[franzdrs]

Hello,

I want to simulate in ANSYS Fluent the drying of droplets (spray drying) using a custom law, since the evaporation law of Fluent is for pure liquid droplets. I have been reading the UDF manual, the Users guide, etc., but a couple of things are not clear to me. Maybe someone with more experience can help.

1. I would have expected that my custom law has to return the evaporation rate in a specified unit (i.e mol/m2-s, or kg/m2-s) and that Fluent would take care of removing the amount of species from the droplet depending on its surface area and particle time step. In this case I would also expect Fluent to use the returned value as a species and mass source for the continuous phase. However, the DEFINE_DPM_LAW does not return a value. Is one supposed to directly and explicitly update the droplet mass by doing mp(t+dt) = mp(t)-Ni*Ap*dt ?

2. If the latter is true and the UDF has to directly update the droplet mass, do I have to write more UDFs to account for the mass and species sources in the continuous phase or is this still done automatically by Fluent? I would expect the second. I think, when one uses the Fluent in-built evaporation law the sources to the continuous phase are taken care of. In the theory guide it says: "The vapor flux given by Equation 16.84 (which is for the evaporation rate) becomes a source of species i in the gas phase species transport equation...", but I am not sure that if one uses a custom law this also is the case.

I am grateful for any guidance.

Franz