Better understanding of film coefficient in Autodesk CFD

[ri.na]

Hello, 

Here is the description of film coefficient in the Autodesk CFD 2017 user guide:

To account for both conduction through the volume as well as convection off the surface, assign a film coefficient boundary condition to the inside surfaces of suppressed walls, windows, or roofs. Assign a value that is the inverse of the R-value for walls or the U factor for windows:

• To simulate a wall with a total effective R value of R19, apply a film coefficient of 0.05 BTU/ft⊃2;/h/R.
• To simulate a window with a U factor of 0.30, apply a film coefficient of 0.30 BTU/ft⊃2;/h/R.

My question is:

The value of thermal conductivity (K) will be defined when we assign a specific material type to a geometric volume. Since we have the geometric model, we know the thickness of the material, thus the model already has the R-value(or U-value) information.So in my understanding that we do not need to assign the film coefficient to account for the conduction through the volume.

But why the user guide says that assign a film coefficient boundary condition to the inside surfaces of suppressed walls, windows, or roofs? By the way, I thought we could only assign the boundary conditions on the outer surfaces of a volume, am I wrong?

[Jon.Wilde]

Film coefficients can only be used on the external surfaces of a model - usually to simulate heat lost from a solid to the air. You could also model the air and then you would not need a film coefficient - they are useful to speed things up if you choose not to model/mesh the air though.

If they are applied on an inside wall, the wall must be suppressed from the mesh. This means that once the mesh is created, the film coefficient is actually on the outermost/external surface again.

Does that make sense?

[duanqa]

Why the film coefficient cannot be applied on an inside wall? 

If I apply the film coefficient on the internal surface of window, and the window is not suppressed from the mesh, what is wrong with the results?

Thank you!

[Jon.Wilde]

I think you would be confusing many things within CFD. How would this then include both conduction and convection? I suspect that it would not be able to work between both.

These should only be used on the external surfaces, to mimic both conduction and convection. Internally they will cause issues.

[duanqa]

Thanks for your reply. But I still don't understand that.

In the heat transfer calculation, setting internal film coefficient, external film coefficient and inside insulation are required (shown in the following figure), so we can know the overall heat transfer U-factor.

If we do not set the internal film coefficient, how can we get the internal film coefficient? Will the convection part be calculated by fluid (air) itself or some other way (solid)? After heat transfer calculation, we clicked the internal surface of the window, I do find the internal film coefficient, how we get this number? Does Autodesk CFD help documents have any description or equation to show this? How can we get this coefficient?

Thank you!

[kavirajvin · Accepted solution]

Hi,

The film coefficient only includes the heat transfer coefficient between air and the solid only when we are not including an air volume outside the geometry of the set up.

It does not include the thermal conductivity of the solid material.

Right?

 .

Thank you in anticipation.