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pras January 21, 2013 19:14

simulation problem -- convection BC
Hello i have a problem with a simulation I was running. I am modeling melting of a salt in a cylinder (a transient problem). The cylinder has a wall thickness. The outside wall of the cylinder has a convection condition with a fluid of temperature ~590K and heat transfer coeff of 19-35 w/m2-k. The inside of the cylinder has salt and air. the salt melts at ~579K. the initial temp is 578K. The cylinder wall material is aluminum. The inside wall has a shadow, as an interface, which is coupled to the inside wall.

I have done 7 cases of cylinders with different dimensions for the same case as above except with an isothermal boundary condition on the outside wall, instead of convection. All the isothermal cases computed fine. I opened the isothermal case file and changed the BC constant temperature to convection with constant h and free stream temperature, and reinitialized.

When I ran the simulation, the energy equation residual was constant line, as with all the other residuals. This behavior lasted during the entire computation. at time = 60 seconds, I checked the static temperature contour; it showed that the highest temperature in the system was 579K (this should be near 590K, especially since a metal with high thermal conductivity is used). I created a integral total surface heat flux monitor at the inside wall of the cylinder. This monitor stated that the heat flux rose from 3.26 to 3.44 and then stayed constant at 3.44 for rest of the computation, which doesn't make sense.

I ran the same case except for a cylinder with no wall thickness, with a convection boundary condition. This case simulated fine and began melting rather quickly.

I do not understand why the convection case for a cylinder with a wall did not compute correctly. No matter what I tried the residuals would always come out as a flat line. The residuals came out fine (as a jigsaw with quick convergence) in the case of a cylinder with a wall with isothermal boundary condition and the case of a cylinder without a wall with a convection boundary condition.

I would greatly appreciate any ideas on what I could do to fix my case for the cylinder with wall with convection boundary condition. I have made a zip file with the case and data files for fluent14 for the cylinder convection cases with and without the wall. I tried to attach them but they are over the size limit. Is there a way I can show the case and data files to help other see my problem?

pras January 22, 2013 21:47

I also have an related question. If I have a convective boundary condition, does that mean I would have to use the conjugate heat transfer method to compute my model? I assumed I can change the boundary condition from isotherml to convection and it would work. But it does not seem that the flux is getting through the wall. When is the appropriate time to use conjugate heat transfer?

pras January 24, 2013 17:59

Wanted to bump this thread, because I did not see any feedback. I wanted to update the problem a little as well: Case 1: cylinder with 0 wall thickness. I impose a convection BC on the outside of cylinder with consant convection coeff, and free stream temp. When computation is done, the flow is correctly solved but the heat transfer is not. The flux monitor on the wall displays ~2 Watts, but it should display ~200 for the convection coeff and free stream temperature I have input (19.55 W/m2-K, and 589 K, while cylinder surface is at 578 K) Case 2: cylinder with 2 millimeter wall thickness. The inside wall is coupled with a shadow wall. A convection BC with constant convec coeff and free stream temp is placed on the outside wall. During computation, flow comes out correct but there is no heat transfer. The heat flux monitor on the inside wall displays ~2-3 Watts when it should be 2 orders of magnitude higher. Heat is not flowing into the cylinder in either Case 1 or 2. This only occurs when I place a convection BC. Case 1 and 2 compute correctly (both the flow and heat) if the BC used is a isothermal rather than convection. When I put convection BC on the wall, is it required that I input the wall thickness and/or enable shell conduction? Does anyone have any ideas about this? I would greatly appreciate any help.

msaeedsadeghi January 26, 2013 00:58

The problem is that you have written too much. Please Write your problem on one or two lines.

pras January 30, 2013 11:41

I am sorry that the posts were verbose.

When all is said and done, When I set a constant free stream temperature and constant convection coefficient boundary condition on the outside wall of my cylinder, I do not have heat flowing into the cylinder. There is no flux coming into cylinder from the convection wall. Due to this there is no temperature rise in my system.

This is what my problem was. I will be uploading another post with pictures ( i will make it simpler about my problem). Hopefully it will be a lot clearer

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