regarding biomass gasification.
 

Hello everyone,

I am doing my thesis on biomass gasification. I am using the Euler-Euler method to simulate the gasification and my model description is below.

Multiphase
Primary phase - gases that participate in the homogenous reaction.
Secondary phase - biomass (granular)
Dia - 0.0005m
Drag model - symalal
Heat transfer between primary and secondary phase - gunn.
Secondary phase - sand (granular)
Dia- 0.0003m
Drag model - symalal
Heat transfer between primary and secondary phase - gunn.
Interfacial area between primary and secondary phases - ia-particle.
between secondary phases - ia-particle.

turbulence
K-Epsilon- dispersed
Standard wall function

Reactions
volumetric - Finite rate / eddy dissipation.
3 homogenous reactions and 5 heterogeneous reactions.

Boundary conditions
Biomass - mass flow inlet - 0.012kg/s @ 800k
Primary air - velocity inlet - 0.035 m/s @ 800k
Secondary air - velocity inlet - 0.015 m/s @ 800k
outlet - pressure outlet
wall - no slip for both primary and secondary phases.

Methods
pressure velocity coupling - phase coupled SIMPLE
Gradient- Least square cell-based.
pressure - second order
momentum, volume fraction, turbulent kinetic energy, dissipation etc - first order.

initialization
Standard initialisation with every phase at room temperature without sand bed.

calculation step size - 0.1 (the large step is due to
No. of iteration - 30

I have first initialized without sand bed with the above-mentioned conditions and ran the simulation for 25 seconds physical time step for the development of combustion. the combustion reactions happened at the bottom of the reactor.

Problems

when I patch the sand bed with the volume fraction of 0.6 and up to 200mm from the bottom of the reactor the temperature of the bottom gradually reduces to inlet temperature i.e 800k. But according to results from the experiment, the reactions happen at the bottom (high temperature) and the top of the reactor should be at low temperature. But in my case after introducing sand bed it happens vice versa. Most of the temperature is near the secondary inlet.

Qestions

How to overcome this phase interaction problem?
What is the heat transfer mechanism between two granular secondary phases? because the heat transfer between primary and secondary phases are defined by Gunn. But I can't able to find the heat transfer mechanism between two granular secondary phases in Ansys fluent guide.

I have attached images of the reactor.

Thank you for your supports and have a nice day.