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Hello MFiX developers and users,
I am simulating a cold bubbling fluidized bed containing two particle-size classes using MFiX 25.2.1 and the TFM approach. The case file is `
20_d06d08p13q27.mfx (13.6 KB)
The main problem is that the spatial distributions of the two solid phases are almost identical. Although the absolute volume fractions differ, the second phase remains approximately three times the first phase throughout most of the bed:
[ \varepsilon_{s,2}\approx3\varepsilon_{s,1} ]
Consequently, the local composition remains close to its initial value:
[ X_1=\frac{\varepsilon_{s,1}} {\varepsilon_{s,1}+\varepsilon_{s,2}}\approx0.25, \qquad X_2\approx0.75. ]
The instantaneous contours and time-averaged axial profiles therefore appear to reproduce the initial 1:3 volume-fraction ratio, with very little size segregation or differential transport.
Experimental configuration represented by the simulation
- Cold air fluidization
- Column internal diameter: 0.060 m
- Column height: 1.40 m
- Initial bed height: 0.1634 m
- Total bed inventory: approximately 800 g
- Fine-particle sieve range: 0.355–0.50 mm
- Coarse-particle sieve range: 0.50–0.71 mm
- Fine/coarse composition: 25%/75%
- Gas flow rate: 27 m³/h
- Superficial gas velocity: 2.654 m/s
- Estimated mixture minimum fluidization velocity: approximately 0.322 m/s
- Fluidization number: (U/U_{mf}\approx8.23)
Particle and initial-condition settings
| Parameter | Solid phase 1 | Solid phase 2 |
|---|---|---|
| Representative diameter | 0.4275 mm | 0.6050 mm |
| Density | 3817 kg/m³ | 3899 kg/m³ |
| Initial volume fraction | 0.11 | 0.33 |
| Initial velocity | 0 m/s | 0 m/s |
| Initial granular temperature | 0 | 0 |
The initial gas volume fraction in the bed is 0.56. A third TFM phase with a diameter of 0.855 mm is still defined in the project, but its initial volume fraction is zero. Thus, MMAX=3, although only phases 1 and 2 are present in this case.
Numerical and model settings
- Solver: MFiX 25.2.1
- Model: 2-D TFM
- Domain: 0.060 m × 1.40 m
- Grid: 30 × 700 cells
- Cell size: approximately 2 mm × 2 mm
- Simulation time: 12 s
- Initial time step: (1.0\times10^{-4}) s
- Minimum/maximum time steps: (10^{-7})–(10^{-2}) s
- Gas–solid drag model: Syamlal–O’Brien
- Solids viscous-stress model: Lun et al. (1984)
- Frictional-stress model: Schaeffer
- Radial distribution function: Lebowitz
- Particle–particle restitution coefficient: 0.9
- Interphase friction coefficient,
C_F: 0.1 - Packed-bed void fraction,
EP_STAR: 0.3688 - Internal friction angle: 30°
- Stress blending: none
- Energy and species equations: disabled
Boundary conditions
- Bottom: gas mass inlet, (U_g=2.654) m/s
- Top: pressure outlet, 101325 Pa
- Side walls: no-slip for the gas
- Solids wall condition: Johnson–Jackson partial slip
BC_JJ_PS=1- Specularity coefficient:
PHIP=0.2 - Particle–wall restitution coefficient: 0.9
- Wall friction angle: 11.3°
I would appreciate advice on the following questions:
I am simulating a binary bubbling fluidized bed using MFiX 25.2.1. The particle diameters are 0.4275 and 0.6050 mm, with densities of 3817 and 3899 kg/m³. Their initial solid volume fractions are 0.11 and 0.33, corresponding to a 1:3 ratio. The gas velocity is 2.654 m/s, the grid is 30 × 700, and the simulation time is 12 s. I use the Syamlal–O’Brien drag model, Lun et al. (1984) solids-stress model, Schaeffer friction model, C_F=0.1, and Johnson–Jackson partial-slip walls.
During the simulation, the two phases satisfy approximately
[ \varepsilon_{s,2}(x,y,t)\approx3\varepsilon_{s,1}(x,y,t), ]
so their normalized instantaneous contours and axial profiles remain almost identical to the initial 1:3 distribution.
Why do the two solid phases with different particle diameters retain almost identical normalized spatial distributions instead of developing relative motion or size segregation?
