Post on 01-Mar-2021
Fine Grid Simulations of Gas SolidsFine-Grid Simulations of Gas-Solids Flow in a Circulating Fluidized Bed
Sofiane Benyahiaf yUS DOE, National Energy Technology Laboratory
Multiphase Flow Science WorkshopMultiphase Flow Science WorkshopAugust 16-18, 2011
Introduction
Two-fluid or continuum model based simulations are l d t d t ti l hcommonly conducted on coarse computational mesh
requiring subgrid model.
Based on some publications1, some doubts still remain if finely resolved two-fluid simulations can capture basic experimental observations of solids hold-up and pressure d fil i idrop profiles in risers.
We will first show some results2 using subgrid models and then attempt to obtain similar results using grid-refined two-fluid simulations with no subgrid corrections.
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1 Lu, B.; Wang, W.; Li, J. Chem. Eng. Sci. 2009; 64; 3437-3447.2 Benyahia, S. AIChE J. doi: 10.1002/aic.12603
Purpose of this presentation
First show that coarse two-fluid simulations can provide bl ith th h l f b id d l (thireasonable accuracy with the help of subgrid models (this
is a continuation of my last year presentation.)
Then, demonstrate that two fluid simulations without subgrid models will predict the correct experimental observations as the grid is refined
Finally, show the necessity to invest more research in the development of subgrid models due to significant computer time requirements to conduct these fine grid simulations.
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Two-fluid simulation conditions1
Two gas‐solids outlets
0.35 m
0.14 m
Process temperature 297 K
Process pressure 1.01 x 105 Pa
Air density ~1.2 kg/m3
0 09 m
Air viscosity 1.8 x 10-5 Pa·s
Gas-phase turbulence length-scale 0.01 m
Solids density 930 kg/m3
0.09 m
60, 90 cells
10.5 m
450, 4500, 10500 cells
Particle diameter 54 micron
Single-particle terminal velocity 0.074 m/s
Particle-particle restitution coefficient 0.9
P ti l ll tit ti ffi i t
2.78 m
Particle-wall restitution coefficient 0.7
Specularity coefficient 0.0001
Particle-particle angle of internal
frictionpi/6
Gas inlet
Two solids side inlets
0.35 m
0.14 meps_mf =0.4
eps = 0.3 , Vs: computed
friction
Particle-wall angle of friction pi/16
Void fraction at maximum packing 0.4
Void fraction at minimum fluidization 0.6
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Gas inlet
Vg = 1.5 m/s1 Lu, B.; Wang, W.; Li, J. Chem. Eng. Sci. 2009; 64; 3437-3447.
Coarse grid simulation resultsDrag model Wen-Yu EMMS Experimental
Solids mass flux (kg/m2s)
205 16.9 14.3( g )
Standard deviation (kg/m2s)
25.2 5.59 -
10EMMS (pressure)
8
EMMS (pressure)
Wen‐Yu (pressure)
experiment (pressure)
EMMS (voidage)
Wen‐Yu (voidage)
4
6Heigh
t (m
)
Wen-Yu drag EMMS drag
2
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g EMMS drag
Time-averaged 0
0.7 0.8 0.9 1Void fraction
Coarse-grid simulation results
There is clearly a need to “correct” the drag force as id i l ti f il t di t th lid h ldcoarse-grid simulations fail to predict the solids hold-up.
There is a significant difference in the computed solids hold-up and the one deduced from pressure-gradient. Note that measurements are based on pressure*.
So even with drag corrections, there is still disagreements between measurements and simulation data, which will be investigated in the next slides…
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*All subsequent results will be based only on our computed pressure gradient data.
Effect of particle-particle friction10
e = 0.9
0 79 ( i l d hi h f i i )8
e = 0.79 (simulated higher friction)
experiment
4
6Heigh
t (m
)
2
0
0.7 0.8 0.9 1Void fraction
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Effect of particle-wall friction10
phi = 0.0001
phi = 0 01
6
8
phi = 0.01
phi = 0.1
experiment
4
6Heigh
t (m
)
2
0
0.7 0.8 0.9 1Void fraction
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3 Li, T; Benyahia, S. AIChE J. doi: 10.1002/aic.12728
Effect of polydispersity10
Monodisperse (EMMS)
Monodisperse (Wen‐Yu)
10dp30
dp60
6
8
)
Polydisperse (EMMS)
Polydisperse (Wen‐Yu)
experiment
6
8dp115
solids hold‐up (polydisperse)
solids hold‐up (monodisperse)
4
6
Heigh
t (m
)
4
6
Heigh
t (m
)
2 2
0
0.7 0.8 0.9 1Void fraction
0
0 1 2 3Scaled solids volume fraction
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Coarse-grid simulation results
There is clearly a need to correct the drag force for both d l di t ith idmono- and poly-disperse systems with a coarse grid.
Next we present results of refined two-fluid simulations using a standard homogeneous drag correlation.
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Fine-grid simulation results
Grid resolutionSolids mass flux
(kg/m2s)
60x450 205
60x4500 200
90x10500 14090x10500
EMMS 16.9
Experimental 14.3
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Coarse 60x450
60x4500 90x10500 or 1 mm2 cells
Fine-grid simulation results
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Summary
This study shows that more accurate results are obtained as the grid is refined, but a grid-converged3 solution was not obtained even with 1 M cells!with 1 M cells!
It is, therefore, necessary to include sub-grid corrections due to the large computational requirements and time needed to conduct fully-resolved simulations.fully resolved simulations.
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3 Wang, J.; van der Hoef, MA.; Kuipers, JAM. Chem. Eng. Sci. 2009; 64; 622-625.