Problems bout CFD model of biomass gasification, Downdraft gasifier
 

I am a Master student study in biomass gasification in Korea. I am trying to make a CFD model of downdraft gasifier using FLUENT 6.3. I have lot problems. I hope you give me some advice about it. Thank you very much.

I tried 2 ways to simulate this gasification process: one phase and multiphase.
In one phase case, wood determined as very small carbon particles. So the carbon particles and air determined as fluid phase. The details you can see in the fluent case file. The model was run in a computer which has 3G RAM and Q6600 cpu for more than 12 hours and iterated about 26,000 times.
The problem is:
1.The simulated result presented that nearly no reactions occurred in the gasifier.
2.If I use a injection of wood inlet, the injection rate will be very small because the wood is drop down very slowly, I cannot simulate the right case use a injection.

In 2 phase case, carbon determined as solid phase and details of model setting as below:
The problem is :
1.In the simulated result, temperature is too high (highest is 5000K), the temperature range in experiment is from 293K~1600K.
2.No reactions occurred in the gasifier as the simulated results.

**Setting of Multiphase (2 phase)
Slover
Slover -Pressure Based
Formulation -implicit
Space -3D
Velocity Formulation -Absolute
Gradient Option -Green-Gauss Cell Based
Porous Formulation -Superficial Velocity
Multiphase
Multiphase Model -Eulerian
Number of phase -2
Energy
-Energy Equation
Viscous Model
Model -k-epsilon (2 eqn)
k-epsilon Model -Realizable
Near-Wall Treatment -Standard Wall Functions
k-epsilon Multiphase Model -Mixture
Model Constants
C2-Epsilon: 1.9, TKE Prandtl Number: 1, TDR Prandtl Number: 1.2, Disperion Prandtl number: 0.75, Energy Prandtl Number: 0.85, Turb. Schmidt Number: 0.7,
Species Model
Model -Species Transport
Reactions -Volumetric & Particle Surface
Options -Inlet Diffusion & Diffusion Energy Source
Turbulent -Chemistry Interaction -Finite-Rate/Eddy-Dissipation
Operation Condition
Pressure: Operating Pressure(pascal)-101325
Gravity: Gravity- Gravitational Acceleration Z(m/s2)=-9.8
Boussinesq Parameters: Operating Temperature (k)=288.16

Dear, some more informations about your case are required
1) Drag Model?
2) Granular Temperature Model: which of the following you have used (Phase Property or Partial Differential Equation)? If Partial Differential equation then which model for GT
3)Granular Viscosity, Granular Bulk Viscosity and Frictional Viscosities Models

Dear ANKW:

Thank you for your help. I am fresh in using FLUENT, just more than 1 month. I am sorry I am not very understand you.

1) drag model.... I think I did not set drag model, what about the drag model of downdreft gasifier?
2) 6 reaction formulas and Arrhenius equation were used in the model
3 particle-surface reactions
2C+O2=2CO
C+H2O=CO+H2
C+CO2=2CO
3 volumetric reactions
2CO+O2=2CO2
CO+H2O<=>CO2+H2
3) Granular Viscosity: I am not clear Granular Viscosity in this model.

Can you give me more advice ? Thank you!

YOURS,
WANG