R. Viskanta

Heat transfer to impinging isothermal gas and flame jets

Abstract Heat transfer characteristics of single and multiple isothermal turbulent air and flame jets impinging on surfaces are reviewed. Both circular and slot two-dimensional jets are considered, and the effect of crossflow on impingement heat transfer is included. The emphasis is on physical phenomena and not on comparison of published empirical correlations or comparisons of theory and experiments. The review focuses on applications in the materials or comparisons of theory and experiments. The review focuses on applications in the materials processing field. In spite of the fact that there are many differences in the jet characteristics (ie, axial velocity and turbulence intensity) of isothermal and flame jets, the stagnation point heat transfer of these different jets can be described in a similar way. Areas needing research attention are also identified.

Experimental determination of the volumetric heat transfer coefficient between stream of air and ceramic foam

Abstract An experimental investigation is described that characterizes heat transfer between a heated air stream and ceramic foam. An apparatus is designed to determine the volumetric heat transfer coefficient between the foam and a stream of air using a single-blow transient technique. Experiments are reported on different mean pore diameter specimens. The governing conservation equations of energy for both the gas and the solid phases with appropriate boundary and initial conditions are solved using a finite-difference procedure. A nonlinear, least-square fit of predicted and measured gas temperatures is used to determine the volumetric heat transfer coefficient between air and ceramic foams. Two different types of ceramic foams are investigated, and the results obtained are compared. Heat transfer coefficient correlations are developed for each different mean pore diameter of ceramic foam and for the range of Reynolds numbers between 12 and 563 covered in the experiments.

Natural convection solid/liquid phase change in porous media

media with natural convection in the melt region. The model is based on volume averaged transport equations, while phase change is assumed to occur over a small temperature range. Experiments are performed in a vertical, square enclosure using gallium and glass beads as the fluid and the porous matrix, respectively. For natural convection, melting and solidification (with initial supercooling), the numerical results show reasonable agreement with the temperature and interface position measurements. Natural convection in the melt as well as heat conduction in the solid is found to considerably influence the interface shape and movement during both the melting and solidification experiments.

Effect of nozzle geometry on local convective heat transfer to a confined impinging air jet

This article reports results on the effects of hyperbolic nozzle geometry on the local heat-transfer coefficients for confined impinging air jets. A thermochromatic liquid-crystal technique is used to visualize and record isotherms on a uniformly heated impingement surface. Experiments are conducted at low nozzle-to-plate spacings (0.25 < HD < 6.0) and Reynolds numbers in the range of 10,000 to 50,000 for two different confined, hyperbolic nozzles. As a reference, results have also been obtained for a confined orifice and are compared with those for the hyperbolic nozzles. The effects of Reynolds number, nozzle-to-plate spacing, and nozzle geometry on the local heat-transfer coefficients are reported and compared with similar experiments for unconfined jets. It is concluded that the local heat-transfer coefficients for confined jets are more sensitive to Reynolds number and nozzle-to-plate spacing than those for unconfined jets.

Mathematical modeling of transport phenomena during alloy solidification

Mathematical modeling of mass, momentum, heat, and species transport phenomena occurring during solidification of metal alloys is reviewed. Emphasis is placed on the incorporation of the effects of the solid structure and the interactions between the solid and liquid phases on a microscopic scale into a (macroscopic) model of the transport phenomena occurring at the system scale. Both columnar and equiaxed growth structures, as well as laminar convection of liquid and solid crystals are considered. The macroscopic conservation equations are introduced via a volume averaging approach and commonly made simplifications are examined. Basic constitutive relations for the phase interactions occurring in alloy solidification are presented. Recent progress in including nucleation, microsegregation, undercooling and other microscopic phenomena in the macroscopic equations is reviewed. The specific areas where future theoretical and experimental research is needed are identified.

Natural convection in vertical enclosures containing simultaneously fluid and porous layers

A numerical and experimental study is reported of natural convection in a vertical rectangular fluid enclosure that is partially filled with a fluid-saturated porous medium. Velocities, stresses, temperatures, and heat fluxes are assumed to be continuous across the fluid/porous-medium interface, and the conservation equations for the fluid and the porous regions are combined into a single set of equations for numerical solution. Thermocouples as well as a Mach-Zehnder interferometer are used to measure temperature distributions and infer fluid flow patterns within the fluid and the porous medium. For various test cells, porous-layer configurations and fluid-solid combinations, the model predictions show excellent agreement with the experimental measurements. It is found that the intensity of natural convection is always much stronger in the fluid regions, while the amount of fluid penetrating into the porous medium increases with increasing Darcy and Rayleigh numbers. The degree of penetration of fluid into the porous medium depends strongly on the porous-layer geometry and is less for a horizontal porous layer occupying the lower half of the test cell. If penetration takes place, the flow patterns in the fluid regions are significantly altered and the streamlines show cusps at the fluid/porous-medium interfaces. For a high effective-thermal-conductivity porous medium, natural convection in the medium is suppressed, while the isotherms bend sharply at the fluid/porous-medium interface.

MELTING AND SOLIDIFICATION OF A METAL SYSTEM IN A RECTANGULAR CAVITY

The role of natural convection on solid-liquid interface motion during melting and solidification of Lipowitz metal in a rectangular cavity was studied. The measurements of both temperature distributions and temperature fluctuations were used as a qualitative indication of the natural convection flow regimes and structures of melt during phase transformation. The measured and predicted solid-liquid interface positions during solidification from below and above, as well as melting from above and below, show reasonably good correspondence. The suppression of natural convection in the Lipowitz metal, which is not taken into account in the model, leads to a slower rate of melting and a higher rate of freezing than that predicted.

Transient combined laminar free convection and radiation in a rectangular enclosure

The mass, momentum and energy-transfer equations are solved to determine the response of a rectangular enclosure to a fire or other high-temperature heat source. The effects of non-participating radiation, wall heat conduction, and laminar natural convection are examined. The results indicate that radiation dominates the heat transfer in the enclosure and alters the convective flow patterns significantly. At a dimensionless time of 5·0 the surface of the wall opposite a vertical heated wall has achieved over 99% of the hot-wall temperature when radiation is included but has yet to change from the initial temperature for pure convection in the enclosure. At the same time the air at the centre of the enclosure achieves 33% and 13% of the hot-wall temperature with and without radiation, respectively. For a hot upper wall the convection velocities are not only opposite in direction but an order of magnitude larger when radiation transfer between the walls is included.

Solid-liquid phase-change heat transfer and interface motion in materials cooled or heated from above or below

Solid-liquid phase-change heat transfer has been studied experimentally and analytically in several different materials (e.g. stearic acid, sodium phosphate dodecahydrate, sodium sulfate decahydrate and n-octadecane which have been suggested as candidates for latent-heat-of-fusion thermal energy storage materials. Solid-liquid interface motion during freezing and melting from above as well as below has been determined in a rectangular test cell suitable for photographic observations. Comparison of experimental data for n-octadecane with predictions based on Neumann and other analyses which account for natural convection heat transfer at the solid-liquid interface show that natural convection in the liquid must be accounted in the prediction of phase-change boundary motion for unstable situations which arise during melting from below and solidification from above.

Experimental studies of combined heat transfer in turbulent mixed convection fluid flows in double-skin-façades

Abstract The current contribution describes the experiments performed in an outdoor test stand for so called “double-skin-facades” at the Technical University of Munich. The purpose of the investigation was to determine the time and local averaged overall heat transfer coefficients for solar radiation augmented turbulent mixed convection flows in transparent vertical channels. The external plate of the vertical channel is formed with horizontally oriented ventilation gratings in the external glass facade, thus providing openings for the air circulation within the gap. In the course of the research, the distance between the external and the internal glass facade was measured for three gaps of 0.3, 0.6 and 0.9 m wide. Using the pressure compensation method, the effective mass flow rates for a given box-window, which is the smallest functional unit of all known double-skin-facade systems, was measured by means of a ventilation duct installed at the outlet of such a box-window. Finally, the experimental data was reduced in terms of an average Nusselt number as a function of an average Archimedes number for several gap distances.