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13.2.6 Shell Conduction Considerations



Introduction


By default, FLUENT treats walls as zero thickness presenting no thermal resistance to heat transfer across them. If a thickness is specified for walls then the appropriate thermal resistance across the wall thickness is imposed, although conduction is considered in the wall in the normal direction, only. There are applications, however, where conduction in the planar direction of the wall is also important. For these applications, you have two options: you can either mesh the thickness or you can use the shell conduction approach. Shell conduction can be used to model thin sheets without the need to mesh the wall thickness in a preprocessor. When the shell conduction approach is utilized, you have the ability to easily switch on and off conjugate heat transfer on any wall. When you specify a thickness for the wall, a material property, and toggle the Shell Conduction switch in the Wall panel, then during the solution process FLUENT automatically grows a layer of prism cells or hex cells for the wall, depending on the type of face mesh that is utilized.

Shell conduction can be used to account for thermal mass in transient thermal analysis problems such as soaking. It can also be used for multiple junctions and allows heat conduction through the junctions. Shell conduction can be applied on boundary walls as well as internal walls.



Physical Treatment


In the case of shell conduction that is applied on a boundary wall, the boundary condition that you specify on the original wall is applied to the outer shell wall and the original wall is treated as a coupled wall (Figure  13.2.4). Note however, that internal emissivity is applied on the inner shell. The shell boundaries (the sides of the shell zone) need boundary conditions as well. If the wall with shell conduction is connected to another wall that has no shell conduction, the shell side will take its boundary condition. The sides will be adiabatic if they are connected to face zones having a boundary condition type other than a "wall". If the shell is connected to another shell of another wall at the boundary (referred to as shell junctions in FLUENT), then a net flux of zero is imposed at the shared boundary shell face zone.

Figure 13.2.4: A Case for Shell Conduction
figure



Limitations of Shell Conduction Walls


The following is a list of limitations for the shell conduction model:



Initialization


Shell zones can be patched using the Patch panel.

Solve $\rightarrow$ Initialize $\rightarrow$ Patch...



Postprocessing


Shell zones can be postprocessed. The shell cell temperature is stored in the Temperature variable (inner surface). If a more detailed analysis of he solid zone and surfaces is required, then you should consider using a layer of solid zones in your model.


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