CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Convergence issues for a 3D Centrifugal pump simulation using ANSYS CFX

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree2Likes
  • 1 Post By ghorrocks
  • 1 Post By highorder_cfd

Reply
 
LinkBack Thread Tools Display Modes
Old   February 28, 2014, 04:00
Default Convergence issues for a 3D Centrifugal pump simulation using ANSYS CFX
  #1
Member
 
Venkat
Join Date: Nov 2009
Posts: 35
Rep Power: 8
enr_venkat is on a distinguished road
Am currently solving a steady state 3D Centrifugal pump problem using ANSYS CFX.

Objective:

To validate the performance curve of pump based on experimental inputs

Domain decomposition:

Stationary domain: Inlet, Outlet and Casing (1 Body)

Rotating domain: Impeller fluid volume surrounding the solid impeller. But solid impeller trace is left within the impeller fluid volume

Domain interface: Between Stationary domain and rotating domain.

Method: SRF and Frozen Rotor with no pitch change

Mesh:

Fine On curvature with skewness of 0.92 and Aspect ratio of 60

Boundary conditions used:

Domain ref pressure = 1 bar.

Inlet: Total pressure = 0 bar

Outlet: Mass flow rate = 0.065 kg/s or 3.9 lpm.

Turbulence intensity: Medium

Heat transfer: none

Rotating domain: 3100 RPM about Global Y

Issues:

Convergence beyond 1E-3 seem to be impossible with lot of wiggles Number of iterations: 700; Criteria: Want to set 1E-6 but I set it to default for first run.

If I consider the heat transfer, I get similar behavior. My idea is to consider the temperature later when I get the desired pressure rise.

I tried various combinations of BCs. Gone through previous threads.

It was so tricky to get the solution converged even if I use thumb rule of 1/2w , 1/4w....as physical timescale. Checked with automatic timescale in the beginning.

After 290 iterations, flow is getting blocked at inlet and outlet with changes in block % ranging from 0.5 % to 67%

Any suggestions or discussions related to this matter is highly appreciated.

Please find attached the current status as images:


Attached Images
File Type: jpg Mass and momentum.jpg (42.7 KB, 52 views)
File Type: jpg Turbulence.jpg (39.9 KB, 31 views)
File Type: jpg User points.jpg (39.8 KB, 30 views)
enr_venkat is offline   Reply With Quote

Old   February 28, 2014, 04:46
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,565
Rep Power: 97
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
ghorrocks is offline   Reply With Quote

Old   October 13, 2015, 23:22
Default
  #3
New Member
 
Suraj Kashyap
Join Date: Sep 2015
Posts: 5
Rep Power: 2
surajkashyap is on a distinguished road
Hi, you have mentioned a thumb-rule to determine the physical timescale. Can you please elaborate? How do I estimate a starting physical timescale given the speed and number of blades in the impeller?
I am analysing a centrifugal pump with 6 guide vanes running at 1440 rpm.
__________________
And if thou gaze long into an abyss, the abyss will also gaze into thee.
surajkashyap is offline   Reply With Quote

Old   October 13, 2015, 23:39
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 12,565
Rep Power: 97
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
The general rule of thumb is a starting point for the time step size is the fluid residence time in the simulation domain (if the flow goes through without gross recirculations).

From there you should adjust it higher or lower as described in the FAQ.

You can also start with a guessed time step size and adjust it from there. This works fine as well in many applications.
surajkashyap likes this.
ghorrocks is offline   Reply With Quote

Old   October 14, 2015, 11:30
Default
  #5
Member
 
Join Date: Jan 2015
Posts: 36
Rep Power: 3
highorder_cfd is on a distinguished road
Hi

First, I would try with a big time step, something like auto timescale with a factor of 10, or physical time scale 1 or 2 sec, to try to remove some unsteady behaviour. Second, I would try to increase (ramp up) the rpm slowly, maybe starting from 1000 rpm.
Finally, I would try it is to rotate the impeller of few degrees. As you are solving with a MRF model, the impeller position will impact on the final solution. A slight change in the configuration might help the convergence.
surajkashyap likes this.
highorder_cfd is offline   Reply With Quote

Old   October 14, 2015, 18:05
Default
  #6
New Member
 
Suraj Kashyap
Join Date: Sep 2015
Posts: 5
Rep Power: 2
surajkashyap is on a distinguished road
@highorder_cfd and @ghorrocks Thanks for the tips. I achieved a much faster convergence this time
__________________
And if thou gaze long into an abyss, the abyss will also gaze into thee.
surajkashyap is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Small cluster configuration for pump simulation at CFX Nevel Hardware 2 April 7, 2014 06:07
centrifugal pump simulation OK but result with 30% error ARohit FLUENT 0 January 1, 2014 02:54
ANSYS CFX simulation: Post CFX error MInXD CFX 3 May 19, 2013 19:24
How the way I can find efficiency of centrifugal pump from CFX tttonggg CFX 2 March 25, 2012 06:29
CFX convergence issues with free surface adenlan CFX 3 September 2, 2011 06:43


All times are GMT -4. The time now is 20:17.