Showing papers in "Journal of Heat Transfer-transactions of The Asme in 1986"

Melting and Solidification of a Pure Metal on a Vertical Wall

Abstract: Etude experimentale de l'ecoulement de convection naturelle dans la phase liquide et de son influence sur le mouvement de l'interface liquide-solide et sur le transfert de chaleur au cours de la fusion et de la solidification de gallium a haute purete sur une paroi verticale

Developing flow and flow reversal in a vertical channel with asymmetric wall temperatures

Abstract: Etude numerique des effets de la poussee hydrostatique sur les parametres hydrodynamiques et thermiques dans l'ecoulement laminaire, vertical ascendant d'un gaz entre plaques paralleles

Use of Melting Point Surface Coatings for Local Convection Heat Transfer Measurements in Rectangular Channel Flows With 90-deg Turns

Abstract: Determination, a l'aide de revetements de surface a point de fusion precis, pour la determination des repartitions des nombres de Nusselt locaux pour le transfert de chaleur locaux dans des coudes a 90° de conduites rectangulaires

Radiative transfer in axisymmetric, finite cylindrical enclosures

Abstract: Modele de transfert radiatif axisymetrique a 2 dimensions pour un milieu heterogene, diffusant de facon anisotrope, emissif, absorbant base sur l'approximation d'harmoniques spheriques du 3eme ordre

Laminar Natural Convection in Shallow Open Cavities

Abstract: Etude experimentale realisee par anemometrie laser Doppler. La cavite est rectangulaire avec une paroi laterale ouverte

Effects of Ambient Pressure on the Instability of a Liquid Boiling Explosively at the Superheat Limit

Abstract: The effect of ambient pressure on the dynamical behaviour of a single droplet (1-2 mm diameter) of volatile liquid boiling explosively at the limit of superheat is studied experimentally and theoretically. In a series of experiments it is shown that the evaporative instability, observed earlier by Shepherd & Sturtevant (1982) during the rapid vapourization of butane droplets at atmospheric pressure, is suppressed at high pressure. Three other fluids (pentane, isopentane, and ether) are tested to establish the generality of the instability and other transient processes previously observed. Direct evidence is obtained showing that during violently unstable boiling small liquid particles are torn from the liquid-vapour interface. This ejection of fine droplets from the evaporating surface produces a mass flux orders of magnitude greater than that characteristic of ordinary boiling. Raising the ambient pressure lowers the superheat attained at the superheat limit, which decreases the vapourization rate. At high pressure boiling consists of normal slow vapourization from a smooth interface. Observed bubble growth rates show reasonable agreement with theory. At intermediate pressures a transitional regime of stability occurs in which a drop initially vapourizes stably for several milliseconds while incipient instability waves develop on the evaporating interface. When only a small amount of liquid remains in the drop in the shape of a thin cap, heat transfer from the surrounding hot host fluid initiates violent boiling at the edge of the liquid cap. The subsequent rapid vapourization generates a radiated pressure field two orders of magnitude larger than during stable boiling, and sets the bubble into violent oscillation. The bubble is subject to the Rayleigh-Taylor instability and rapidly disintegrates into a cloud of small bubbles. Lowering the ambient pressure decreases the time delay between nucleation and onset of unstable boiling. For example, in ether at atmospheric pressure the instability is triggered less than 8 µsec after nucleation, shortly after the smooth vapour bubble contacts the droplet surface. Heterogeneous nucleation spreads out along the surface of the drop while disturbances (with a length scale of 100 µm) distort the unstably evaporating interface within the drop, substantially enhancing the vapourization rate. At early times, droplets torn from the evaporating surface evaporate before the instability-driven jet impinges upon the surrounding fluid, bulging the bubble surface. The last portion of liquid in a drop boils particularly violently and droplets ejected from the evaporating interface at this time remain intact to splatter the bubble surface. At subatmospheric pressures the most rapid vapourization occurs and temperature gradients within a drop produce spatial variations in vapourization rate. The Landau mechanism for the instability of laminar flames is adapted to the case of evaporation to investigate the effects of variable ambient pressure. A spherical version of the theory, applicable before the vapour bubble contacts the droplet surface, predicts absolute stability at atmospheric pressure. At later times the spherical constraint is inappropriate and planar theory yields results in general agreement with observation. Differences in fluid properties make some fluids more prone to instability than others. The product of the maximum growth rate with the time interval the interface is predicted to be linearly unstable measures the susceptibility to instability. For practical estimates it is suggested that a value of 3 of this parameter be taken as the lower limit for instability. The sensitivity of the instability to temperature suggests that small temperature nonuniformities may be responsible for quantitative departures of the behaviour from predictions.

Heat Conduction in a Moving Semi-infinite Solid Subjected to Pulsed Laser Irradiation

Abstract: Heat conduction in a moving semi-infinite body subject to laser irradiation is considered. The body of knowledge of exact analytical solutions for Gaussian laser irradiation is expanded to include pulsed lasers, and laser beams that penetrate into the body with exponential decay. For applications with complicated geometries (laser melting and evaporation), a simple integral method, based on one-dimensional diffusion is presented and its range of validity determined.

Radiative Transfer With Dependent Scattering by Particles: Part 2—Experimental Investigation

Abstract: Dependent radiative scattering by particles is experimentally investigated using plane-parallel cells containing latex spheres of 11, 2, and 0.08 μm diameter dispersed in an air or water matrix. The dependent scattering efficiencies and the bidirectional transmittance and reflectance were measured and compared with analytical results. The close-packed 2-μm spheres, which were expected to show dependent scattering from the previous criterion, gave results identical to independent scattering. Measured dependent scattering efficiencies of the small particles tested decrease with increasing particle volume fraction and were compared with those predicted by the theoretical investigation. The bidirectional transmittance and reflectance of dependent scattering were compared with those of independent scattering with the same number of spheres within the test cells. Several different patterns of dependent transmittance and reflectance appeared depending on the optical thickness. Finally, a newly proposed regime map bounding independent and dependent scattering is compared with the present and previous experimental data.

Heat Transfer Around Sharp 180-deg Turns in Smooth Rectangular Channels

Abstract: Measured Nusselt numbers are presented for forced convection within and around sharp 180 degree turns in smooth channels of rectangular cross section. Separately determined top wall, bottom wall, and side wall values are presented individually along with azimuthal averages. The geometry of the channels and connecting turn is characterized by parameters W*, the ratio of upstream and downstream channel widths; 0*, the non-dimensional channel depth; and H*, the nondimensional clearance at the tip of the turn. Results from nine combinations of these parameters are presented at several values of channel Reynolds number to illustrate the effect of turn geometry on the heat transfer distributions.

Structure of Laminar Coflow Methane–Air Diffusion Flames

Abstract: Measured temperature and composition profiles are reported for a number of flames. Implications concerning flame structure are deduced, with emphasis on soot formation and on correlation involving conserved scalars.

Momentum and Heat Transfer on a Continuous Moving Surface

Abstract: Determination du transfert de chaleur et de quantite de mouvement dans la couche limite laminaire sur une surface mobile avec une vitesse de surface arbitraire et une temperature de surface non uniforme

Correlations for laminar mixed convection flows on vertical, inclined, and horizontal flat plates

Abstract: Local Nusselt numbers for laminar mixed convection flows along isothermal vertical, inclined, and horizontal flat plates are presented for the entire mixed convection regime for a wide range of Prandtl numbers, 0.1 ≤ Pr ≤ 100. Simple correlation equations for the local and average mixed convection Nusselt numbers are developed, which are found to agree well with the numerically predicted values and available experimental data for both buoyancy assisting and opposing flow conditions. The threshold values of significant buoyancy effects on forced convection and forced flow effects on free convection, as well as the maximum increase in the local mixed convection Nusselt number from the respective pure convection limits, are also presented for all flow configurations. It is found that the buoyancy or forced flow effect can increase the surface heat transfer rate from pure forced or pure free convection by about 20 percent.

Numerical and experimental study of turbulent heat transfer and fluid flow in longitudinal fin arrays

Abstract: Etude numerique et experimentale du transfert de chaleur d'un ensemble d'ailettes longitudinales paralleles a un courant turbulent d'air passant entre les ailettes

The Influence of Coupled Molecular Diffusion on Double-Diffusive Convection in a Porous Medium

Abstract: The effect of coupled molecular diffusion on double-diffusive convection in a horizontal porous medium is studied using linear and nonlinear stability analyses. In the case of linear theory, normal mode analysis is employed incorporating two cross diffusion terms. It is found that salt fingers can form by taking cross-diffusion terms of appropriate sign and magnitude even when both concentrations are stably stratified. The conditions for the diffusive instability are compared with those for the formation of fingers. It is shown that these two types of instability will never occur together. The finite amplitude analysis is used to derive the condition for the maintenance of fingers. The stability boundaries are drawn for three different combinations of stratification and the effect of permeability is depicted.

The Formulation of Spray Combustion Models: Resolution Compared to Droplet Spacing

Abstract: Formulations of the governing equations for burning air/fuel spray mixtures are discussed. The subsystem of equations describing liquid properties is formulated in three ways: Eulerian description, Lagrangian description, and probabilistic or distribution function description. The relationships among these approaches and the relative advantages and disadvantages are discussed. The reduction of numerical error, the ability to resolve multivalued solutions, and the ability to achieve resolution a scale smaller than droplet spacing lead to a preference of the Lagrangian method over the Eulerian method. However, when resolution is desired on a scale smaller than the average droplet spacing, the location of each droplet is known only in a statistical manner. The advantages of a probabilistic formulation in this case are evaluated.

Binary Fluid Convection and Double-Diffusive Convection in a Porous Medium

Abstract: Analyse numerique du transfert de chaleur et de masse par convection dans une couche horizontale d'un milieu poreux sature par un melange binaire

Mean-Parameter Modeling of Oscillating Flow

Abstract: Apres etude de la solution exacte pour l'ecoulement laminaire oscillant incompressible entre plaques paralleles avec un gradient de temperature longitudinal, les equations de quantite de mouvement et d'energie sont refondues en termes de vitesse moyenne et de temperature

Simultaneous Heat and Mass Transfer Accompanied by Phase Change in Porous Insulation

Abstract: Analyse de l'accumulation et de la migration de l'humidite dans une isolation thermique poreuse. Formulation rigoureuse du processus de transfert de chaleur et de masse. Resolution des equations en utilisant un schema numerique diphasique permettant de determiner les regimes de condensation et les facteurs influant sur le champ de temperatures

Scaling of Plain and Externally Finned Heat Exchanger Tubes

Abstract: Etude experimentale de la variation des coefficients de transfert de chaleur d'un tube de cuivre, d'un tube d'acier doux et d'un tube d'acier a ailettes externes dans des conditions d'entartrage par une solution de carbonate de calcium soumis a un flux thermique constant pendant des periodes de 70 h. Presentation de l'influence de la vitesse de l'ecoulement sur la vitesse d'entartrage pour les 3 tubes et comparaison des resultats experimentaux avec le modele de Hasson

An Inverse Finite Element Method for the Analysis of Stationary Arc Welding Processes

Abstract: An inverse finite element computer code was developed to facilitate the experimental analysis of two-dimensional stationary arc welding processes. The method uses transient temperature data from thermocouples inbedded in the solid region of the work piece to determine through a Newton-Raphson interpolation procedure the transient position of the solid-liquid interface and the transient temperature distribution in the solid region of the work piece. The accuracy of the method was demonstrated through comparison with results obtained with a direct finite element code and through comparison with experiments.

Evaporation of a Water Droplet Deposited on a Hot High Thermal Conductivity Surface

Abstract: Many studies have been performed to quantify the vaporization process for both single droplets and multiple-droplet arrays impacting on hot surfaces. For the studies found in the published literature, the full span of the droplet vaporization processes is usually reported. These would include evaporation, nucleate boiling, film boiling, and Leidenfrost transition. The present investigation is concerned with the evaporation of a droplet on a hot surface The study reports detailed results for spital and temporal variation of the heat flux at the exposed surface of the droplet, and for temporal variation of the droplet volume. Limiting the study to evaporation implies that conditions are maintained under which nucleate boiling is fully suppressed.

Condensation Effects in a Fibrous Insulation Slab

Abstract: Etude detaillee de l'interaction entre la vitesse de condensation, la temperature, le contenu de liquide et les champs de densite de vapeur