[Q] High mach number in combustion
 

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?

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

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.

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.

Geometry and mesh information in Solver header
 

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 |
+----------------------+---------------+--------------+--------------+

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.

Thank you very much indeed, I will do that.