Step 6: Boundary Conditions

Boundary Conditions

  For this problem, you need to set the boundary conditions for three boundaries: the velocity inlet and the two outflows. Since this is a mixture multiphase model, you will set the conditions at the velocity inlet that are specific for the mixture (i.e., conditions that apply to all phases) and also conditions that are specific to the primary and secondary phases.

1.   Set the boundary conditions at the velocity inlet ( velocity-inlet-4) for the mixture.

Boundary Conditions velocity-inlet-4 Edit...

(a)   Select Intensity and Length Scale from the Specification Method drop-down list.

(b)   Retain the default value of 10 ${\rm\%}$ for Turbulent Intensity.

(c)   Enter 0.025 ${\rm m}$ for Turbulent Length Scale.

(d)   Click OK to close the Velocity Inlet dialog box.

2.   Set the boundary conditions at the velocity inlet ( velocity-inlet-4) for the primary phase (water).

Boundary Conditions velocity-inlet-4

(a)   Select water from the Phase drop-down list.

(b)   Click Edit... to open the Velocity Inlet dialog box.

i.   Retain the default selection of Magnitude, Normal to Boundary from the Velocity Specification Method drop-down list.

ii.   Retain the default selection of Absolute from the Reference Frame drop-down list.

iii.   Enter 1.53 ${\rm m/s}$ for Velocity Magnitude.

iv.   Click OK to close the Velocity Inlet dialog box.

3.   Set the boundary conditions at the velocity inlet ( velocity-inlet-4) for the secondary phase (air).

Boundary Conditions velocity-inlet-4

(a)   Select air from the Phase drop-down list.

(b)   Click Edit... to open the Velocity Inlet dialog box.

i.   Retain the default selection of Magnitude, Normal to Boundary from the Velocity Specification Method drop-down list.

ii.   Retain the default selection of Absolute from the Reference Frame drop-down list.

iii.   Enter 1.6 ${\rm m/s}$ for Velocity Magnitude.

  In multiphase flows, the volume rate of each phase is usually known. Volume rate divided by the inlet area gives the superficial velocity, which is the product of the inlet physical velocity and the volume fraction. When you have two phases, you must enter two physical velocities and the volume fraction of the secondary phase. Here it is assumed that bubbles at the inlet are moving with faster physical speed and their relative velocity with respect to water is $1.6 - 1.53 = 0.07~m/s$.

iv.   Click the Multiphase tab and enter 0.02 for Volume Fraction.

v.   Click OK to close the Velocity Inlet dialog box.

4.   Set the boundary conditions at outflow-5 for the mixture.

Boundary Conditions outflow-5

(a)   Select mixture from the Phase drop-down list.

(b)   Click Edit... to open the Outflow dialog box.

i.   Enter 0.62 for Flow Rate Weighting.

ii.   Click OK to close the Outflow dialog box.

5.   Set the boundary conditions at outflow-3 for the mixture.

Boundary Conditions outflow-3 Edit...

(a)   Enter 0.38 for Flow Rate Weighting.

(b)   Click OK to close the Outflow dialog box.