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-   -   Natural convection in water (http://www.cfd-online.com/Forums/star-ccm/132056-natural-convection-water.html)

CneSpaulding March 25, 2014 05:39

Natural convection in water
 
Hi guys,

Here is the case: I want to modelize the water natural convection.
The geometry is quite simple: it's a parallelepiped rectangle.
A surface heatsink represented by a quarter circle in a corner ( the center coordinate is (0,0,0).

MESH: Polyhedral Mesher
Prism Layer Mesher
Surface Remesher

BC: All the surfaces are wall except the 2 vertical walls between the surface are symetrics.

The physics models I used are:

Boussinesq
Constant density
Coupled Flow/energy
Gradients
Gravity
Laminar
3D
Liquid

I tried steady and unstead but the calculation do not converges. The main problem is the Z- momentum (him residuals states at around 1-10)

I saw that replacing the two walls and the top by outlet pressure instead of walls could gives results, but when I tried I had reversed flows..

Anyone can tell me wich are my mistakes and which parameters I have to modifiate?

Thanks for reading

ping March 25, 2014 10:16

natural convection is in real life an unsteady process so i would be surprise if you can get a converged solution in steady state.
so i would be running unsteady and testing a few time steps and checking the courant number and dumping scene images to check for reasonable flows.
also when using the physics you have mentioned be careful to set the coef of expansion of the fluid since star-ccm+ sets this to 0. for some reason it is not in the material database.

CneSpaulding March 25, 2014 11:25

It's working now for some reasons!
Thanks for your answer.

enayath May 5, 2014 14:09

Ping,

Could you please explain what did you do to fix it?
what is you Ra number?

How did you understand that it converged?
just by checking the residuals or you defined some points/lines inside your domain and look for a converged of averaged value for the parameter you defined...

Thank you,
Hooman

ping May 7, 2014 06:07

you had better ask CneSpaulding about how he solved his case.

but you should always have some relevant reports being monitored and plotted to help judge convergence, even for steady state cases.

CneSpaulding May 13, 2014 08:25

Hi,

Sorry for the late answer, I'm quite busy this month.

I used the following parameters:

Implicit Unsteady
Boussinesq
Coupled Flow/Energy
The 2 walls are adiabatic

And I used a better mesh.

I just checked the residuals plot, when I saw that all the parameters converged under 10-5 I decided to stop the calculation.

greg_2666 July 8, 2015 11:14

I still seem to be really struggling with this.

The fluid looks like its heating up its just not rising...

I have the models set up like above, what am I missing?

enayath July 8, 2015 11:17

greg_2666,

What's the problem exactly?
Hooman

greg_2666 July 9, 2015 04:18

So all it is, is a simple model of a server immersed in water. It is a 90 cm board held vertically in the fluid. It has two hot chips towards the bottom which should heat the liquid up next to it and in turn the water should rise.

The models activated are:

Boussinesq, Constant density, coupled energy/flow, gradients, gravity, K-e, liquid (H2O), steady, 3d, laminar, 2 layer wall treatment

The board is 90cm high and I have made a block on the surface and subtracted it to give the fluid region.

I set the temps of the two chips (modeled as 2 10x10 squares) at 320K and set the reference temp at 288K.

I've just realised i don't have a heat sink, i've only set the wall temperatures of the fluid block at 278K

I'm not seeing a heat transfer from the heat source to the water... I think it is not conducting heat from the source into the water, I've checked the properties and they all seem fine..


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