Learning Forum

[sjyang]

What is the difference between calculating the total heat transfer rate in the Fluxes section and calculating the total surface heat flux in the Wall Fluxes section of Surface Integrals in ANSYS Fluent?

 

 

When calculating the total heat transfer rate on a mass-flow-inlet surface in the Fluxes section, a non-zero value is returned. However, when calculating the total surface heat flux in the Surface Integrals section, it returns zero. I am curious to know why these results differ.

[Rob]

How do the two values compare on a wall?

[sjyang]

Hi, thank you for your response.

For a wall boundary condition(isothermal temperature wall condition), the surface integral of the total surface heat flux and the total heat transfer rate in the flux report yield the same value. 

However, as I mentioned, for a mass-flow-inlet(isothermal temperaure fluid condition), these two values are different. It is weird that the two values differ for the mass-flow-inlet. Could it be that only conduction heat transfer is considered in the surface integral of the total surface heat flux?

[Rob]

So you're wondering why a wall function doesn't agree with a flux definition on an inlet? 

[sjyang]

Yes, that's correct.

[Rob]

The clue is in the definition: WALL function. There's no flow through a wall. 

[sjyang]

I gave you the wrong answer on October 11th, and I sincerely apologize. What I'm actually curious about is not why a wall function doesn't agree with a flux definition on an inlet, but rather why the surface integral of the total surface heat flux and the total heat transfer rate in the flux report output different values at the inlet.

I am attaching a screenshot of the phenomenon. Please take it into consideration. Thank you

 

[The surface integral of the total surface heat flux]

[The total heat transfer rate in the flux report]

[Rob]

It's a WALL flux. It's not designed to deal with convection. What is the inlet temperature?

[sjyang]

Thank you for your reply. The inlet temperature is 700°C. 

As I understand your reply, is it correct that the surface integral of the total surface heat flux is a function only available for walls, and should I use the total heat transfer rate in the flux report to know the heat flux at the inlet?

[Rob]

Yes, that's why it's in the Wall Fluxes section.  Use the Flux reports for all surfaces when judging where heat goes.  Note, you may see a non-zero heat flux on the inlet based on it's temperature and material properties.