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fiqs March 12, 2010 06:11

Heat Transfer simulation: No convergence problem
Hi. I am a newbie to this forum and also CFX. Thank God I found this forum. It really helps me a lot to enhance my knowledge in CFD as well as for troubleshooting.

The Model Setup

I am currently running a simulation to study the heat stratification on pipe surface of a 3m cylindrical pipe. It's a steady state simulation. The model has 2 domain, 1 is the fluid that flows inside the pipe and also a solid domain (the pipe wall thickness). The boundary conditions are, inlet, outlet, wall and interface which allows heat transfer. I assign a fixed value for the inlet temperature and also the flowrate (kg/s) for both inlet and outlet. The flow is turbulence (I use SST model) and it also has radiation (I use Monte Carlo model: Surface-to-surface mode)

The problem

After running about 3000 iterations, I got convergence for mass & momentum and domain imbalances is very low which is presume is a good result. However, there's no convergence on the average temperature of the inlet, the fluid that flows inside and also the pipe wall. Then I increase the number of iterations but to no avail. It seems, the longer the iterations, the more temperature of those 3 decreasing, as if it doesn't achieve the steady state condition.

I hope I explained the problem clear enough and really, really appreciate if any of you can help me out. I'd be more than happy to clarify further if there's a need. Thank you very much.

ghorrocks March 14, 2010 18:55

The Monte Carlo radiation model can be hard to converge. Do you need Monte Carlo? Unless you need the detailed path model which Monte Carlo includes the discrete transfer model will do the trick but be far easier and quicker to converge.

JDA April 21, 2010 15:47

One other thing to try is to include a conservation target in the solver control just below residual target. Best wishes.

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