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Old   November 14, 2011, 07:39
Question Axial compressor calculation steps?
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Dear All. I'm a newbie in CFX calculations and i need your advices concerning calculation of axial compressor stage in CFX.
I calculated axial turbine before and everything was OK. But in compressor (Stage 37), when i set the same boundary conditions (P_total_in, T_total_in and P_static_out) the flow goes in wrong direction. Solver puts wall on 100% of inlet. The only way i managed to calculate it right is by setting of "supersonic" in outlet conditions.

Could you please tell me what i'm doing wrong and what are the right steps of compressor calculation should be?

TIA, Oleg.
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Old   November 14, 2011, 07:43
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rpm is 17188.7 or -17188.7?
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Old   November 14, 2011, 07:48
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-17188.7 rpm
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Old   November 14, 2011, 07:49
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whats inlet and outlet pressure? and flow direction (-1 axial, 0 for other directions)?
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Old   November 14, 2011, 08:05
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Inlet total pressure 101.4 kPa, outlet static pressure = 138 kPa. I'm not sure what you mean on "flow direction". The flow goes in reverse direction comparing to how it shoud be (from outlet to inlet)
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Old   November 14, 2011, 08:08
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are you using rotor or complete stage. If rotor then 138 KPa is very high value, beyond the stall point!! Even with stator case, it is still too high value, have start with 101325 Pa and gradually increase in increment of 5000
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Old   November 14, 2011, 08:14
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really, i'm not sure that i'm doing as i suppose to do for compressor.

dear Far. Maybe there's some specific BC or loss model need to be set for compressor calculation? Is the task formulation that i use right and compressor should calculate OK with such scope boundary conditions an the probles is just in me?
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Old   November 14, 2011, 08:19
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Quote:
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are you using rotor or complete stage. If rotor then 138 KPa is very high value, beyond the stall point!! Even with stator case, it is still too high value, have start with 101325 Pa and gradually increase in increment of 5000
I'm using comlete stage.

Thanks for the advise. While increasing pressure on 5000 Pa what should i control as convergense parameter? Mass flow or maybe something else? in other words when should i decide that I can inrease pressure?
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Old   November 14, 2011, 08:19
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Simple. Just use 101325 pa as inlet pressure (or profile at later stages) and outlet pressure 101325, then gradually increase static pressure at outlet to

105000
110000
115000
120000
122500
125000
127000
128000
129000
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Old   November 14, 2011, 08:33
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I understood about increasing outlet pressure step by step. But what parameter should i control to decide that it is the right time to increase pressure? e.g. i'm doing calculation for P_stat_out=101325, controlling what parameter i can decide that it's time for increasing presure up to 105000? Mass flow rate stabilize? or maybe some of convergense parameters go lower than some specific value?
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Old   November 14, 2011, 16:32
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If the inlet pressure is about 101kPa and the outlet pressure is about 130kPa then you should be using a reference pressure of 101kPa and an inlet pressure of 0kPa, outlet of 29kPa. If you do not use a reference pressure you will have more round-off error and that can lead to convergence problems.

Are you using a reference pressure?
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Old   November 15, 2011, 04:33
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Quote:
Originally Posted by ghorrocks View Post
If the inlet pressure is about 101kPa and the outlet pressure is about 130kPa then you should be using a reference pressure of 101kPa and an inlet pressure of 0kPa, outlet of 29kPa. If you do not use a reference pressure you will have more round-off error and that can lead to convergence problems.

Are you using a reference pressure?
No i dont. I'll try.

Thank you for advice.
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Old   November 15, 2011, 06:35
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Dear ghorrocks.

I have a question concerning total pressure ratio. Can i plot total pressure at stage oultet while solver running the calculation in new monitor?
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Old   November 15, 2011, 07:34
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Could you please post some pics of your domain and mesh. Also post information about total no of nodes in domain, any information about the interface between rotor and stator.
Any how, compressor flows are more difficult to handle than the turbine and you need to handle it by putting little load at start-up (in terms of rpm and pressure at outlet) and then ramp-up to desired value. Also search the forum for older posts regarding the same issue
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Old   November 15, 2011, 08:53
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Thanks Far!
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Old   November 15, 2011, 09:33
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Quote:
Originally Posted by Far View Post
Could you please post some pics of your domain and mesh. Also post information about total no of nodes in domain, any information about the interface between rotor and stator.
Number of nodes 229118. All interfaces are set as "Stage". The picture of domain is attached.
Attached Images
File Type: jpg St 37 domain.jpg (59.5 KB, 43 views)
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Old   November 15, 2011, 23:32
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This should also be noted as the no. of nodes increases compressor simulation tend to numerically stall at the higher pressure ratio than for coarse mesh. Therefore it is good idea to refine mesh further and also check the solution at lower back pressure for the current mesh.

Moreover which turbulence model you are using? What is Y+ in domain? Since appropriate Y+ should be used for each model.

Other things to be checked are (important to solution convergence and accuracy): aspect ratio, max and min angle, expansion rate
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Old   November 16, 2011, 05:29
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Quote:
Originally Posted by Far View Post
This should also be noted as the no. of nodes increases compressor simulation tend to numerically stall at the higher pressure ratio than for coarse mesh. Therefore it is good idea to refine mesh further and also check the solution at lower back pressure for the current mesh.

Moreover which turbulence model you are using? What is Y+ in domain? Since appropriate Y+ should be used for each model.

Other things to be checked are (important to solution convergence and accuracy): aspect ratio, max and min angle, expansion rate
I'm using the SST model. On blade surface maximum Yplus is 200, on nozzle 100.
+--------------------------------------------------------------------+
| Domain Name | Orthog. Angle | Exp. Factor | Aspect Ratio |
+----------------------+---------------+--------------+--------------+
| | Minimum [deg] | Maximum | Maximum |
+----------------------+---------------+--------------+--------------+
| Rotor | 41.5 ok | 6 ok | 736 ok |
| Stage in | 85.7 OK | 1 OK | 7 OK |
| Stator | 46.0 ok | 40 ! | 52 OK |
| Global | 41.5 ok | 40 ! | 736 ok |
+----------------------+---------------+--------------+--------------+
| | %! %ok %OK | %! %ok %OK | %! %ok %OK |
+----------------------+---------------+--------------+--------------+
| Rotor | 0 <1 100 | 0 <1 100 | 0 2 98 |
| Stage in | 0 0 100 | 0 0 100 | 0 0 100 |
| Stator | 0 <1 100 | <1 1 99 | 0 0 100 |
| Global | 0 <1 100 | <1 <1 100 | 0 1 99 |
+----------------------+---------------+--------------+--------------+
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Old   November 16, 2011, 07:16
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Use K-epsilon (also use lower pressure at outlet as discussed earlier)
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Old   November 16, 2011, 07:19
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Quote:
Originally Posted by Far View Post
Use K-epsilon (also use lower pressure at outlet as discussed earlier)
Thank you. I'll try.
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