Modeling joule heating in a platinum tip plate for glass forehearth

In glass forehearth applications, clients want to model the platinum tip of a bushing block for a glass forehearth. The platinum tip has several holes through which molten glass fibers are drawn. The platinum tips also carry voltages through them which result in joule heating of the platinum and the glass surrounding it. The purpose of the joule heating is to ensure uniform temperature in the molten glass fibers. Typically, this would require specifying a voltage difference across two walls of the platinum tip plate and then solving for the voltage as a scalar and use an energy source term to calculate the amount of joule heating as a function of the voltage gradient.

By default, Fluent does not solve for a scalar either in a wall/wall-shadow zone or a solid zone.

The porous media model in Fluent 6.1 can be used to create a thin fluid zone representing the platinum tip. The porosity of this zone can be specified to reflect the actual openings in the tip plate, typically a small area ratio. To represent the glass flow vertically downward, the porous media resistance can be set to high values in the x and y directions and a lower value in the z direction.

The scalar equation for voltage should be set up without the convective part and the energy source term in the porous media should be set with a UDF to calculate the joule heating. It is important to set the electrical conductivity for the scalar (voltage) to be volume weighted by the conductivity of the glass and platinum, much like the thermal conductivity calculation for porous media.

The energy source term in the porous media should be calculated as the product of the effective electrical conductivity multiplied by the square of the voltage gradient. The electrical conductivity can be defined as a function of temperature, if necessary, in a separate UDF.

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