What should be the inlet boundary condition for rocket biomass cook stove
 

I am pursuing PhD, working on biomass combustion in rocket biomass cookstove. While performing simulation in fluent i am facing problem in inlet boundary condition. I have solid fuel and air at single inlet going inside combustion chamber. I know mass flow of solid fuel but air flow is natural convection flow so mass flow of air is unknown. so what inlet condition should be given. I have to varry the inlet area also as amount of fuel is varrying.
I am attaching the image and computational domain of cookstove for reference.

If anyone have any idea they can contact me on surajghiwe@gmail.com
someone help me !!!!!