13.2.5 Shell Conduction Considerations

Introduction

By default, ANSYS 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 enable Shell Conduction in the Wall dialog box, then during the solution process ANSYS 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 inner surface and the original wall is treated as a coupled wall (Figure  13.2.4). Note however, that internal emissivity is applied at the outer surface. 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 attached wall has shell conduction, the common sides at the junction will be coupled.

Figure 13.2.4: A Case for Shell Conduction

Limitations of Shell Conduction Walls

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

• Shells cannot be created on non-conformal interfaces.

• Shell conduction cannot be used on moving wall zones.

• Shell conduction cannot be used with FMG initialization.

• Shell conduction is not available for 2D.

• Shell conduction is available only when the pressure-based solver is used.

• Shell conduction cannot be used with the non-premixed or partially-premixed combustion model.

• When used in conjunction with the Discrete Ordinates (DO) radiation model, shell conducting walls cannot be semi-transparent and should not be used with the DO/Energy Coupling Method.

• Shell conducting walls cannot be split or merged. If you need to split or merge a shell conducting wall, you will need to turn off the Shell Conduction option for the wall (in the Wall dialog box, perform the split or merge operation, and then enable Shell Conduction for the new wall zones.

• The shell conduction model cannot be used on a wall zone that has been adapted. If you want to perform adaption elsewhere in the computational domain, be sure to use the mask register described in Section 27.11.1. This will ensure that adaption is not performed on the shell conducting wall.

• Fluxes at the ends of a shell conducting wall are not included in heat balance reports. These fluxes are accounted for correctly in the ANSYS FLUENT solution, but are not listed in the flux report.

Initialization

Shell zones can be patched using the Patch dialog box.

Solution Initialization 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 the solid zone and surfaces is required, then you should consider using a layer of solid zones in your model.