Wall Calculator Heat Flux in AutoDesk Simulation CFD

[Anonymous]

Hello,

I am hoping for some feedback regarding the use of the wall calculator in determining a heat flux through a surface. Because of problems with a more complicated model, I have setup a very simple model to run some tests. The model includes 3 layers of material with dimensions 50mm width x 50 mm Length x 5 mm thickness.

I am running a steady state model, have applied a "hot face" of 250C to one surface, and a thin film coeficient of 10 W/m2-K to the opposite and "cold face" surface. For reference, the model automatically assumes that other exterior walls with no boundary conditions are adiabatic.

My question involves the use of the wall calculator to determine heat fluxes through different materials. The results are inconsistent.

SITUATION 1: Model Returns correct Heat Flux when using wall calculator with hot and cold faces of assembly

When I select the hot and cold faces of the assembly to measure a net flux through the assembly, the flux calculator seems to work correctly and returns an overall 0 Watt flux , as should be expected in the steady state operation(i.e., roughly 7.5 Watts entering on the hot face; 7.5 Watts leaving on the cold face):

SITUATION 2: Model shows a net (non-zero) flux on middle layers inside the assembly.

HOWEVER, suppose that I want to measure the flux penetration on the middle layer only (i.e., I have removed the outer layers and am calculating flux on only the middle layer). The layer - in the steady state model - should return an overall 0 Watt heat flux, just like the first calculation. That is, at steady state, the flux coming in should equal the flux leaving this layer. This does not happen! WHY>?

Again, this is a simplified version of the actual problem I am experiencing. But, with this type of inconsistency in such a simple model, I don't know how I can expect the more complicated model to report the correct values/calculations!

Does anyone have any insight as to why this might be happening, and/or how the wall flux calculator works?

Thanks in advance for anyone who has experienced a similar problem or knows how to help!

[AstroJohnPE]

This knowledge base posting is not directly related to your topic: http://help.autodesk.com/view/SCDSE/2014/ENU/ It is related to calculating the average temperature at an inlet or outlet. But the posting does say "The wall calculation tool should only be used on walls.", so I wonder if that is your problem. The interface between two parts is not a wall.

The knowledge base posting goes on to say "The correct method to measure the average temperature at an opening is with the bulk calculator." I do not know if that can be used for the heat fluxes, but it would be something to try.

[Anonymous]

Hi John,

Thanks for the reply. I appreciate the insight.

However, I seem to have found an AutoDesk help tutorial that indicates an interior wall (interface) is acceptable... The caveat lies in that the interface is fluid-solid interface, not a solid-solid like in the example I provided.

Maybe the wall calculator will not work - as you mentioned - between two interior, solid walls? For reference, the tutorial can be found in the Essential Skill Videos and is called "How to Asses wall Results".

Unfortunately, no details are provided on the limitations of the calculator when using it in this regard.

Thanks again for the reply,

Megan