bluuebunny/crossref_metadata_2025 huggingface dataset:
Onset of Convection in a Porous Medium With Internal Heat Generation
Authors: Gasser R. D. , Kazimi M. S.
Published: November 2010

The conditions leading to onset of thermal convection in a horizontal porous layer are determined analytically using the method of linear stability of small disturbances. The lower boundary is treated as a rigid surface and the upper boundary as a free surface. The critical internal and external Rayleigh numbers are determined for both stabilizing and destabilizing boundary temperatures. The predicted critical external Rayleigh number in the limit of no heat generation is in agreement with the critical number predicted for a porous medium heated from below.
Transient Behavior of a Solid Sensible Heat Thermal Storage Exchanger
Authors: Szego J. , Schmidt F. W.
Published: November 2010

The transient response characteristics of a solid sensible heat storage exchanger which interacts with two energy transporting fluids are presented. The storage unit is composed of a series of large aspect ratio rectangular channels for the fluids, separated by slabs of the heat storage material. The hot and cold fluids flow in counter current fashion, in alternate channels so that each slab of storage material is in contact with both fluids. The entire system is considered to be initially in equilibrium at a uniform temperature, a step change in the inlet temperature of one of the fluids is imposed, and the thermal response of the unit is predicted until steady state conditions are reached. The response of the storage exchanger to an arbitrary time variation of one of the fluids’ inlet temperature may be obtained using superposition.
Influence of Pressure on Film Boiling Heat Transfer
Authors: Hesse G. , Sparrow E. M. , Goldstein R. J.
Published: November 2010

Experiments on film boiling of carbon dioxide were performed covering the range of pressures from the triple point to the critical point. Measurements were also made at supercritical pressures. Three different heating wire sizes were employed with diameters of 0.0508, 0.1, and 0.4 mm. The boiling curves, plotted in terms of heat flux and temperature difference, were found to be pressure dependent, with a more marked dependence for smaller diameter heating wires. The role of pressure level was exhibited in greater detail in a presentation in which the heat transfer coefficient is plotted against pressure at fixed values of heat flux. The most rapid variations of the heat transfer coefficient with pressure occur in the neighborhoods of the triple and critical points, with relatively gradual variations in evidence in the intermediate range of pressures. The curves of heat transfer coefficient versus pressure take on minimum values at the triple and critical points. Nusselt numbers evaluated from the experimental data agree satisfactorily with available predictive equations. Photographs of the vapor separation patterns revealed that with increasing pressure, the bubble columns which break away from the vapor film successively evolve into vapor columns and vapor sheets. As the critical pressure is approached, the height of the sheet diminishes.
Natural Convection in Enclosed Spaces—A Review of Application to Solar Energy Collection
Authors: Buchberg H. , Catton I. , Edwards D. K.
Published: November 2010

A useful solar-thermal converter requires effective control of heat losses from the hot absorber to the cooler surroundings. Based upon the theory and some experimental measurements it is shown that the spacing between the tilted hot solar absorber and successive glass covers should be in the range 4–8 cm to assure minimum gap conductance. Poor choice of spacing can significantly affect thermal conversion efficiency, particularly when the efficiency is low or when selective black absorbers are used. Recommended data for gap Nusselt number are presented as a function of the Rayleigh number for the high aspect ratios of interest in solar collector designs. It is also shown that a rectangular cell structure placed over a solar absorber is an effective device to suppress natural convection, if designed with the proper cell spacing d, height to spacing ratio L/d and width to spacing ratio W/d needed to give a cell Rayleigh number less than the critical value.
The Effect of Heater Size, Location, Aspect Ratio, and Boundary Conditions on Two-Dimensional, Laminar, Natural Convection in Rectangular Channels
Authors: Chu H. H.-S. , Churchill S. W. , Patterson C. V. S.
Published: November 2010

The effect of localized heating in rectangular channels was studied by solving the partial differential equations for the conservation of mass, momentum, and energy numerically using an unsteady state formulation and the alternating-direction-implicit method. The heating element was a long, horizontal, isothermal strip located in one, otherwise-insulated vertical wall. The opposing wall was maintained at a lower uniform temperature and the upper and lower surfaces were insulated or maintained at the lower temperature. Computations were carried out for Pr = 0.7, 0 ≤ Ra ≤ 105, a complete range of heater widths and locations and a wide range of aspect ratios. Flow visualization studies and comparison with prior computed results for a limiting case confirm the validity of the computed values. The computed rates of heat transfer and circulation provide guidance for locating heaters or coolers.
Augmentation of Laminar Flow Heat Transfer in Tubes by Means of Twisted-Tape Inserts
Authors: Hong S. W. , Bergles A. E.
Published: November 2010

Heat transfer coefficients for laminar flow of water and ethylene glycol in an electrically heated metal tube with two twisted-tape inserts were determined experimentally. The Nusselt number for fully developed flow was found to be a function of tape twist ratio, Reynolds number, and Prandtl number. These Nusselt numbers were as much as nine times the empty tube constant property values. The correlation of these data is in fair agreement with the only available analytical predictions. The friction factor is affected by tape twist only at high Reynolds numbers, in accordance with analytical predictions. The performance of these augmented tubes is compared with that of empty tubes under similar heating conditions.
Numerical Study of the Velocity and Temperature Fields in a Flow-Through Reservoir
Authors: Oberkampf W. L. , Crow L. I.
Published: November 2010

The fluid dynamic and temperature fields in a reservoir are numerically simulated by means of a finite difference procedure. The flow in the rectangular reservoir is assumed to be two-dimensional in a vertical plane. Inflow is allowed at the surface on one end of the reservoir and outflow occurs at any depth on the opposite end. The reservoir inflow is set at a given temperature and velocity so as to simulate the thermal discharge from a power generating facility. At the surface of the reservoir wind shear, short wave and long wave radiation, evaporation, and convective heat transfer are taken into account. The effects of inflow/outflow, wind shear, and heat transfer on the reservoir are discussed.
Thermal Instabilities in Discharging Gas Reservoirs
Authors: Johnston S. C. , Dwyer H. A.
Published: November 2010

The flow structure in a discharging gas reservoir has been studied using schlieren cinematography. Effects due to discharge orientation, gas species, and vessel wall vibration have been observed. Initially, the internal flow structure has been found to be controlled by the mass sink, with streamlines going in approximate straight lines from the vessel wall to the exit orifice. At later time, free convection dominates and recirculation patterns are established. Discharge orientation can have a dramatic effect on the free convection flow field as well as on the growth and stability of the wall thermal diffusion layer. A jet-like structure (initiated at very early time and believed to be caused by wave effects) was observed to extend from the vessel wall diametrically opposite the exit orifice. When the discharge was parallel to but in the opposite direction of the gravitational vector, this jet-like structure was found to cause an instability in the lower wall diffusion layer. Vessel wall vibration resulted in a standing cellular acoustic wave pattern in the gas which, depending on discharge orientation, caused a violent instability. A model for temperature variation in the thermal diffusion layer is discussed and a numerical solution is given. Results of the predicted thermal diffusion layer histories are compared to data. A discussion of vessel orientation effects on critical Rayleigh number is given.
Active Sites in Boiling
Authors: Singh A. , Mikic B. B. , Rohsenow W. M.
Published: November 2010

Effects of cavity size, shape, and their population on nucleation characteristics of a surface were investigated. A theoretical model has been developed for the stability of a cylindrical cavity in boiling considering the wetting characteristics of the fluid and the transient inertial, viscous, and heat transfer effects. For cavities having small depth/diameter ratio and boiling with organics (low contact angle) the model predicts higher superheats than those predicted by the static equilibrium criterion, and as depth/diameter ratio becomes large, the static criterion successfully predicts the required superheat. The predictions of the model are consistent with the experimental observations made on the natural and artificial cavities made by laser. A qualitative theoretical approach has been presented to predict the population of active sites at different superheats for a given fluid and surface.
A Numerical Solution of Entrance Region Heat Transfer in Plane Couette Flow
Authors: El-Ariny A. S. , Aziz A.
Published: November 2010

A numerical procedure has been developed to solve the problem of entrance region heat transfer in steady, plane Couette flow of an incompressible viscous fluid. The formulation includes the effects of additional pressure gradient and viscous dissipation. The analysis leads to an eigenvalue problem which is solved numerically by an adaptation of Rutishauser technique. Numerical results are presented for two sets of boundary conditions: (i) fixed but different temperatures at the lower and upper plates and (ii) fixed temperature at the lower plate and zero flux at the upper plate. The effects of additional pressure gradient and viscous dissipation on the spatial development of temperature profile and Nusselt number are shown. For (i), Bruin neglected viscous dissipation and obtained an analytical solution of the energy equation. However, due to the difficulty in computing higher eigenvalues, the solution was truncated to a few terms. Besides avoiding this difficulty, the present approach offers computational simplicity and yields highly accurate results. A comparison of present results with those of Bruin shows that the latter are significantly in error. To confirm the accuracy of the numerical procedure, the method is tested for slug flow model which admits simple analytical solution. Excellent agreement is exhibited between numerical and analytical results throughout the entrance region.
Natural Convection From a Vertical Surface to a Thermally Stratified Fluid
Authors: Chen C. C. , Eichhorn R.
Published: November 2010

Results are presented of a study of natural convection from an isothermal finite plate immersed in a stable thermally stratified fluid. An analytical solution to the problem is obtained by using the local nonsimilarity method. Theoretical local and overall heat transfer coefficients are given for Pr = 0.7 and 6.0. Velocity and temperature profiles are given for Pr = 6.0. The actual experimental configuration was a vertical copper cylinder enclosed in a cube with rigid walls. Heat transfer data are correlated with the measured ambient thermal gradient. Visual studies of the flow field are also discussed. Excellent agreement is achieved between analysis and experiment.
The Onset of Thermohaline Convection in a Linearly-Stratified Horizontal Layer
Authors: Wright J. H. , Loehrke R. I.
Published: November 2010

The convective stability of a horizontal layer of water with salt and heat addition from below was studied experimentally. The layer was bounded above and below by porous metal plates through which heat and salt could diffuse. A well-mixed region of warm, salty water below the lower plate and another of cooler, fresher water above the upper plate set the temperature and concentration difference for the intervening quiescent layer. With a fixed, constant concentration gradient established between the plates the temperature difference was slowly increased until convective motions were observed. The instability boundary for this system lies within the unstable region predicted by linear theory for a horizontal layer with constant gradients and stress-free boundaries and approaches the linear boundary at high Rayleigh numbers.
A Numerical Analysis of Heat Transfer to Fluids Near the Thermodynamic Critical Point Including the Thermal Entrance Region
Authors: Schnurr N. M. , Sastry V. S. , Shapiro A. B.
Published: November 2010

A two-dimensional numerical method has been developed to predict heat transfer to near critical fluids in turbulent flow through circular tubes. The analysis is applicable to the thermal entry region as well as fully developed flows. Agreement with experimental data for water at 31.0 MN/m2 is quite good. A correlation in the form of the heat flux parameter of Goldmann was found to be satisfactory for water at that pressure. Results are presented in graphical form which apply to a wide range of heat fluxes, mass velocities, and tube diameters. Preliminary results in the entrance region show that film coefficients remain well above the corresponding fully developed values for a larger distance downstream than would be the case with a constant property fluid. This effect becomes more pronounced as the heat flux is increased.
Heat Transfer From Spheres in the Naturally Turbulent, Outdoor Environment
Authors: Kowalski G. J. , Mitchell J. W.
Published: November 2010

Heat transfer coefficients from spheres were measured in a naturally turbulent, outdoor environment and compared to measurements made in wind tunnels at various turbulent intensities. The experiments were performed using spheres of three different diameters placed at different heights relative to the ground surface. The time-average values of Nusselt number and Reynolds number were obtained over 5-min time periods. Over the Reynolds number range 2000–35,000, the value of the Nusselt number obtained outdoors was up to 2.2 times the values determined in low turbulent intensity wind tunnels. The data, averaged for each sphere at each height, showed an average enhancement of the Nusselt number that decreased with height from 1.8 to 1.1. The heat transfer enhancement was found to be essentially independent of Reynolds number and correlated with the ratio of height above the surface to the sphere diameter. Standard micrometeorological theory was used to estimate the turbulence intensity as a function of height above the ground. The enhancement in heat transfer was found to be mainly correlated with turbulence intensity. The results for all but the lowest placed spheres are in substantial agreement with results using artificially induced turbulence.
Spectral Absorption of Water Vapor and Carbon Dioxide Mixtures in the 2.7 Micron Band
Authors: Saido K. , Giedt W. H.
Published: November 2010

An investigation of the spectral absorption characteristics of water vapor and carbon dioxide mixtures in the 2.7 micron band is described. The absorption of black body radiation by gas mixtures in the central region of a high temperature furnace was measured. The optical path length of 30 cm was bounded by NaCl windows mounted in water-cooled holders. The temperature was essentially uniform in the central one-third of the test cell, and decreased to between 400 800 K at the ends. Representative spectral transmissivity distributions obtained for central temperatures ranging from 1000 to 2200 K and pressures from 0.25 to 3 atm for four different mixture ratios of water vapor and carbon dioxide are presented. Results showed the interaction effect on spectral transmissivity to be greatest at the band center where the maximum change from around 0.40 to 0.48 was observed for approximately equimolar mixtures at a total pressure of 0.5 atm. Since this occurs over a limited wave number range, spectral transmissivities of such mixtures can be acceptably approximated by ignoring the interaction. Spectral distributions predicted from published data for spectral H2O and CO2 absorption coefficients and line half-width to spacing ratios are in good agreement with measured distributions.
Heat/Mass Transfer Characteristics for Flow in a Corrugated Wall Channel
Authors: Goldstein Leonardo , Sparrow E. M.
Published: November 2010

Experiments based on the naphthalene sublimation technique were carried out to determine the local and average transfer characteristics for flow in a corrugated wall channel. The range of the experiments encompassed the laminar, transition, and low-Reynolds-number turbulent regimes. Local mass transfer measurements were made both in the spanwise (i.e., cross stream) and streamwise directions, and overall transfer rates were also determined. The experiments demonstrated the existence of a variety of complex transfer processes and related fluid flow phenomena. These included secondary flows and associated spanwise mass transfer variations, suppression of the secondary flow by counteracting centrifugal forces, and destruction of the secondary flow by the onset of turbulence. Flow separation on the leeward facets of the corrugated wall caused a sharp decrease in the local transfer rates, but relatively high transfer rates were in evidence in the reattachment region. In the laminar range, the average transfer coefficients for the corrugated wall channel were only moderately larger than those for a parallel-plate channel. On the other hand, in the low-Reynolds-number turbulent regime, the wall corrugations were responsible for an increase of nearly a factor of three in the average coefficient compared with the smooth wall channel.
A Mathematical Solution for Gas-to-Surface Radiative Exchange Area for a Rectangular Parallelepiped Enclosure Containing a Gray Medium
Authors: Becker H. B.
Published: November 2010

The transfer of heat in a gas filled enclosure can be calculated by the zone method put forward by Hottel and Cohen [1, 2]. For a shape which can be divided into cylindrical or cubic zones, tabulated and graphical data on exchange areas are available, however, insufficient data exist for subdivision into rectangular zones. The present contribution provides a mathematical solution for the gas-to-surface exchange area between a rectangular parallelepiped gas volume and an adjacent face. The rules for manipulating and scaling of exchange areas are reviewed, and their application to the zoning of a rectangular furnace chamber into concentric rectangular volume elements is demonstrated.
A Theoretical and Experimental Study of Radiation–Convection Interaction in a Diffusion Flame
Authors: Negrelli D. E. , Lloyd J. R. , Novotny J. L.
Published: November 2010

An experimental and theoretical investigation of the interaction of gaseous thermal radiation with natural convection was made for a laminar methane-air diffusion flame in the lower stagnation region of a horizontal porous cylinder. The exponential wide-hand gas radiation model was employed in this nonhomogeneous (nonuniform in temperature and composition) problem through the use of scaling techniques. Using a numerical scheme, the compressible energy, flow, and species-diffusion equations were solved simultaneously with and without the radiative component. In the experiment, methane was blown uniformly from the surface of the porous cylinder, setting up (upon ignition) a diffusion flame within the free-convection boundary layer. Using a Mach-Zehnder interferometer and a gas chromatograph, temperature and composition measurements were obtained along the stagnation line. Excellent agreement was found between the results based on the nongray wide-band model and the experimental data. Furthermore, it was found that the wide-band model yielded results that were superior to those results that excluded radiation-interaction effects. Thus, this study demonstrates that the exponential wide-band model can be accurately applied to nonhomogeneous combustion situations in order to account for the radiation-convection interactions.
Role of Taylor Instability on Sublimation of a Horizontal Slab of Dry Ice
Authors: Dhir V. K. , Castle J. N. , Catton Ivan
Published: November 2010

Sublimation of a horizontal slab of dry ice (≃ 190 K) placed beneath a pool of warm water or benzene (278–340 K) has been observed experimentally. Data for the heat transfer coefficient have been obtained in both steady and quasi-static states. The heat transfer coefficient for this pseudo film boiling process is found to be strongly dependent on the pool temperature. In the temperature range of stable film boiling, the heat transfer coefficient depends on the laminar or turbulent nature of the gas film. However, when the pool temperatures are such that a stable film can no longer be maintained, and the overlying liquid starts to freeze at the interface, the heat transfer coefficient data are correlated with the parameter cpΔTf/hsf for the liquid. Post-experiment visual observations of the dry ice surface show the presence of valleys and ridges arranged in a nearly square array spaced about one Taylor wavelength apart. An application of the present study to the fast reactor hypothetical accident situations in which a pool of molten fuel may be formed on horizontal steel surfaces is discussed.
Transient Convective Heat Transfer for Laminar Boundary Layer Flow With Effects of Wall Capacitance and Resistance
Authors: Wang R. C. C. , Chung B. T. F. , Thomas L. C.
Published: November 2010

Transient forced convective heat transfer from a laminar boundary layer flow over a flat plate with appreciable thermal capacity and resistance is studied analytically. In the analysis, the flow is assumed to be steady and incompressible and the solid plate is subjected to a uniform step heat input at the lower surface. The integral method is utilized to reduce systems of nonlinear partial differential equations to a single integro-differential equation in terms of interfacial temperature which is then solved with the aid of finite difference technique. Numerical results for the fluid-solid interface temperature, heat transfer coefficient, and temperature distributions within the fluid and solid are presented. Some limiting solutions are found to agree well with the results of the previous theoretical analyses.
A Three-Dimensional Rigid-Lid Model for Closed Basins
Authors: Sengupta S.
Published: November 2010

A numerical rigid-lid model for wind driven circulation and temperature fields in closed basins has been developed. The horizontal momentum equations each include the non-steady, non-linear inertia, Coriolis, pressure gradient and all three viscous terms. The energy equation includes the non-steady, convective and all three diffusion terms. The hydrostatic and Boussinesq approximations have been used. A Poisson equation derived from the vertically integrated horizontal momentum equations has been used as the predictive equation for surface pressure. An iterative scheme with normalization has been developed to solve the Poisson equation for pressure with Neumann boundary conditions. A vertically normalized system of equations which maps variable depth domains to a constant depth has been used. The model has been applied to a pond located near Cleveland, Ohio. The effect of topography and buoyancy on wind driven circulation has been investigated. The relative importance of the terms in the transport equations has been analyzed.
Determination of Void Fraction, Incipient Point of Boiling, and Initial Point of Net Vapor Generation in Sodium-Heated Helically Coiled Steam Generator Tubes
Authors: U¨nal H. C.
Published: November 2010

Void fraction was measured with high-speed photography in a 26.7 m and a 40.1 m long, sodium-heated helically coiled steam generator tube of 0.018 m ID. The ratio of coil diameter to tube diameter was 38.9. The operating conditions for the tests were as follows: Pressure: 4–18 MN/m2, mass velocity: 429–1518 kg/m2s, heat flux: 0.013–0.42 MW/m2, outlet subcooling: 0.3–12.5 K, outlet steam quality: 0.000032–0.075. For vapor volumetric rate ratios greater than 0.4, the so-called distribution parameter is not affected by centrifugal forces, and is equal to 0.875. For vapor volumetric rate ratios smaller than 0.4, this parameter is affected by centrifugal forces and the aforesaid ratio. The incipient point of boiling and initial point of net vapor generation were determined with high-speed photography in the aforementioned 26.7 m long helical coil for the following range of operating conditions: Pressure: 4–18 MN/m2, mass velocity 757–1518 kg/m2s, heat flux: 0.082–0.413 MW/m2, outlet subcooling: 4.4–12.5 K. The data were correlated by using both the average and local values of the operating conditions.
Numerical Method for Multi-Dimensional Freezing Problems in Arbitrary Domains
Authors: Saitoh T.
Published: November 2010

This paper presents a simple numerical method for solving two and three-dimensional freezing problems with arbitrary geometries. The change of variable method introduced by Landau for the one-dimensional problem is extended to the multi-dimensional using an independent variable which takes constant values at the boundary and the freezing front. Example calculations were performed for the Stefan type freezing problem in regular squares, triangles, and ellipses. Then some of the results were compared with the experimental ones that were obtained for the constant cooling rate.
On the Analogy Between the Calorimeter Problem and Some Granulate-Fluid Exchange Processes
Authors: Kern J. , Hemmings J. W.
Published: November 2010

The solution to a generalized version of the classical calorimeter problem approximates the behavior of certain continuous exchange processes between particulate solids and a fluid. For modeling purposes, the much simpler energy-balance solution has obvious advantages, although it only holds for negligible intraparticle resistance and thus predicts too short contact times. An error diagram provides a quantitative comparison between those two results as a function of the two pertinent system parameters.
The Effect of Longitudinal Heat Conduction on Crossflow Heat Exchanger
Authors: Chiou J. P.
Published: November 2010

A numerical method to determine the effectiveness of the crossflow heat exchanger, accounting for the effect of the two-dimensional longitudinal heat conduction through the exchanger wall structure in the directions of fluid flows, is presented. The exchanger effectiveness and its deterioration due to the conduction effect have been calculated for various design and operating conditions of the exchanger. The results indicate that the thermal performance deterioration of the exchanger may become significant for some typical applications.
Free Convection across Inclined Air Layers with One Surface V-Corrugated
Authors: ElSherbiny S. M. , Hollands K. G. T. , Raithby G. D.
Published: November 2010

Experimental measurements are presented for free convective heat transfer across inclined air layers, heated from below, and bounded by one V-corrugated plate and one flat plate. The measurements covered three values for the ratio, A, (average plate spacing to V-height), namely, A = 1, 2.5 and 4. It also covered angles of inclination with respect to the horizontal, θ, of 0, 30, 45 and 60 deg, and a range in Rayleigh number of 10 < Ra < 4 × 106. The study proves, both theoretically and experimentally, that the free convective heat transfer is essentially the same, regardless of whether the V-corrugated plate is above or below. It was found that for the same average plate spacing, L, the convective heat losses across air layers bounded by one V-corrugated and one flat plate are greater than those for two parallel flat plates by up to 50 percent for the range studied. Experimental results are given as plots of Nusselt number versus Rayleigh number. A correlation equation is given for Nusselt number, Nu, as a function of A, θ and Ra.
Transient Free Convection from a Suddenly Heated Horizontal Wire
Authors: Parsons J. R. , Mulligan J. C.
Published: November 2010

Experimental data are presented for the transient free convective heat transfer from a horizontal wire in air subjected to a step change in heat rate. An unbalanced Wheatstone bridge is described which allows simultaneous observation of wire temperature and heat rate, under transient as well as steady-state conditions. An “overshoot” of the steady-state is shown to occur in the transient decay of the Nusselt number, and the occurrence, magnitude, and duration of this phenomenon is shown to depend upon the Rayleigh number. Simple stability theory is shown to explain the delay in the convective process which is associated with the overshoot in heat transfer. Steady-state data are also presented and are shown to agree well with proposed low Rayleigh number correlations.
Measurement of Buoyant Jet Entrainment from Single and Multiple Sources
Authors: Davis L. R. , Shirazi M. A. , Siegel D. L.
Published: November 2010

An experimental investigation was conducted to determine the dilution characteristics of single and multiple port buoyant discharges typical of modern natural and mechanical draft cooling towers. Simultaneous measurements of velocity and tracer concentration profiles were taken at various downstream locations in the three-dimensional plumes discharged into a stagnant ambient using a hot film anemometer and conductivity probe. The number of discharge ports was varied from one to seven. Discharge densimetric Froude numbers were varied from 1.5 to infinity. Numerical integration of the profiles gave dilution, tracer conservation, and momentum fluxes. The effect of reducing Froude number was to increase entrainment considerably. Increasing the number of discharge ports reduced the rate of entrainment. In multiple port discharges the shape of the plume changed from an elongated configuration to nearly axisymmetric within the first 20–30 diameters of discharge.
Corresponding States Correlations of the Extreme Liquid Superheat and Vapor Subcooling
Authors: Lienhard J. H. , Karimi A. H.
Published: November 2010

A variety of corresponding states correlations are made for the maximum or minimum nucleation temperatures obtained in many experiments by many workers. These correlations reveal: 1) That the best correlation occurs when the reduced liquid superheat is plotted against the reduced saturation temperature; and 2) that liquid superheat data correlate well and appear to fall near the spinodal line, while vapor subcooling data correlate poorly and fall far from the spinodal line. It is suggested that the published vapor subcooling results have been inferred in invalid ways from the data. It is therefore not presently possible to know whether existing vapor nucleation temperatures are actually far from the spinodal line, or incorrect.
An Experimental and Analytical Study of a Unique Wet/Dry Surface for Cooling Towers
Authors: Bentley J. M. , Snyder T. K. , Glicksman L. R. , Rohsenow W. M.
Published: November 2010

An advanced wet/dry heat transfer surface has been developed for power plant cooling towers eliminating the need for conventional dry surface. Hot water to be cooled is channeled down grooves in the surface; the balance of the surface is dry and acts like a fin. The cooling air passes over the surface in cross-flow. Since the grooves occupy only a small fraction of the surface area, a majority of the heat transfer is by sensible heat transfer. In the experimental apparatus the wet surface area was five percent of the total area and the heat transfer by evaporation varied between 20 and 40 percent of the total heat transfer. An analytical model indicated that the yearly water consumption of a cooling tower with the new wet/dry surface would be less than half that of a conventional wet cooling tower and fog plumes would be eliminated.
Effect of Solid Properties and Contact Angle in Dropwise Condensation and Evaporation
Authors: Sadhal S. S. , Plesset M. S.
Published: November 2010

For droplets condensing on or evaporating from a solid surface, the effects of the solid material properties and the droplet contact angle are analyzed by solving the steady heat-conduction equation for a geometry consisting of a spherical segment droplet on a semi-infinite solid. In obtaining the solution it is assumed that heat is transported to or from the solid only through the droplet and that there is perfect thermal contact at the solid-liquid interface. With suitable boundary conditions at the liquid-vapor interface some exact and approximate solutions are found and then used to obtain the overall heat flow through the droplet.
Back-Melting of a Horizontal Cloudy Ice Layer with Radiative Heating
Authors: Seki N. , Sugawara M. , Fukusako S.
Published: November 2010

This paper is concerned with the melting of a horizontal ice layer sticking to a substrate by using halogen lamps, which are comparatively short wave radiation sources. This radiation in the visible and infrared spectral range may be employed to remove ice from structures subject to atmospheric icing. It is concluded that the behavior of radiation transfer in a cloudy ice layer depends a great deal on the density of the cloudy ice including air bubbles which produce the scattering of radiation. Also the phenomenon of back-melting caused by radiant energy penetrating through the ice layer is observed. Moreover, it is shown that the melting rate of an ice layer can be predicted numerically by using the band model of extinction coefficient for cloudy ice assumed in this study.
Combined Heat and Mass Transfer in Regenerators with Hygroscopic Materials
Authors: Holmberg R. B.
Published: November 2010

Simultaneous heat and mass transfer in periodic-flow heat and mass exchangers, or regenerators, with hygroscopic matrix materials has been analyzed. The coupled heat and mass transfer equations are derived for boundary-layer controlled heat and mass transfer and include longitudinal heat conduction in the matrix. A numerical method of the finite-difference type is applied to the steady-state performance. Results for a water vapor-air mixture show the influence of matrix heat and moisture capacities on temperature and humidity efficiencies. Distributions of air temperature, air humidity, matrix temperature and adsorbent moisture content are calculated in drying as well as recovery operations.
Effect of Stabilizing Thermal Gradients on Natural Convection in Rectangular Enclosures
Authors: Ostrach S. , Raghavan C.
Published: November 2010

An experimental investigation is described of the effect of stabilizing thermal gradients on natural convection in silicone oils in rectangular enclosures with different aspect ratios. The Prandtl numbers are of the order of 105, Grashof numbers range up to 20, and the aspect ratios are 1 and 3. The thermal boundary conditions are established by imposing different temperatures on opposite walls of the enclosure so that there is simultaneous horizontal and vertical heat flow. The effect of stabilizing temperature gradients on flow established by horizontal gradients and the effect of horizontal temperature gradients on a stably stratified fluid are studied for ranges of the parameters. Streamline patterns are observed at steady-state and velocity profiles are calculated from streamline data and extrapolated with approximate theoretical calculations. It is found that the flow generated by a horizontal gradient is retarded by a stabilizing thermal gradient. The reduction is shown as a function of the relevent parameters. For the range of variables investigated complete stabilization of the fluid driven by a horizontal gradient does not seem possible by means of a vertical gradient. The steady state flow patterns obtained do not depend on the manner in which the flow is started, i.e., on the order in which the temperature differences are imposed.
The Determination of the Thermal History in a One-Dimensional Freezing System by a Perturbation Method
Authors: Rubinsky B. , Cravalho E. G.
Published: November 2010

A perturbation method is employed to determine the thermal history of a biological organ frozen with a constant cooling rate at its outer surface. Solutions were obtained for the time history of the propagating phase front and the distribution of cooling rates within the organ. By means of a one-dimensional analysis, a new dimensionless group has been found to correlate the range of cooling rates in a frozen organ of complex shape. The analytical model has been confirmed by experimental measurements on a one-dimensional freezing system. The results of this treatment are useful in the design and analysis of experiments dealing with freezing in one dimension, particularly in the preservation of organs by freezing.
Forced Convection Heat Transfer on Heated Bottom Surface of a Cavity
Authors: Yamamoto H. , Seki N. , Fukusako S.
Published: November 2010

Experiments to measure the heat transfer characteristics for various cavities situated at a duct-wall were performed. Flow visualization, measurements of pressure and temperature distributions on the heated bottom surface of cavity were carried out. It was observed that the effects of main flow stream, reattachment of separated flow, and vortex flow in the cavity on heat transfer unexpectedly large. It was found that heat transfer did not always decrease monotonously with an increase of aspect (depth-width) ratio D/W, in the flow range of laminar to turbulent. Correlations between Num and Rew were made in laminar and turbulent heat transfer ranges.
Convective Heat Transfer of Laminar Droplet Flow in Thermal Entrance Region of Circular Tubes
Authors: Yao Shi-chune
Published: November 2010

Connective heat transfer of laminar droplet flow is calculated numerically for the thermal entrance region of circular tube with constant wall temperature. The heat transfer contribution of saturated droplets in the superheated vapor stream is considered as distributed heat sink. In the thermal entrance region, the size and the population density of the droplets are considered as constants. The heat transfer of droplet flow is found to be considerably higher than that of single phase flow. The effects of the droplet characteristics and the wall superheat to the convective heat transfer are studied. Fundamental differences of heat transfers in single phase flow and droplet flow are revealed.
Heat Transfer Characteristics for Inline and Staggered Arrays of Circular Jets with Crossflow of Spent Air
Authors: Metzger D. E. , Florschuetz L. W. , Takeuchi D. I. , Behee R. D. , Berry R. A.
Published: November 2010

Heat transfer characteristics were measured for two-dimensional arrays of jets impinging on a surface parallel to the jet orifice plate. The impinging flow was constrained to exit in a single direction along the channel formed by the jet plate and the heat transfer surface. Both mean Nusselt numbers and streamwise Nusselt number profiles are presented as a function of Reynolds number and geometric parameters. The results show that significant periodic variations occur in the streamwise Nusselt number profiles, persisting downstream for at least ten rows of jet holes. Both channel height and hole spacing can have a significant effect on the streamwise profiles, smoothed across the periodic variations. Where significant differences exist, inline hole patterns provide better heat transfer than staggered ones, particularly downstream. These and other effects of the geometric parameters are presented and discussed.
Local and Average Heat Transfer Characteristics for Turbulent Airflow in an Asymmetrically Heated Tube
Authors: Knowles G. R. , Sparrow E. M.
Published: November 2010

Turbulent airflow experiments were performed with a specially fabricated test section tube which facilitated nonuniform heat transfer around the tube circumference. Heating was accomplished by passing electric current axially through half the tube wall (subtending a 180 deg arc), while the other half of the wall was not directly heated. Measurements were made both in the thermal entrance region and the fully developed region, and the Reynolds number was varied from about 4400 to 64,000. The results of the experiments underscored the strong interaction between circumferential tube-wall conduction and fluid convection when a gas flowing in a tube is heated nonuniformly around its circumference. The effects of the wall conduction were shown to be significant at low Reynolds numbers but diminished as the Reynolds number increased. Owing to the circumferential nonuniformities, the thermal development was much slower than that for a uniformly heated pipe flow. By use of a suitably defined circumferential average heat transfer coefficient, the present fully developed results agreed well with a literature correlation for uniformly heated flows. At any cross section, the local coefficients varied around the tube circumference, with the smallest value at the mid-point of the heated arc. Buoyancy effects at low Reynolds numbers were investigated and found to be undetectably small.
Natural Convection Heat Transfer in Moderate Aspect Ratio Enclosures
Authors: Meyer B. A. , Mitchell J. W. , El-Wakil M. M.
Published: November 2010

Local and average heat transfer coefficients for natural convection between parallel plates separated by slats to create enclosures of moderate aspect ratio have been experimentally determined using an interferometric technique. The effects of Rayleigh number, tilt and slat angle, and aspect ratio on the Nusselt number have been determined. The Rayleigh number range tested was up to 7 × 104, and the aspect ratio (ratio of enclosure length to plate spacing) varied between 0.25 and 4. The angles of tilt of the enclosure with respect to the horizontal were 45, 60 and 90 deg. Slat angles of 45, 60, 90 and 135 deg were studied. The results obtained in a previous investigation [1] for aspect ratios of 9 to 36 are included to show continuity. The results indicate that the convective heat transfer is a strong function of the aspect ratio for aspect ratios less than 4. For aspect ratios in the range of 0.5 to 4, spacers between the plates increase, rather than decrease, natural convection heat transfer compared to that for long enclosures. Slat angles less than 90 deg (i.e., oriented downward) reduce convective heat transfer.
Effects of Particle-Size and Temperature Difference on Mist Flow over a Heated Circular Cylinder
Authors: Nishikawa N. , Takase H.
Published: November 2010

An analytical study is performed to evaluate the surface heat flux and the boundary layer structure over an isothermally heated circular cylinder subjected to evaporating particlevapor flow in forced convection. The governing equations are solved by the local nonsimilarity method and by the Goertler-type series solution. The applicability of both methods is supported by a comparison between the obtained results and with existing solutions for particle-free, single phase flow. Numerical results for the velocity, temperature, and particle size profiles are presented for single component steam-droplets mixture with oncoming velocity 5 m/s at the saturation temperature 100°C. For the number density 5 × 108 numbers/m3 the droplet diameter at free stream is chosen as 10, 30, and 60 μm. For each cases, the wall-to-free stream temperature difference are chosen as 100, 200, and 350 K. Some results are also shown for cases where the free stream droplet diameter is 1, 3 or 5 μm for the fixed value of droplet number density equated 5 × 1011 m−3 and of the wall-to-free stream temperature difference as 100 K. The results show that the enhancement of heat transfer rate is comparatively small in cases where there is large temperature difference but distinctly large in cases involving large particles.
The Influence of Surface Roughness on the Mechanism of
Friction
Authors: Tsukizoe T. , Hisakado T.
Published: November 2010

A study was made of surface roughness effects on dry friction between two metals, assuming that the asperities are cones of the slopes which depend on the surface roughness. The theoretical explanations were offered for coefficients of friction of the hard cones and spheres ploughing along the soft metal surface. A comparison of calculated values based on these with experimental data shows good agreement. Moreover, theoretical discussion was carried out of surface roughness effects on dry friction between two metal surfaces on the basis of the analyses of the frictional mechanism for a hard slider on the metal surface. The theoretical estimation of the coefficient of friction between two metal surfaces can be carried out by using the relations between the surface roughness and the slopes of the asperities, and the coefficient of friction due to the adhesion at the interface. The experiments also showed that when two metal surfaces are first loaded normally and then subjected to gradually increasing tangential forces, real area of contact between them increases and the maximum tangential microslip of them increases with the increase of the surface roughness.
Elastohydrodynamic Lubrication of Roller Bearings
Authors: Skurka J. C.
Published: November 2010

Roller bearing endurance tests have been run on groups of bearings with L10 and L50 lives established by Weibull analysis. Bearing roller path surface finish, shaft speed, lubricant viscosity and lubricant temperature were varied. Relevant tapered roller and cylindrical roller bearing life data was selected from prior tests for comparison. Weibull plots show the effect of the variables on life and a graph comparing the ratio of L10 test life to the calculated life and the ratio of elastohydrodynamic lubricant film thickness to composite rolling contact surface finish is given. An empirical equation to predict the effect of varying lubricant and surface finish conditions on fatigue life is given.
Elastohydrodynamic Effects in a Clearance Seal
Authors: Wang N. M. , Kamal M. M.
Published: November 2010

An elastohydrodynamic solution for a high-pressure, low-clearance metal seal is presented. The fluid flow is assumed to satisfy Reynolds equation of hydrodynamic lubrication, and the deformation of the shaft and the seal is governed by the linear theory of elasticity. The viscosity of the fluid is assumed to have an exponential dependence on the pressure, while the density of the fluid is a linear function of the pressure. Closed-form solutions are obtained for two asymptotic limiting cases: (i) when the length of the seal is much greater than the radius of the shaft, and (ii) when it is much less. For intermediate ratios of the seal length to shaft radius, solutions are obtained numerically and examples are given to show the effect of seal length on the rate of mass flow.
An Extended Variational Formulation for Isothermal Gas-Dynamic
Lubrication
Authors: Hays D. F.
Published: November 2010

An extended variational formulation is derived which can be applied to problems in isothermal compressible flow where the medium obeys the perfect gas law. The Reynold’s approximations for fluid film lubrication have been applied to this variational formulation to obtain an expression that is applicable to the field of gas-dynamic lubrication. This variational formulation for gas-dynamic lubrication has as its Euler-Lagrange equation, the Reynolds’ equation of lubrication for a perfect gas. The extended variational formulation is shown to differ from the classical Lagrangian expression with respect to its form and to its applicability to nonlinear problems.