Hi everyone!

I am a very very new user of Fluent and am running a simulation of a simple 3D slugging fluidized bed on ANSYS Fluent 2019 R2. I have tried looking around the forum but there does not appear to be any fast solution to my troubles. The following are the "settings" I used in my simulation.

Geometry

- Cylindrical riser of height 2m and diameter 0.1m.

Meshing

- Face mesh with 1e-04m

Models

- Eulerian model

Materials

- Fluid-air (primary phase) --> properties left very close to default values
- Fluid-sand (secondary phase) --> granular
- diameter: 1.5e-04m
- density: 2660 kg/m3
- viscosity: left as air viscosity as I understand the value is unimportant for Two-Fluid Models in Fluent.

Boundary Conditions

- inlet:
- gas phase:
- velocity: 0.04638m/s in (0,1,0) direction

- particulate phase:
- velocity: 0m/s

- gas phase:
- outlet
- default

- wall
- gas phase:
- non-slip

- particulate phase
- specified shear: 0.5 coefficient

- gas phase:

Initialisation

- hybrid initialisation
- Patching: x,z = (-5e-2, 5e2); y = (0,1), solid volume fraction = 0.6.

Autosave every 100 time steps

Calculation

- time step: 0.0001s
- maximum iterations per time step = 500
- convergence criteria: 1e-3

My problem is this: initially, everything was converging very quickly each time step. However, after approx 1000+ time steps the continuity residual started increasing while the other residuals continued to converge reasonably.

I read from some papers that the grid size used should 3-4 times the diameter of the particles, but when I tried this scale the meshing took an unreasonably long amount of time and I settled for a much coarser mesh size.

May I know how I can resolve the divergent continuity residual issue?