Dt<dt_min and sum of solid phase mass fraction is not equel one error

MFiX
#1

I simulate the gasifier with a diameter of 0.5 m and a length of 5 m. what should be the grid size. I work in 2D. The particle diameter is 0.5 mm and the mesh is 20 X 200. I get the error dt <dt_min continuously. dt_max is at the level of 10 ^ -5. I’m getting smaller, but the error continues. First, the sum of solid phase mass fractions gives an error not equal to 1, then it gives the error dt <dt_min. Can you help me.
Errors occur in 0.5-0.6 seconds of the simulation.
@YupengXu gXu

#2

means that the simulation is not converging.

probably means that something is wrong with your chemical reactions. Make sure they are balanced, conserve mass, etc…

1 Like
#3

I have done this work without any problem in a 0.05 m diameter and 0.5 m long gasifier. However, when I did scale up, it started to give these errors. There is no problem in chemical reactions. Increasing the number of mesh or decreasing the dtmax value does not solve the problem. What else could be the cause of the problem.

#4

How much is your maximum iterations in numerics tab? maybe you can increase this value because of convergence problem. Sometimes the number of iterations may not be enough.

Secondly you can disable “Detect stall” and maybe “Enable max wall time” at Run tab. Because mfix tries to converge by changing the time step (mostly tries to reduce) and dt<dt_min is occur.

For example, even if you enter 500 iterations, it can decrease the time step when it becomes 100 iterations.

#5

It’s all done. The problem still continues. I did not know that I would encounter such a problem when I scale up. I had no problems on a lab scale. This problem occurred when I enlarged the gasifier and capacity. Lowering dtmax or increasing mesh is not the solution. It gives error in 0.5 seconds of simulation. What else should be done. Can you help me. @YupengXu @onlyjus @gaoxi

#6

Could you share you case files?
Did you run in a debug mode to check your code?

#8

Few things needs to be considered,

  1. For 2D simulation, make sure all the parameters scaled correctly, especially the boundary conditions.
  2. Since it is the biomass gasification simulation with TFM, so the momentum equation for all 3 phases, also the species equation for gas and biomass should be set as .TRUE.
  3. Make sure the reaction rate expression in the usr_rates are properly defined. https://mfix.netl.doe.gov/doc/mfix/17.1.3/chemical_reactions.html
    “All TFM gas and solids phase reactions as well as homogeneous gas phase reactions for DEM simulations are to be included in usr_rates.f . Reaction rates defined in usr_rates.f must have units of reacted moles per time per volume (i.e., moles/sec/cm3 for CGS units and kmoles/sec/m3 for SI units).”
  4. Make sure the reaction rates expression parameters are written correctly and also the squence of the reactions, usually the drying happens first, then pyrolysis, the gasification reaction.