[dibs87jg]

Dear all,


I am doing a CFD analysis of an Eckardt pump impeller and I am having problems setting up my case in Fluent in order to find convergence. I must say I am a newbie in this field, this is my 2nd CFD work.


I modeled and meshed in Gambit one impeller passage. I built a structured grid with only hexaedrals mesh elements. The geometry is pretty complex and I need some accuracy near the blades so it led me to a high aspect ratio (99.8) and a skew value of 0.88. No negative values found on my mesh.


My problem is a compressible and steady flow. The conditions given to my problem are

• Total pressure at the inlet is 101300 Pa
• Total temperature is 288.1 K
• Adiabatic
• Mass flow rate is 4.54Kg/s
• The impeller is spinning at 14000 rpm
• Static pressure at the outlet is 163300 Pa
• Ideal gas

My boundary conditions are:

• Pressure inlet for the inlet
• Pressure oulet for the outlet
• Wall for the blades and casing

I am using a density-based solver, K-Epsilon turbulence model, Ideal gas for the air, and a single reference frame.


First of all, when I import my .msh file into Fluent and I check it, I get a warning about my high aspect ratio but I am trying to run some iterations anyway. I don’t know if my aspect ratio is too high or acceptable. I can remesh my problem and get an aspect ratio of 50 but it will increase the computational time a lot.


I am using the atmospheric pressure as the operating pressure, and hence, the pressure imputed at the inlet is -25 Pa and the pressure imputed at the outlet is 61975. Discretization is 1st order upwind everything, and everything else set as default.


With all these settings, I initialize the problem and It does not even start iterating, it pops up some errors: "Divergence detected, reducing courant number… divergence detected in AMG solver… Error".


I’ve tried changing the inlet boundary condition to mass flow inlet and it iterates but my problem keeps diverging.


I’ve also tried solving my problem as stationary and then increasing the rotational velocity progressively but I don’t get too far and it takes me days to get to 1000 rpm…


I’ve got some errors such; “Time step reduced in X cells due to excessive temperature change”, “Temperature limited to X”, “Turbulent viscosity limited to viscosity ratio of X”…


Do you have any advices, tips, comments I can rely on to keep on working? My last hope is remeshing my geometry but I would like to avoid that.
http://img5.imageshack.us/i/impeller.jpg/

http://img199.imageshack.us/i/periodic2.jpg/



Thanks,
Sincerely
Javier

[dibs87jg]

Hello again.



I’ve tried to remesh my geometry and I made it simpler. I got a skew value of 0.77 and an aspect ratio of 27, much better than last time.


My divergence problem remains the same and fluent keeps displaying the same error (time step reduced… turbulent viscosity limited to… ) while iterating. I am trying and trying different settings but I cannot find convergence.


I am running out of options. Can anyone help me?

[lovejerryloveme]

Hi,

I got the same problem for my case and still trying to fix it. Have you solved it finally? Maybe you can post your solution or suggestions for further discussion, thanks.

jerry