centreVoidFraction

Syntax

Defined in couplingProperties dictionary.

voidfractionModel centre;
centreProps
{
    alphaMin         scalar;
    volScale         scalarList;
    volScaleGlobal   scalar;
}

alphaMin = minimum limit for voidfraction (default: 0.3)
volScale = scaling (per type) of the particle volume to account for porosity or agglomerations. (default: 1 for each particle type)
volScaleGlobal = volScale factor to apply for all particle types. This conveniently sets all elements of the volScale list to the same value. (default: 1)

Examples

voidfractionModel centre;
centreProps
{
    alphaMin     0.1;
    volScale     ( 1. );
}

Description

The centre voidFraction model calculates the voidfraction in a CFD cell accounting for the volume of the particles whose centres are inside the cell. Consequently, all particle volume is accounted for in the CFD cell which contains the particle center. This procedure is a good model for very small ratios of particle diameter to CFD cell size but becomes inaccurate for in case of large particles (compared to the CFD cell size), where it may lead to very low voidfraction values in CFD cells. It is also clear that this model cannot create a smooth transition of voidfraction when a particle crosses from one CFD cell to another.

_images/voidfraction_center_divided.png — Figure 1: Comparison of center (top) and divided (bottom) voidfraction model. The center model accounts for particle volume in the CFD cell where particle center point is located.

The resulting voidfraction can be limited to a lower value by the alphaMin keyword which discards excess particle volume.

The particle volume occupied in the CFD domain can be adjusted by the parameter volScale (referred to as $S$ ), using $V_\mathrm{p} = d^3 \pi / 6 S$ .

Note

The divided model yields more accurate results in case of large particles and for particles at cell boundaries by distributing the particle volume also across neighboring CFD cells.

Restrictions

None.