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Andreas February 5, 2002 07:59

conjugate heat transfer problem
 
Dear CFD users,

I have a conjugate heat transfer problem in a complex internal flow geometry. The analysis procedure I have used is to keep the convection study separated from the conduction study, due to limitations in computational resources, and also due to the fact that mechanical integrity analysis will anyway be performed separately (with a different code) for this problem.

For the post processing of the convection study I have up till now used T_ref = Tbulk = a global temperature, which I know will not work if the fluid heat is entering and exiting the solid at the same time (at different locations) due to negative HTC's. The benefit of this approach is however that the grid dependence of the HTC is eliminated and the solution is roughly independent of the wall boundary condition (I use an "average" constant temperature at the wall). The other way would be to use the approach that is standard in most CFD codes; the near wall node temperature as a reference temperature (T_ref = T_nwn). But then the solution is probably grid dependent and surely strongly dependent on the wall boundary condition (i.e. specified temperature level in my case). What would You prefer?

regards Andreas


Robin February 5, 2002 18:22

Re: conjugate heat transfer problem
 
Hi Andreas,

A heat transfer coefficient is typically used to calculate the heat flux at a surface in your FEA code. So if you can enter the heat flux directly, do so.

Otherwise, you want to calculate a heat transfer coefficient which, when used properly, will return the proper heat flux at the surface of you FEA model.

If you use a local heat transfer coefficient that uses the near wall temperature, you will also have to include the local temperature distribution. If you were to do so, then use a constant reference temperature, your heat fluxes will be completely meaningless. This is more work than necessary.

A better approach is to calculate the heat transfer coefficient using a bulk reference temperature. Your bulk temperature could be anything, but typically you would want to choose an inlet temperature. In this way, you can provide the FEA boundary condition with a single reference temperature. The added advantage to this apprach is the ability to see the effect of different bulk temperatures, although the farther you get from the value used in you CFD analysis, the more error you will introduce.

Lastly, if you are not providing your FEA code with a profile boundary condition for the heat transfer coefficient, you should then calculate your heat transfer coefficient from the total heat flux, area averaged temperature, and a bulk reference temperature.

Personally, I would do a CHT analysis, then output the temperature in the solid from the Post processor. The FEA code will calculate the temperature seperately anyway. The added overhead for the CFD solution is very small, since you are only solving the energy equation in this region.

Best regards, Robin


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