USING DISCRETE PHASE WITH FLUENT
The first thing to do is to define a model for the discrete phase.
Here we can see the window of discrete phase modeling.
The tracking parameters are Max. Number of steps and Length scale.
The Max Number of Steps is the maximum number of time steps used to compute a single particle trajectory. So, if you want to track a particule on a long time, you must increase this parameter.
The length scale L controls the integration time step used to integrate the equations of motion of the particle.Its equivalent to the distance that the particle will travel before its motion equation are solved again and its trajectory is updated.
Dt = L / (up + uc)
where 
up is the velocity of the particle

uc is the velocity of the continuous phase.
The stochastic model
It used for turbulent flows.
The Number of tries : An input of zero compute the particle trajectory based on the mean continuous phase velocity field, ignoring the effects of turbulence on the particle trajectories. An input of one or greater includes turbulent velocity fluctuations in the particle force balance. One trajectory calculation are performed if you input one, two if you input two , etc ...
You must then define the injection properties.
Use principal windows of Fluent :
DEFINE > INJECTIONS....
Choose create a new injection ....
You can see the panel below :
We can choose the name of the injection , the injection type, the particle type, and the point properties.
The particle type is inert, because there are no chemical reactions and no combustion.
Injection type : You should create a single injection when you want to specify a single value for each of the initial conditions (see Point properties). Create a group injection when you want to define a range for one or more of the initial conditions. We didn't use the surface and the random injection type.
Number of particle streams is the number of streams you want to see.
Point properties : You must provide for the discrete phase calculations are the initial conditions that define the starting positions, velocities, and other parameters for each particle stream. These initial conditions provide the starting values for all the dependant discrete phase variables that describe the instantaneous conditions of an individual particle.
You have the position, the velocities , the characteristic diameter and the temperature of the first and the last particles.
You must now choose the material. This material can be defined in the DEFINE > MATERIALS menu.
Here you can choose an inert particle and change the physical properties as you want.
To see the track of particles click on the name of the injection you have done (here Injection  1) then click on Display. Note that you can display all the parameters you want (Particle variables , etc ...)