Internal flow simulation

Kishore

i'm simulating internal flow through a duct which delivers it to a chamber where the flow splits into two and exits. when i'm running 3-d segregated implicit solver with spallart-allmaras model, the residuals of x,y,z velocity keep oscillating and never converge.. when i removed the chamber and simulated it only for duct, it converges... what can be the possible problem? either grid or boundary conditions? while it is solving , it says "reverse flow in __ faces" and "turbulent viscosity exceeds the limiting value"... what could be the possible cause of this..

your help in fixing this problem will be appreciated.. if u need further info regarding boundary conditions i can give..