[daviqperez]

Hello everyone, I am doing the simulation of a mixing tank for industry purpose, however, I have multiple doubts about the setup.

The geometry is the following:

TankGeometry.png

The idea is to simulate the actual conditions of the tank which run at a velocity of 44 rpm and have 40-50% concentration in the bottom, 30-40% in the middle, and 0.1-2% in the top. I have the water mass flow inlet (3.6 kg/s), and the solid mass flow inlet (0.5 kg/s), however, I do not have any data on the inlet's geometries or outlet information, so I decided to do the simulation in a close system patching an initial volume which was about 27% of the total volume (corresponding to 40% concentration).

TankInitialization.png

Here I patched the particle's volume with a vof of 1, however, the packing limit was setup at 0.63. The set-up I used was an Euleria-Eulerian model, with a granular model active, the diameter particle was 100 um with a density of 1800 kg/m3, I chose the gidaspow and Lun-et al model for granular viscosity and bulk viscosity, respectively. I also use the gidaspow model in drag coefficient. The movement of the impeller where simulated using the MRF model. Gravity was activated and also turbulence was resolved using the K-epsilon model with dispersed turbulence multiphase model active.

I first did a steady-state simulation using a simple solver and a pseudo transient solver, and both solutions were very different. I only drop residual to 1e-3, if residuals drop to 1e-5, the steady solution keeps changing. In the case of coupled the residual gets flat near 2e-3 for continuity and does not converge.

SteadySimple.pngSteadycoupled.png

Right now, I am doing a transient simulation using simple solver, i'm solving using an adaptive time scale with CFL of 1, but the maximum time step is 5e-4. This is the water counter after 20 seconds.

TransientSimple.png

So, my questions are:

1. Is the set-up okay for steady/transient simulation and the objectives? Or should I go with sliding mesh in the transient case?
2. Why does the solution change between simple/coupled solvers?
   
3. Although I understand the problem is indeed transitory, I was thinking that a steady solution was possible to get the concentrations, I wrong here?
4. With the transient simulation the big problem is that it will take forever to simulate the process, any tricks here to accelerate the process?