The following model is in two stages. The first is a coarse grid check model to determine whether an existing incinerator secondary chamber could comply with modern legislation on residence time, i.e. provide for 2 seconds minimum transit time above 850°C.
The shape of the unit comprised a large cylinder, with an inlet offset from the axis at one end, and a tangential exhaust at the other. This type of design suffers from the problem of stagnation in the main cylinder, and short circuiting, where the high speed incoming flow hits the rear wall and splashes out, with a proportion going straight out of the exhaust.
The initial CFD model showed that this was indeed the case, and that the unit would need modifications in order to meet the new requirements.
The picture at left is the original chamber design. Hot gases from the primary (combustion) chamber enter on the axial connection and leave on the tangetial connection at bottom right.
On the right is a CFD plot of the gas flow showing the high velocity stream of gases and the largely stagnant region in the bottom of the chamber.
At the bottom right is the residence time contour with superimposed velocity vectors in black.
Residence time is just over 1 second due to short-circuiting of the gases.
Above is the residence time at the outlet plane which is substantially less than the 2 second requirement.