[xpqiu]

Hi, every one

I am working on OpenFOAM-2.1.1, I need to simulate a gas-solid two phase flow with twoPhaseEulerFoam, the geometry of my simulation is a 2d-rectangular, bottom is inlet, top is outlet, and side patches are walls.
gas phase velocity BC is set as follows:

"Ub"

boundaryField
{
    walls
    {
        type            fixedValue;
        value           uniform (0 0 0);
    }
    outlet
    {
       type             zeroGradient;
    }
    inlet
    {
        type               fixedValue;
        value              uniform (0 0.20 0);
    }
    frontAndBackPlanes
    {
        type               empty;
    }
}

For solid phase, I need the solid exitting the top outlet recirculate back to the bottom inlet, so I used groovyBC for inlet of solid phase velocity, as follows:

"Ua"

boundaryField
{
    walls
    {
        type               fixedValue;
        value              uniform (0 0 0);
    }
    outlet
    {
        type               zeroGradient;
    //type                 inletOutlet;
    //inletValue           uniform (0 0 0 );
    //value                uniform (0 0 0 );
    }
    inlet
    {
        //type               fixedValue;
        //value              uniform (0 0 0);
    type                 groovyBC;
    valueExpression      "-inVel*normal()"
    value                uniform ( 0 0 0 );
    variables (
        "A=sum(area());"
        "outFlow{outlet}=sum(Ua&normal()*area()*alpha);"
        "myFlow=outFlow/alpha;"
        "inVel=myFlow/A;"
    );

    }
    frontAndBackPlanes
    {
        type               empty;
    }
}

where "alpha" is the volume fraction of solid phase.

Boundary condition for alpha are set as:


"alpha"

boundaryField
{
    inlet
    {
        type            fixedValue;
        value           uniform 0.2;
    }
    outlet
    {
        type              zeroGradient;
    }
    walls
    {
        type            zeroGradient;
    }
    frontAndBackPlanes
    {
        type            empty;
    }

Note that I used a fixed inlet solid phase volume fraction.


The simulation worked fine at the beginning, but as the solid phase approached the top outlet, and started to recirculated back to bottom inlet, things got strange. It seems that the mass conservation of solid phase is violated. Here are some of my logs:

At the beginning , total solid phase volume fraction is:

Dispersed phase volume fraction = 0.2925  Min(alpha) = 0  Max(alpha) = 0.6

But at Time = 8.65s, solid phase approached at the top outlet, and total solid phase volume fraction started to increase:

Time = 8.65

DILUPBiCG:  Solving for alpha, Initial residual = 0.00712221, Final residual = 6.46064e-11, No Iterations 3
Dispersed phase volume fraction = 0.292501  Min(alpha) = 0.0141432  Max(alpha) = 0.565962

from that on, total solid phase volume fraction increased monotonically, and reached 0.293174 at time 20.4s :

Time = 20.4

DILUPBiCG:  Solving for alpha, Initial residual = 0.00496807, Final residual = 1.18516e-11, No Iterations 3
Dispersed phase volume fraction = 0.293174  Min(alpha) = 0.0697732  Max(alpha) = 0.513855

I monitored the flux of solid phase at outlet and inlet with "simpleSwakFunctionObjects" , here is the code in the controlDict file:

massConservationTest
    {
        type patchExpression;
        outputControlMode   outputTime;
    patches (
        inlet
        outlet
        );
    verbose true;
    expression "(Ua & normal())*alpha*area()";
    accumulations (
        sum
    );
    }

And the flux at inlet and outlet are always equal in value and opposite in direction.

       Time       flux of inlet       flux of outlet
          8.65     -0.000376597          0.000376597
          8.7     -0.000382609          0.000382609
         8.75     -0.000377901          0.000377901
          8.8     -0.000374998          0.000374998
         8.85     -0.000373855          0.000373855
          8.9     -0.000377711          0.000377711
         8.95     -0.000387632          0.000387632
            9     -0.000393048          0.000393048
         9.05      -0.00039903           0.00039903
          9.1     -0.000403242          0.000403242
         9.15     -0.000376861          0.000376861
          9.2     -0.000323375          0.000323375
         9.25     -0.000147949          0.000147949
          9.3      -0.00010581           0.00010581
         9.35     -0.000138232          0.000138232
          9.4     -0.000138196          0.000138196
         9.45     -4.15751e-05          4.15751e-05
          9.5     -2.36378e-05          2.36378e-05
         9.55      -9.4021e-05           9.4021e-05
          9.6     -8.63874e-05          8.63874e-05
         9.65     -8.27537e-05          8.27537e-05
          9.7     -9.60432e-05          9.60432e-05
         9.75     -0.000152863          0.000152863
          9.8     -0.000159683          0.000159683
         9.85     -0.000191661          0.000191661
          9.9      -0.00022118           0.00022118
         9.95     -0.000152557          0.000152557
           10     -0.000189653          0.000189653
        10.05     -0.000233514          0.000233514
         10.1     -0.000197505          0.000197505
        10.15     -0.000214339          0.000214339
         10.2     -0.000235228          0.000235228
        10.25     -0.000221006          0.000221006
         10.3     -0.000226268          0.000226268
        10.35      -0.00025188           0.00025188
         10.4     -0.000264869          0.000264869
        10.45     -0.000206712          0.000206712
         10.5     -0.000236892          0.000236892
        10.55     -0.000204415          0.000204415
         10.6     -6.89294e-05          6.89294e-05
        10.65     -7.52851e-05          7.52851e-05
         10.7     -9.99905e-05          9.99905e-05
        10.75     -6.23968e-05          6.23968e-05
         10.8      8.60961e-05         -8.60961e-05
        10.85     -4.74735e-05          4.74735e-05
         10.9     -7.44399e-05          7.44399e-05
        10.95     -3.77192e-05          3.77192e-05
           11     -1.52147e-05          1.52147e-05

It seems that the flux on both inlet and outlet are reasonable, and flux on inlet is exactly conserved with flux on outlet, but why the total volume fraction of solid phase is not conserved ?

Thanks a lot for any hints