bluuebunny/crossref_metadata_2025 huggingface dataset:
A Model for Correlating Flow Boiling Heat Transfer in Augmented Tubes and Compact Evaporators
Authors: Kandlikar S. G.
Published: May 2008

The additive model for the convective and nucleate boiling components originally suggested by Bergles and Rohsenow (1964) for subcooled and low-quality regions was employed in the Kandlikar correlation (1990a) for flow boiling in smooth tubes. It is now extended to augmented tubes and compact evaporators. Two separate factors are introduced in the convective boiling and the nucleate boiling terms to account for the augmentation effects due to the respective mechanisms. The fin efficiency effects in the compact evaporator geometry are included through a reduction in the nucleate boiling component over the fins due to a lower fin surface temperature. The agreement between the model predictions and the data reported in the literature is within the uncertainty bounds of the experimental measurements.
Turbulent Free Convection Heat Transfer to Drag-Reducing Fluids From Arbitrary Geometric Configurations
Authors: Nakayama A. , Shenoy A. V.
Published: May 2008

The problem of turbulent free convection heat transfer from curved surfaces to drag-reducing fluids has been investigated using the Nakayama-Koyama solution methodology. The surface wall temperature is allowed to vary in the streamwise direction in an arbitrary fashion and calculations are carried out for the turbulent free convection about the horizontal circular cylinder and the sphere for the sake of illustration.
Measurements of Velocity and Turbulence in Vertical Axisymmetric Isothermal and Buoyant Jets
Authors: Peterson J. , Bayazitoglu Y.
Published: May 2008

The current study examines the transition region of axisymmetric isothermal and buoyant jets of low Reynolds number, directed vertically upward into a stagnant, unstratified ambient. The region in which measurements were obtained allows examination of two types of transition occurring in the jet: from nozzle exit dominated to fully developed, and from momentum to buoyancy-dominated flow. Isothermal velocity data were acquired using a two-channel laser-Doppler anemometer for Reynolds numbers ranging from 850 to 7405. The buoyant cases studied had Froude numbers ranging from 12 to 6425 and Reynolds numbers from 525 to 6500. In each case data were taken from 5 to 44 nozzle diameters downstream. Curve fit approximations of the data were developed by assuming polynomial similarity profiles for the measured quantities. Each profile was individually curve fit because in the transition region under consideration the flow field is not necessarily similar. Profile constants were then curve fit to determine profile variation as a function of nozzle exit parameters and downstream location. These allow prediction of the downstream velocity flow field and turbulent flow field as a function of the Reynolds number, Froude number, and density ratio at the nozzle exit. Profile width and entrainment increased at low Reynolds number. Axial and radial velocity fluctuations were found to increase at low Reynolds number. The buoyant cases studied were found to have lower velocity fluctuations and significantly lower Reynolds stresses than isothermal cases of similar Reynolds number.
Critical Heat Flux in Horizontal Tube Bundles in Vertical Crossflow of R113
Authors: Leroux K. M. , Jensen M. K.
Published: May 2008

The critical heat flux (CHF) on a single tube in a horizontal bundle subject to an upward crossflow of R113 has been studied in three bundle geometries. Effects of local quality, mass flux, pressure, and bundle geometry on the CHF were investigated. The shapes of the CHF-quality curves display three distinct patterns, which progress from one to another as mass flux increases. At low mass fluxes, the CHF data monotonically decreased with increasing quality. At intermediate mass fluxes with increasing quality, the CHF data initially decreased to a relative minimum, then increased to a relative maximum, and finally began to decrease again as the higher qualities were reached. At high mass fluxes, as quality increased, the CHF rose gradually from the zero quality value to a maximum and then began to decrease. For all mass fluxes, the zero-quality CHF points clustered around an average value, which varied slightly with test section geometry. Mechanisms for the CHF condition are suggested.
Droplet Condensation in Rapidly Decaying Pressure Fields
Authors: Peterson P. F. , Bai R. Y. , Schrock V. E. , Hijikata K.
Published: May 2008

Certain promising schemes for cooling inertial confinement fusion reactors call for highly transient condensation in a rapidly decaying pressure field. After an initial period of condensation on a subcooled droplet, undesirable evaporation begins to occur. Recirculation within the droplet strongly impacts the character of this condensation-evaporation cycle, particularly when the recirculation time constant is of the order of the pressure decay time constant. Recirculation can augment the heat transfer, delay the onset of evaporation, and increase the maximum superheat inside the drop by as much as an order of magnitude. This numerical investigation identifies the most important parameters and physics characterizing transient, high heat flux droplet condensation. The results can be applied to conceptual designs of inertial confinement fusion reactors, where initial temperature differences on the order of 1500 K decay to zero over time spans the order of tens of milliseconds.
Surface Roughness and Its Effects on the Heat Transfer Mechanism in Spray Cooling
Authors: Pais M. R. , Chow L. C. , Mahefkey E. T.
Published: May 2008

In the spray cooling of a heated surface, variations in the surface texture influence the flow field, altering the maximum liquid film thickness, the bubble diameter, vapor entrapment, bubble departure characteristics, and the ability to transfer heat. A new method for determining and designating the surface texture is proposed, and the effects of surface roughness on evaporation/nucleation in the spray cooling flow field studied. A one-dimensional Fourier analysis is applied to determine experimentally the surface profile of a surface polished with emery paper covering a spectrum of grit sizes between 0.3 to 22 μm. Heat transfer measurements for liquid flow rates between 1 to 5 l/h and air flow rates between 0.1 to 0.4 l/s are presented. Maximum heat fluxes of 1200 W/cm2 for the 0.3 μm surface at very low superheats were obtained.
Transient Radiation Properties of a Subgrid Scale Eddy
Authors: Gore J. P. , Jang J. H.
Published: May 2008

Analysis of transient radiative cooling of a strongly radiating turbulent diffusion flame is described. The specific objective is to study the possibility of including realistic radiative cooling effects in transient simulations of turbulent reacting flows. The laminar flamelet concept is used to estimate all scalar properties other than temperature and density. These are obtained from transient evolutions for total enthalpy in conjunction with the state relationships. The equation is numerically solved in the mixture fraction-time space. Results show aflame structure involving a relatively cold soot layer on the fuel-rich side consistent with recent experimental data.
Thermal Resonance Under Frequency Excitations
Authors: Tzou D. Y.
Published: May 2008

The resonance phenomenon of thermal waves is studied in this work. Under the excitation of a body heat source oscillating at a certain frequency, both the temperature and the heat flux waves are found excitable to resonance. The physical conditions necessary for this unusual behavior to occur are obtained in terms of the resonance amplitude and the modal frequency. It has been shown that thermal resonance is a high-mode phenomenon. Only the wave modes with a modal frequency exceeding a critical value of 0.6436f, with f being the critical frequency of the solid medium, may present resonance. This unique behavior is attributed to the partition of the over- and underdamped waves oscillating in time. The critical mode numbers governing (i) the transition from an over- to an underdamped mode, and (ii) the occurrence of thermal resonance phenomenon, are derived analytically. A length parameter, the relaxation distance extended from the concept of relaxation time, is found to dominate these critical mode numbers. Last, the thermal wave speed is related to the resonance frequency, which provides an alternative approach to determining the thermal wave speed in solids.
Thermal Contact Conductance of a Bone-Dry Paper Handsheet/Metal Interface
Authors: Seyed-Yagoobi J. , Ng K. H. , Fletcher L. S.
Published: May 2008

An apparatus was constructed for determination of the thermal contact conductance for a paper handsheet/metal interface and for measurement of the effective thermal conductivity of handsheet samples. Bone-dry Bleached Southern Mixed Kraft hand-sheets with a water retention value of 1.832 were used to study the effect of pressure on thermal contact conductance and to measure the effective thermal conductivity of samples at various sheet density levels. A regression model describing the interface thermal contact conductance as a function of pressure and basis weight was derived. The contact conductance increases with increasing pressure or with decreasing basis weight. At a pressure of 2.3 kPa, the value of the interface contact conductance for the bone-dry samples considered ranges from approximately 97 W/m2K for a sheet of 348.7 g/m2 basis weight to 200 W/m2K for a sheet of 68.0 g/m2 basis weight. For pressures near 300 kPa, these values increase to 146 and 452 W/m2K, respectively. The effective thermal conductivity of the handsheet samples was derived from measured values of overall joint conductance and interface contact conductance. The results indicate that the thermal conductivity of the bone-dry samples increases with increasing sheet density, ranging from 0.14 W/mK to 0.70 W/mK for sheet densities of 90 kg/m3 to 500 kg/m3, respectively, for the samples considered.
Turbulent Heat Transfer Augmentation Using Microscale Disturbances Inside the Viscous Sublayer
Authors: Kozlu H. , Mikic B. B. , Patera A. T.
Published: May 2008

We report here on an experimental study of heat transfer augmentation in turbulent flow. Enhancement strategies employed in this investigation are based on the near-wall mixing processes induced in the sublayer through appropriate wall and near-wall streamwise-periodic disturbances. Experiments are performed in a low-turbulence wind-tunnel with a high-aspect-ratio rectangular channel having either (a) two-dimensional periodic microgrooves on the wall, or (b) two-dimensional microcylinders placed in the immediate vicinity of the wall. It is found that micro-disturbances placed inside the sublayer induce favorable heat-transport augmentation with respect to the smooth-wall case, in that near-analogous momentum and heat transfer behavior are preserved; a roughly commensurate increase in heat and momentum transport is termed favorable in that it leads to a reduction in the pumping power penalty at fixed heat removal rate. The study shows that this favorable performance of microcylinder-equipped channel flows is achieved for microcylinders placed inside y+ ≃20, implying a dependence of the optimal position and size on Reynolds number. For microgrooved channel flows, favorable augmentation is obtained for a wider range of Reynolds numbers; however, optimal enhancement still requires a matching of geometric perturbation with the sublayer scale.
Splattering and Heat Transfer During Impingement of a Turbulent Liquid Jet
Authors: Lienhard J. H. , Liu X. , Gabour L. A.
Published: May 2008

Splattering and heat transfer due to impingement of an unsubmerged, fully turbulent liquid jet is investigated experimentally and analytically. Heat transfer measurements were made along a uniformly heated surface onto which a jet impacted, and a Phase Doppler Particle Analyzer was used to measure the size, velocity, and concentration of the droplets splattered after impingement. Splattering is found to occur in proportion to the magnitude of surface disturbances to the incoming jet, and it is observed to occur only within a certain radial range, rather than along the entire film surface. A nondimensional group developed from inviscid capillary disturbance analysis of the circular jet successfully scales the splattering data, yielding predictive results for the onset of splattering and for the mass splattered. A momentum integral analysis incorporating the splattering results is used to formulate a prediction of local Nusselt number. Both the prediction and the experimental data reveal that the Nusselt number is enhanced for radial locations immediately following splattering, but falls below the nonsplattering Nusselt number at larger radii. The turbulent heat transfer enhancement upstream of splattering is also characterized.
Heat Transfer in Crossflow Over Cylinders Between Two Parallel Plates
Authors: Kundu D. , Haji-Sheikh A. , Lou D. Y. S.
Published: May 2008

The heat transfer coefficient and pressure drop are measured for laminar and turbulent incompressible flow over an in-line cylinder array (eight copper tubes) between two parallel plates. Data are given using two different aspect ratios for the intermediate range of the Reynolds number between 220 to 2800. A criterion is defined for flow transition from laminar to turbulent. The pressure and heat transfer data are compared to numerically computed data obtained for laminar flow and the results exhibit reasonably good agreement.
Laminar Heat Transfer and Fluid Flow in the Entrance Region of a Rotating Duct With Rectangular Cross Section: The Effect of Aspect Ratio
Authors: Jen T.-C. , Lavine A. S.
Published: May 2008

A numerical calculation has been performed for the simultaneously developing laminar velocity and temperature fields in the entrance region of an isothermal rectangular duct rotating about an axis perpendicular to the duct axis. The present paper particularly addresses the effect of the aspect ratio of the rectangular duct. The vorticity-velocity method with the power law scheme is employed to solve the governing equations for the flow and heat transfer. It is seen that pairs of counterrotating vortices appear in the cross section of the duct. The number of vortex pairs depends on the aspect ratio of the duct. Under some circumstances, the number of vortex pairs varies with axial position. The numerically calculated stability boundary points for small aspect ratio show excellent agreement with the neutral stability curve, which was obtained by linear stability analysis. Friction factor and Nusselt number are presented as functions of axial position, and it is seen that they are strongly affected by the vortices. Comparisons with existing theoretical and experimental results are also presented.
Evaluation of Total Emittance of an Isothermal Nongray Absorbing, Scattering Gas-Particle Mixture Based on the Concept of Absorption Mean Beam Length
Authors: Yuen W. W. , Ma A.
Published: May 2008

A general methodology to evaluate the total emittance of an isothermal, nongray, isotropically scattering particle-gas mixture is illustrated. Based on the concept of absorption mean beam length (AMBL), the methodology is demonstrated to be computationally efficient and accurate. As an illustration, the total emittance of a slab containing carbon particles and CO2 is evaluated. The nongray extinction coefficient and scattering albedo of carbon particles are calculated based on Mie theory and the available index of refraction data. The narrow-band fixed-line-spacing model (Edwards et al., 1967) is used to characterize the nongray spectral absorption coefficient of CO2. Numerical data show that the combined nongray and scattering effects are quite significant. For particles with moderate and large radius (say, ≥1 μm), ignoring the effect of scattering can lead to error in the prediction of total emittance by more than 20 percent. The no-scattering results also yield incorrect qualitative behavior of the total emittance in terms of its dependence on the mixture temperature and particle concentration. The accuracy of many of the existing predictions of total emittance of gas-particle mixtures that ignore the scattering effect is thus highly uncertain.
Analysis of Rewetting for Surface Tension Induced Flow
Authors: Peng X. F. , Peterson G. P.
Published: May 2008

An analytical investigation was conducted to determine the rewetting characteristics of thin, surface tension driven liquid films over heated plates as a function of the fluid properties, the film thickness, and the applied heat flux. Analytical expressions for the maximum sustainable heat flux and the rewetting velocity were developed for both flat and grooved plates and were compared with data from previous investigations. The results indicated good agreement for low film velocities; however, at high velocities the experimental data deviated significantly from the theoretical predictions. It was hypothesized that this deviation was due to the presence of liquid sputtering near the liquid front. To compensate for this liquid sputtering, the expressions for maximum sustainable heat flux and rewetting velocity were modified using an empirical correction factor developed from the data of previous thin film thickness investigations. The resulting modified expressions were found to compare very favorably with available experimental data over a large range of flow conditions and velocities.
An Experimental Study of Heat and Mass Transfer During Drying of Packed Beds
Authors: Lee W. C. , Plumb O. A. , Gong L.
Published: May 2008

An experimental study has been conducted to provide a data base for drying packed beds of granular, nonhygroscopic materials. Experimental results for drying rate, saturation distribution, temperature distribution, and surface saturation are reported for drying glass beads under carefully documented drying conditions. Capillary pressure for both imbibition and drainage was measured for the glass beads, whose size ranged from 65 μm to 450 μm. The drying results demonstrate that, contrary to available model predictions, porous materials do not necessarily exhibit saturation gradients that always increase with distance from the drying surface. Under certain conditions the capillary potential is sufficient to create an internal drying front. The measurements of surface saturation are the first to be reported. They are utilized to speculate on the reasons for the failure of drying models to compare well with experiment without adjusting the convective heat or mass transfer coefficients.
A Three-Dimensional Analysis of Particle Deposition for the Modified Chemical Vapor Deposition (MCVD) Process
Authors: Lin Y. T. , Choi M. , Greif R.
Published: May 2008

A study has been made of the deposition of particles that occurs during the modified chemical vapor deposition (MCVD) process. The three-dimensional conservation equations of mass, momentum, and energy have been solved numerically for forced flow, including the effects of buoyancy and variable properties in a heated, rotating tube. The motion of the particles that are formed is determined from the combined effects resulting from thermophoresis and the forced and secondary flows. The effects of torch speed, rotational speed, inlet flow rate, tube radius, and maximum surface temperature on deposition are studied. In a horizontal tube, buoyancy results in circumferentially nonuniform temperature and velocity fields and particle deposition. The effect of tube rotation greatly reduces the nonuniformity of particle deposition in the circumferential direction. The process is chemical-reaction limited at larger flow rates and particle-transport limited at smaller flow rates. The vertical tube geometry has also been studied because its symmetric configuration results in uniform particle deposition in the circumferential direction. The “upward” flow condition results in a large overall deposition efficiency, but this is also accompanied by a large “tapered entry length.”
Flow and Morphological Conditions Associated With Unidirectional Solidification of Aqueous Ammonium Chloride
Authors: Magirl C. S. , Incropera F. P.
Published: May 2008

Using a 27 percent aqueous ammonium chloride solution as a transparent analog, shadowgraph and dye injection techniques have been employed to observe flow and morphological conditions associated with unidirectional solidification (UDS) from below. Dendritic crystals, which initially form at the cold surface, reject lighter, solute deficient fluid, and the attendant instability is manifested by finger-type double-diffusive convection phenomena. As a two-phase (solid/liquid), or mushy, region grows from the bottom surface, vertical channels develop in the mushy region, and solutal plumes that emanate from the channels are characterized primarily by an ascending, oscillatory motion and secondarily by wisps of fluid, which detach and descend from bends in the plumes. In time, double-diffusive convection layers also form in the melt. From a numerical simulation of the process, it is concluded that the channels originate from perturbations at the liquid interface, which cause localized remelting and create conditions conducive to development of the channels.
Spaces Filled With Fluid and Fibers Coated With a Phase-Change Material
Authors: Lim J. S. , Fowler A. J. , Bejan A.
Published: May 2008

Thermally active fabrics and insulations consist of fibers coated with phase-change materials and surrounded by air. This paper constructs a homogeneous porous medium model for melting and solidification in spaces filled with thermally active fibers. Three basic configurations are analyzed numerically: one-dimensional conduction, one-dimensional convection, and two-dimensional natural convection due to heating or cooling from the side. Concrete means for calculating the time of complete melting or solidification are reported as functions of the important dimensionless groups that govern each configuration. It is shown that the change of phase in a space filled with coated fibers and air differs fundamentally from the corresponding phenomena in a porous medium saturated with a phase-change material.
The 1991 Max Jakob Memorial Award Lecture: Image-Forming Optical Techniques in Heat Transfer: Revival by Computer-Aided Data Processing
Authors: Mayinger F.
Published: May 2008

Increasing possibilities of computer-aided data processing have fostered a revival of image-forming optical techniques in heat and mass transfer as well as in fluid dynamics. Optical measuring techniques can provide comprehensive and detailed information on the formation of phase interfaces, particle movement, or the size distribution of droplet swarms. Holographic interferograms contain full information, not only about boundary layers restricting transport processes, but also on local coefficients of heat and mass transfer. Laser-induced fluorescence promotes a better understanding of combustion processes by conveying insights into the concentration and the temperature in and around a flame. For describing complicated phenomena in fluid dynamics or in heat transfer by computer programs, global experimental information is not sufficient. Optical techniques provide local data without disturbing the process and with a high temporal resolution. By using the results of optical measuring techniques, it is possible to improve computer programs that describe physical processes. Optical techniques are also very sensitive touchstones for checking the quality of such programs.
Thermal Response of Rolling Components Under Mixed Boundary Conditions: An Analytical Approach
Authors: Ulysse P. , Khonsari M. M.
Published: May 2008

An analytical solution for the steady-state temperature distribution in a cylinder undergoing uniform heating and nonuniform cooling is presented. The method of solution is a Fourier integral transform technique. The analysis shows that the Neumann series resulting from an integral equation can be well represented by a first-order approximation when the Peclet number is large. Furthermore, it is shown that the ratio of the Biot number to the square root of the Peclet number of the cooling zones is found to play an important role in governing the thermal response of the cylinder surface. The predicted results for the circumferential temperature distribution are compared to published experimental measurements for hot rolling and also existing analytical solutions for special cases. The agreement is found to be very good. By an appropriate superposition technique, the analysis presented may be easily extended to various heat sources and convective cooling zones at different locations of the cylinder surface.
Forced Convection Heat Transfer From Microstructures
Authors: Ma S. W. , Gerner F. M.
Published: May 2008

For many microstructures, which utilize forced convection cooling, the average thickness of the thermal boundary layer is of the same order as the length of the heated element. For these cases, thermal boundary layer theory is invalid. The elliptic energy equation for steady, two-dimensional incompressible flow over a finite flat plate with insulated starting and ending lengths is analyzed utilizing matched asymptotic expansions. A conventional Blasius technique transforms the energy equation into an elliptic-to-parabolic equation. A new technique is used that treats the boundary layer solution as the outer expansion of the elliptic-to-parabolic equation. The inner expansion, or leading-edge equation, is found by stretching to independent variables simultaneously. Trailing-edge effects are considered using superposition methods. A first-order composite formula is constructed based on the outer and inner expansions, which is uniformly valid over the entire surface of the plate. With the aid of statistics, a correlation is developed for the average Nusselt number Nu¯l=0.6626Pr1/3Rex0+l1/21−x0x0+l3/42/31+0.3981(x0/l)0.5987Pr0.3068Rex00.4675for0.5≤Pr≤100,x0/l≤50,andRex0≥100 where x0 and l represent the lengths of the insulated starting section and the heated element, respectively. This correlation is accurate to within 2 percent as compared with the entire composite solution.
Heat Transfer and Pressure Drop Correlations for Twisted-Tape Inserts in Isothermal Tubes: Part II—Transition and Turbulent Flows
Authors: Manglik R. M. , Bergles A. E.
Published: May 2008

Thermal-hydraulic design correlations are developed to predict isothermal f and Nu for in-tube, turbulent flows with twisted-tape inserts. Experimental data taken for water and ethylene glycol, with y = 3.0, 4.5, and 6.0, are analyzed, and various mechanisms attributed to twisted tapes are identified. Tube blockage and tape-induced vortex mixing are the dominant phenomena that result in increased heat transfer and pressure drop; for loose- to snug-fitting tapes, the fin effects are insignificant. The limiting case of a straight tape insert correlates with the hydraulic-diameter-based smooth tube equation. Tape twist effects are thus isolated by normalizing the data with the asymptotic predictions for y = ∞, and the swirl effects are found to correlate with Re and l/y. The validity of the final correlations is verified by comparing the predictions with previously published data, which include both gases and liquids, under heating and cooling conditions and a wide range of tape geometries, thereby establishing a very generalized applicability. Finally, correlations for laminar (presented in the companion Part I paper) and turbulent flows are combined into single, continuous equations. For isothermal f, the correlation describes most of the available data for laminar-transition-turbulent flows within ±10 percent. For Nu, however, a family of curves is needed due to the nonunique nature of laminar-turbulent transition.
Influence of Unsteady Wake on Heat Transfer Coefficient From a Gas Turbine Blade
Authors: Han J.-C. , Zhang L. , Ou S.
Published: May 2008

The effect of unsteady wake on surface heat transfer coefficients of a gas turbine blade was experimentally determined using a spoked wheel type wake generator. The experiments were performed with a five-airfoil linear cascade in a low-speed wind tunnel facility. The cascade inlet Reynolds number based on the blade chord was varied from 1 to 3 × 105. The wake Strouhal number was varied between 0 and 1.6 by changing the rotating wake passing frequency (rod speed and rod number), rod diameter, and cascade inlet velocity. A hot-wire anemometer system was located at the cascade inlet to detect the instantaneous velocity, phase-averaged mean velocity, and turbulence intensity induced by the passing wake. A thin foil thermocouple instrumented blade was used to determine the surface heat transfer coefficients. The results show that the unsteady passing wake promotes earlier and broader boundary layer transition and causes much higher heat transfer coefficients on the suction surface, whereas the passing wake also significantly enhances heat transfer coefficients on the pressure surface. The blade heat transfer coefficients for a given Reynolds number flow increase with the wake Strouhal number by increasing the rod speed, rod number, or rod diameter. For a given wake passing frequency and rod diameter, the blade heat transfer coefficients decrease with decreasing Reynolds number, although the corresponding wake Strouhal number is increased. The results suggest that both the Reynolds and Strouhal numbers are important parameters in determining the blade heat transfer coefficients in unsteady wake flow conditions.
Prediction and Measurement of the Thermal Conductivity of Amorphous Dielectric Layers
Authors: Goodson K. E. , Flik M. I. , Su L. T. , Antoniadis D. A.
Published: May 2008

Thermal conduction in amorphous dielectric layers affects the performance and reliability of electronic circuits. This work analyzes the influence of boundary scattering on the effective thermal conductivity for conduction normal to amorphous silicon dioxide layers, kn,eff. At 10 K, the predictions agree well with previously reported data for deposited layers, which show a strong reduction of kn,eff compared to the bulk conductivity, kbulk. A steady-state technique measures kn,eff near room temperature of silicon dioxide layers fabricated using oxygen-ion implantation (SIMOX). The predictions and the SIMOX data, which agree closely with kbulk, show that boundary scattering is not important at room temperature. Lower than bulk conductivities of silicon dioxide layers measured elsewhere near room temperature must be caused by interfacial layers or differences in microstructure or stoichiometry.
Experimental Study on Convective Heat Transfer for Turbulent Flow in a Square Duct With a Ribbed Rough Wall (Characteristics of Mean Temperature Field)
Authors: Hirota M. , Fujita H. , Yokosawa H.
Published: May 2008

This paper presents experimental results concerning a time-mean temperature field obtained in forced convection heat transfer for a turbulent flow through a square duct with a ribbed rough bottom wall. The secondary flow pattern in the duct is reflected in the distribution of the local Nusselt number, the values of which on the smooth walls of the rough duct are 1.71~1.97 times those of the smooth duct. In the upper half cross section near the upper smooth wall opposite the bottom ribbed rough wall, the profile of the mean temperature distribution is similar to that of the primary flow velocity distribution, and the validity of the temperature inner law was confirmed. However, in the lower half cross section near the bottom ribbed rough wall, the dissimilarity between the mean velocity and the mean temperature fields becomes pronounced, and the inner law is not valid for mean temperature distributions. The mechanism of the heat transfer near the ribbed rough wall was examined based on the transport equations of turbulent shear stress and turbulent heat flux.
Film-Cooling From Holes With Compound Angle Orientations: Part 1—Results Downstream of Two Staggered Rows of Holes With 3d Spanwise Spacing
Authors: Ligrani P. M. , Wigle J. M. , Ciriello S. , Jackson S. M.
Published: May 2008

Experimental results are presented that describe the development and structure of flow downstream of two staggered rows of film-cooling holes with compound angle orientations. With this configuration, holes are spaced 3d apart in the spanwise direction, inclined at 35 deg with respect to the test surface when projected into the streamwise/normal plane, and inclined at 30 deg with respect to the test surface when projected into the spanwise/normal plane. Results are presented for an injectant to free-stream density ratio near 1.0, and injection blowing ratios from 0.5 to 1.50. Comparisons are made with measurements from two other configurations to determine: (1) the effects of hole angle orientation for constant spanwise hole spacing, and (2) the effects of spanwise hole spacing when the hole angle orientation is maintained constant. Results from the first comparison show that the compound angle injection configuration provides significantly improved film-cooling protection compared to a simple angle configuration for the same spanwise hole spacing, normalized streamwise location x/d, and blowing ratio m, for x/d<60. At x/d>60, spanwise-averaged adiabatic effectiveness data downstream of the two configurations generally cover about the same range. Results from the second comparison show that spanwise-averaged effectiveness values are 25 to 40 percent higher when 3d spanwise hole spacing is employed compared to 3.9d spanwise hole spacing for the same m and x/d, for x/d<40. At x/d>40, differences between the two configurations range from 12 to 30 percent. Results from all configurations studied show that spanwise-averaged iso-energetic Stanton number ratios cover approximately the same range of values and show roughly the same trends, ranging between 1.0 and 1.25. In particular, Stf/St0 values increase with m at each x/d, and show little variation with x/d for each value of m tested.
Heat Exchange Effectiveness and Pressure Drop for Air Flow Through Perforated Plates With and Without Crosswind
Authors: Kutscher C. F.
Published: May 2008

Low-porosity perforated plates are being used as absorbers for heating ambient air in a new type of unglazed solar collector. This paper investigates the convective heat transfer effectiveness for low-speed air flow through thin, isothermal perforated plates with and without a crosswind on the upstream face. The objective of this work is to provide information that will allow designers to optimize hole size and spacing. In order to obtain performance data, a wind tunnel and small lamp array were designed and built. Experimental data were taken for a range of plate porosities from 0.1 to 5 percent, hole Reynolds numbers from 100 to 2000, and wind speeds from 0 to 4 m/s. Correlations were developed for heat exchange effectiveness and also for pressure drop. Infrared thermography was used to visualize the heat transfer taking place at the surface.
Structure of Round, Fully Developed, Buoyant Turbulent Plumes
Authors: Dai Z. , Tseng L.-K. , Faeth G. M.
Published: May 2008

An experimental study of the structure of round buoyant turbulent plumes was carried out, emphasizing conditions in the fully developed (self-preserving) portion of the flow. Plume conditions were simulated using dense gas sources (carbon) dioxide and sulfur hexafluoride) in a still air environment. Mean and fluctuating mixture fraction properties were measured using single-and two-point laser-induced iodine fluorescence. The present measurements extended farther from the source (up to 151 source diameters) than most earlier measurements (up to 62 source diameters) and indicated that self-preserving turbulent plumes are narrower, with larger mean and fluctuating mixture fractions (when appropriately scaled) near the axis, than previously thought. Other mixture fraction measurements reported include probability density functions, temporal power spectra, radial spatial correlations and temporal and spatial integral scales.
Complete Transient Two-Dimensional Analysis of Two-Phase Closed Thermosyphons Including the Falling Condensate Film
Authors: Harley C. , Faghri A.
Published: May 2008

A transient two-dimensional thermosyphon model is presented that accounts for conjugate heat transfer through the wall and the falling condensate film. The complete transient two-dimensional conservation equations are solved for the vapor flow and pipe wall, and the liquid film is modeled using a quasi-steady Nusselt-type solution. The model is verified by comparison with existing experimental data for a low-temperature thermosyphon with good agreement. A typical high-temperature thermosyphon was then simulated to examine the effects of vapor compressibility and conjugate heat transfer.
Influence of Mainstream Turbulence on Heat Transfer Coefficients From a Gas Turbine Blade
Authors: Zhang L. , Han J.-C.
Published: May 2008

The influence of mainstream turbulence on surface heat transfer coefficients of a gas turbine blade was studied. A five-blade linear cascade in a low-speed wind tunnel facility was used in the experiments. The mainstream Reynolds numbers were 100,000, 200,000, and 300,000 based on the cascade inlet velocity and blade chord length. The grid-generated turbulence intensities at the cascade inlet were varied between 2.8 and 17 percent. A hot-wire anemometer system measured turbulence intensities, mean and time-dependent velocities at the cascade inlet, outlet, and several locations in the middle of the flow passage. A thin-foil thermocouple instrumented blade determined the surface heat transfer coefficients. The results show that the mainstream turbulence promotes earlier and broader boundary layer transition, causes higher heat transfer coefficients on the suction surface, and significantly enhances the heat transfer coefficient on the pressure surface. The onset of transition on the suction surface boundary layer moves forward with increased mainstream turbulence intensity and Reynolds number. The heat transfer coefficient augmentations and peak values on the suction and pressure surfaces are affected by the mainstream turbulence and Reynolds number.
Thermal and Mechanical Effects in the Spreading of a Liquid Film Due to a Change in the Apparent Finite Contact Angle
Authors: Wayner P. C.
Published: May 2008

A new physical model for the spreading dynamics of fluids with an apparent finite contact angle on solid substrates is presented. The model is based on the premise that both interfacial intermolecular forces and temperature control change-of-phase heat transfer and (therefore) motion in the moving contact line region. Classical change-of-phase kinetics and interfacial concepts like the Kelvin–Clapeyron, Young–Dupre, and augmented Young–Laplace equations are used to compare the effects of stress (change in apparent dynamic contact angle) and temperature (superheat). Explicit equations are obtained for the velocity, heat flux, and superheat in the contact line region as a function of the change in the apparent contact angle. Comparisons with experimental data demonstrate that the resulting interfacial model of evaporation/condensation not only describes the “apparently isothermal” contact line movement in these systems at 20°C but also describes the substrate superheat at the critical heat flux.
Review: The Use of Kirchhoff’s Method in Computational Aeroacoustics
Authors: Lyrintzis A. S.
Published: May 2008

A comprehensive review of the use of Kirchhoff’s method in computational aeroacoustics is given. Kirchhoff’s integral formulation allows radiating sound to be evaluated based on quantities on an arbitrary control surface S if the wave equation is assumed outside. The control surface S is assumed to include all the nonlinear flow effects and noise sources. Thus only surface integrals are needed for the calculation of the far-field sound. A numerical CFD method can be used for the evaluation of the flow-field solution in the near-field and thus on surface S. Kirchhoff’s integral formulation has been extended to an arbitrary, moving, deformable piecewise-continuous surface. The available Kirchhoff formulations are reviewed and various aeroacoustic applications are given. The relative merits of Kirchhoff’s method are also discussed.
Investigation of Fully Developed Turbulent Flow in a Straight Duct With Large Eddy Simulation
Authors: Su M. D. , Friedrich R.
Published: May 2008

Large eddy simulations have been performed in straight ducts with square cross section at a global Reynolds number of 49,000 in order to predict the complicated mean and instantaneous flow involving turbulence-driven secondary motion. Isotropic grid systems were used with spatial resolutions of 256 * 642. The secondary flow not only turned out to develop extremely slowly from its initial conditions but also to require fairly high resolution. The obtained statistical results are compared with measurements. These results show that the large eddy simulation (LES) is a powerful approach to simulate the complex turbulence flow with high Reynolds number. Streaklines of fluid particles in the duct show the secondary flow clearly. The database obtained with LES is used to examine a statistical turbulence model and describe the turbulent vortex structure in the fully developed turbulent flow in a straight duct.
Hydrodynamics and Erosion Modeling of Fluidized Bed Combustors
Authors: Li Chunyuan , Zakkay Victor
Published: May 2008

Hydrodynamics and erosion of Fluidized Bed Combustors (FBCs) containing tubes have been analyzed through the use of the hydrodynamic model, which has been developed at New York University, utilizing a CRAY Y-MP/832 supercomputer and a CONVEX C210 minisupercomputer. Particle velocity fields and bed dynamics including bubble formation and motion and bed expansion and collapse were analyzed for high pressure cold and hot beds. The cold beds were studied by (a) optimizing tube models for simulating a circular tube, (b) varying the distance from distributor plate to the tubes, (c) changing tube size, and (d) using vertical tubes. Atmospheric and pressurized fluidized bed combustors were also simulated and compared to each other for bubble size, bubble frequencies, particle jets, and tube erosion. In particular, the computation revealed bubble coalescence which causes high particle jets. Threshold velocity has been analyzed utilizing Hertz contact theory and the maximum shear theory (Tresca). Tube erosion was predicted utilizing the Finnie erosion model and the modified Finnie model, which considers the threshold velocity and compared to each other. Computed results show that the distance from distributor to the tubes, tube size, tube orientation and operating pressure have significant effect on bed dynamics and tube erosion. The threshold velocity is found to significantly influence the prediction of erosion.
Flow of Newtonian and Non-Newtonian Fluids in a Concentric Annulus With Rotation of the Inner Cylinder
Authors: Nouri J. M. , Whitelaw J. H.
Published: May 2008

Mean velocity and the corresponding Reynolds shear stresses of Newtonian and non-Newtonian fluids have been measured in a fully developed concentric flow with a diameter ratio of 0.5 and at a inner cylinder rotational speed of 300 rpm. With the Newtonian fluid in laminar flow the effects of the inner shaft rotation were a uniform increase in the drag coefficient by about 28 percent, a flatter and less skewed axial mean velocity and a swirl profile with a narrow boundary close to the inner wall with a thickness of about 22 percent of the gap between the pipes. These effects reduced gradually with bulk flow Reynolds number so that, in the turbulent flow region with a Rossby number of 10, the drag coefficient and profiles of axial mean velocity with and without rotation were similar. The intensity of the turbulence quantities was enhanced by rotation particularly close to the inner wall at a Reynolds number of 9,000 and was similar to that of the nonrotating flow at the higher Reynolds number. The effects of the rotation with the 0.2 percent CMC solution were similar to those of the Newtonian fluids but smaller in magnitude since the Rossby number with the CMC solution is considerably higher for a similar Reynolds number. Comparison between the results of the Newtonian and non-Newtonian fluids with rotation at a Reynolds number of 9000 showed similar features to those of nonrotating flows with an extension of non-turbulent flow, a drag reduction of up to 67 percent, and suppression of all fluctuation velocities compared with Newtonian values particularly the cross-flow components. The results also showed that the swirl velocity profiles of both fluids were the same at a similar Rossby number.
Discrete Vortex Simulation of Pulsating Flow Behind a Normal Plate
Authors: Sung Hyung Jin , Kim Young Nam , Hyun Jae Min
Published: May 2008

A numerical study is made of the separated flow behind a flat plate. The plate is placed normal to the direction of the approach flow. The oncoming freestream velocity contains a pulsating part, U∞ = U0(1 + A0cosfpt). The temporal behavior of vortex shedding patterns is scrutinized over broad ranges of the two externally specified parameters, i.e., the pulsation amplitude (A0≤ 0.6), and the dimensionless pulsation frequency, (fp≤0.32). A version of the discrete vortex method is utilized. The variable-position nascent vortex technique is applied, and it proves to be adequate for pulsating approach flows. The numerical results clearly capture the existence of lock-on when fp exceeds a threshold value. The modulation of vorticity shedding is also detected when fp is reasonably low. The influence of A0 on the flow characteristics is examined in detail. As A0 increases to a moderate value (e.g., A0≤0.6), an appreciable broadening is seen of the range of fp for which lock-on occurs. Based on the numerical results, three characteristic flow modes in the wakes are identified. These findings are qualitatively consistent with the existing flow-visualization studies for a cylinder.
Effect of Surface Deformations on Contact Conductance
Authors: Williamson M. , Majumdar A.
Published: May 2008

This study experimentally investigates the influence of surface deformations on contact conductance when two dissimilar metals are brought into contact. Most relations between the contact conductance and the load use the surface hardness to characterize surface deformations. This inherently assumes that deformations are predominantly plastic. To check the validity of this assumption, five tests were conducted in the contact pressure range of 30 kPa to 4 MPa, with sample combinations of (I) smooth aluminum-rough stainless steel, (II) rough aluminum-smooth stainless steel, (III) rough copper-smooth stainless steel, (IV) smooth copper-rough stainless steel, and (V) smooth aluminum-smooth stainless steel. The experimental results of tests I, II, and IV indicate that the conductance of the first load-unload cycle showed hysteresis, suggesting that the plastic deformation was significant. However, for subsequent load cycles, no conductance hysteresis was observed, implying that elastic deformation was predominant. In contrast, no conductance hysteresis was observed for all load-unload cycles of tests III and V. Therefore, the surface deformation for this combination was always predominantly elastic. In practical applications where plastic deformation is significant for the first loading, mechanical vibrations can produce oscillating loads, which can finally lead to predominance of elastic deformation. Comparison of the results of tests II and V show that even though plastic deformation was significant for the first loading of a rough aluminum surface, elastic deformation was always predominant for the smoother aluminum surface