Showing papers by "Raymond Viskanta published in 2001"

Jet Impingement Boiling From a Circular Free-Surface Jet During Quenching: Part 1—Single-Phase Jet

Abstract: A proposed technique for controlling jet impingement boiling heat transfer involves injection of gas into the liquid jet. This paper reports results from an experimental study of boiling heat transfer during quenching of a cylindrical copper specimen, initially at a uniform temperature exceeding the temperature corresponding to maximum heat flux, by a two-phase (water-air), circular free-surface jet. The second phase is introduced as small bubbles into the jet upstream of the nozzle exit. Data are presented for single-phase convective heat transfer at the stagnation point, as well as in the form of boiling curves, maximum heat fluxes, and minimum film boiling temperatures at locations extending from the stagnation point to a radius of ten nozzle diameters. For void fractions ranging from 0.0 to 0.4 and liquid-only velocities between 2.0 and 4.0 m/s (11,300≤Re d,fo ≤22,600), several significant effects are associated with introduction of the gas bubbles into the jet. As well as enhancing single-phase convective heat transfer by up to a factor of 2.1 in the stagnation region, addition of the bubbles increases the wall superheat in nucleate boiling and eliminates the temperature excursion associated with cessation of boiling. The maximum heat flux is unaffected by changes in the void fraction, while minimum film boiling temperatures increase and film boiling heat transfer decreases with increasing void fraction. A companion paper (Hall et al., 2001) details corresponding results from the single-phase jet.

Two-dimensional combined conduction and radiation heat transfer: comparison of the discrete ordinates method and the diffusion approximation methods

Abstract: Heat transfer by combined conduction and radiation in a two-dimensional semitransparent medium has been investigated. The discrete ordinates method (DOM) and the Rossland diffusion approximation are used to analyze radiative transfer. Glass is considered as an example of a radiation absorbing and emitting medium, and the spectral dependence of the absorption coefficient on wavelength is accounted for. The results predicted by the DOM are in good agreement with those based on the one-dimensional integral formulation. However, when the opacity of the medium is large, the DOM suffers from the numerical smearing, which distorts the radiative flux distribution. The diffusion approximation greatly underpredicts the temperature and flux distributions in the medium, particularly when the thickness or the opacity of the medium is small. The predictions of the diffusion approximation are only reasonable for the thick layer. Hence, the approximation should be used with extreme caution to obtain quantitatively accura...

Analysis of heat transfer during glass forming

Abstract: A thermal model is described which is intended to simulate internal heat transfer in glass being cooled by the mold and plunger after pressing. The heat transfer analysis in glass accounts for the spectral nature of radiation, the dependence of the thermophysical properties of glass on temperature and the contact heat transfer between mold and glass as well as plunger and glass during and after pressing. Heat exchange between glass and mold as well as plunger across a very small gap by contact conduction and thermal radiation are also accounted for. To assess the utility of the Rosseland diffusion approximation for radiative transfer, the results are compared with those based on rigorous formulation of radiative transfer. Numerical solutions have been obtained for typical conditions simulating symmetric and asymmetric cooling as well as cyclic operation, and the results are presented and discussed. During the dwell time thermal contact conduction between the glass and the mold as well as plunger is the dominant heat extraction mechanism from the glass. Results show that radiation from the surface of the glass plays a relatively small part in the heat extraction process, but radiation from the interior of the glass is much more significant but less important than thermal contact conduction.