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 EshitaPal June 2, 2013 09:03

Boundary condition for transient natural convection

1 Attachment(s)
Hello,

I am using Openfoam 2.2 and trying to simulate natural circulation in a tank with multiple inlets at the bottom and multiple outlets at the top. As I run the transient buoyantBoussinesqPimpleFoam solver, the flow reverses very quickly, and the water starts flowing out of the inlet. Are my boundary conditions appropriate? Thanks.

U file

dimensions [0 1 -1 0 0 0 0];

internalField uniform (0 0 0);

boundaryField
{

pipe
{
type fixedValue;
value uniform (0 0 0);
}

inlet1
{
type pressureInletOutletVelocity;
value uniform (-0.1414 0 0);
}
inlet2
{
type pressureInletOutletVelocity;
value uniform (-0.130636565 -0.054111437 0);
}
inlet3
{
type pressureInletOutletVelocity;
value uniform (-0.099984898 -0.099984898 0);
}
outlet
{
}
symmetry
{
type symmetryPlane;
}

wall
{
type fixedValue;
value uniform (0 0 0);
}

}

p_rgh file

dimensions [0 2 -2 0 0 0 0];

internalField uniform 0;

boundaryField
{
pipe
{
type buoyantPressure;
rho rhok;
value uniform 0;
}
inlet1
{
type totalPressure;
p0 uniform 51.993; // this is equal to g*(height of the tank)
U U;
phi phi;
rho rhok;
psi none;
gamma 1;
value uniform 0;
}
inlet2
{
type totalPressure;
p0 uniform 51.993;
U U;
phi phi;
rho rhok;
psi none;
gamma 1;
value uniform 0;
}
inlet3
{
type totalPressure;
p0 uniform 51.993;
U U;
phi phi;
rho rhok;
psi none;
gamma 1;
value uniform 0;
}
outlet
{
type buoyantPressure;
rho rhok;
value uniform 0;
}
symmetry
{
type symmetryPlane;
}

wall
{
type buoyantPressure;
rho rhok;
value uniform 0;
}

}

 fredo490 June 2, 2013 10:58

Question, why using buoyantBoussinesqPimpleFoam ? Do you have any heat transfer ? Is it to consider the Gravity ?

Is your flow really driven by the total pressure ? The pressure you have set looks quite small (especially for water).

 EshitaPal June 2, 2013 11:27

The tubes inside the geometry has surface heat flux b.c. There is a total heat generation of 2MW in a volume of 198 m3. I do consider gravity for the natural convection flow generated. The idea is to generate natural convection flow that will exit through the outlets at the top.

The working fluid is water. I am not very sure about the pressure condition. The tank is part of a natural circulation loop. The flow should thus be generated due to the heat transfer from the hot tubes. The pressure is not the driving force.

 fredo490 June 2, 2013 11:48

My mistake, I forgot your topic tittle while I was reading it :D

Well, I don't think that a pressureInletOutletVelocity is suitable in your case. Personally, I would have used a simple "zeroGradient" for the velocity boundaries. But I don't know if it is enough, you can give it a try.

To me, your pressure boundaries are ok but not the velocity. If your problem persists, you can try to increase slightly the pressure at the inlet to "force" the fluid to go up and once the global flow behavior is catch, you can return to the original pressure value.

 EshitaPal June 2, 2013 12:15

Thank you. I shall try if this works.

Dear fredo490, I ahve similar problem, natural convection in channel (with inlet and oultlet), I have a problem to specify the boundary conditions for p and p-rgh (inlet and outlet)
I need help

 JR22 August 14, 2013 12:04

If anybody is still interested, I just saw that there was a discussion on natural convection in the 2013 OpenFoam Workshop:
http://www.openfoamworkshop2013.org/...aeKim-OFW8.tar

I also had some problems with getting buoyantBoussinesqPimpleFoam to converge. I changed buoyantPressure BCs for fixedFluxPressure and the problem started converging. Give it a try.

 Kanarya April 29, 2015 10:51

Hi I am using buoyantBoussinesqSimpleFoam for heat transfer in single pipe..so I have one inlet and outlet and wall..
I have BC for pressure as following:
PHP Code:

``` dimensions      [0 2 -2 0 0 0 0];internalField   uniform 0;boundaryField{    inlet    {        type            zeroGradient;    }    outlet    {                type fixedValue;        value \$internalField;                  }    wall    {        type            zeroGradient;    }}  ```
and for p_rgh

PHP Code:

``` dimensions      [0 2 -2 0 0 0 0];internalField   uniform 0;boundaryField{    wall{type  fixedFluxPressure;rho rhok;value uniform 0;}outlet{type  fixedFluxPressure;rho rhok;value uniform 0;}inlet{type fixedValue;value uniform 0;} }  ```
for T

PHP Code:

``` dimensions      [0 0 0 1 0 0 0];internalField   uniform 363.15;boundaryField{    wall    {         type            fixedValue;        value           uniform 343.15;    }    inlet    {        type            fixedValue;        value           uniform 363.15;    }    outlet    {        type            zeroGradient;    }}  ```
and alpha

PHP Code:

``` dimensions      [0 2 -1 0 0 0 0];internalField   uniform 0;boundaryField{    wall    {        type            alphatJayatillekeWallFunction;        Prt             0.85;        value           uniform 0;    }    inlet    {        type            calculated;            }    outlet    {       type            calculated;    }}  ```
I have a laminar flow and i am getting results but they seems like wrong...
Can you see any mistake in the BC?