CFD Online Discussion Forums - Modeling Non-Premixed fuel burning in a cylindrical reactor

Hi,
I am trying to model burning of solid fuel in air inside a cylindrical reactor. The geometry I created is two cylinders of diameters 1cm and 2cm respectively and of equal length 5 cm at the same origin. I filled the smaller cylinder with the fuel and the surrounding bigger cylinder with air. The boundaries specified for both the cylinders are:
1. Smaller cylinder:
The circular faces are given as walls and the third face as mass flow inlet
2. Bigger cylinder:
The circular faces are given as outflows and the third face is given as a wall.

I have set the boundary conditions as:
1. The outer wall has a heat input of ~9x10^6 W/m^3 and at an initial temperature of 800K
2. The mass flow inlet is set at a mass flow rate of 0.0001 kg/s.
3. All the other surfaces are set at room temperature.

I initialized the solver from the outer wall at 800K. The problem I have is the solution diverges after 30-35 iterations. I am having difficulty in trying to figure out where the problem lies in. I tried to define the finite rate reaction at the interface of fuel and air, but I don't know if my way of defining is hampering with the model and the solution is diverging because of that.

I also looked into the following links to get some answers but was unsuccessful

http://www.fluentusers.com/training/index.htm

http://www.fluentusers.com/fluent/do...help/index.htm

It would be helpful if I can get some answers regarding the above model including how to define a chemical reaction at an interface of fuel and air.

Thanks,
Varun Sagi