Coupling of drying process and flow simulation
 

Hi, I would like to explain my case and what I want to do to see if my ideas are ok or if there is any (or many) flaws in them.
I have done some airflow simulations (Fluent) in hay round bales to predict the air distribution inside them, which affects decisively the drying of moist bales. The simulations were done in steady state as I had not expected any transient behaviour. I used the porous media model to represent the bale. Given the resulting flow field inside a round bale, I would like now to try and perform drying simulations, which involve heat and mass transfer. I have not found a way to do this in Fluent, at least when the wet solid which is to be dried by the flowing air is a porous medium. My idea is to employ UDFs to incorporate the drying model in Fluent and use Fluent to simulate the drying process under the steady flow field in the bale. I am yet to find how to do this, but that is the idea. Since drying is a transient process, this would change the problem for a steady to unsteady. However, since the flow field influences how the drying process occurs but not the other way around, the transient part would still be the drying simulation only.
I think that, if I were to start the flow/drying simulation from the beginning the smart thing to do would be to run the simulation of the airflow alone until the solution has converged, and only then activate the drying part. Am I correct?
Now the thing is, in real life the drying of a round bale can take from 8 hours to several days depending on different factors. I do not need to run a simulation for the whole drying, a couple of hours would do. The question is, if the flow field is steady, do I need to worry about the size of the timestep from the side of the flow field? Should the size of the timestep be selected according to what is acceptable from the drying simulation point of view?