Browsing by Author "Viskanta, Raymond"

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    Analysis of heat transfer during glass forming
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2001) Viskanta, Raymond; Lim, Jongmook
    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 condidons 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.
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    Comparison of the diffusion approximation and the discrete ordinates method for the investigation of heat transfer in glass
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1999) Lee, Kong Hoon; Viskanta, Raymond
    Heat transfer by combined conduction and radiadon has been investigated in a one-dimensional glass layer. The layer is semitransparent to radiadon and the dependence of the absorption coefficient on wavelength is accounted for. The discrete ordinates method and the diffusion approximation are used to analyze radiative transfer. The differencing scheme for the discrete ordinates method has been investigated to obtain the constant total heat flux distribution across the glass layer as required by the energy conservation equation, and the best uniformity is obtained with the diamond scheme. The results predicted by the discrete ordinates method are in good agreement with those based on the exact (integral) equation formulation of radiative transfer. The diffusion approximation greatly underpredicts the temperature and heat flux distributions in the glass layer when the thickness or the opacity of the layer is small. The predictions of the diffusion approximation are only reasonable for thick glass layers. This approximation should be used with extreme caution to obtain quantitatively accurate results.
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    Effect of radiation on the melting of glass batch
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1983) Viskanta, Raymond; Wu, Xigi
    A model has been constructed to predict the temperature distribution and heat transfer in a glass batch blanket. On the basis of appropriate assumptions a mathematical model of the basic physical processes is developed which simulates the effects of the individual factors on the complex conversion process. The model is described by a partial differential equation in the unreacted batch and a nonlinear integrodifferential equation in the fused batch/molten glass layer. The solution of the equations together with the appropriate initial boundary and Interface conditions is obtained numerically. The model is used to determine the effect of radiation transfer in fused batch/molten glass layer and on the melting time of unpreheated and preheated batch. The effects of changes in the thermal conditions, blanket thicknesses, spectral absorption coefficient for the glass and glass melting model are investigated. Effet du rayonnement sur la fusion du mélange vitrifiable On a établi un modèle permettant de prévoir la distribution de température et le tranfert de la chaleur dans un tapis de composition. En partant d'hypothèses appropriées, on a développé un modèle mathématique des processus physiques de base, qui simule les effets des facteurs individuels sur le processus complexe de transformation. Le modèle est décrit par une équation différentielle partielle pour ce qui concerne le mélange non transformé, et par une équation intégrale différentielle non linéaire pour ce qui concerne le couche mélange fondu/fonte de verre. On obtient une résolution numérique des équations ainsi que des conditions initiales appropriées aux limites et à l'interface. Le modèle est utilisé pour déterminer l'effet du transfert par rayonnement dans la couche mélange fondu/fonte de verre sur la durée de fusion du mélange préchauffé ou non. On examine l'influence de variations des conditions thermiques, de l'épaisseur du tapis de composition, du coefficient d'absorption spectrale du verre et du modèle de fusion du verre. Strahlungseinfluß auf das Einschmelzen von Glasgemenge Es wurde ein Modell für die Vorhersage der Temperaturverteilung und der Wärmeübertragung in einem Glasgemengeteppich erstellt. Ausgehend von geeigneten Annahmen wurde ein mathematisches Modell der grundlegenden physikalischen Vorgänge entwickelt, das die einzelnen auf den Umwandlungsprozeß einwirkenden Größen berücksichtigt. Das Modell wird beschrieben durch eine partielle Differentialgleichung im Gemenge, in dem noch keine Reaktionen stattgefunden haben, und eine nichtlineare Integrodifferentialgleichung in der Schicht aus verschmolzenem Gemenge und geschmolzenem Glas. Die Lösung der Gleichungen unter Berücksichtigung der entsprechenden Anfangs-, Rand- und Übergangsbedingungen wird mit numerischen Verfahren erhalten. Das Modell wird angewendet zur Bestimmung des Einflusses der Strahlungsübertragung in der Schicht aus geschmolzenem Gemenge und Glasschmelze auf die Schmelzzeit von nicht vorgewärmtem und vorgewärmtem Gemenge. Der Einfluß von Änderungen der thermischen Verhältnisse, der Dicke des Gemengeteppichs, des spektralen Absorptionskoeffizienten für das Glas und das Glasschmelzmodell wird untersucht.
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    Melting behavior of continuously charged loose batch blankets in glass melting furnaces
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1986) Ungan, Aydin; Viskanta, Raymond
    The paper reports on numerical methodology for predicting the melting of a two-dimensional, axially fed glass batch blanket. In the absence of rheological data for commercial glass batch mixtures, it is assumed that each grain within batch acquires a horizontal velocity, which is constant and equal to that of charging speed throughout the transformation process. The vertical motion of batch is obtained from Newton's first law of motion by ignoring all the internal shear stresses in the vertical direction. During the transformation, the chemical reactions are simulated using modified form of Arrhenius equation. The gas percolation within the blanket is considered and the flow is predicted using Darcy's law, and heat exchange between gas and solid phase is accounted for. The formation of molten glass layer on the top of the blanket is modeled by appropriate transport equations, and the penetration of radiation from the combustion space incident on this semitransparent layer is approximated by spectral, two-flux equations. The system of equations is solved using the marching method of Patankar and Spalding [16]. The model is utilized to generate some sample results under idealized conditions. Accordingly, the heat flux at the top surface is assumed to be due to an effective gas-crown temperature. At the bottom surface, the heat flux from the melt was simulated by imposing a constant heat flux over the entire batch. A baseline condition is set for each parameter and parametric calculations were performed for a range of physically plausible conditions to determine the effects of the bottom heat transfer rate and preheating on the batch melting characteristics.
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    On the diffusion approximation for radiation transfer in glass
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1985) Viskanta, Raymond; Song, Tae-Ho
    The paper examines the validity of the diffusion approximation for radiative transfer in semitransparent materials. As an example, heat transfer by combined conduction and radiation is analyzed in a plane layer of material bounded by an opaque substrate and free space. The layer is heated radiatively and convectively. The radiation transfer problem is formulated rigorously as well as using a diffusion approximation for radiative transfer. As an example, numerical results are obtained for container glass and are used to establish the range of validity of the diffusion approximations. For small thickness (L = 0,1 m) of the layer the diffusion approximation underpredicts the temperature levels and temperature gradients at the cooled boundary. For large physical thicknesses (L = 1 m) the diffusion approximation yields reasonable predictions for the temperature distribution near the cooled boundary but overpredicts the temperature gradients near the heated boundary. The diffusion and modified diffusion approximations overpredict the heat transfer rate across the layer.
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    Recovery of dynamic temperature distribution in glass from remotely sensed spectral emission data
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1974) Hommert, P. J.; Viskanta, Raymond
    A thermal remote sensing method for recovering the transient temperature distribution in glass from remotely sens-ed spectral emission data is studied. An analytical model which relates the emerging spectral intensity of radiation to the tem-perature distribution, spectral optical properties, and radiation reflection and transmission characteristics of the interfaces is formulated. The temperature profile is expressed in the form of a finite series of LEGENDRE polynomials. This profile is determined by an optimization scheme, which by iteratively solving the expressions for the emerging intensity, recovers the distribution that best fits the spectral emission data. The accuracy and validity of the method was evaluated by measuring spectral emission from a glass plate while it cooled by radiation and free convection from an initial uniform temperature (800 K). Using the spectral emission data obtained under dynamic conditions temperature profiles were re-covered. The recovered surface temperatures were a maximum of one and one-half percent higher than those measured with thermocouples. It was found that when lower order poly-nomial approximations were used the recovered and analytically predicted profiles agreed within 1 %. Recovered profiles using more than two unknowns deviated from the predicted ones primarily due to uncertainties in optical property data. Sensitivity studies indicated that improvement in these higher order approximations could be obtained by adjusting optical property data and by proper selection of wavelengths at which spectral emission is measured. Calcul de la distribution dynamique des températures dans le verre à partir de données relatives à l'émission spectrale enregistrées à distance Une methode thermique d'enregistrement à distance de la distribution temporaire des températures dans le verre à partir de données relatives à l'émission spectrale recueillies à distance est étudiée. Un modèle analytique est développé, qui met en relation l'intensité spectrale du rayonnement émergent avec la distribution des températures, les propriétés optiques spectrales, la reflexion du rayonnement et les caractéristiques de transmission des interfaces. Le profil de température est exprimé sous la forme d'une serie finie de polynômes de LEGENDRE. Ce profil est déterminé par un schéma d'optimisation qui, par résolution itérative des expressions de l'inten-sité émergente, fournit la distribution la mieux adaptée aux données d'émission spectrale. La précision et la validité de la méthode sont évaluées en mesurant l'émission spectrale d'une plaque de verre au cours de son refroidissement par rayonnement et convection libre à partir d'une température initiale uniforme (800 K). En utilisant les données fournies par l'émission spectrale en regime dynamique, an obtient des profils de température. Les températures superficielles ainsi obtenues excèdent au maximum de 11/2% celles mesurées à l'aide de thermocouples. On constate qu'en utilisant des approximations polynömiales d'ordre inférieur, les divergences entre les profils obtenus et ceux prédits par le modèle analytique sont inférieures à 1 % Les profils obtenus en utilisant plus de deux inconnues s'écartent des profils calculés par suite, surtout, d'incertitudes dans les données concernant les propriétés optiques. Des études de sensibilité indiquent qu'une amélioration de ces approximations d'ordre supérieur pourrait être obtenue en adaptant les données relatives aux propriétés optiques et en sélectionnant correctement les longueurs d'ondes auxquelles l'émission spectrale est mesurée. Berechnung der dynamischen Temperaturverteilung in Glas durch ferngesteuerte Messung spektraler Emissionswerte Untersucht wird ein Verfahren zur Ermittlung der instationären Temperaturverteilung in Glas durch Messung der spektralen Emissionswerte. Die angegebene analytische Gleichung setzt die auftretende spektrale Strahlungsintensität zur Temperaturverteilung, zu den spektraloptischen Eigenschaften sowie zu den Reflexions- und Transmissionseigenschaften der Grenzflächen in Beziehung. Das Temperaturprofil wird durch eine endliche Reihe von LEGENDRE-Polynomen ausgedrückt. Durch ein Näherungsverfahren wird dann dasjenige Temperaturprofil bestimmt, das bei iterativer Lösung der Ausdrücke für die auftretende Intensität die beste übereinstimmung mit den spektralen Emissionswerten ergibt. Die Genauigkeit und damit die Gültigkeit der Methode wurde durch die Messung der spektralen Intensität an einer Glasplatte geprüft, die sich durch Strahlung und freie Konvektion von einer anfangs gleichmäßig verteilten Temperatur (800 K) abkühlte. Mit den spektralen Emissionswerten, die unter instationären Bedingungen gemessen wurden, wurden die Temperaturprofile errechnet. Die Oberflächentemperaturen waren maximal 1,5 % höher als die mit den Thermoelementen gemessenen Temperaturen. Wurde eine Näherung mit Polynomen niederer Ordnung verwendet, so stimmten die berechneten und die analytisch erhaltenen Temperaturprofile bis auf 1 % überein. Wurden Profile mit mehr als zwei Variablen berechnet, so wichen sie besonders auf Grund der ungenauen optischen Daten von den analytisch bestimmten Profilen ab. Diese Näherung mit Polynomen höherer Ordnung konnte dadurch verbessert werden, daß die optischen Daten korrigiert sowie geeignete Wellenlängen für die Messung der spektralen Emissionswerte ausgewählt wurden.
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    Spectral remote sensing of the dynamic temperature distribution in glass plates
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1993) Field, Robert E.; Viskanta, Raymond
    Dynamic temperature distributions in 3.71, 6.76, and 11.68 mm thick float glass test plates were experimentally determined using both the spectral remote sensing method and thermocouples fused in the plates. The measurements were made while the test plates cooled, from an initial temperature of approximately 550 °C, by radiation and natural convection in the laboratory ambient. Temperatures determined using the spectral remote sensing method are compared with those from the thermocouples fused in the glass and with theoretical predictions to demonstrate the accuracy and limitations of the spectral method. The agreement between this method, thermocouple measurements, and theoretical predictions show the spectral method can be used to measure the temperature to within approximately ± 0.5 % at the front surface, ± 1 % at the center plane, and ± 5 % at the rear surface of a 6.76 mm thick float glass plate. The accuracy of the reconstructed temperature distribution is a function of plate thickness and is limited by uncertainties in both the spectral absorption coefficient of the glass and by the accuracy with which the emerging spectral radiance can be measured.
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    Three-dimensional flow and thermal structure in glass melting furnaces : Part II. Effects of batch blanket and bubbles
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Pilon, Laurent; Zhao, Guochang; Viskanta, Raymond
    This paper is the second part of a parametric study of the flow and thermal structures in glass melting furnaces with a throat. The effects of the following parameters are discussed: a) the batch velocity, b) the melting temperature, c) the submerged depth of the batch, d) the wall heat losses, and e) the thickness of glass melt containing gas bubbles under the batch. The study indicates that the partially submerged batch and heat losses through the refractories have a strong impact on both the longitudinal and spanwise flow patterns of the molten glass. These physical phenomena must be accounted for if one wants to realistically simulate the natural convection circulation of the molten glass in the bath.
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    Three-dimensional flow and thermal structures in glass melting furnaces : Part I. Effects of the heat flux distribution
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Pilon, Laurent; Zhao, Guochang; Viskanta, Raymond
    This paper presents a study of the flow and thermal structures in the molten glass bath of a typical glass melting furnace with a throat but without air bubblers or electric boosting. Different separate effects on the flow structure of the glass melt are simulated, but only the glass melt is considered. The net heat flux distribution is imposed at the combustion space/glass melt interface, and its effects on the flow and thermal structures of the glass melt are analyzed in a systematic manner by changing the heat flux distribution while keeping the total heat input to the glass bath constant. The main purpose of the work is to evaluate the capability of the furnace operators to control the glass flow and temperature fields by adjusting the firing in the combustion space. The physical phenomena affecting the flow structure in the glass melt are analyzed and discussed in detail. The major results of the study indicate that a) the heat flux distribution has no significant effect on the flow structure of the glass melt under the batch blanket where several Rayleigh Benard cells develop in the spanwise direction, b) a heat flux gradient in the longitudinal direction is required to generate two recirculation loops in this direction, and c) a steep heat flux gradient in the refining part of the tank significantly increases the size of the refining recirculation loop near the front wall.
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    Three-dimensional numerical modeling of circulation and heat transfer in a glass melting tank Part 1. Mathematical formulation
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1987) Ungan, Aydin; Viskanta, Raymond
    This paper presents numerical methodology to simulate the transport processes in a glass melting tank. The model also considers the most widely used glass circulation enhancement schemes such as air bubblers and electric boosting. Refining process is simulated by predicting the size distribution and number density of gas bubbles within the glass melt. The gas bubbles are allowed to grow and dissolve within the melt. The diffusion of gas is restricted to a single component, and bubble rupture and collision are neglected. Contribution of gas bubbles to the buoyancy force is also accounted for. The numerical method of solution employed is summarized.
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    Three-dimensional numerical modeling of circulation and heat transfer in a glass melting tank Part 2. Sample simulations
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1987) Ungan, Aydin; Viskanta, Raymond
    In part 1 of the companion paper the mathematical formulation and numerical solution procedure was developed for the simulation of the transport processes within a glass melting tank. In this paper the utility of the model, as coupled to the batch and combustion space submodels, is demonstrated by presenting and discussing sample numerical results.