammability limits of the H2–air–steam mixture in the ice condenser upper plenum region of a pressurized water reactor (PWR) ice condenser containment during postulated large loss of coolant accident (LOCA) conditions indicate that combustion may be suppressed beyond the downward flammability limit (8 percent H2 by volume).
Determination of Convective Heat Flux on Steady-State Heat Transfer Surfaces With Arbitrarily Specified Boundaries
Authors: Wiedner B. G. , Camci C.
Published: February 2008

The present study focuses on the high-resolution determination of local heat flux distributions encountered in forced convection heat transfer studies. The specific method results in an uncertainty level less than 4 percent of the heat transfer coefficient on surfaces with arbitrarily defined geometric boundaries. Heat transfer surfaces constructed for use in steady-state techniques typically use rectangular thin foil electric heaters to generate a constant heat flux boundary condition. There are also past studies dealing with geometrically complex heating elements. Past studies have either omitted the nonuniform heat flux regions or applied correctional techniques that are approximate. The current study combines electric field theory and a finite element method to solve directly for a nonuniform surface heat flux distribution due to the specific shape of the heater boundary. Heat generation per unit volume of the surface heater element in the form of local Joule heating is accurately calculated using a finite element technique. The technique is shown to be applicable to many complex convective heat transfer configurations. These configurations often have complex geometric boundaries such as turbine endwall platforms, surfaces disturbed by film cooling holes, blade tip sections, etc. A complete high-resolution steady-state heat transfer technique using liquid crystal thermography is presented for the endwall surface of a 90 deg turning duct. The inlet flow is fully turbulent with an inlet Re number of 360,000. The solution of the surface heat flux distribution is also demonstrated for a heat transfer surface that contains an array of discrete film cooling holes. The current method can easily be extended to any heat transfer surface that has arbitrarily prescribed boundaries.
A Velocity and Length Scale Approach to k–ε Modeling
Authors: Kwon O. , Ames F. E.
Published: February 2008

This paper describes a velocity and length scale approach to low-Reynolds-number k–ε modeling, which formulates the eddy viscosity on the normal component of turbulence and a length scale. The normal component of turbulence is modeled based on the dissipation and distance from the wall and is bounded by the isotropic condition. The model accounts for the anisotropy of the dissipation and the reduced length of mixing in the near wall region. The kinetic energy and dissipation rate were computed from the k and ε transport equations of Durbin (1993). The model was tested for a wide range of turbulent flows and proved to be superior to other k–ε based models.