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legoman March 9, 2012 10:44

Heat Transfer Problem
 
Hello,

I have a simple problem to solve, however my results are not what I am expecting. It is the flow through a simple shell and tube heat exchanger and I have simplified the problem by taking the annular case between the two walls of the tube and outside shell. The wall of the tube is kept at a constant 700^C and the shell wall is at 300^C.

I want to find the established temperature gradient across air flow between the tube and shell. The gradient only seems to be present across the first row of mesh and then at a constant temperature for the rest of the radial distance.

I am wondering if there is a heat transfer setting when defining the fluid? Or is the problem within the mesh?

I have spent hours messing around with settings and thought it was time to ask for some help!!! Thanks in advance.

Dave

robboflea March 9, 2012 11:46

Hi Dave,

I anticipate that I'm not sure whether I can help you with your problem but, reading your question I thought that it's not clear to me what's exactly your problem.
I think it would be helpful if you attach some pictures. Also what do you mean by saying "I am wondering if there is a heat transfer setting when defining the fluid?"

Cheers

Rob

duri March 9, 2012 15:21

Dave,

Thermal gradients occurs only at thermal boundary layer and the thickness of this boundary layer is related to prandtl number. What's your prandtl number?

legoman March 12, 2012 06:29

2 Attachment(s)
Thanks for the reply, sorry about the lack of uinderstanding, maybe these pictures will try to communicate across my problem.

From my manual calculations, the temperature changes from 700^C at the bottom wall to 300^C at the top wall. The bulk fluid velocity is 50m/s flowing from left to right. The two walls are assumed to have Q=0.

The picture shows the temperature change in the first row of mesh cells, then a constant 300^C for the rest of the annular space. This is why I thought it could be an error with my mesh settings but thats only a guess.

Thanks again

robboflea March 12, 2012 10:51

Your results seem to be perfectly consistent with your boundary conditions. You have a 300^C flow flowing between two surfaces kept at 300^C (top) and 700^C (bottom). A thermal boundary layer is present close to the bottom wall due to the temperature difference between the flow and the surface and this boundary layer fully develops after some distance from the inlet.
The only thing you should ask yourself is if the boundary conditions you posed are in agreement with the physics of the problem you're trying to model.
Particularly you're saying that the two walls are assumed to have Q=0 but this is not true! First: you cannot specify both temperature and heat transfer as thermal boundary conditions on a surface: one excludes the other!
Second: a heat transfer from the bottom surface to the fluid exists and you can see it from the fact that the fluid is changing temperature!

laurentb May 10, 2012 11:00

Hi Dave,

Do you use an inflation method to mesh your boundary layer ? It's recommended to use a groth rate lower than 1.2 and it will be better if the last boundary layer cell have quite the same size as cells in the bulk.

Regards

Laurent


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