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Strange pressure drop behavior

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Old   July 22, 2020, 17:54
Question Strange pressure drop behavior
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Hi Foamers,

recently, I have been working on some 3D 90 degree bends. I investigated two cases:

1: With constant diameter D1 (from inlet to outlet which means inlet pipe of D1, 90 degree bend of D1, then outlet pipe of D1)
2: With bigger diameter (D2>D1) at the inlet, then reducing to the same diameter at the outlet (which means inlet pipe of D2, 90 degree bend of D2, reducer from D2 to D1 after the bend, then outlet pipe of D1)

I expected the second geometry to have lower pressure drop; however, simulation results are showing opposite of that...

Some details: simpleFoam, incompressible, turbulent (tried kEpsilon, realizableKE, kOmegaSST all giving similar answers), used wall functions.

Any ideas would help...

Thanks
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Old   July 22, 2020, 20:38
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Ardalan
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Hi,
How much is your pressure drop?
Would you post your p file?
Try a finer mesh and check the results if you are sure about your p BCs!
Ardalan
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Old   July 23, 2020, 12:58
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Quote:
Originally Posted by Ardali View Post
Hi,
How much is your pressure drop?
Would you post your p file?
Try a finer mesh and check the results if you are sure about your p BCs!
Ardalan
Dear Ardalan,

Thanks for your swift reply.
They are 292 and 492 Pa for Case 1 and Case 2, respectively.

Sure, here is the p file:

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v1812                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      binary;
    class       volScalarField;
    arch        "LSB;label=32;scalar=64";
    location    "0/refrigerant";
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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


internalField   uniform 0;

boundaryField
{
    Inlet
    {
        type            zeroGradient;
        value           uniform 0;
    }
    Outlet
    {
        type            fixedValue;
        value           uniform 0;
    }
    Wall
    {
        type            zeroGradient;
        value           uniform 0;
    }
}


// ************************************************************************* //
I also tried with several meshes, still similar results...

Last edited by mm66; July 23, 2020 at 14:42.
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Old   July 23, 2020, 20:05
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Hi,
You say that the pressure drop for a nozzle in larger than that in a pipe! right?
It should be like this. That is not strange. The nozzle decreases the pressure and increases the velocity.

Ardalan
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Old   July 24, 2020, 10:59
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Quote:
Originally Posted by Ardali View Post
Hi,
You say that the pressure drop for a nozzle in larger than that in a pipe! right?
It should be like this. That is not strange. The nozzle decreases the pressure and increases the velocity.

Ardalan
Hi Ardalan,

Please note that the first case is a simple pipe with a 90 degree bend. The second case is the same but the inlet and bend have larger diameters. So I expect the second case to have lower pressure drop. My hand calculations also support this but not OpenFOAM...

Thanks,
MJ
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Old   July 24, 2020, 14:59
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As you have smaller outlet, the pressure decreases and the velocity increases in the outlet. Your boundary condition at outlet is set to zero so the simulation increases the pressure in the reset of the domain to keep the outlet to zero.
Anyhow, it increases the pressure gradient in the domain.


In a constant diameter pipe, pressure loss is due to the friction. In a nozzle or a sudden contraction, the dissipation will be added to the friction too. If you change anything in a constant diameter pipe you will have higher loss.
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Old   July 24, 2020, 15:38
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Quote:
Originally Posted by Ardali View Post
As you have smaller outlet, the pressure decreases and the velocity increases in the outlet. Your boundary condition at outlet is set to zero so the simulation increases the pressure in the reset of the domain to keep the outlet to zero.
Anyhow, it increases the pressure gradient in the domain.


In a constant diameter pipe, pressure loss is due to the friction. In a nozzle or a sudden contraction, the dissipation will be added to the friction too. If you change anything in a constant diameter pipe you will have higher loss.
Thanks for the reply Ardalan.
Mmmm, the outlet size is the same for both cases. It's the inlet size that increases in the second case. So I expect the second case to have lower pressure drop...
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