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Old   January 20, 2010, 20:46
Default Heat transfer rate calculation
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Kevin Kerrigan
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I am doing an analysis on a finned tube channel and getting some really good temperature profile plots of the natural convection air flow within the vertical flat plate channel and around the horizontal tube which pass through it. I am using only a quarter of the model due to symmetry within the structure. I have meshed and domained a solid fin model and a fluid air channel model upon which my boundary conditions and a GGI interface are applied. I am using an opening boundary condition on the outer surfaces of the air channel and adiabatic BCs for those on the fin (which is very thin).

My question relates to the solution set which I have tried to find in the ANSYS help files. I understand how the software computes the temperature at each point (i think), but I'm having trouble understanding how the heat transfer coefficient, h, and the heat transfer rate are determined. I am using a no slip condition between the air interfacing the fin surface and would have expected to have come across something about Fourriers law for the start of the wall temperature generation, but couldn't find anything - what should I be looking for in the manual?

I know that the answer is somewhere in the user guide, but I've exhausted my vocabulary trying to search through it and have come up with nothing more than greater confusion. Can someone please help me?
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Old   January 21, 2010, 17:30
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Glenn Horrocks
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Is your flow laminar or turbulent? If laminar the temperature boundary is just the wall temperature. If turbulent it is either the wall temperature (when integrating to the wall) or evaluated with wall functions. For details see the boundary conditions section in the documentation, especially on wall functions.
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Old   February 11, 2010, 13:25
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Kevin Kerrigan
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Thanks for getting back to me Glenn.

My flow is definitely laminar and I can see from the solver theory guide that for such a flow the temperature boundary is taken from the fixed temperature input that I had applied initially. However, the guide does not state how such temperature values are used in order to determine the heat flux, and further, how the average heat flux over a specified boundary surface (such as the interface between fluid and solid in my case) is determined. Is there a numerical integration performed by ANSYS CFX over the surface in question. I have been able to plot the wall heat flux using a volume plot in the fluid domain, so I can see that there are definitely results for local heat flux values, but I cannot find out how these are computed - can you explain this?

Many Thanks
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Old   February 11, 2010, 19:35
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Heat transfer is calculated from the control volume boundary faces - the wall just becomes a boundary face. This is part of the numerical algorithm of the solver. Look into a CFD textbook and the documentation for further details - people write textbooks on this subject so I can't answer it in a post on the forum.

Intergration of the heat flux over a surface to get a total value is a post processing operation and can be done in a number of ways. See CFD-Post for details.
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