Heat exchanger simulation diverges on buoyantBoussinesqSimpleFoam
 

I am trying to simulate the shell side flow of a double pipe heat exchanger using buoyantBoussinesqSimpleFoam on Openfoam 2.2.0. If I increase the velocity at inlet or make the meshing fine, then the simulation diverges. Can anyone suggest why?

boundary conditions:
inlet: const vel and zeroGrad pressure
outlet: atmospheric pressure and zeroGrad velocity
tube wall: const T and no slip
shell wall: zeroGrad T and no slip
turbulence model: kEpsilon
Scheme: upwind