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Old   July 6, 2012, 09:46
Default Fan simulation divergence
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Ayothi Ramanathan
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Dear all,
I am doing FD fan simulation in Ansys 13 (domain has four individual part mesh). The aim of project is to develop the performance curve for different position of Inlet Guide Vane ( flow regulating system). Readings are getting by changing mass flow rate (36.9, 42.7, 48.9, 53.5, 58.8, 64.2 and 69.9 kg/s). I have finished the simulation for all mass flow rates at IGV fully opened condition. While doing simulation at IGV 30 degree inclination, the simulation got diverged for mass flow 53.5 kg/s onwards. First three mass flow runs finished normally. I have set k-epsilon turbulence model. Later I changed SST model and checked and the same problem arises. Even, I tried coarse mess to converge the simulation. I don't know the approach to proceed further. Finally, the result stopped after getting diverged for long time.
Thanks in advance
snap of coarse mesh simulation with SST model-momentum and continuity monitor (http://www.cfd-online.com/Forums/att...1&d=1341581085)
snap of coarse mesh with SST model - mass imbalance of individual domain (http://www.cfd-online.com/Forums/att...1&d=1341581189)
snap of coarse mesh with k-epsilon model - mass imbalance of individual domain (http://www.cfd-online.com/Forums/att...1&d=1341581259)
snap of coarse mesh simulation with k-epsilon model-momentum and continuity monitor (http://www.cfd-online.com/Forums/att...1&d=1341581283)
snap of coarse mesh simulation with k-epsilon turbulence monitor (http://www.cfd-online.com/Forums/att...1&d=1341581301)
Image of IGV (http://www.cfd-online.com/Forums/att...1&d=1341582197)
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Old   July 7, 2012, 06:58
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Sainath
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Are you trying to model it as a Steady case? Is your residual giving a bumpy characteristic?


What are your boundary conditions? Any idea of the stalling conditions?
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Old   July 7, 2012, 10:08
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Quote:
Originally Posted by sainath.s@tdps.co.in View Post
Are you trying to model it as a Steady case? Is your residual giving a bumpy characteristic?
Thank you to reply sainath. Yes, I am doing it as Steady case. I have noticed that for inclination 15 deg there is no kind of problem happened for max mass flow. But, using 30 deg as well 60 deg it is getting diverged after the residual around 2.7e-04.

Quote:
Originally Posted by sainath.s@tdps.co.in View Post
What are your boundary conditions? Any idea of the stalling conditions?
Total pressure inlet and mass flow rate outlet. No, stalling condition won't be to use here. I just take the pressure value and develop the graphical representation of performance(mass flow versus pressure & efficiency).


Thanks again.

Last edited by ayothicfd; July 7, 2012 at 10:12. Reason: forgot to enter some details.
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Old   July 8, 2012, 07:46
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Glenn Horrocks
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Is the flow compressible? Is it choking?
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Old   July 8, 2012, 21:22
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Hello Glenn,
Thank you. The flow is incompressible and isothermal flow. The pressure ratio is not much high so that it is considered as incompressible flow in fan (given as best practices guide in help manual).
Thanks again.
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Old   July 8, 2012, 21:29
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OK, thanks for teh clarification.

Have you been through this FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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Old   July 9, 2012, 04:16
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Quote:
Originally Posted by ayothicfd View Post
Thank you to reply sainath. Yes, I am doing it as Steady case. I have noticed that for inclination 15 deg there is no kind of problem happened for max mass flow. But, using 30 deg as well 60 deg it is getting diverged after the residual around 2.7e-04.


Total pressure inlet and mass flow rate outlet. No, stalling condition won't be to use here. I just take the pressure value and develop the graphical representation of performance(mass flow versus pressure & efficiency).


Thanks again.

Is there a artificial wall getting created at the inlet? If yes, then try changing your inlet to an opening boundary condition.

If no, Try changing your boundary condition to a mass flow inlet and a static pressure outlet. Just give this a try!!!1

Regarding your stalling conditions, there may be chances that your fan is entering a stalling region at those IGV conditions. Have you checked the pressure ratios at those points?

Your performance curve tells you the region of stall. It is very important to determine the stalling region. There may be different outflows at same delta P.

Are you trying to generate different curves at various positions of IGV or is it just to change the pressure at the outlet?
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Old   July 10, 2012, 09:54
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Quote:
Originally Posted by sainath.s@tdps.co.in View Post
Is there a artificial wall getting created at the inlet? If yes, then try changing your inlet to an opening boundary condition.

If no, Try changing your boundary condition to a mass flow inlet and a static pressure outlet. Just give this a try!!!1
There is no recirculation at inlet. So I changed boundary condition as you suggested to check. It is also getting diverged and stopped.

Quote:
Originally Posted by sainath.s@tdps.co.in View Post
Regarding your stalling conditions, there may be chances that your fan is entering a stalling region at those IGV conditions. Have you checked the pressure ratios at those points?

Your performance curve tells you the region of stall. It is very important to determine the stalling region. There may be different outflows at same delta P.

Are you trying to generate different curves at various positions of IGV or is it just to change the pressure at the outlet?
Thank you for your suggestion. My aim is to generate different curves at various positions of IGV. I have more doubts on the significance of stalling condition. It tends to apply an opposite power(force or torque) to resist impeller rotation(tends to stop). Is that the region before stalling we have to use the fan? or any technical significance behind stalling condition?
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Old   July 11, 2012, 06:01
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In fans, stalling means that your fan is rotating but is not giving any flow. Just like Glen had mentioned "Choking". In fans and pumps you call it "stall".

Tell me your fan OD and ID(Excluding HUB),blade inlet and outlet angles, fan width at ID and OD and rpm.

Your mass flow rates are too big at the exit. Provide me the details above, let me check it out.
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Old   July 11, 2012, 18:23
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Compressible flow choking is very different to stall. Do not confuse the two. But it has been said that this is a low pressure ratio model so it will not be choking.
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Old   July 13, 2012, 08:41
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Quote:
Originally Posted by sainath.s@tdps.co.in View Post
Tell me your fan OD and ID(Excluding HUB),blade inlet and outlet angles, fan width at ID and OD and rpm.

Your mass flow rates are too big at the exit. Provide me the details above, let me check it out.
My geometry details for your notice.
Type : Forward curved blade fan
Speed : 1485 rpm
Mass flow rate : range between 37.48-69.9 kg/s (37.48,42.83,48.18,53.54,58.89,64.25,69.6 kg/s)
Impeller ID & OD : 1362 mm & 2530 mm
Width at ID & OD : 169 mm & 206.8 mm
Blade inlet & outlet angle : 37 deg & 41 deg

One more thing is that what could be the acceptable convergence criteria for this kind of problem statement?
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Old   July 14, 2012, 05:36
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Do a convergence sensitivity analysis and determine it for your application. Also the CFX documentation has a good guide for convergence criteria.
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Old   July 16, 2012, 00:11
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Sr.No. P Q*Q 2 Q L
[IN.WG] [KCFM] [CFM] [KW]
1 95.235 0.0000 0 0.00
2 94.971 38.0105 6165 159.55
3 94.180 152.0419 12331 306.83
4 92.861 342.0942 18496 441.85
5 91.014 608.1674 24661 564.60
6 88.640 950.2616 30826 675.09
7 85.738 1368.3768 36992 773.32
8 82.308 1862.5129 43157 859.28
9 78.351 2432.6701 49322 932.97
10 73.866 3078.8482 55487 994.40
11 68.854 3801.0472 61653 1043.57
12 63.314 4599.2667 67818 1080.47
13 57.246 5473.5071 73983 1105.11
14 50.651 6423.7684 80148 1117.49
15 43.528 7450.0505 86314 1117.60
16 35.878 8552.3535 92479 1105.44
17 27.700 9730.6773 98644 1081.02
18 18.994 10985.0220 104809 1044.34
19 9.761 12315.3876 110975 995.39
20 0.000 13721.7741 117140 934.18


This is your fan's output. I believe the width at inlet and outlet is reverse in what you have typed (inlet<outlet). I guess it is wrong. the outlet width has to be lower than the inlet.

Let me know it you wanna know more.
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Old   July 17, 2012, 05:33
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Mr. Sainath,
Thank you so much for giving the details. You were right. I entered the details about the width of blade at ID and OD by mistake.
I have some difficulty in understanding your output. Could you please explain it clearly? Anyhow, I have understood something that values under CFM column specifies volume flowrate and values under KW specifies Power (not clear whether Input power or output power). Could you please explain me how to correlate my results with your output?
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Old   July 17, 2012, 05:41
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The values below IN WG is your pressure along corresponding CFM.
The column order is
Sr.No.
P
Q*Q
Q
L

L is the output power. Plot Q vs P and you will get your curve for the rpm given.
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Old   July 17, 2012, 22:12
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Hi Sainath,
Thank you. So you have given the value of pressure in inch of water column and volume flowrate in cfm. From your results, I have a doubt that the fan will not operate the flow after 110975 cfm (52.35 kg/s). Is it right?
I have been getting this problem while working on IGV 30 deg inclination only. Even it is getting results while working on full open and IGV 15 deg inclination (upto massflow 69.6 kg/s). If I should not run the mass flow beyond 52.35 kg/s previous runs itself I have to recognize the same problem.
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Old   July 18, 2012, 01:16
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You are right.
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Old   July 18, 2012, 10:07
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Mr. Sainath,
I am not clearly understood that why the simulation getting stopped while working on the mass flowrate above 52.35 kg/s at IGV 30 deg and more than that? I don't know why it is not giving the same kind of behaviour at IGV full open and 15 deg inclination(at both conditions, the simulation converged upto mass flowrate 69.6 kg/s). Could you please clarify my doubt?
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Old   July 20, 2012, 05:53
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Mr. Sainath
I have tried the same simulation to check the initial guess by setting boundary condition as total pressure inlet and static pressure outlet. But it is also stopped for IGV 30 deg inclination. I don't know the reason why it is getting diverged and stopped
Thank you.
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