CFD Online Discussion Forums - multiphaseEulerFoam error when adjusting dispersed group

I have a simulation which has dispersed phase "air1" and "particle" with phaseProperties like below.

Code:





air1

{

type            pureIsothermalPhaseModel;



    diameterModel   velocityGroup;



    velocityGroupCoeffs

    {

        populationBalance    air3;



        shapeModel           spherical;



        sizeGroups

        (

            f1 {dSph  4e-3; value0.4;}

            f2 {dSph  5e-3; value0.2;}

            f3 {dSph  6e-3; value0.4;}



        );

    }



    residualAlpha   1e-8;

}



particle

{

type            pureIsothermalPhaseModel;



    diameterModel   velocityGroup;



    velocityGroupCoeffs

    {

        populationBalance    air4;



        shapeModel           spherical;



        sizeGroups

        (

            f4 {dSph  7e-3; value0.2;}

            f5 {dSph  8e-3; value0.6;}

            f6 {dSph  9e-3; value0.2;}

        );

    }



    residualAlpha   1e-8;

}

then i want to change the particle phase size groups into smaller one, so i changed its residualAlpha as well into this setting

Code:

particle

{

type            pureIsothermalPhaseModel;



    diameterModel   velocityGroup;



    velocityGroupCoeffs

    {

        populationBalance    air4;



        shapeModel           spherical;



        sizeGroups

        (

            f4 {dSph  7e-5; value0.2;}

            f5 {dSph  8e-5; value0.6;}

            f6 {dSph  9e-5; value0.2;}

        );

    }



    residualAlpha   1e-11;

}

now the simulation become divergent with this error

https://www.cfd-online.com/Forums/at...1&d=1647987792

What should I replace/change to make the simulation run well? I've seen the titania synthesis in tutorial have smaller phase size and can be executed with no problem. What should I consider in setting the phaseProperties? thanks in advance