bluuebunny/crossref_metadata_2025 huggingface dataset:
The Unsteady Pressure Field in a High Specific Speed Centrifugal Pump Impeller—Part II: Transient Hysteresis in the Characteristic
Authors: Kaupert Kevin A. , Staubli Thomas
Published: February 2008

Hysteresis in a pump characteristic results from instability phenomena involving complex three dimensional flow with recirculation. The unsteady flow field on the top and bottom branches of a hysteresis loop in a high specific speed (ωs = 1.7) centrifugal pump characteristic was experimentally evaluated. A hypothesis for recirculation zones and prerotation as power dissipaters is proposed for explaining the discrepancy in the pressure and shaft power hysteresis. The experimental investigation was performed in both the rotating and stationary frame. In the rotating frame 25 miniature pressure transducers mounted in an impeller blade passage were sampled with a telemetry system. In the stationary frame a fast response probe was implemented. The changing impeller flow field manifested itself between the two branches of the hysteresis with increasing stochastic pressure fluctuations. Using this information the position, size, and strength of the impeller recirculation was quantitatively determined. Theoretically the rate of change of useful hydraulic power in the hysteresis regime during transient pump operation was found to be a function of throttling rate. Quasi-steady behavior existed for slow throttling, |dφ/dt| < 0.005 s−1. A second-order nonlinear dependence on the throttle rate was determined for the change of useful flow power during the commencement/cessation of the impeller recirculation.
Effect of Electrostatic Field on Film Rupture
Authors: Gorla Rama Subba Reddy , Byrd Larry W.
Published: February 2008

Nonlinear thin film rupture has been analyzed by investigating the stability of films under the influence of a nonuniform electrostatic field to finite amplitude disturbances. The dynamics of the liquid film is formulated using the Navier-Stokes equations including a body force term due to van der Waals attractions. The effect of the electric field is included in the analysis only in the boundary condition at the liquid vapor interface. The governing equation was solved by finite difference method as part of an initial value problem for spatial periodic boundary conditions. The electric field stabilizes the film and increases the time to rupture when a long wavelength perturbation is introduced.
The Design of a Droplet Ejector
Authors: Grassia P.
Published: February 2008

A model is developed describing a novel ink jet printer driven by a piezoelectric component used to eject a fluid droplet through a rubber valve. The model is analyzed to address specific printer design issues. Rapid droplet production and efficient conversion of piezoelectric energy to droplet kinetic energy are ensured by suitable choices of the ejector geometry and the voltage step used to produce the droplet. A parameter regime is found in which a resonance prevents the valve from closing properly, and this particular regime must be avoided for correct printer operation. By choosing a suitable voltage signal after the production of an ink droplet, the device is returned rapidly to its initial or quiescent state.
Geometry Effects on Free Surface Vorticity Flux
Authors: Peck Bill , Sigurdson Lorenz
Published: February 2008

Effects of geometry on the flux of vorticity from a free surface are discussed. Special attention is paid to situations where curvature-dependent contributions to the vorticity flux can be neglected. The geometry of vortex lines embedded in the surface is discussed in this context. These results show that vortex lines can be straight and geometry-induced vorticity flux is produced; conversely vortex lines can be curved and no geometry-induced vorticity flux is produced. A convenient method for assessing vorticity flux from a steady surface based on Gaussian curvature is derived.
Turbulent Flow in a Rotating Two Pass Smooth Channel
Authors: Hsieh Shou-Shing , Chen Ping-Ju , Chin Hsiang-Jung
Published: February 2008

Laser-Doppler anemometry has been applied to approximately 2-D turbulent air flow in a rotating 2 pass channel of square cross section. The axis of rotation is normal to the axis of the duct, and the flow is radially outward/inward. The duct is of finite length and the walls are isothermal. Smooth channels are experimentally conducted with rotational speeds of 100, 200, and 300 rpm with ReH = 5000 and 10,000. The main features of the flow, flow separation and mean velocity as well as turbulent intensity at particular location along the downstream are presented. The measured flow field is found to be quite complex, consisting of secondary cross-stream and radially outward flows due to the Coriolis effects and centrifugal forces.
A Modified Model for Diffusion in Second-Moment Turbulence Closures
Authors: Straatman Anthony G.
Published: February 2008

A study has been carried out to determine the relative roles of the three diffusion sub-processes contained in the Lumley (1978) diffusion model. The three sub-processes are described as being the production of turbulent transport, the third-order pressure-velocity process, which regulates the relative magnitudes of the turbulent transport components, and the pressure-diffusion. The present work describes a unique method for calibrating the model based on an analysis of zero-mean-shear turbulence. On the basis of the analysis, and using recent direct numerical simulation and experimental data, the coefficients in the Lumley (1978) model are modified such that the model gives the correct behavior in the diffusive limit. The modified model was then validated by carrying out CFD predictions for three benchmark flows of engineering interest. The modified Lumley (1978) diffusion model has two clear advantages over the more commonly used Daly and Harlow (1970) model. First, unlike the Daly and Harlow (1970) model, the modified Lumley (1978) model is mathematically correct and second, it was shown in the present validation that the modified Lumley (1978) model gives the most consistently reasonable predictions.
Re-Entrant Jet Modeling of Partial Cavity Flow on Two-Dimensional Hydrofoils
Authors: Dang J. , Kuiper G.
Published: February 2008

A potential based panel method is developed to predict the partial cavity flow on two-dimensional hydrofoil sections. The Dirichlet type dynamic boundary condition on the cavity surface and the Neumann type kinematic boundary condition on the wetted section surface are enforced. A re-entrant jet cavity termination model is introduced. A validation is accomplished by comparing the present calculations with cavitation experiments of a modified Joukowsky foil and a NACA 66(MOD) a = 0.8 section.
Re-Entrant Jet Modeling of Partial Cavity Flow on Three-Dimensional Hydrofoils
Authors: Dang J. , Kuiper G.
Published: February 2008

A potential-based lower-order surface panel method is developed to calculate the flow around a three-dimensional hydrofoil with an attached sheet cavity the leading edge. A Dirichlet type dynamic boundary condition on the cavity surface and a Neumann boundary condition on the wetted surface are enforced. The cavity shape is initially assumed and the kinematic boundary condition on the cavity surface is satisfied by iterating the cavity length and shape. Upon convergence, both the dynamic boundary condition and the kinematic boundary condition on the cavity surface are satisfied, and a re-entrant jet develops at the cavity closure. The flow at the closure of the cavity and the mechanism of the re-entrant jet formation is investigated. Good agreement is found between the calculated results and MIT’s experiments on a 3-D hydrofoil.
Study of Several Jets Impinging on a Plane Wall: Visualizations and Laser Velocimetry Investigations
Authors: Bernard A. , Brizzi L.-E. , Bousgarbie`s J.-L.
Published: February 2008

Different experimental techniques have been used in order to describe the flow pattern generated by fifteen jets impinging on a plane wall. The spreading over method revealed the jet influence on the impinged surface, particularly reattachment and detachment lines. The laser sheet visualizations show complex vortical structures such as the ground vortex and secondary rolling-ups near the target plate have been deduced. Velocity measurements have been realised to confirm these observations and to specify the flow pattern. A schema of the flow near the impinged wall is thus drawn.
Three-Dimensional Linear Analysis of Rotating Cavitation in Inducers Using an Annular Cascade Model
Authors: Watanabe Satoshi , Yokota Kazuhiko , Tsujimoto Yoshinobu , Kamijo Kenjiro
Published: February 2008

A three-dimensional linear analysis of rotating cavitation is carried out using an annular cascade model. The purpose is to investigate three-dimensional effects of cavitation, including the radial change of peripheral velocity and the hub/tip ratio of the impeller, on rotating cavitation in inducers. A semi-actuator disk method is employed. It is assumed that the mean stream surface does not radially shift across the impeller. It is shown that there are many modes of instabilities corresponding to rotating cavitation with various radial modes travelling forward and backward. The 0th radial modes are found to correspond to the forward and backward travelling modes predicted by a previous two-dimensional analysis (Tsujimoto et al, 1993). The three-dimensionality of the geometry has the effects of diminishing the amplifying region of these fundamental modes. It is also shown that the amplifying regions of higher radial modes may be larger than those of 0th radial modes. However, it is shown that the three-dimensionality of cavitation does not significantly affect rotating cavitation.
Numerical Computation of Shock Waves in a Spherical Cloud of Cavitation Bubbles
Authors: Wang Yi-Chun , Brennen Christopher E.
Published: February 2008

The nonlinear dynamics of a spherical cloud of cavitation bubbles have been simulated numerically in order to learn more about the physical phenomena occurring in cloud cavitation. A finite cloud of nuclei is subject to a decrease in the ambient pressure which causes the cloud to cavitate. A subsequent pressure recovery then causes the cloud to collapse. This is typical of the transient behavior exhibited by a bubble cloud as it passes a body or the blade of a ship propeller. The simulations employ the fully nonlinear continuum bubbly mixture equations coupled with the Rayleigh-Plesset equation for the dynamics of bubbles. A Lagrangian integral method is developed to solve this set of equations. It was found that, with strong bubble interaction effects, the collapse of the cloud is accompanied by the formation of an inward propagating bubbly shock wave. A large pressure pulse is produced when this shock passes the bubbles and causes them to collapse. The focusing of the shock at the center of the cloud produces a very large pressure pulse which radiates a substantial impulse to the far field and provides an explanation for the severe noise and damage potential in cloud cavitation.
Monitoring Fluidization Dynamics for Detection of Changes in Fluidized Bed Composition and Operating Conditions
Authors: van Ommen J. Ruud , Schouten Jaap C. , van den Bleek Cor M.
Published: February 2008

In many industrial applications of gas-solids fluidized beds, it is worthwhile to have an on-line monitoring method for detecting changes in the hydrodynamics of the bed (due for example to agglomeration) quickly. In this paper, such a method, based on the short-term predictability of fluidized bed pressure fluctuations, is examined. Its sensitivity is shown by experiments with small step changes in the superficial gas velocity and by experiments with a gradual change in the particle size distribution of the solids in the bed. Furthermore, it is demonstrated that the method is well able to indicate if a stationary hydrodynamic state has been reached after a change in the particle size distribution (a ‘grade change’).
Identification of Pool Boiling Heat Transfer Mechanisms From a Wire Immersed in Saturated FC-72 Using a Single-Photo/LDA Method
Authors: Ammerman C. N. , You S. M. , Hong Y. S.
Published: February 2008

A unique method to determine the vapor volumetric flow rate above a heated wire utilizing a single photograph and laser-Doppler anemometry is developed and discussed. The volumetric flow rate is combined with additional analyses to determine the overall contributions to the total heat flux from four nucleate boiling heat transfer mechanisms (latent heat, natural convection, Marangoni flow, and microconvection). This method is applied to a 75-μm wire immersed in a saturated, highly wetting liquid (FC-72). Latent heat is identified as the dominant mechanism in the fully developed nucleate boiling regime.
Thickness of the Liquid Film Formed by a Growing Bubble in a Narrow Gap Between Two Horizontal Plates
Authors: Moriyama K. , Inoue A.
Published: February 2008

An experiment was performed to measure the thickness of the liquid film formed by a growing flattened bubble in a narrow gap whose width ranged from 0.1 to 0.4 mm. High-speed photographs were also taken to measure the bubble growth velocity in order to validate an investigation into the mechanism of the liquid film formation. From the experimental results, it was clarified that the liquid film thickness was controlled by the viscous boundary layer thickness or the capillary number according to whether the Bond number was greater or smaller than 2.
Experimental Investigation of Interfacial Thermal Conductance for Molten Metal Solidification on a Substrate
Authors: Wang G.-X. , Matthys E. F.
Published: February 2008

Experiments have been conducted to quantify the interfacial thermal conductance between molten copper and a cold metallic substrate, and in particular to investigate the heat transfer variation as the initial liquid/solid contact becomes a solid/solid contact after nucleation. A high heat transfer coefficient during the earlier liquid cooling phase and a lower heat transfer coefficient during the subsequent solid splat cooling phase were estimated through matching of model calculations and measured temperature history of the sample. The dynamic variations in the interfacial heat transfer resulting from the solidification process were quantified for splat cooling and were found to be affected by the melt superheat, the substrate material, and the substrate surface finish.
Numerical and Experimental Investigation of Interface Bonding Via Substrate Remelting of an Impinging Molten Metal Droplet
Authors: Amon C. H. , Schmaltz K. S. , Merz R. , Prinz F. B.
Published: February 2008

A molten metal droplet landing and bonding to a solid substrate is investigated with combined analytical, numerical, and experimental techniques. This research supports a novel, thermal spray shape deposition process, referred to as microcasting, capable of rapidly manufacturing near netshape, steel objects. Metallurgical bonding between the impacting droplet and the previous deposition layer improves the strength and material property continuity between the layers, producing high-quality metal objects. A thorough understanding of the interface heat transfer process is needed to optimize the microcast object properties by minimizing the impacting droplet temperature necessary for superficial substrate remelting, while controlling substrate and deposit material cooling rates, remelt depths, and residual thermal stresses. A mixed Lagrangian–Eulerian numerical model is developed to calculate substrate remelting and temperature histories for investigating the required deposition temperatures and the effect of operating conditions on remelting. Experimental and analytical approaches are used to determine initial conditions for the numerical simulations, to verify the numerical accuracy, and to identify the resultant microstructures. Numerical results indicate that droplet to substrate conduction is the dominant heat transfer mode during remelting and solidification. Furthermore, a highly time-dependent heat transfer coefficient at the droplet/substrate interface necessitates a combined numerical model of the droplet and substrate for accurate predictions of the substrate remelting. The remelting depth and cooling rate numerical results are also verified by optical metallography, and compare well with both the analytical solution for the initial deposition period and the temperature measurements during droplet solidification.
Effects of Heating Mode on Steady Axisymmetric Thermocapillary Flows in Microgravity
Authors: Kamotani Y. , Chang A. , Ostrach S.
Published: February 2008

Steady thermocapillary flows in cylindrical containers with unit aspect ratio (radius = depth) are studied numerically and by scaling analysis in order to investigate how the velocity and thermal fields are influenced by the method of heating. Two heating modes are considered: heating by a uniform temperature cylindrical heater placed along the centerline (called the CT heating) and heating by an external radiative source (the CF heating). The steady velocity and temperature fields are computed numerically under various conditions for various Prandtl number fluids. The effects of those two types of heating on the flows are discussed. The results are also found to support the scaling laws derived herein for the characteristic velocity, length, and Nusselt number. The main thermocapillary driving force region is fixed by the heating zone area in the CF heating but it generally varies with the flow in the CT heating.
Experimental Studies and Correlations of Radially Outward and Inward Air-Flow Heat Transfer in a Rotating Square Duct
Authors: Kuo C. R. , Hwang G. J.
Published: February 2008

Experiments were conducted to investigate the convective heat transfer of radially outward and inward air flows in a uniformly heated rotating square duct. The interior duct surfaces, constructed by fiberglass-reinforced plastic, were plated with separated film heaters for distinguishing the local wall heat transfer rate. The duct hydraulic diameter, the actively heated length, and the mean rotation radius are 4, 120, and 180 mm, respectively. In the experiments, the parameters were the throughflow Reynolds number, Re = 1,000∼15,000; the rotation number, Ro = 0∼0.32; and the rotational buoyancy parameter, Ra* = 0∼0.5. For the outward flow the Coriolis-induced cross-stream secondary flow strongly enhanced the heat transfer on the leading edge. But for the radially inward flow the trend was reversed. When the throughflow Reynolds number was increased, the rotating-buoyancy decreased, then increased the heat transfer for the outward flow; however, the rotating-buoyancy always increased the heat transfer for the inward flow. The heat transfer data are correlated for the outward and inward flows for the ranges of parameters under study.
Experimental Investigation of Natural Convection Heat Transfer in Volumetrically Heated Spherical Segments
Authors: Asfia F. J. , Frantz B. , Dhir V. K.
Published: February 2008

External cooling of a light water reactor vessel by flooding of the concrete cavity with subcooled water is one of several management strategies currently being considered for accidents in which significant relocation of core material is predicted to occur. At present, uncertainty exists with respect to natural convection heat transfer coefficients between the pool of molten core material and the reactor vessel wall. In the present work, experiments were conducted to examine natural convection heat transfer in internally heated partially filled spherical pools with external cooling. In the experiments, Freon-113 was contained in a Pyrex bell jar, which was cooled externally with subcooled water. The pool was heated using a 750 W magnetron taken from a conventional microwave. The pool had a nearly adiabatic free surface. The vessel wall temperature was not uniform and varied from the stagnation point to the free surface. A series of chromel–alumel thermocouples was used to measure temperatures in both steady-state and transient conditions. Each thermocouple was placed in a specific vertical and radial location in order to determine the temperature distribution throughout the pool and along the inner and outer walls of the vessel. In the experiments, pool depth and radius were varied parametrically. Both local and averages heat transfer coefficients based on pool maximum temperature were obtained. Rayleigh numbers based on pool height were varied from 2 × 1010 to 1.1 × 1014. Correlations for the local heat transfer coefficient dependence on pool angle and for the dependence of average Nusselt number on Rayleigh number and pool depth have been developed.
Flow and Heat Transfer Due to a Buoyant Ceiling Jet Turning Downward at a Corner
Authors: Kapoor K. , Jaluria Y.
Published: February 2008

An experimental investigation has been carried out on the flow and heat transfer characteristics of a horizontal buoyant ceiling jet that turns downward at a corner to yield a vertical negatively buoyant wall flow. Such flow situations are frequently encountered in thermal energy storage, in electronic systems, and in room fires. However, not much work has been done to understand the basic mechanisms governing such flows, particularly the flow near the corner. In this study, a two-dimensional jet of heated air is discharged adjacent to the lower surface of an isothermal horizontal plate. An isothermal vertical plate is attached at the other end of the horizontal surface, making a right angle corner. The vertical penetration distance of the resulting downward flow is measured and is related to the inflow conditions, particularly to the temperature and velocity at the jet discharge. This penetration distance is found to increase as the distance between the discharge location and the corner is reduced and also as the relative buoyancy level in the inlet flow is decreased. Velocity and temperature measurements are also carried out over the flow region. These indicate that the ceiling flow separates from the horizontal surface just before reaching the corner and then reattaches itself to the vertical wall at a finite distance vertically below the corner. The local surface heat flux measurements show a minimum in the heat transfer rate before the turn, along with a recovery in the heat transfer rate after the turn and the existence of a small recirculation zone near the corner. The net entrainment into the flow and heat transfer rate to the solid boundaries are also measured and correlated with the jet discharge conditions.
Evidence of a Convective Instability Allowing Warm Water to Freeze in Less Time Than Cold Water
Authors: Maciejewski P. K.
Published: February 2008

This experimental study explores the possibility that warm water may freeze in less time than cold water due to natural convection alone, i.e., in the absence of significant cooling by evaporation. This possibility is rooted in the following two hypotheses: (1) The Rayleigh number associated with a sample of warm water may exceed a critical value above which the convective motions within the water sample may become turbulent and enhance the rate of convective cooling, and (2) the inversion of the flow field that is expected to occur in the vicinity of maximum density, i.e., at 4°C, will occur at different points in the cooling process for identical samples of water at different levels of initial temperature and result in an enhanced rate of convective cooling after the flow field inversion for those cases at higher levels of initial temperature that enter the flow field inversion with higher kinetic energy. The results of this study establish that, under certain circumstances, a sample of water that is initially warm will freeze in less time than an identical sample of water that is initially cold due to natural convection alone.
Surface Temperature Measurement Using Infrared Radiometer by Applying a Pseudo-Gray-Body Approximation: Estimation of Radiative Property for Metal Surface
Authors: Inagaki T. , Okamoto Y.
Published: February 2008

The visualization technique using an infrared radiometer has been widely used to estimate a two-dimensional temperature distribution on a surface. We have previously proposed various characteristics of the radiative properties for nonmetal surfaces on which the gray-body approximation can be used. However, the gray-body approximation can not be applied to glossy metal surfaces, because the influence of the reflected energy on the temperature measurement becomes severe. A pseudo-gray-body approximation is therefore proposed to estimate a temperature field on metal surfaces and its applicability is confirmed using an infrared radiometer with three detection wavelength bands.
Correlation of Measured and Computed Radiation Intensity Exiting a Packed Bed
Authors: Jones P. D. , McLeod D. G. , Dorai-Raj D. E.
Published: February 2008

The spectral and directional distribution of radiation intensity is measured, using a direct radiometric technique, at the exposed boundary of a packed bed of stainless steel spheres. The purpose of these measurements is to provide an experimental data base of radiation intensity with which to correlate intensity field solutions of the radiative transfer equation in participating media. The bed is considered to be one-dimensional, is optically thick, and has measured constant-temperature boundary conditions. Intensity exiting the bed is numerically simulated using a discrete ordinates solution to the radiative transfer equation, with combined mode radiation-conduction solution of the coupled energy conservation equation. Radiative properties for the bed are computed using the large size parameter correlated scattering theory derived by Kamiuto from the general theory of dependent scattering by Tien and others. The measured intensity results show good agreement with computed results in near-normal directions, though agreement in near-grazing directions is poor. This suggests that either radiative transfer near the boundaries of this medium might not be adequately represented by a continuous form of the radiative transfer equation, or that the properties derived from correlated scattering theory are insufficient. In either case, development of a more detailed radiation model for spherical packed beds appears warranted.
A Numerical Study of Local and Average Natural Convection Nusselt Numbers for Simultaneous Convection Above and Below a Uniformly Heated Horizontal Thin Plate
Authors: Chambers B. , Lee Tien-Yu T.
Published: February 2008

Numerical simulations were conducted to determine local and average natural convection Nusselt numbers for uniformly heated horizontal plates with convection occurring simultaneously from upper and lower surfaces. Plate width and heating rate were used to vary the modified Rayleigh number over the range of 86 to 1.9 × 108. Upper surface Nusselt numbers were found to be smaller than corresponding lower surface Nusselt numbers. The local Nusselt number was largest at the plate edge and decreased towards the plate center for both surfaces. This variation followed approximately a minus 1/3-power law variation with the non-dimensionalized x coordinate on the upper surface for modified Rayleigh numbers greater than 104, and a minus 1/9-power law variation on the lower surface for all modified Rayleigh numbers. Comparative simulations were also performed for upward and downward facing uniformly heated plates (single sided convection). For these cases, Nusselt numbers on the upward facing plates were larger than for downward facing plates.
Nucleate Pool Boiling Heat Transfer Coefficients of Distilled Water (H2O) and R-134a/Oil Mixtures From Rib-Roughened Surfaces
Authors: Hsieh Shou-Shing , Weng Chun-Jen
Published: February 2008

Measurements of pool-boiling heat transfer coefficients in distilled water and R-134a/oil mixtures with up to 10 percent (by weight) miscible EMKARATE RL refrigeration lubricant oil are extensively studied for a smooth tube and four rib-roughened tubes (rib pitch 39.4 mm, rib height 4 mm, rib width 15 mm, number of rib element 8, rib angle 30 deg–90 deg). Boiling data of pure refrigerants and oil mixtures, as well as the influences of heat flux level on heat transfer coefficient, are presented and discussed. A correlation is developed for predicting the heat transfer coefficient for both pure refrigerants and refrigerant-oil mixtures. Moreover, boiling visualizations were made to broaden our fundamental understanding of the pool boiling heat transfer mechanism for rib roughened surfaces with pure refrigerants and refrigerant-oil mixtures.
A Stochastic Lagrangian Model for Near-Wall Turbulent Heat Transfer
Authors: Mazumder S. , Modest M. F.
Published: February 2008

The modeling of near-wall turbulent heat transfer necessitates appropriate description of near-wall effects, namely, molecular transport, production of turbulence by inhomogeneities, and dissipation of the temperature fluctuations by viscosity. A stochastic Lagrangian model, based on the velocity-composition joint probability density function (PDF) method, has been proposed. The proposed model, when compared with experimental and direct numerical simulation (DNS) data, overdamps the dissipation of the temperature fluctuations in the inertial sublayer, but reaches the correct limit at the wall. The performance of the model has also been compared to the standard k-ε and the algebraic Reynolds stress model (ARSM) for both constant heat flux and constant temperature boundary conditions at large Reynolds numbers. The Lagrangian nature of the model helps eliminate numerical diffusion completely.
Direct Numerical Simulation of Unstably Stratified Turbulent Channel Flow
Authors: Iida O. , Kasagi N.
Published: February 2008

Direct numerical simulations of the fully developed horizontal channel flow under unstable density stratification were carried out to investigate interactive shear and buoyancy effects on the turbulent momentum and heat transport. As the Grashof number is increased, buoyant thermal plumes are generated. The large-scale thermal convection involving the thermal plumes diminishes the quasi-coherent streamwise vortices, which are known to play a major role in the transport mechanism of near-wall turbulence. The destruction of the streamwise vortices result in the increased bulk mean velocity and the decreased turbulent friction coefficient. The vertical fluid motion of thermal plumes drastically changes the transport mechanism of the Reynolds shear stress. The thermal plumes are spatially aligned in the streamwise direction, and the low-speed streaks and vortical structures are concentrated in the region where the thermal plume starts to rise. The Prandtl number effects on the turbulent kinetic energy are also studied when the thermal plumes emerge.
Numerical Prediction of Transitional Characteristics of Flow and Heat Transfer in a Corrugated Duct
Authors: Yang L. C. , Asako Y. , Yamaguchi Y. , Faghri M.
Published: February 2008

The numerical prediction of transitional characteristics of fluid flow and heat transfer in periodic fully developed corrugated duct is carried out by using a Lam-Bremhorst low Reynolds number turbulence model. Computations were performed for Prandtl number of 0.7, in the Reynolds number range of 100 to 2500, for corrugation angles of θ = 15 and 30 deg, and for three interwall spacings. The predicted transitional Reynolds number is lower than the value for the parallel plate duct and it decreases with increasing corrugation angle. Experiments were also performed for pressure drop measurements and for flow visualization and the results were compared with the numerical predictions.
Experimental Investigation of Thermal and Hydrodynamic Development Regions for Drag-Reducing Surfactant Solutions
Authors: Gasljevic K. , Matthys E. F.
Published: February 2008

The reductions in friction and heat transfer exhibited by a surfactant solution in the entry region of a circular pipe were measured and analyzed, with special attention paid to the relationship between the local heat transfer and friction. Two entrance configurations were used, a cone contraction and wire mesh plugs used as a device for velocity profile flattening. Both the simultaneous development of temperature and velocity profiles and the development of temperature profile with hydrodynamically predeveloped flow were studied. Interestingly, the local heat transfer measurements for surfactant solutions matched very well a correlation developed for polymer solutions, but for surfactants the development of the heat transfer and velocity profiles appear coupled, unlike what is thought to happen for polymer solutions. The development patterns appear to be independent of velocity and entrance type at low disturbance levels. At high disturbance levels, however, some striking changes in the fluid itself, likely due to temporary micellar structure degradation by high local shear stress in the inlet region, were observed as well, and quantified.
Heat Transfer Enhancement—The Encouragement and Accommodation of High Heat Fluxes
Authors: Bergles A. E.
Published: February 2008

This review considers the many techniques that have been developed to enhance convective heat transfer. After introducing the techniques, the applications to most of the modes of heat transfer (single-phase forced convection, including compound techniques, pool boiling, convective boiling/evaporation, vapor-space condensation, and convective condensation) are described. Comments are offered regarding commercial introduction of this technology and the generations of heat transfer technology; advanced enhancement represents third-generation heat transfer technology.
Development of Multicellular Solutions in Natural Convection in an Air-Filled Vertical Cavity
Authors: Wakitani S.
Published: February 2008

Consideration is given to the multiple solutions of two-dimensional natural convection in a vertical air-filled tall cavity with differentially heated sidewalls. Numerical simulations are carried out for a wide range of Rayleigh numbers from the onset of the steady multicellular flow, through the reverse transition to the unicellular pattern, to the unsteady multicellular flow. The dependence of the flow structure on the initial condition is clarified from the simulations by means of starting from a motionless and isothermal state, and gradually increasing or decreasing the Rayleigh number.
Thermal Conductivity Measurements in Printed Wiring Boards
Authors: Graebner J. E. , Azar K.
Published: February 2008

The effective thermal conductivity κ of multilayer printed wiring boards (PWBs) has been measured for heat flowing in a direction either parallel (κ∥) or perpendicular (κ⊥) to the plane of the board. The conductivity of the glass/epoxy insulating material from which the boards are manufactured is anisotropic (κ∥ge ≈ 3 × κ⊥ge) and nearly three orders of magnitude smaller than the conductivity of copper. This large difference between glass/epoxy and copper produces extremely high anisotropy in PWBs that contain continuous layers of copper. For such boards, values of the board-averaged conductivity in the two directions can differ by a factor of ~100 or more. The value of κ∥ is found to depend on the ratio of the total thickness of continuous layers of copper to the total thickness of glass/epoxy, while it depends hardly at all on the amount of copper circuitry visible on the surface.
Solving an Inverse Heat Conduction Problem by a “Method of Lines”
Authors: Elde´n L.
Published: February 2008

We consider a Cauchy problem for the heat equation in the quarter plane, where data are given at x = 1 and a solution is sought in the interval 0 < x < 1. This inverse heat conduction problem is a model of a situation where one wants to determine the surface temperature given measurements inside a heat-conducting body. The problem is ill-posed in the sense that the solution (if it exists) does not depend continuously on the data. In an earlier paper we showed that replacement of the time derivative by a difference stabilizes the problem. In this paper we investigate the use of time differencing combined with a “method of lines” for solving numerically the initial value problem in the space variable. We discuss the numerical stability of this procedure, and we show that, in most cases, a usual explicit (e.g., Runge-Kutta) method can be used efficiently and stably. Numerical examples are given. The approach of this paper is proposed as an alternative way of implementing space-marching methods for the sideways heat equation.
Turbulent Transport Measurements in a Heated Boundary Layer: Combined Effects of Free-Stream Turbulence and Removal of Concave Curvature
Authors: Kestoras M. D. , Simon T. W.
Published: February 2008

Turbulence measurements for both momentum and heat transport are taken in a boundary layer over a flat recovery wall downstream of a concave wall (R = 0.97 m). The boundary layer appears turbulent from the beginning of the upstream, concave wall and grows over the flat test wall downstream of the curved wall with negligible streamwise acceleration. The strength of curvature at the bend exit, δ99.5/R, is 0.04. The free-stream turbulence intensity (FSTI) is ~8 percent at the beginning of the curve and is nearly uniform at ~4.5 percent throughout the recovery wall. Comparisons are made with data taken in an earlier study, in the same test facility, but with a low FSTI (~0.6 percent). Results show that on the recovery wall, elevated FSTI enhances turbulent transport quantities such as −uν and νt in most of the outer part of the boundary layer, but near-wall values of νt remain unaffected. This is in contrast to near-wall νt values within the curve which decrease when FSTI is increased. At the bend exit, decreases of −uν and νt due to removal of curvature become more profound when FSTI is elevated, compared to low-FSTI behavior. Measurements in the core of the flow indicate that the high levels of cross transport of momentum over the upstream concave wall cease when curvature is removed. Other results show that turbulent Prandtl numbers over the recovery wall are reduced to ~0.9 when FSTI is elevated, consistent with the rise in Stanton numbers over the recovery wall.
Boundary Layer Transition Under High Free-Stream Turbulence and Strong Acceleration Conditions: Part 1—Mean Flow Results
Authors: Volino R. J. , Simon T. W.
Published: February 2008

Measurements from heated boundary layers along a concave-curved test wall subject to high (initially 8 percent) free-stream turbulence intensity and strong (K = (ν/U∞2) dU∞/dx) as high as 9 × 10−6) acceleration are presented and discussed. Conditions for the experiments were chosen to roughly simulate those present on the downstream half of the pressure side of a gas turbine airfoil. Mean velocity and temperature profiles as well as skin friction and heat transfer coefficients are presented. The transition zone is of extended length in spite of the high free-stream turbulence level. Transitional values of skin friction coefficients and Stanton numbers drop below flat-plate, low-free-stream-turbulence, turbulent flow correlations, but remain well above laminar flow values. The mean velocity and temperature profiles exhibit clear changes in shape as the flow passes through transition. To the authors’ knowledge, this is the first detailed documentation of a high-free-stream-turbulence boundary layer flow in such a strong acceleration field.
Velocity and Temperature Profiles in Turbulent Boundary Layer Flows Experiencing Streamwise Pressure Gradients
Authors: Volino R. J. , Simon T. W.
Published: February 2008

The standard turbulent law of the wall, devised for zero pressure gradient flows, has been previously shown to be inadequate for accelerating and decelerating turbulent boundary layers. In this paper, formulations for mean velocity profiles from the literature are applied and formulations for the temperature profiles are developed using a mixing length model. These formulations capture the effects of pressure gradients by including the convective and pressure gradient terms in the momentum and energy equations. The profiles which include these terms deviate considerably from the standard law of the wall; the temperature profiles more so than the velocity profiles. The new profiles agree well with experimental data. By looking at the various terms separately, it is shown why the velocity law of the wall is more robust to streamwise pressure gradients than is the thermal law of the wall. The modification to the velocity profile is useful for evaluation of more accurate skin friction coefficients from experimental data by the near-wall fitting technique. The temperature profile modification improves the accuracy with which one may extract turbulent Prandtl numbers from near-wall mean temperature data when they cannot be determined directly.
Effects of Partial Inlet Blockages on High-Velocity Flow Through a Thin Rectangular Duct: Experimental and Analytical Results
Authors: Stovall T. K. , Crabtree J. A. , Felde D. K. , Farquharson G. , Park J. E.
Published: February 2008

The Advanced Neutron Source (ANS) reactor was designed to provide a research tool with capabilities beyond those of any existing reactors. One portion of its state-of-the-art design required high velocity fluid flow through narrow channels between the fuel plates in the core. Experience with previous reactors had shown that fuel plate damage could occur if debris became lodged at the entrance to these channels. Such debris disrupts the fluid flow to the plate surfaces and could prevent adequate cooling of the fuel. Preliminary ANS designs addressed this issue by providing an unheated entrance length for each fuel plate so that any flow disruption would have time to recover before reaching the heated portions of the fuel plates further downstream. As part of the safety analysis, the adequacy of this unheated entrance length was assessed using both analytical models and experimental measurements. The Flow Blockage Test Facility (FBTF) was designed and built to conduct experiments in an environment closely matching the ANS channel geometry. The FBTF permitted careful measurements of both heat transfer and hydraulic parameters. In addition to these experimental efforts, a thin rectangular channel was modeled using the FLUENT computational fluid dynamics computer code. The numerical results were compared to the experimental data to benchmark the hydrodynamics of the model. After this comparison, the model was extended to include those elements of the safety analysis difficult to measure experimentally. These elements included the high wall heat flux pattern and variable fluid properties.
The Effects of Prandtl Numbers on Local and Average Convective Heat Transfer Characteristics in Helical Pipes
Authors: Xin R. C. , Ebadian M. A.
Published: February 2008

To investigate the effects of the Prandtl number and geometric parameters on the local and average convective heat transfer characteristics in helical pipes, experiments with three different fluids—air, water, and ethylene glycol—were carried out on five uniformly heated helical pipes. The test sections were made from 22.9 mm I.D. and 10.2 mm I.D. 304 stainless steel pipes. The ratios of the pipe diameter and pitch to coil diameter (d/D and b/D) ranged from 0.0267 to 0.0884 and 0.20 to 2.56, respectively. The peripheral and average, fully developed Nusselt numbers were evaluated in the experiments. Experimental findings indicate that after two turns (X > 2) the temperature distributions along the wall are almost parallel to the linear fluid bulk temperatures, and all dimensionless peripheral wall temperatures are nearly identical, implying that both the flow and temperature distribution within the helical pipes are fully developed. These results reveal that the peripheral Nusselt number varies significantly for higher Prandtl numbers and Dean numbers in the laminar flow region. A new set of empirical expressions for the average fully developed Nusselt number has therefore been regressed based on the present data and some data from previous investigations. No obvious effects of the coil pitch or torsion were observed in the scope of this investigation.
Experimental Study of the Effect of Transverse Oscillation on Convection Heat Transfer From a Circular Cylinder
Authors: Cheng C.-H. , Chen H.-N. , Aung W.
Published: February 2008

The present study is concerned with heat transfer characteristics of forced convective flows over a transversely oscillating cylinder. The effect of oscillation of the cylinder on heat transfer coefficient and flow pattern is evaluated by experimental measurement. A modified transient test method and a flow visualization technique are employed to determine the heat transfer coefficient and to observe the flow pattern, respectively, for various dimensionless oscillation frequencies (Sc), dimensionless oscillation amplitudes (A/D), and Reynolds numbers (Re). The ranges of these parameters considered in this study are 0 ≤ Sc ≤ 0.65, 0 ≤ A/D ≤ 0.628, and 0 ≤ Re ≤ 4000. Results show that the heat transfer coefficient can be significantly increased by the oscillation of the cylinder. Two effects that enhance the heat transfer performance are found, namely, the lock-on effect and the turbulence effect. A maximum of 34 percent increase in heat transfer is found within the parameter ranges considered in this study. Agreement of the present data is found with previous numerical studies; however, an existing approximate method, which ignores the lock-on phenomenon but which is commonly used to estimate the effect of oscillation on heat transfer, is found to be inapplicable in this study.
Buoyancy Induced Convection in a Narrow Open-Ended Annulus
Authors: Vafai K. , Desai C. P. , Iyer S. V. , Dyko M. P.
Published: February 2008

Results from a combined experimental and numerical investigation of buoyancy driven flow and heat transfer in a narrow annular gap between co-axial, horizontal cylinders are presented in this work. The annulus is open at both ends through which the ambient fluid can interact with the fluid inside the gap. In the experimental study, a constant heat flux was utilized to simulate buoyancy induced convection in an open ended annular cavity with a low gap to inner cylinder radius ratio; local surface temperature measurements were made to determine heat transfer characteristics of the convective flow. The heat transfer results are correlated by Nu = 0.134(Ra*)0.264 for the range of Rayleigh numbers considered (7.09 ×108 ≤ Ra* ≤ 4.76 × 109) in the experiments. In the numerical investigation, solutions to the three-dimensional time-averaged (Reynolds) steady-state equations of fluid motion and heat transfer were obtained using a finite element analysis. Results of the conjugate study including the local temperature distributions, heat transfer coefficients, and the flow field showing the interactions between the ambient and cavity flow fields agree favorably with experimental results. An investigation was also carried out to study the effect of axial length and the gap width of the annulus. A correlation for the average Nusselt number as a function of Rayleigh number, axial length and gap width has been obtained. The present work provides, for the first time, an experimental and numerical study of turbulent buoyancy induced flows in a narrow open-ended annulus.