Relationship between Velocity and Flow Rate in DPM

When using DPM model, if you want to track the particle trajectory in Ansys Fluent, setting both "Velocity" and "flow rate" can sometimes lead to confusion. This solution will clarify their meanings.

In general, velocity and flow rate are related with the following formula:
Flow rate = density * velocity * cross-sectional area
If the area of the spray nozzle and the density values are known, by setting the "flow rate" the velocity is found automatically.
If the "velocity" is given, the "flow rate" can be obtained.

For DPM, why must both "velocity" and "flow rate" be defined? Does the spray nozzle need to meet the above relationship?
When the fluid is continuous, the above equation relating velocity with mass flow rate must be satisfied as the fluid occupies the entire cross sectional area of the flow path or nozzle. However, since the spray nozzle is a device to form droplets, the supplied liquid is broken into form of droplets. These droplets are discontinuous and may not occupy the entire cross section of the nozzle. So, while the mass flow rate is same as that of continuous fluid, velocity of the droplets differs from the velocity calculated based on the above expression.
If the flow rate is supplied to the atomizer and the velocity is calculated based on the cross-sectional area, generally calculated velocity will be significantly different than reality. Usually, you need to set the velocity obtained from the experiments when certain mass flow rate is used.

For more details, refer to this document.