high pressure ratio

keeper · July 29, 2010, 04:45

hello, I have a problem with centrifugal compressor boundary conditions setting. I have to check 4 points on compressor characteristics and I have already set boundary conditions for 3 points and I got reasonable results from all of them. But the same settings for point with highest pressure ratio and mass flow doesn't provide results with the same deviation

. Please, does anybody have an advice??

ghorrocks · July 29, 2010, 08:11

Have you done mesh, time step and convergence sensitivity checks?

keeper · July 29, 2010, 08:21

No I haven't. Pls could you write me down, where could I find it?? I would be very grateful to you.

keeper · July 29, 2010, 09:33

All conditions you mentioned are set well for other three cases but the last one doesn't work. And I don't know if it is due to high pressure ratio or anything else. Relative pressure on inlet should be 0 Pa but it's about 90 kPa... That's why all other check variables are out of deviation..

ghorrocks · July 29, 2010, 19:50

The concept of a sensitivity check is simple. Vary the parameter (convergence tolerance, mesh size, time step size) and see if it gives you the same answer to within a tolerance you are happy with. Make sure you make increments between parameters big enough to be effective - I double or halve the time step size or mesh element length (not the node count!) and change the convergence tolerance by a factor of 10.

The Journal of Fluids Engineering has an editorial policy which goes into more detail:
http://journaltool.asme.org/Template...umAccuracy.pdf

And for the key textbook in this field go for "Computational Fluid Dynamics" by Roache.

ghorrocks · July 29, 2010, 19:51

And yes, it is quite possible that a simulation set up which works well for one situation does not work well for another. The flow may have entered a new regime and the numerics have to be re-tuned to accurately capture that.

keeper · July 30, 2010, 10:08

Thank you very much. That is exactly my problem, I have to find new set up which works well for different flow regime. I will keep on trying

keeper · July 30, 2010, 10:52

Could be problem in interface?? First I set it up on diameter 281mm which is 1mm from impeller. Do you think that interface on maybe 301mm could solve this problem??

ghorrocks · August 1, 2010, 07:26

Why don't you try it and find out for yourself?

keeper · August 2, 2010, 05:59

I have large mesh and it takes a lot of time to get results. That is why I asked you this question. I thought you have some experience with this type of case... Anyway thx

ghorrocks · August 2, 2010, 08:33

You have not explained your case in much detail so cannot answer. But in all honesty, working it out for yourself is to best way. That way you know from experience and that is always better.

July 29, 2010, 04:45	high pressure ratio	#1
keeper Senior Member Join Date: Jul 2010 Posts: 152 Rep Power: 16	hello, I have a problem with centrifugal compressor boundary conditions setting. I have to check 4 points on compressor characteristics and I have already set boundary conditions for 3 points and I got reasonable results from all of them. But the same settings for point with highest pressure ratio and mass flow doesn't provide results with the same deviation. Please, does anybody have an advice?? Last edited by keeper; July 29, 2010 at 05:21.

July 29, 2010, 08:21		#3
keeper Senior Member Join Date: Jul 2010 Posts: 152 Rep Power: 16	No I haven't. Pls could you write me down, where could I find it?? I would be very grateful to you. Last edited by keeper; July 29, 2010 at 09:32.

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July 29, 2010, 08:11		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Have you done mesh, time step and convergence sensitivity checks?

July 29, 2010, 09:33		#4
keeper Senior Member Join Date: Jul 2010 Posts: 152 Rep Power: 16	All conditions you mentioned are set well for other three cases but the last one doesn't work. And I don't know if it is due to high pressure ratio or anything else. Relative pressure on inlet should be 0 Pa but it's about 90 kPa... That's why all other check variables are out of deviation..

July 29, 2010, 19:50		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	The concept of a sensitivity check is simple. Vary the parameter (convergence tolerance, mesh size, time step size) and see if it gives you the same answer to within a tolerance you are happy with. Make sure you make increments between parameters big enough to be effective - I double or halve the time step size or mesh element length (not the node count!) and change the convergence tolerance by a factor of 10. The Journal of Fluids Engineering has an editorial policy which goes into more detail: http://journaltool.asme.org/Template...umAccuracy.pdf And for the key textbook in this field go for "Computational Fluid Dynamics" by Roache.

July 29, 2010, 19:51		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	And yes, it is quite possible that a simulation set up which works well for one situation does not work well for another. The flow may have entered a new regime and the numerics have to be re-tuned to accurately capture that.

July 30, 2010, 10:08		#7
keeper Senior Member Join Date: Jul 2010 Posts: 152 Rep Power: 16	Thank you very much. That is exactly my problem, I have to find new set up which works well for different flow regime. I will keep on trying

July 30, 2010, 10:52		#8
keeper Senior Member Join Date: Jul 2010 Posts: 152 Rep Power: 16	Could be problem in interface?? First I set it up on diameter 281mm which is 1mm from impeller. Do you think that interface on maybe 301mm could solve this problem??

August 1, 2010, 07:26		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Why don't you try it and find out for yourself?

August 2, 2010, 05:59		#10
keeper Senior Member Join Date: Jul 2010 Posts: 152 Rep Power: 16	I have large mesh and it takes a lot of time to get results. That is why I asked you this question. I thought you have some experience with this type of case... Anyway thx

August 2, 2010, 08:33		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You have not explained your case in much detail so cannot answer. But in all honesty, working it out for yourself is to best way. That way you know from experience and that is always better.