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Convergence centrifugal compressor with cfx solver manager

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Old   March 25, 2021, 17:05
Default Convergence centrifugal compressor with cfx solver manager
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Andrea
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good evening,
I state that I am new to the forum and I have just entered the world of CFD analysis (this is the first analysis I carry out in the case of compressible fluid at high speed).
I am simulating a centrifugal compressor in Ansys CFX to obtain the performance curves with the following characteristics:

Processed fluid: Ideal gas air 30 ° C with constant cp equal to 1005 (j * kg) / K
Total temperature at the entrance: 288.15 K
Total pressure at the inlet: 105 kPa
Rotation speed of the impeller: 13476 rpm
Turbulence parameters are specified at the inlet section in terms of turbulence intensity
and turbulent viscosity ratio, which are set equal to 5% and 10, respectively.
Mass flow rate at the exit of the volute: 7.2 kg / s (mass flow rate BEP = 7.5 kg / s already simulated).
Rotor-stator interaction model between rotor and stator domains: frozen rotor
All walls are adiabatic and no slip

Turbulence's model: k-epsilon with wall function scalable
Energy model: Total energy includes viscous work term
Advection scheme: high resolution
Turbulence numerics: First Order
Time scale control: Auto timescale
Min iteration: 1
Max iteration: 1000000000
Timescale factor: 1.0
Residual target: 1E-10
Residual type: RMS
Global dynamic model control: On

Mesh: 13863334 elements

I am currently simulating the system with a mass flow rate of 7.2 kg / s imposed at the exit and I do not understand if I am reaching convergence (this analysis has been started at 8112 steps and is now at 11389 steps, the residual peak shows the change in flow rate, for the steps prior to 8112 steps the flow rate was 7.5 kg / s and it took over 5000 steps to reach convergence): as you can see, for the flow rate of 7.5 kg / s the convergence was reached (also if very slowly) with the imbalances below 1%, for the current capacity of 7.2 kg / s residuals continue to fluctuate and also the imbalances and do not seem stable: I would like to understand what could be the problem of this and how I can have a convergence.

I attach photos of the residues and imbalances.
Thank you.
Attached Images
File Type: jpg Immagine 2021-03-25 230111.jpg (81.6 KB, 3 views)
File Type: jpg Immagine 2021-03-25 230134.jpg (62.1 KB, 2 views)
File Type: jpg Immagine 2021-03-25 230149.jpg (71.5 KB, 1 views)
File Type: jpg Immagine 2021-03-25 230224.jpg (117.3 KB, 1 views)
File Type: jpg Immagine 2021-03-25 230244.jpg (67.4 KB, 1 views)
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Old   March 26, 2021, 08:50
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Nitesh Attal
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Quote:
Originally Posted by Andrea 240192 View Post
good evening,
I state that I am new to the forum and I have just entered the world of CFD analysis (this is the first analysis I carry out in the case of compressible fluid at high speed).
I am simulating a centrifugal compressor in Ansys CFX to obtain the performance curves with the following characteristics:

Processed fluid: Ideal gas air 30 ° C with constant cp equal to 1005 (j * kg) / K
Total temperature at the entrance: 288.15 K
Total pressure at the inlet: 105 kPa
Rotation speed of the impeller: 13476 rpm
Turbulence parameters are specified at the inlet section in terms of turbulence intensity
and turbulent viscosity ratio, which are set equal to 5% and 10, respectively.
Mass flow rate at the exit of the volute: 7.2 kg / s (mass flow rate BEP = 7.5 kg / s already simulated).
Rotor-stator interaction model between rotor and stator domains: frozen rotor
All walls are adiabatic and no slip

Turbulence's model: k-epsilon with wall function scalable
Energy model: Total energy includes viscous work term
Advection scheme: high resolution
Turbulence numerics: First Order
Time scale control: Auto timescale
Min iteration: 1
Max iteration: 1000000000
Timescale factor: 1.0
Residual target: 1E-10
Residual type: RMS
Global dynamic model control: On

Mesh: 13863334 elements

I am currently simulating the system with a mass flow rate of 7.2 kg / s imposed at the exit and I do not understand if I am reaching convergence (this analysis has been started at 8112 steps and is now at 11389 steps, the residual peak shows the change in flow rate, for the steps prior to 8112 steps the flow rate was 7.5 kg / s and it took over 5000 steps to reach convergence): as you can see, for the flow rate of 7.5 kg / s the convergence was reached (also if very slowly) with the imbalances below 1%, for the current capacity of 7.2 kg / s residuals continue to fluctuate and also the imbalances and do not seem stable: I would like to understand what could be the problem of this and how I can have a convergence.

I attach photos of the residues and imbalances.
Thank you.



Hello,



Looks like you have posted in the wrong forum. This forum is for CONVERGE s/w from Convergent Science Inc. Using CONVEGRE you will be able to perform CFD simulations of Compressors, Turbines, Engines, Combustors and many more applications.



Please write to sales@convergecfd.com to get intouch with our sales team.


Best,
Nitesh
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