Lowering Maximum Residuals
 

I am simulating an axial flow compressor. As my steady state simulation has had difficulty in reaching the convergence, I used CFD Post to identify the location of maximum residuals as shown. The location of maximum W momentum residual is located close to the blade and flow separation has occurred.
I have checked mesh at this location and edge length ratio, minimum and maximum angles and element volume ratio are fine.
Is there any way to lower the residual in this location? Do I have to simulate in transient due to the boundary layer separation?
I would appreciate any comments regarding this.

Why do you say your mesh quality is fine? Unless you try a better quality mesh you cannot know for sure. The guidelines in the documentation are just that - guidelines only, and your case may be different.

But it is highly likely that in this case the separation is transient and wants to move about a bit. The only way of capturing this behaviour is with a transient simulation.

I have tested coarse and fine mesh, different time steps, different turbulence models but still the convergence near to stall is impossible.
My computation domains are bellmouth inlet, igv, rotor, stator, casing treatment and outlet.
As the maximum residual and the separation occur in the stator, will coarsening mesh just in stator change the amount of residual?
Will the pressure ratio of the compressor change if I use a coarser mesh for stator?
I need to obtain the stall point among different casing treatment configurations so I think calculating the pressure ratio is enough for this stage.

I ran a transient simulation over two rotor pitch and 100 time steps. This was one of my first transient simulations.
I used Transient blade row approach and profile transformation.
For the first case, I did not include casing treatment. This is the solid casing case where other casing treatment builds will be compared to this one to calculate possible stall margin improvement.
The reason why I am running transient simulation is that due to the flow separation in stator the residuals did not reduce and I was unable to obtain convergence. So I was unable to calculate the pressure ratio.
I need to simulate behind this point to obtain pressure ratio and decide whether stall inception has occured or no.
The available experimental data shows that the stall point is less than the last mass flow that I can simulate in steady state.
I changed the interface between the igv and the rotor and the rotor to the stator to transient blade row. I am going to use this type of interface between the rotor and the casing treatment. Is it correct? Should I use transient blade row for both igv-rotor and rotor-stator?
After 100 time steps, outlet mass averaged total pressure which has been shown with green color does not show a periodic behaviour.
Can I take this pressure as converged solution?
Do I have to average this outlet total pressure over all 100 time steps?

Modelling near stall is challenging and introduces new challenges which need to be considered. If good models near the stall point are required I suggest you do some literature review to see what is recommended there.