5.2.4 DEFINE_PB_NUCLEATION_RATE

You can use the DEFINE_PB_NUCLEATION_RATE macro if you want to define your own particle nucleation rate. The function is executed at the beginning of every time step.

Usage

DEFINE_PB_NUCLEATION_RATE(name, cell, thread)

Argument Type	Description
char name	UDF name
cell_t cell	Cell index
Thread *thread	Pointer to the secondary phase thread
Function returns
real

There are three arguments to DEFINE_PB_NUCLEATION_RATE: name, cell, and thread. You will supply name, the name of the UDF. cell and thread are variables that are passed by the ANSYS FLUENT solver to your UDF. Your UDF will need to return the real value of the nucleation rate.

Example

Potassium chloride can be crystallized from water by cooling. Its solubility decreases linearly with temperature. Assuming power-law kinetics for the nucleation rate,

where $K_n = 4 \times 10^{10}$ particles/m $^3$-s and $N_n = 2.77$.

/************************************************************************
UDF that computes the particle nucleation rate
*************************************************************************/

#include "udf.h"
#include "sg_pb.h"
#include "sg_mphase.h"

 DEFINE_PB_NUCLEATION_RATE(nuc_rate, cell, thread)
{
  real J,  S;
  real Kn = 4.0e10; /* nucleation rate constant */
  real Nn = 2.77; /* nucleation law power index */
  real T,solute_mass_frac,solvent_mass_frac, solute_mol_frac,solubility;
  real solute_mol_wt, solvent_mol_wt;

  Thread *tc = THREAD_SUPER_THREAD(thread); /*obtain mixture thread */
  Thread **pt = THREAD_SUB_THREADS(tc);     /* pointer to sub_threads */
  Thread *tp = pt[P_PHASE];                 /* primary phase thread */

  solute_mol_wt = 74.55; /* molecular weight of potassium chloride */
  solvent_mol_wt = 18.; /* molecular weight of water */
  solute_mass_frac = C_YI(cell,tp,0);
  /* mass fraction of solute in primary phase (solvent) */

  solvent_mass_frac = 1.0 - solute_mass_frac;
  solute_mol_frac = (solute_mass_frac/solute_mol_wt)/
  ((solute_mass_frac/solute_mol_wt)+(solvent_mass_frac/solvent_mol_wt));

    T = C_T(cell,tp);   /* Temperature of primary phase in Kelvin */

  solubility = 0.0005*T-0.0794;
  /* Solubility Law relating equilibrium solute mole fraction to Temperature*/

  S = solute_mol_frac/solubility; /* Definition of Supersaturation */

 if (S <= 1.)
      {
        J = 0.;
      }
   else
      {
        J = Kn*pow((S-1),Nn);
      }
   return J;
}

Note that the solubility and the chemistry could be defined in a separate routine and simply called from the above function.