Recirculation near volute throat causing divergence in FineOpen

Greetings,

I have been running a volute mesh independent study of a volute+impeller simulation, with water as the fluid - I have been increasing the cell count in hexpress and running the simulations with the same boundary conditions. Thus far, convergence has been achieved with coarse, medium and semi-medium meshes. However, I have encountered issues with the fine mesh:

Repeated mass flow residual patterns have been recurring and consequently, the simulation has not converged, seen in image 1. 


I noticed recirculation zones in the fine mesh simulation which was prior to the volute throat /tongue area, seen by the swirling velocity vectors:


Zoomed in:

A low velocity zone was located in this area seen by the light blue (the volute outlet is actually extended, but appears cut off in this screenshot):



The same boundary conditions were used from the converged coarse/medium/semi-medium meshes, therefore I suspected the issue might lie in the fine mesh as it is the only aspect that changed. Additionally, lowering the CFL number, trying outlet boundary conditions without backflow and other attempts did not yield converged results for the fine simulation. Therefore I went on to the fine mesh to adjust.

The fine mesh had an expansion ratio of 6.4  thats larger than the recommended 5, so I improved the mesh to get a new expansion ratio of 4.9 and am running the simulation again with this mesh. However, I would like to know if there is anything else I can try , given that recirculation only occurs with the fine mesh?

Kind regards,
Joseph