Two years ago, Fluent began to focus on application of CFD in the glass industry. This initiative began with understanding the glass industry needs and continuously improving our capabilities to meet those needs. Our software products now offer capabilities to model glass furnaces, refiners, forehearths, fiberization, float glass operations, spout bowls, lehrs and annealing chambers. Several success stories from the industry have emerged, a selection of which are presented here.

"The models serve as a basis for more geometrically complex analyses that will allow us to optimize process improvements before time and money are spent on experimental testing."

Dr. Philip Burnside 
PPG Industries, Inc.

Heating Automotive  Windshields During Forming

In automotive windshield production, glass pieces held in position with a steel frame ride through the furnace on rollers while being heated by electric coils from the top and bottom. Spectral radiation through the semi-transparent glass medium, coupled with convection and conduction, heats the glass. Dr. Philip Burnside of PPG Industries, Inc. used the newly implemented Discrete Ordinates Model (DOM) in FLUENT 5 to simulate the heating of glass within a furnace. DOM includes the ability to handle semi-transparency and spectral radiation.

Steady and transient 2-D and 3-D calculations were performed to simulate a stationary piece of glass on a frame and a continuous sheet of glass moving through a furnace. The temperature calculations from FLUENT 5 were validated experimentally for different sets of process conditions and were within a few degrees of the measured values for both the stationary and continuous sheet.

Temperature contours of a continuous glass sheet in a furnace with two heating zones. FLUENT 5 predictions were within a few degrees of measured values.

Float Glass Forming

The process of float glass forming transforms a molten glass pool into a solid ribbon. The molten glass is conveyed on a molten tin bath and is stretched laterally by machines to enhance ribbon width while it simultaneously cools and solidifies. The placement of overhead heating and cooling units is crucial to produce high optical quality (i.e.,very uniform thickness).

The float glass forming has been successfully modeled by Dr. Rajiv Tiwary and Dr. C.K. Edge of PPG Industries, Inc. using FIDAP. The simulation results provide insight into the process, including width and thickness profiles, longitudinal and lateral stresses and residence times. Excellent agreement between the FIDAP results and plant measurements was obtained. The model was subsequently used for several parametric design studies.

FIDAP's prediction of ribbon thickness in float glass forming agreed well with field data and provided useful insight during parametric design studies.

Modeling Front End Systems

It is critical for a glass delivery system (front end system) that bridges the melter and forming to insure that glass is conditioned to the requirements of the forming operations while maintaining the highest quality. An improperly designed front end system can cause a number of problems, including poor glass quality and inhomogeneity in the glass thermal profiles. CFD has become an important tool for glass manufacturers to guide and optimize such system designs.

Recently, Dr. Christopher Jian of Owens Corning utilized FLUENT to help in the decision making processes in engineering, operations and business. The temperatures of the front end calculated using FLUENT were in excellent agreement with the measured field data. This established confidence in FLUENT for predictive purposes. FLUENT was then used to optimize the design, and the glass manufacturer was able to achieve significant cost savings.

Vertical temperature profile in a glass delivery system.FLUENT simulation helped to optimize the design and save operating costs. Plot courtesy of Owens Corning.

Coupling Combustion and Glass Tanks

Owens Corning also has successfully modeled the coupling between the combustion space and the glass tank, with forced bubbling and electrical boosting, using FLUENT 5. Two separate submodels, one for the combustion space and another for the glass tank, were coupled by passing heat flux and temperature boundary conditions on the glass surface. The batch area of the glass tank was not modeled explicitly. Owens Corning was able to take advantage of the newly implemented forced bubbling and electrical boosting subroutines in FLUENT 5.

Glass Fiber Drawing

Molten glass is drawn through a nozzle by gravity and attenuated by tension from a winder to form fiberglass. Large free surface deformation and heat transfer characterizes the process of drawing. The ratio of attenuated fiber cross sectional area to initial cross sectional area is defined as the draw ratio. The high draw-down ratio (1:10,000 and higher) makes it critical to understand the attenuation process as it influences fiber quality and productivity.

Recently, single glass fiber drawing was simulated by Dr. Bruno Purnode of Owens Corning using POLYFLOW. POLYFLOW has unique free surface remeshing algorithms that allow it to handle large draw ratios, calculate the fiber surface deformation while accounting for the highly non-linear viscosity relationship, surface tension, and special heat transfer laws. Several flow rates and process conditions of drawing were investigated. The fiber diameter in the axial (draw) direction was compared with the experimental data in literature. The POLYFLOW results at a draw ratio of 1:19,024 show excellent agreement with the experiment data.

Temperature in the combustion chamber and near the bubblers in the glass tank (offset, right) during coupled calculation of combustion and glass tank.

Fiber radius computed using POLYFLOW; comparison to experimental data. Plot courtesy of the American Ceramic Society.

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