[Q] High mach number in combustion

Whitebear · December 11, 2019, 00:39

During calculation in CFX coal combustion problem, I met high mach number like, 8.5E+2. If there is no mesh problem then what can be possibly cause this? Does anyone have this problem?

ghorrocks · December 11, 2019, 05:37

Crazy high Mach number means divergence is only a few iterations away, and divergence usually means you get a floating point error. So the FAQ on floating point error is what to look at: https://www.cfd-online.com/Wiki/Ansy...do_about_it.3F

Whitebear · December 12, 2019, 20:23

Thank you very much indeed. Diverge happened at 43 iterations. I found boiler wall temp and A/F(inlet air and inlet fuel ratio) sometimes make high mach number. The prblem is I cannot change them because it measured in the field.

ghorrocks · December 13, 2019, 03:44

There are many possible causes for this problem. If you want us to help you, you will need to provide more information. For instance post an image of what you are modelling, your mesh and your output file.

Whitebear · December 13, 2019, 08:23

The number of node is 14million. If I put inflation then 37millilon.
Below mesh statistics doesn't have inflation layers.

+--------------------------------------------------------------------+
| Mesh Statistics |
+--------------------------------------------------------------------+
| Domain Name | Orthog. Angle | Exp. Factor | Aspect Ratio |
+----------------------+---------------+--------------+--------------+
| | Minimum [deg] | Maximum | Maximum |
+----------------------+---------------+--------------+--------------+
| Boiler | 33.9 ok | 32 ! | 21 OK |
+----------------------+---------------+--------------+--------------+
| | %! %ok %OK | %! %ok %OK | %! %ok %OK |
+----------------------+---------------+--------------+--------------+
| Boiler | 0 <1 100 | <1 1 99 | 0 0 100 |
+----------------------+---------------+--------------+--------------+

Opaque · December 13, 2019, 10:11

If I were to do it, here would be my steps (assuming steady-state simulation),

1 - Setup your case to write backup files including equation residuals

2 - Set the frequency of the files such you can study from when the solution starts to diverge. Say, one timestep before you got the Mach number message, and two more if possible.

3 - Be certain you variables such as Pressure, Temperature, Mach Number in those files as well.

4 - Post-process them looking for: where is the maximun residual for each equation located, is there a sign of temperature going out of the bounds in the problem (too cold for example), etc.

Then based on your findings try to extrapolate what may be the source of the problem: mesh quality, too large timestep, boundary conditions incompatibility, etc.

Whitebear · December 15, 2019, 21:14

Thank you very much indeed, I will do that.

December 11, 2019, 00:39	[Q] High mach number in combustion	#1
Whitebear Member Jinwhan Ryuk Join Date: Feb 2013 Location: South Korea Posts: 91 Rep Power: 13	During calculation in CFX coal combustion problem, I met high mach number like, 8.5E+2. If there is no mesh problem then what can be possibly cause this? Does anyone have this problem?

December 11, 2019, 05:37		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Crazy high Mach number means divergence is only a few iterations away, and divergence usually means you get a floating point error. So the FAQ on floating point error is what to look at: https://www.cfd-online.com/Wiki/Ansy...do_about_it.3F __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

December 13, 2019, 03:44		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	There are many possible causes for this problem. If you want us to help you, you will need to provide more information. For instance post an image of what you are modelling, your mesh and your output file. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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December 12, 2019, 20:23		#3
Whitebear Member Jinwhan Ryuk Join Date: Feb 2013 Location: South Korea Posts: 91 Rep Power: 13	Thank you very much indeed. Diverge happened at 43 iterations. I found boiler wall temp and A/F(inlet air and inlet fuel ratio) sometimes make high mach number. The prblem is I cannot change them because it measured in the field.

December 13, 2019, 10:11		#6
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	If I were to do it, here would be my steps (assuming steady-state simulation), 1 - Setup your case to write backup files including equation residuals 2 - Set the frequency of the files such you can study from when the solution starts to diverge. Say, one timestep before you got the Mach number message, and two more if possible. 3 - Be certain you variables such as Pressure, Temperature, Mach Number in those files as well. 4 - Post-process them looking for: where is the maximun residual for each equation located, is there a sign of temperature going out of the bounds in the problem (too cold for example), etc. Then based on your findings try to extrapolate what may be the source of the problem: mesh quality, too large timestep, boundary conditions incompatibility, etc.

December 15, 2019, 21:14		#7
Whitebear Member Jinwhan Ryuk Join Date: Feb 2013 Location: South Korea Posts: 91 Rep Power: 13	Thank you very much indeed, I will do that.