In addition to the simple injection types described in Injection Types in the User's Guide, Ansys Fluent also provides more complex injection types for sprays describing primary breakup phenomena. For most types of injections, you will need to provide the initial diameter, position, and velocity of the particles. For sprays, however, there are models available to predict the droplet size and velocity distributions.

All of the atomization models use physical atomizer parameters, such as orifice diameter and mass flow rate, to calculate initial droplet size, velocity, and position.

For realistic atomizer simulations, the droplets must be randomly distributed, both spatially through a dispersion angle and in their time of release. For other types of injections in Ansys Fluent (nonatomizer), all of the droplets are released along fixed trajectories at the beginning of the time step. The atomizer models use stochastic trajectory selection and staggering to attain a random distribution. Further information on staggering can be found in section Staggering of Particles in Space and Time in the User’s Guide.

Stochastic trajectory selection is the random dispersion of initial droplet directions. All of the atomizer models provide an initial dispersion angle, and the stochastic trajectory selection picks an initial direction within this angle. This approach improves the accuracy of the results for spray-dominated flows. The droplets will be more evenly spread among the computational cells near the atomizer, which improves the coupling to the gas phase by spreading drag more smoothly over the cells near the injection. Source terms in the energy and species conservation equations are also more evenly distributed among neighboring cells, improving solution convergence.

Five atomizer models are available in Ansys Fluent to predict the spray characteristics from knowledge of global parameters such as nozzle type and liquid flow rate. You can choose them as injection types and define the associated parameters in the Set Injection Properties dialog box, as described in Injection Types in the User's Guide. Details about the atomizer models are provided below.