Problems with stiff chemical solvers

[wangjinjing]

Hello,everyone!
I’m trying to implement a process where biomass falls from the top of the fluidised bed and mixes with the bed material for combustion, but I’m having some problems:
1.When the stiff chemical solver is enabled, the biomass particles no longer fall down, but are separated on the top(as shown in the figure). And with the stiff chemical solver ticked, the simulation time increases considerably. I would like to ask if there are any parameters in the stiff chemical solver that need to be adjusted?
2. The incoming hot air stream does not seem to heat the room temperature biomass particle.
Here is my procedure, hopefully someone can help me out with my difficulties, thanks a lot!

b0709.mfx (28.9 KB)
usr_rates.f (4.5 KB)
usr_rates_des.f (6.2 KB)

[HangZhou]

1. The issue with the biomass partilces is not the problem with stiff solver. Please try to set p_ref to 0. In your case, p_ref=101325Pa and pressure in the initial condition is 101325Pa, which will leads to a zero Pa pressure (101325-p_ref)/p_scale=0, in the bed.
   For the parameters in stiff solver, you can adjust them in the Chemistry pane, including the minimum reaction rates for fluid and solid, minimum mass fraction you want to consider, tolerance and the maximum step the solver could take.If the problem is not stiff, turning off the stiff solver should work as well.
   

   

2. The hot air is heating the biomass. It is just slow, depending on the heat capacity of the biomass and the heat of reactions. It increases from 300 to 300.11K within 0.05s. You can see obvious temperature change in the gas phase.