In heat transfer simulations, it is often important to model the thermal effects of the walls bounding a fluid,
but it may not be necessary to mesh them. In Fluent there are three options for modelling heat transfer in walls.
The first option is to mesh the thickness of the wall and assign it as a solid zone.
This is the most thorough approach, and you can always choose this option,
but if the wall is relatively thin compared to its length or width, there is often no difference in accuracy compared with other options
and meshing thin walls as solids can sometimes be very challenging.
In cases like that, there might also be better options: In the second option, you can assign a wall thickness
and material in the boundary conditions panel.
When you do this, Fluent will account for one dimensional conduction in the normal direction across the wall.
For one dimensional conduction in a wall, you can look in just about any heat transfer handbook, and find that the heat flux,
q, is given by the thermal conductivity, times delta T divided by the wall thickness.
So the reason you have to enter the material in the panel is to tell Fluent what to use for the thermal conductivity.
As an aside, when you enter a wall thickness, you can also define a volumetric heat source in the wall.
Normally you would know the heat source in Watts, so to find the volume of the heat source, get the
area of the wall from the surface integrals panel and multiply by the thickness entered in the boundary conditions panel.
You can also use expressions to define the wall heat generation and that might be an easier way to convert a known value in Watts
to the required value in Watts per meter cubed
There is a third option for modeling heat transfer in the wall.
If you enable “Shell Conduction” in the boundary conditions panel,
Fluent will create one or more layers of virtual cells, as shown in the picture,
and it will compute conduction both across the wall and parallel to the wall.
After turning on shell conduction, use the Edit button to define the shells.
You can define one or more layers of shells, and like in the second option, for each shell you define, the thickness and the material
have to be entered, and you can also define volumetric heat generation.
On the topic of wall conduction, there is one additional feature I want to bring to your attention.
In the case here, there are only a few walls and you can easily go to each wall one-by-one
and define its 1D thin wall or 3d Shell conduction wall properties.
But it is not unusual for industrial cases to have tens or hundreds of walls, and then the setup procedure can become more complicated.
For cases like that, you can use the Conduction Manager feature to quickly and easily define all of your wall conduction
settings. Using the Conduction Manager, which can be found in the Physics tab,
you can enable wall thickness and shell conduction for multiple walls at the same time,
and assign common settings without having to open any individual boundary conditions panels.
To see how it works, previously in this demo, I have defined “wall_board” as a shell conduction zone.
When I open Conduction Manager, you can see that it is listed under 3d-conduction-shell-walls,
while all the other boundaries are listed as ordinary walls, which means walls without any defined thickness.
So let’s say you want to assign shell conduction for all the walls of the board.
First I will use the filter text field and some wildcards to select those walls.
In case you are thinking why not just select them directly in the panel without using the filter, for sure it can be done that way too,
and it would be easy here since there are just a few walls. I just wanted to draw attention to the filter because it can be helpful in working
with complex cases which have large numbers of walls.
When I click Settings, you can see that “wall_board” is the master zone, so the entries in the panel have been pre-filled
with its settings. If necessary, you can change the master zone, but I don’t want to do that here.
Clicking OK will apply the same settings to all the selected walls, or it is also possible to change settings before clicking OK,
so for instance if I just found out that the thickness was actually supposed to be 2 millimeters,
I can change that here. Now, imagine I also want to apply a 1d thickness to some of the other walls.
I will just select the outer walls of the fluid zone and click Settings again.
In the panel, I will enter a thickness and a material, but I will not select Shell Conduction.
After then after clicking OK, you can see the walls are listed as 1d-conduction-thin-walls in the Conduction Manager.
So the conduction manager allows you to quickly see which walls are defined as conduction zones,
both 3d-shell walls and 1d-thin walls, and also which ones are not defined as conduction zones.
You can see the Read and Write buttons at the bottom of the panel.
These allow you to write the conduction settings for all of the walls to a CSV file for use in other cases,
or also to define the settings for the walls in the current case by reading in a CSV file.
You can also edit these CSV files in a spreadsheet program which is sometimes a convenient way to provide values.
So just to summarize, there are three options for handling conduction in walls.
You can mesh the wall as a solid zone, you can define a thickness and a material, or you can activate shell conduction
and Fluent will create one or more layers of virtual cells.