bluuebunny/crossref_metadata_2025 huggingface dataset:
Uneven Wall Heat Flux Effect on Local Heat Transfer in Rotating Two-Pass Channels With Two Opposite Ribbed Walls
Authors: Hsieh Shou-Shing , Liu Wei-Jen
Published: February 2008

The influence of rotation and uneven heating condition on the local heat transfer coefficient in rotating, two-pass rib-roughened (rib height e/DH ≈ 0.17 − 0.20; rib pitch p/e = 5) rectangular channels with cross-sectional aspect ratios of 1 and 1.5 were studied for Reynolds numbers from 5000 to 25,000 and rotation numbers from 0 to 0.6152. Regionally averaged Nusselt number variations along the duct have been determined over the trailing and leading surfaces for two pass channels. In general, Coriolis-induced secondary flows are shown to enhance local heat transfer over the trailing (leading) surface in the first (second) pass compared to a duct without rotation. Centrifugal buoyancy is shown to influence the heat transfer response with heat transfer being imposed on both leading and trailing surfaces as the wall-to-bulk fluid temperature difference is increased with other controlling parameters fixed. Results also indicate a slight decrease in heat transfer coefficient for an increase in passage aspect ratio. Results are compared with previous studies. It is found that the results agree quite well with those reported by other works for two-pass flow channels.
Experimental Investigation on Pulsating Horizontal Heating of a Water-Filled Enclosure
Authors: Antohe B. V. , Lage J. L.
Published: February 2008

Experimental results of the natural convection generated by the time periodic horizontal heating of a square cross-section enclosure filled with water are reported. A pulsating (on/off) heat flux is delivered to the heating wall of the enclosure, with the opposite wall cooled by a high thermal capacitance system. All other surfaces are insulated. Heating periods from 32 to 1600 seconds and cycle-averaged heat-flux based Rayleigh numbers from 2.5 × 108 to 1.0 × 109 are considered. Results presented in terms of time series, phase-plane portraits, and cyclic evolution of surface-averaged cooling and heating wall temperatures illustrate the main characteristics of the evolution to periodic regime. Also presented are the cycle-averaged heat transfer coefficient versus heating period, and the corresponding average Nusselt number versus Rayleigh number for various heating frequencies. These results, which support published theoretical and numerical analysis, indicate that by tuning the heating period properly, the heat transfer across an enclosure can be enhanced. The results also reveal that short heating periods hinder the convection within the enclosure, in general (e.g., for Ra = 7.5 × 108 and a period of 32 s the heat transfer coefficient is 13 percent smaller than the steady heating value). The sensitivity of the transport phenomenon to pulsating heat is shown to depend strongly on Ra. Finally, a correlation for estimating the maximum heat transfer coefficient, derived from the experimental results, is presented.
Comparison of Several Heat Transfer Enhancement Technologies for Gas Heat Exchangers
Authors: Andrews M. J. , Fletcher L. S.
Published: February 2008

A comparative study about the performance of several enhanced heat transfer technologies for gas heat exchangers is presented. A Reynolds number range from 100 to 50,000 is considered for plate heat exchangers and the tube side of shell-and-tube heat exchangers. A volumetric performance measure has been adopted to evaluate the comparative performance of widely different technologies. The performance parameter, based on the heat transfer rate per unit pumping power, is suitable for different geometries, Reynolds numbers, and fluid properties. Modern technologies can achieve significant heat transfer enhancement, but comparison reveals that recent advances offer only marginal improvements that are often associated with more complex technology. Care must be exercised in choosing a technology because the best performing one is not necessarily the preferred choice since construction, retrofit, and maintenance costs may significantly alter the economic viability. However, there is an intrinsic interest in the comparative performance of very different technologies. Our performance evaluation indicates an upper limit may exist for single-mode convective heat transfer enhancement and compound enhancement may exceed this limit.
Trajectories and Deposition of Silica Particles on Cylinders in Crossflow With and Without a Magnetic Field
Authors: Chan S. H. , Moussa B.
Published: February 2008

To investigate magnetic effects on silica scale deposition, two cylindrical probes with and without magnet insert were placed in a crossflow fluid solution containing dissolved silica, iron ion, and NaCl salt. The differences in silica deposition are reported. To explain the differences on the basis of first principles, a Lagrangian analysis is presented to predict the silica colloidal particle trajectories and deposition onto the probes under the influence of nonuniform magnetic field force and other relevant surface forces including London–van der Waals forces, viscous force, and added mass force. The results of the predication show that the particles made of pure silica can be deflected away from the magnetic probe. However, the particles made of silica–iron can be attracted toward the probe, more so at the magnetic poles than at the midsection of the magnetic probe. These and other results presented can explain all the qualitative differences of scale deposit found experimentally on magnetic and nonmagnetic probes.
New Radiative Analysis Approach for Reticulated Porous Ceramics Using Discrete Ordinates Method
Authors: Hendricks T. J. , Howell J. R.
Published: February 2008

A novel radiative modeling technique using discrete ordinates has been developed for reticulated porous ceramics (RPCs) from experimental measurements of transmittance and reflectance on small samples of partially stabilized zirconium oxide (PS ZrO2) and oxide-bonded silicon carbide (OB SiC). The new technique defines and quantifies the direct transmittance fraction, fdt, of a reticulated porous ceramic, demonstrates how it redefines the extinction process, and creates a new effective extinction coefficient, Kλ,eff. This ultimately produces a modified form of the radiative transfer equation (RTE) and an innovative discrete ordinates formulation to solve the RTE unique to RPCs. The direct transmittance modeling approach has been compared to a more conventional homogeneous modeling approach, in which the direct transmittance effects are essentially ignored and the RPC is treated as a homogeneous lump of material. The two modeling approaches yield identical results in predicting small test sample reflectances and transmittances. The direct transmittance technique does demonstrate explicitly, through a unique relationship between absorption coefficients in the two modeling approaches, the importance of scattering processes in enhancing the absorption mechanism in RPCs. It can also be an important secondary modeling technique that imposes additional parameter constraints in an inverse analysis to help refine derived radiative coefficients.
Saturable Absorption During High-Intensity Laser Heating of Liquids
Authors: Longtin J. P. , Tien C.-L.
Published: February 2008

The interaction of high-intensity, short-pulse laser radiation with liquids is fundamental to many contemporary technologies. At low laser intensities, the classical model of absorption and heating applies, which assumes a constant absorption coefficient and no dependence on intensity. As the intensity increases, however, many molecules are promoted to excited states, whose absorption properties differ from those of the ground state, and the absorption of the bulk liquid is altered. This phenomenon, called saturable absorption, results in intensity-dependent absorption, heating, and temperature distributions that can deviate significantly from classical absorption. This work investigates the thermal aspects of saturable absorption during laser heating of liquids. A microscopically based model of the radiation absorption and heating processes is presented. Model solutions are discussed and compared with experiment for a contemporary saturable absorbing liquid. Simple engineering criteria and relevant applications are then discussed.
Heater Orientation Effects on Pool Boiling of Micro-Porous-Enhanced Surfaces in Saturated FC-72
Authors: Chang J. Y. , You S. M.
Published: February 2008

Experiments are performed to understand the effects of surface orientation on the pool boiling characteristics of a highly wetting fluid from a flush-mounted, micro-porous-enhanced square heater. Micro-porous enhancement was achieved by applying copper and aluminum particle coatings to the heater surfaces. Effects of heater orientation on CHF and nucleate boiling heat transfer for uncoated and coated surfaces are compared. A correlation is developed to predict the heater orientation effect on CHF for those surfaces.
Modeling of Nonequilibrium Surface Melting and Resolidification for Pure Metals and Binary Alloys
Authors: Wang G.-X. , Matthys E. F.
Published: February 2008

A model was developed for surface melting and resolidification of both pure metal and binary alloy substrates. Nonequilibrium kinetics are introduced in the model to account for the departure from thermodynamic equilibrium at the solid/liquid interface. The modeled problem involves a moving boundary with both heat and solute diffusion and is solved by an implicit control volume integral method with solid/liquid interface immobilization by coordinate transformation. To illustrate the model capabilities, we have analyzed laser-induced surface melting of pure metals (Al, Cu, Ni, Ti) and dilute Al–Cu alloys, and some typical results are presented. The computation results show some large solid overheating and melt undercooling effects, which result from the high heat flux and the slow kinetics. Large interface velocity variations are also seen during the process, depending on the substrate material and laser flux. Complex interface velocity variations during the earlier stages of resolidification were also predicted for the alloys, and result from interactions between the several physical mechanisms involved. Results on interface temperatures, solute concentrations, and nonequilibrium partition coefficients are also presented.
A Unified Correlation for the Prediction of Heat Transfer Coefficients in Liquid/Solid Fluidized Bed Systems
Authors: Jamialahmadi Mohammad , Malayeri Mohammad Reza , Mu¨ller-Steinhagen Hans
Published: February 2008

In recent years, there has been considerable interest in the development of solid-liquid fluidized bed heat exchangers for efficient utilization of energy and for the control of heat transfer surface fouling. However, the design for optimum heat transfer remains uncertain and essentially empirical. In this study, a data bank containing a large number of measured heat transfer coefficients over a wide range of operational parameters (solid and liquid phase physical properties and heat transfer surface configurations) are compared with the prediction of almost all available correlations in the literature. A unified model for the prediction of heat transfer coefficients for wall-to-bed and immersed heater-to-bed systems is presented which predicts the published experimental data with good accuracy.
Surface Ripple in Electron-Beam Welding Solidification
Authors: Wei P. S. , Chang C. Y. , Chen C. T.
Published: February 2008

The occurrence of ripples on the workpiece surface after solidification in electron-beam welding or melting is experimentally and analytically investigated. The maximum accelerating voltage and welding current of the electron-beam welder are 60 kV and 50 mA, respectively, while the workpieces are Al 1100 and SS 304. The average pitches and amplitudes of surface ripples are measured for different beam powers and welding speeds. Using a scale analysis to account for heat transfer and fluid flow induced by a negative temperature-dependent surface tension gradient in the molten pool the variations of ripples with dimensionless beam power, Marangoni, Peclet, Prandtl, Stefan, and Biot numbers are found for the first time. The predicted results show good agreement with experimental data.
Prediction and Measurement of Temperature Fields in Silicon-on-Insulator Electronic Circuits
Authors: Goodson K. E. , Flik M. I. , Su L. T. , Antoniadis D. A.
Published: February 2008

Field-effect transistors (FETs) in conventional electronic circuits are in contact with the high-thermal-conductivity substrate. In contrast, FETs in novel silicon-on-insulator (SOI) circuits are separated from the substrate by a thermally resistive silicon-dioxide layer. The layer improves the electrical performance of SOI circuits. But it impedes conduction cooling of transistors and interconnects, degrading circuit reliability. This work develops a technique for measuring the channel temperature of SOI FETs. Data agree well with the predictions of an analytical thermal model. The channel and interconnect temperatures depend strongly on the device and silicon-dioxide layer thicknesses and the channel–interconnect separation. This research facilitates the thermal design of SOI FETs to improve circuit figures of merit, e.g., the median time to failure (MTF) of FET–interconnect contacts.
Heat Transfer Within a Steel-Reinforced Porous Concrete Slab Subjected to Microwave Heating
Authors: Li W. , Ebadian M. A. , White T. L. , Foster D.
Published: February 2008

The concrete decontamination and decommissioning (D&D) process using microwave technology is investigated theoretically in this paper. A one-dimensional uniform plane wave is assumed for the microwave propagation and the microwave field and the power dissipation within the concrete. Also, by considering the effects of the microwave frequencies, a one-dimensional model of unsteady heat and mass transport in the concrete is developed to evaluate variations of the temperature and pressure distributions with the heating time. The effects of the microwave frequency (f), microwave power intensity (Qo,avg), the thermal boundary conditions on the front wall, and the concrete porosity (Φ) on the microwave power dissipation (Qd,avg), temperature (T), pressure (P), and evaporation rate (Δm) distributions are all investigated in the present model. Finally, the effects of the presence of a steel reinforcement and its location on the microwave power dissipation, and the temperature and inner steam pressure distributions are discussed accordingly.
Introduction to the Method of Average Magnitude Analysis and Application to Natural Convection in Cavities
Authors: Lykoudis P. S.
Published: February 2008

The method of Average Magnitude Analysis is a mixture of the Integral Method and the Order of Magnitude Analysis. The paper shows how the differential equations of conservation for steady-state, laminar, boundary layer flows are converted to a system of algebraic equations, where the result is a sum of the order of magnitude of each term, multiplied by a weight coefficient. These coefficients are determined from integrals containing the assumed velocity and temperature profiles. The method is illustrated by applying it to the case of drag and heat transfer over an infinite flat plate. It is then applied to the case of natural convection over an infinite flat plate with and without the presence of a horizontal magnetic field, and subsequently to enclosures of aspect ratios of one or higher. The final correlation in this instance yields the Nusselt number as a function of the aspect ratio and the Rayleigh and Prandtl numbers. This correlation is tested against a wide range of small and large values of these parameters.
Three-Dimensional Natural Convection in an Enclosure With an Internal Isolated Vertical Plate
Authors: Yang M. , Tao W. Q.
Published: February 2008

Natural convection in a cubic enclosure with an internal isolated heated vertical plate was investigated both experimentally and numerically. The internal plate was suspended under the lower surface of the enclosure top wall and electrically heated. The six enclosure walls were at a lower constant temperature. The plate average Nusselt number and the air temperature in the enclosure symmetry plane were experimentally determined in the range of Ra = 8 × 104 to 5 × 107. Numerical simulations of laminar natural convection in the same configuration were performed. The agreement between the test data and the numerically predicted values is reasonably good, with a maximum deviation in plate average Nusselt number of 12.3 percent and that in air temperature of 9.4 percent. Detailed temperature and velocity distributions in four cross sections were presented for the case of Ra = 6.57 × 105.
The First Instability Mechanism in Differentially Heated Cavities With Conducting Horizontal Walls
Authors: Janssen R. J. A. , Henkes R. A. W. M.
Published: February 2008

A classical configuration in thermal engineering is the rectangular cavity that is differentially heated over two opposing vertical walls. In this paper, the instability mechanism responsible for the transition from steady to time-periodic flow in both two and three-dimensional cavities with perfectly conducting horizontal walls is studied. For both air (Pr = 0.71) and water (Pr = 7.0), the instability is a thermal instability resulting from an unstable stratification in the boundary layers along the horizontal cavity walls. The frequency is in good agreement with the frequency predicted using Howard’s model (1966). For air, the perturbations arise at fixed depths in the cavity whereas for water they travel along the hot and cold walls of the cavity.
Unsteady Three-Dimensional Natural Convection in an Inclined Air Slot With a Hexagonal Honeycomb Core
Authors: Asako Y. , Yamaguchi Y. , Yamanaka T. , Faghri M.
Published: February 2008

Unsteady three-dimensional natural convection heat transfer in an inclined air slot with a hexagonal honeycomb enclosure is investigated numerically. The numerical methodology is based on an algebraic coordinate transformation technique that maps the hexagonal cross section onto a rectangle. The transformed governing equations are solved with a control volume discretization scheme using a fully implicit method with time. The computations are performed for inclination angles in the range of 60 to 80 deg for Ra = 104, and in the range of 45 to 80 deg for Ra = 105, for Prandtl number of 0.7, and for a fixed aspect ratio of H/L = 5. A conductive thermal boundary condition for the honeycomb side walls is considered. Both periodic and nonperiodic oscillating solutions are obtained depending on the inclination angle and Rayleigh number. The complex flow patterns are presented in form of particle trajectory maps and are compared with the flow visualization results using microcapsulated liquid crystals.
Computation of Turbulent Flow in Mixed Convection in a Cavity With a Localized Heat Source
Authors: Papanicolaou E. , Jaluria Y.
Published: February 2008

A numerical simulation of the turbulent transport from an isolated heat source in a square cavity with side openings is presented in this work. The openings allow an externally induced air stream at ambient temperature to flow through the cavity and, thus, mixed convection arises. Results for the turbulent regime are obtained, by employing a suitable, high-Reynolds-number from of the K–E turbulence model. A stream function-vorticity mathematical formulation is used, along with the kinetic energy and dissipation rate equations and an expression for the eddy viscosity. A time-marching scheme is employed, using the ADI method. The values of the Reynolds number Re, associated with the external flow, and the Grashof number Gr, based on the heat flux from the source, for which turbulent flow sets in are sought. Two typical values of the Reynolds number are chosen, Re = 1000 and Re = 2000, and turbulent results are obtained in the range Gr = 5 × 107 – 5 × 108. For both values of Re, the average Nusselt number over the surface of the source is found to vary with Gr in a fashion consistent with previous numerical and experimental results for closed cavities, while the effect of Re in the chosen range of values was small.
Thermal Analysis of In-Situ Curing for Thermoset, Hoop-Wound Structures Using Infrared Heating: Part I—Predictions Assuming Independent Scattering
Authors: Chern B.-C. , Moon T. J. , Howell J. R.
Published: February 2008

A curing process for unidirectional thermoset prepreg wound composite structures using infrared (IR) in-situ heating is investigated. In this method, the infrared energy is from all incident angles onto the composite structure to initiate the curing during processing. Due to the parallel geometry of filaments in wound composite structures, the radiative scattering coefficient and phase function within the structure depend strongly on both the wavelength and the angle of incidence of the IR incident radiation onto the fibers. A two-dimensional thermochemical and radiative heat transfer model for in-situ curing of thermoset, hoop-wound structures using IR heating is presented. The thermal transport properties that depend on the process state are also incorporated in the analysis. A nongray, anisotropic absorbing, emitting, and scattering unidirectional fibrous medium within a matrix of nonunity refractive index is considered. The temperatures and degrees of cure within the composite during processing are demonstrated numerically as a function of the configuration of IR heat source, nondimensional power input, mandrel winding speed, and size of wound composite. Comparison between the numerical result and experimental data is presented.
Thermal Analysis of In-Situ Curing for Thermoset, Hoop-Wound Structures Using Infrared Heating: Part II—Dependent Scattering Effect
Authors: Chern B.-C. , Moon T. J. , Howell J. R.
Published: February 2008

The volume fraction of the fibers present in commercial filament wound structures, formed from either epoxy-impregnated tapes (“prepreg”) or fiber strands pulled through an epoxy bath, approaches 60 percent. Such close-packed structures are near the region that may cause dependent scattering effects to be important; that is, the scattering characteristics of one fiber may be affected by the presence of nearby fibers. This dependent scattering may change the single-fiber extinction coefficient and phase function, and thus may change the radiative transfer in such materials. This effect is studied for unidirectional fibers dispersed in a matrix with nonunity refractive index, and with large size parameter (fiber diameter to wavelength ratio) typical of commercial fiber–matrix composites. Only the case of radiation incident normal to the cylinder axes is considered, as this maximizes the dependent effects. The dependent extinction efficiency is found by solving the dispersion relations for the complex effective propagation constant of the composites. An estimation of this dependent scattering effect on the infrared in-situ curing of thermoset-hoop-wound structures is also conducted. It is found that the wave interference effect is significant for S-glass/3501-6 composite, and neglect of this effect tends to overestimate the temperature and cure state within the materials during IR in-situ curing.
A Theoretical Model for Flow Boiling CHF From Short Concave Heaters
Authors: Galloway J. E. , Mudawar I.
Published: February 2008

Experiments were performed to enable the development of a new theoretical model for the enhancement in CHF commonly observed with flow boiling on concave heaters as compared to straight heaters. High-speed video imaging and photomicrography were employed to capture the trigger mechanism for CHF for each type of heater. A wavy vapor layer was observed to engulf the heater surface in each case, permitting liquid access to the surface only in regions where depressions (troughs) in the liquid–vapor interface made contact with the surface. CHF in each case occurred when the pressure force exerted upon the wavy vapor–liquid interface in the contact regions could no longer overcome the momentum of the vapor produced in these regions. Shorter interfacial wavelengths with greater curvature were measured on the curved heater than on the straight heater, promoting a greater pressure force on the wavy interface and a corresponding increase in CHF for the curved heater. A theoretical CHF model is developed from these observations, based upon a new theory for hydrodynamic instability along a curved interface. CHF data are predicted with good accuracy for both heaters.
Heat Transfer in Excimer Laser Melting of Thin Polysilicon Layers
Authors: Xu X. , Grigoropoulos C. P. , Russo R. E.
Published: February 2008

A pulsed KrF excimer laser with nanosecond pulse duration is used for surface melting of thin polycrystalline silicon films. The velocity of the moving phase boundary during melting and solidification, the maximum melting depth, as well as the melting duration are experimentally determined by combined optical and electrical methods. A melting interface tracking model is used to calculate the melt front propagation and the transient temperature field in the semiconductor. A phase-change model, which allows the occurrence of melting and solidification at temperatures other than the equilibrium melting temperature, is employed in the numerical calculation. The effect of interfacial superheating/undercooling is discussed.
Investigation of Non-Darcian Forced Convection in an Asymmetrically Heated Sintered Porous Channel
Authors: Hwang G. J. , Wu C. C. , Chao C. H.
Published: February 2008

A study of non-Darcian forced convection in an asymmetric heating sintered porous channel is carried out to investigate the feasibility of using this channel as a heat sink for high-performance forced air cooling in microelectronics. A volume-averaging technique is applied to obtain the macroscopic equations with the non-Darcian effects of no-slip boundary, flow inertia, and thermal dispersion. Local non-thermal-equilibrium is assumed between the solid and the fluid phases. The analysis reveals that the particle Reynolds number significantly affects the solid-to-fluid heat transfer coefficients. A wall function is introduced to model the transverse thermal dispersion process for the wall effect on the lateral mixing of fluid. The local heat transfer coefficient at the inlet is modeled by a modified impinging jet result, and the noninsulated thermal condition is considered at exit. The numerical results are found to be in good agreement with the experimental results in the ranges of 32 ≤ Red ≤ 428 and q = 0.8 ~ 3.2 W/cm2 for Pr = 0.71.
Design of Experiments Using Uncertainty Information
Authors: Emery A. F. , Fadale T. D.
Published: February 2008

The process of parameter estimation and the estimated parameters are affected not only by measurement noise, which is present during any experiment, but also by uncertainties in the parameters of the model used to describe the system. This paper describes a method to optimize the design of an experiment to deduce the maximum information during the inverse problem of parameter estimation in the presence of uncertainties in the model parameters. It is shown that accounting for these uncertainties affects the optimal locations of the sensors.
Experimental Investigation of the Laminar Flow Heat Transfer Enhancement in a Small-Scale Square Duct With Aqueous Carbopol Solutions
Authors: Lin Cheng-Xian , Ko Shao-Yen , Tsou F. K.
Published: February 2008

This paper presents results of an experimental study on the heat transfer enhancement in laminar flow of non-Newtonian fluids, aqueous Carbopol-934 solutions through a small-scale square duct. The square duct is a top-wall heated configuration with a hydraulic diameter of 0.4 cm. The aqueous Carbopol solutions examined are those neutralized, and have a polymer concentration range of 1000–2000 wppm. It is shown that the enhanced heat transfer behavior of the Carbopol solutions within low Reynolds number range is different from that within relatively high Reynolds number range. There exists a limiting polymer concentration, Cmax, at which the non-Newtonian fluid possesses the maximum ability to enhance heat transfer. If the polymer concentration becomes too high, the minimum Reynolds number required to enhance heat transfer increases with the increasing polymer concentration.
Optimal Spacing Between Pin Fins With Impinging Flow
Authors: Ledezma G. , Morega A. M. , Bejan A.
Published: February 2008

This is an experimental numerical and theoretical study of the heat transfer on a pin-finned plate exposed to an impinging air stream. The pin fins are aligned with the air approach velocity. The base plate and the fin cross section are square. It is demonstrated experimentally that the thermal conductance between the plate and the air stream can be maximized by selecting the fin-to-fin spacing S. Next, a simplified numerical model is used to generate a large number of optimal spacing and maximum heat transfer data for various configurations, which differ with respect to fin length (H), fin thickness (D), base plate size (L), fluid type (Pr), and air velocity (ReL). Finally, the behavior of the optimal spacing data is explained and correlated theoretically based on the intersection of asymptotes method. The recommended correlations for optimal spacing, Sopt/L ≅ 0.81 Pr−0.25 ReL−0.32, and maximum thermal conductance, (q/ΔT)max/kaH ≅ 1.57 Pr0.45 ReL0.69 (L/D)0.31, cover the range D/L = 0.06 − 0.14, H/L = 0.28−0.56, Pr = 0.72−7, ReD = 10−700, and ReL = 90−6000.
Local Heat Transfer in Rotating Smooth and Ribbed Two-Pass Square Channels With Three Channel Orientations
Authors: Dutta S. , Han J.-C.
Published: February 2008

This paper presents experimental heat transfer results in a two-pass square channel with smooth and ribbed surfaces. The ribs are placed in a staggered half-V fashion with the rotation orthogonal to the channel axis. The channel orientation varies with respect to the rotation plane. A change in the channel orientation about the rotating frame causes a change in the secondary flow structure and associated flow and turbulence distribution. Consequently, the heat transfer coefficient from the individual surfaces of the two-pass square channel changes. The effects of rotation number on local Nusselt number ratio distributions are presented. Heat transfer coefficients with ribbed surfaces show different characteristics in rotation number dependency from those with smooth surfaces. Results show that staggered half-V ribs mostly have higher heat transfer coefficients than those with 90 and 60 deg continuous ribs.
Unsteady Heat Transfer in Baffled Channels
Authors: Wang G. , Stone K. , Vanka S. P.
Published: February 2008

In this paper, the enhancement of heat transfer due to unsteady flow in channels with in-line and staggered baffles is investigated through the numerical solution of the governing unsteady fluid flow and energy equations with periodicity in the stream wise direction. For the inline configuration, the flow becomes naturally unsteady at a critical Reynolds number (Q/v) around 110. For the staggered case, this value is around 200. Significant increases in heat transfer rate are observed once the flow becomes unsteady. Results for several Reynolds numbers up to 500 are presented. The present results can be valuable to the design and operation of compact heat exchangers used in process industry.
Natural Convection Heat Transfer in Horizontal Rod-Bundle Enclosures
Authors: Keyhani M. , Dalton T.
Published: February 2008

Natural convection heat transfer in enclosed horizontal N × N arrays (N = 3, 5, and 7) of electrically heated rods with a pitch-to-diameter ratio (P/d) of 1.35 has been experimentally investigated. Each array was positioned in an isothermal square enclosure with a width-to-diameter ratio (W/d) of 20.6. Pressurized air or helium was used as the working fluid. It was observed that the bottom-tow rods were relatively insensitive to increases in the array size, as they exhibit only slight temperature variations, but the top-row rods demonstrated substantial temperature increases. Natural convection correlations in the form of Nusselt number (Nud) as a function of modified Rayleigh number (Rad*) were obtained for each rod in each array. The correlations cover three flow regimes of conduction, transition, and convection in the range of 6.45 < Rad* < 3.08 × 105. A generalized enclosure Nusselt number was correlated as a function of enclosure modified Rayleigh number and the array size (N). Comparison of the data with previous numerical prediction showed that this correlation may be readily used to obtain a conservative estimate of the maximum temperature in the arrays with N = 3, 5, 7, and 9.
Flow and Mixed Convection Heat Transfer in a Divergent Heated Vertical Channel
Authors: Gau C. , Huang T. M. , Aung W.
Published: February 2008

This paper concerns an experimental study of the mixed convection flow and heat transfer inside a divergent channel formed by two plane walls. One of the side walls is oriented vertically and is heated uniformly, and the opposite wall is tilted at an angle of 3 deg with respect to the vertical position and is insulated. The ratio of the height to wall spacing at the flow inlet, which is at the smaller opening of the channel is 15. The Reynolds number of the main forced flow ranges from 100 to 4000 and the buoyancy parameter, Gr/Re2, varies from 0.3 to 907. Flow reversal is found to occur for both assisted and opposed convection. The effect of channel divergence on the occurrence and structure of the reversed flow and the heat transfer is presented and discussed. It is found that the divergence of the channel decelerates the mainstream such that flow reversal is initiated at a much lower buoyancy parameter. The adverse pressure gradient tends to push the reversed flow upstream and leads to a deeper penetration of the reversed flow into the channel The destabilization effect of the divergent channel can lead to breakdown of vortices and to transition to turbulent flow. This can significantly enhance the heat transfer. Temperature fluctuation measurements at different locations are used to indicate oscillations and fluctuations of the reversed flow. The effect of the buoyancy parameter on the Nusselt number and the reversed flow structure is discussed. The average Nusselt number is determined and correlated in terms of relevant nondimensional parameters for pure forced and mixed convection, respectively.
A Multi-objective Optimization of Radiative Fin Array Systems in a Fuzzy Environment
Authors: Chung B. T. F. , Zhang B. X. , Lee E. T.
Published: February 2008

This study analyzes and optimizes a new design for a four-fin radiating fin array system in a multi-objective fuzzy optimization environment. The conflicting objectives of minimizing both weight and horizontal size are considered simultaneously in the fuzzy optimization logic. A fuzzy feasible domain is constructed by considering all of the fuzzy sets individually defined by the fuzzy objectives and constraints. The system is optimized by maximizing the fuzzy decision function. A systematic procedure of the fuzzy optimization is discussed in detail.
Transition Boiling Curves in Saturated Pool Boiling From Horizontal Cylinders
Authors: Ungar E. K. , Eichhorn R.
Published: February 2008

Pool boiling heat flux versus wall superheat boiling curves were obtained for horizontal 3.18-mm-dia thin-walled brass tubes heated by an internal high-speed flow of ethylene glycol. The boiling liquids were saturated n-pentane, R-113, acetone, methanol ethanol, benzene, and isopropanol. Boiling results include nucleate and transition boiling in all the test liquids, but film boiling was achieved only with methanol. The measured peak heat fluxes are well correlated by available predictions. The methanol experiments clearly display two transition boiling curves, one obtained on increasing the cylinder temperature from nucleate boiling, the other on decreasing the cylinder temperature from film boiling. For the cases in which the highest cylinder temperature reached only into the transition regime, a single transition boiling curve resulted.
Optimizing and Predicting CHF in Spray Cooling of a Square Surface
Authors: Mudawar I. , Estes K. A.
Published: February 2008

Spray cooling of a hot surface was investigated to ascertain the effect of nozzle-to-surface distance on critical heat flux (CHF). Full cone sprays of Fluorinert FC-72 and FC-87 were used to cool a 12.7 × 12.7 mm2 surface. A theoretical model was constructed that accurately predicts the spray’s volumetric flux (liquid volume per unit area per unit time) distribution across the heater surface. Several experimental spray sampling techniques were devised to validate this model. The impact of volumetric flux distribution on CHF was investigated experimentally. By measuring CHF for the same nozzle flow rate at different nozzle-to-surface distances, it was determined CHF can be maximized when the spray is configured such that the spray impact area just inscribes the square surface of the heater. Using this optimum configuration, CHF data were measured over broad ranges of flow rate and subcooling, resulting in a new correlation for spray cooling of small surfaces.
Study on Sublimation Phenomenon of Horizontal Frost Layer Exposed to Forced Convection Air Flow and Radiant Heat
Authors: Inaba H. , Imai S.
Published: February 2008

The present paper deals with a new defrosting method for application to a cold heat exchanger by means of a frost sublimation phenomenon. The frost sublimation phenomenon occurs under the condition of temperature below the triple point of water (0.01°C, 0.6105 kPa). The present experimental study has focused on the heat and mass transfer processes at a horizontal frost layer in a moist airflow. It is found that the mass flux from the frost layer increases with the increase of water vapor mass concentration difference between the frost and main moist air flow, main air flow velocity, and the infrared radiant heat intensity. Nondimensional correlations for mass transfer, heat transfer, and defrost completion time have been derived.
Fluid Flow Effects in Evaporation From Liquid–Vapor Meniscus
Authors: Khrustalev D. , Faghri A.
Published: February 2008

A mathematical model of the evaporating liquid–vapor meniscus in a capillary slot has been developed. The model includes two-dimensional steady-state momentum conservation and energy equations for both the vapor and liquid phases, and incorporates the existing simplified one-dimensional model of the evaporating microfilm. The numerical results, obtained for water, demonstrate the importance of accounting for the fluid flow in calculating the effective evaporative heat transfer coefficient and the superheat of the vapor over the liquid–vapor meniscus due to the heat transfer from the heated wall. With higher heat fluxes, a recirculation zone appears in the vapor near the heated wall due to extensive evaporation in the thin-film region of the liquid–vapor meniscus.
Quintuple Hot-Wire Measurements of the Turbulence Structure in Confined Swirling Flows
Authors: Holza¨pfel F. , Lenze B. , Leuckel W.
Published: February 2008

The novel quintuple hot-wire measurement technique was used to perform detailed measurements of the mean velocities and Reynolds stresses in an isothermal model combustion chamber at two different levels of swirl. The measured flow quantities are analyzed and described in detail where the emphasis is put on typical swirl-related effects as well as the interaction of rotation and turbulence dynamics. The results provide a well-documented data base for the development and validation of turbulence closures. They also serve to improve understanding of specific characteristics of swirl flows.
Turbulent Boundary Layers Subjected to Multiple Strains
Authors: Schwarz Andreas C. , Plesniak Michael W. , Murthy S. N. B.
Published: February 2008

Turbomachinery flows can be extremely difficult to predict, due to a multitude of effects, including interacting strain rates, compressibility, and rotation. The primary objective of this investigation was to study the influence of multiple strain rates (favorable streamwise pressure gradient combined with radial pressure gradient due to convex curvature) on the structure of the turbulent boundary layer. The emphasis was on the initial region of curvature, which is relevant to the leading edge of a stator vane, for example. In order to gain better insight into the dynamics of complex turbulent boundary layers, detailed velocity measurements were made in a low-speed water tunnel using a two-component laser Doppler velocimeter. The mean and fluctuating velocity profiles showed that the influence of the strong favorable pressure augmented the stabilizing effects of convex curvature. The trends exhibited by the primary Reynolds shear stress followed those of the mean turbulent bursting frequency, i.e., a decrease in the bursting frequency coincided with a reduction of the peak Reynolds shear stress. It was found that the effects of these two strain rates were not superposable, or additive in any simple manner. Thus, the dynamics of the large energy-containing eddies and their interaction with the turbulence production mechanisms must be considered for modeling turbulent flows with multiple strain rates.
An Experimental Analysis of Vortex Trapping Techniques
Authors: Riddle Todd W. , Wadcock Alan J. , Tso Jin , Cummings Russell M.
Published: February 2008

An experimental verification is presented for the theoretical vortex trapping concept. A suction orifice located along one wall of a water channel test section was used to simulate a point sink to trap spanwise vortices downstream of a backward-facing step and between two parallel fences. Results from the backward-facing step geometry indicated an increase in the sink strength required to hold a vortex as the sink is positioned closer to the step, closely following previous theoretical predictions made using conformal mapping. The experimental data also showed reasonable agreement with the theoretical position for optimum vortex trapping. Flow visualization has shown a three-dimensional cross-stream effect due to bending of the forced-vortex core by suction. Results from the dual-fence geometry, on the other hand, verified the ability to use a lower level of suction for vortex trapping when compared with the backward-facing step.
The Effect of Permeability Variations on the Flow in a Heterogeneous Porous Channel Subject to Rotation
Authors: Vadasz Peter , Havstad Mark A.
Published: February 2008

A significant effect of permeability variations on the three-dimensional fluid flow in a heterogeneous porous channel subject to rotation is presented. The results of a numerical solution to the governing equations confirm for the more general case the conclusions from earlier analytical investigations, which suggest that permeability functions be classified corresponding to whether their variation is monotonic or not, and to whether their vertical gradient is positive or not. Unicellular and multiple vortex solutions are obtained for the secondary flow in the plane perpendicular to the imposed axial flow, while their direction is dictated by the corresponding class of permeability function as applicable. The impact of rotation on the imposed axial flow is shown to be significant as well, leading to different axial flow fields depending again on the class of permeability function used. In particular, the rotation impacts significantly in creating axial flow deficiencies in some regions on the cross section.
Vortex Detachment and Reverse Flow in Pulsatile Laminar Flow Through Axisymmetric Sudden Expansions
Authors: Tavoularis S. , Singh R. K.
Published: February 2008

Incompressible, steady and pulsatile flows in axisymmetric sudden expansions with diameter ratios of 1:2.25 and 1:2.00 have been simulated numerically over the ranges of time-averaged bulk Reynolds number 0.1 ≤ Re ≤ 400 and Womersley number 0.1 ≤ W ≤ 50. For steady flow, the calculated recirculation zone length increased linearly with an increase in Re, in good agreement with earlier experiments. For pulsatile flows, particularly at higher values of W, the recirculation zone length correlated strongly with the acceleration of the flow and not with the instantaneous Reynolds number; it increased during the deceleration phase and decreased during the acceleration phase. The computed mean velocity and reattachment length were in general agreement with published experimental data. At relatively low W, the computed near-wall, reverse flow region extended along the full domain over part of the cycle, similarly to that in the experiments. At low values of W, the vortex rings created at the expansion remained attached and oscillated back and forth; for an intermediate range of W, they detached and moved downstream; at relatively high W, these vortices became, once more, attached.
Study on Side-Jet Injection Near a Duct Entry With Various Injection Angles
Authors: Liou Tong-Miin , Liao Chin-Chun , Chen Shih-Hui , Lin Hsin-Ming
Published: February 2008

Turbulent flowfields resulting from an oblique jet injecting from a rectangular side-inlet duct into a rectangular main duct with an aspect ratio 3.75 without a forced crossflow are presented in terms of laser-Doppler velocimetry measurements. The main focuses are the effects of the side-jet angle (θ) and side-jet flow rate (Qs) on the mass entrainment upstream of side-jet port and the flowfield in the rectangular duct. The side-inlet angles investigated were 30, 45, and 60 deg and Reynolds numbers based on the air density, rectangular duct height and bulk mean velocity were in the range of 7.1 × 103 to 3.6 × 104 corresponding to Qs values of 1 × 103 to 5 × 103 L/min. The present study suggests the presence of a critical side-injection angle θc (30 ≤ θc ≤ 45 deg) above which a recirculation zone appears in the rectangular duct, whereas below which the recirculation zone is absent. For the more tangential angle (θ = 30 deg), almost as much fluid is entrained into the main duct as was injected from the side jet. The mean flow field in the rectangular duct is found to be a weak function of the Reynolds-number for the range of Qs investigated. In addition, a simple linear correlation between the mass entrainment upstream of the side-jet port and side-injection angle is obtained. Complementary flow visualizations and numerical computations with an algebraic Reynolds stress model were also performed. The discrepancies between measured and computed results are documented.
Hydrodynamic Characteristics of a Mechanical Pectoral Fin
Authors: Kato Naomi
Published: February 2008

This paper describes the use of a mechanical pectoral fin as a new device for maneuvering and stabilizing an underwater vehicle. The mechanical pectoral fin consists of two servo-motors generating a feathering motion and a rowing motion. The hydrodynamic characteristics of the device were analyzed experimentally and theoretically. The mechanical pectoral fin generates a thrust force in a range of phase differences between both motions. The unsteady vortex-lattice method, which takes into account the effects of viscosity, reasonably expresses the unsteady forces acting on the mechanical pectoral fin.
Influence of the Flatness Ratio of an Automotive Torque Converter on Hydrodynamic Performance
Authors: Ejiri E. , Kubo M.
Published: February 2008

Automotive torque converters have recently been designed with an increasingly narrower profile for the purpose of achieving a smaller axial size, which also translates into weight savings. Four torque converters with different flatness ratios were manufactured and tested in order to evaluate the change in their overall performance, including efficiency, stall torque ratio and torque transmission capacity. The experimental results show that the overall performance deteriorates when the flatness ratio is reduced to less than about 0.2. The internal flow characteristics of the torque converters were also investigated by numerical analysis using a CFD code. The computational results indicate that the main cause of this performance deterioration is a reduction in pump efficiency, which is attributed to increases in shock loss in the inlet region, separation loss in the fore half region, and friction loss in the exit region.