22.12 Setting Initial Conditions for the Discrete Phase

For liquid sprays, a convenient representation of the droplet size distribution is the Rosin-Rammler expression. The complete range of sizes is divided into an adequate number of discrete intervals; each represented by a mean diameter for which trajectory calculations are performed. If the size distribution is of the Rosin-Rammler type, the mass fraction of droplets of diameter greater than is given by

 (22.12-1)

where is the size constant and is the size distribution parameter. Use of the Rosin-Rammler size distribution is detailed in Section  22.12.1.

The primary inputs that you must provide for the discrete phase calculations in FLUENT 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 of the dependent discrete phase variables that describe the instantaneous conditions of an individual particle, and include the following:

• position ( , , coordinates) of the particle

• velocities ( , , ) of the particle

Velocity magnitudes and spray cone angle can also be used (in 3D) to define the initial velocities (see Section  22.12.1). For moving reference frames, relative velocities should be specified.

• diameter of the particle,

• temperature of the particle,

• mass flow rate of the particle stream that will follow the trajectory of the individual particle/droplet, (required only for coupled calculations)

• additional parameters if one of the atomizer models described in Section  22.8 is used for the injection

 When an atomizer model is selected, you will not input initial diameter, velocity, and position quantities for the particles due to the complexities of sheet and ligament breakup. Instead of initial conditions, the quantities you will input for the atomizer models are global parameters.

These dependent variables are updated according to the equations of motion
(Section  22.2) and according to the heat/mass transfer relations applied (Section  22.9) as the particle/droplet moves along its trajectory. You can define any number of different sets of initial conditions for discrete phase particles/droplets provided that your computer has sufficient memory.

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