Problem  Natural Convection
Hello guys!
I am facing a transient cooling problem: The physics of the problem is as follows: a body of 320K organic material should be cooled by natural convection in air at 283K. Experimentally the cooling of this solid body takes place in 5 hours. I am performing simulations in FLUENT 19.2. but I can not get satisfactory results in relation to the time versus temperature plot. I'm using 100 time steps with 180s in size. Can anyone help me with this? 
Just a question: 1000 timesteps with 180s is 50 hours, not 5. Is it mistype or mistake?

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Put some solution monitors, probably your solution isn't converged at all at each timestep. 
Sorry
Yes! i messed up
There are 100 time steps with 180s in size.:) 
This is one of the problems!
I do not have data of heat flow, besides the geometry and properties of the material the data that I have are those described above. That is, I want to check if the body really cools at the given time. The input data are only the temperature of the solid, the domain and the total time for cooling. 
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In the problem in question I only need to impose deltaT and estimate an appropriate time step, this is what I am not succeeding! 
If in this case, when finding a time step with the ideal size and that it is very small, could I increase the number of steps so that it covers all the physical time imposed by the real problem? That is my fundamental question.

Yes. You cannot pick arbitrarily your timestep size. You only need a temporal resolution of 180s because you only want to see 100 temperature points in time for comparison. But you need to pick one small enough that makes numerical sense for the solver. Yes you can always just increase the number of timesteps to get the desired total time.
I'll give you a warning though. You might be surprised that you will need a very small timestep size and very large number of timesteps. The physics are not complex, but this is a computationally expensive problem because the heat diffusion happens at very small length and time scales. Also I don't follow what your domain is. Is it only the solid with a convective boundary condition or a conjugate problem (solid and fluid domain)? 
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Exactly LuckyTran! The problem is not complex, the case studied is the conjugate (heat transfer between a solid and a fluid). Follow the figure below.https://www.cfdonline.com/Forums/at...1&d=1548445257

Okay, cool. So just check to make sure your solution is converged every timestep. And keep shrinking the timestep until it does.
A fast easy check is to get the Courant number (it's a field in Fluent). Your Courant number should be low (less than 10, ideally less than 0.5 everywhere). Choose a small enough timestep size so that you can get the Courant number around this low. If your Courant number is too big, you flow solution will not be accurate or stable. The Biot and Fourier number for the solid will be even smaller than this Courant number, but you do not have stability problems to worry about over there. I anticipate this problem needing a very long calculation time. It might be cool to try it anyway. If it takes too long, one thing you can do is do a steady problem with the full domain. This lets you get the heat transfer coefficients on the surface of the solid body. Then you can run another simulation with only the solid body with the convection BC being applied. Of course you will miss out on nonlinearities due to problem coupling. 
Thank you very much for the help! I had many doubts about this case! During all this time working on this few gave me tips as valuable as you!
Thank you! 
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