I have to model combustion in industrial furnace and I decided to model a section of the furnace (2d problem). When I set the boundary condition for air inlet I can't decide if I have to choose "velocity inlet" or "mass flow inlet". In the first case, my 2d simulation doesn't respect the mass balance of the real furnace; in the second case my 2d simulation doesn't respect the effective flow velocity in the furnace. What I have to do? Is it a limitation of 2d simulation in comparison with 3d simulation? I hope my question were clear… thanks

A two-dimensional analysis assumed a unit depth, so the velocity inlet does respect the mass balance. To use a mass flow inlet, you need to reduce the mass flow accordingly. For example, if you have 100 kg/s entering a 10 m deep furnace and your 2-D model is built in meters, you need to enter 10 kg/s.