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Too low temperature at combustor outlet

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Old   February 4, 2012, 07:13
Default Too low temperature at combustor outlet
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Hello everyone,

I have problem with my CFD simulation. I have to model a combustion process in turbine engine combustor.


Here are some information about simulation:

ANSYS Fluent 13.0

- pressure based
- realizable k-epsilon
- partially premixed combustion of c12h23
- DPM with 4 continuous phase iterations per DPM iteration, other properties defaults.
- Cone injection, droplet, kerosene-liquid, evaporating species: c12h23
- mesh is 5 mln tetra (I also tried 6 and 20 mln tetra, but final problem was always the same)


I run calculations in 3 steps:

1. Calculating only air flow
2. Turning on DPM
3. Calculating combustion by patching progress variable equal 1 to whole combustor.

Im monitoring temperature at combustor outlet (t_min, t_mean and t_max).
Calculated and experimentally measured average temperature (t_mean) at outlet is about 1180K

Problem starts when I "ignite" mixture.

After some iterations temperature "is trying" to converge at 1180K - great! ( temperature distribution in whole chamber is also correct )
but after more iterations, temperature drops to value of about 600K. Temperature distribution in chamber is also not good then.

It is clearly shown at plots of residuals and outlet temperature.





Here are some info from FLUENT:

Code:
  iter continuity x-velocity y-velocity z-velocity     energy          k    epsilon      fmean       fvar    premixc      t_max     t_mean      t_min     time/iter
  1628 2.4042e-03 1.7096e-04 2.8642e-04 1.6927e-04 1.0230e-04 1.8258e-04 5.7209e-04 9.0740e-05 3.4400e-04 4.9054e-05 6.3157e+02 6.1549e+02 5.9406e+02  2:22:29  549

 turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 98345 cells 
  1629 3.9247e-03 1.6888e-04 2.0046e-04 1.6883e-04 5.1275e-05 1.3198e-04 4.2686e-04 5.5448e-05 1.9339e-04 4.4105e-05 6.3146e+02 6.1542e+02 5.9401e+02  2:30:19  548

 turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 96157 cells 
  1630 4.6650e-03 5.5767e-04 6.3399e-04 5.5584e-04 4.6992e-05 1.4767e-04 3.7893e-04 5.4586e-05 1.6977e-04 4.3964e-05 6.3145e+02 6.1549e+02 5.9421e+02  2:23:44  547

 turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 97903 cells 
  1631 3.3313e-03 2.9818e-04 3.3710e-04 2.9924e-04 7.1853e-05 1.6899e-04 5.4183e-04 6.5568e-05 2.7728e-04 4.7080e-05 6.3134e+02 6.1541e+02 5.9416e+02  2:20:16  546

DPM Iteration ....

 turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 98144 cells 
number tracked = 100, escaped = 0, aborted = 0, trapped = 0, evaporated = 100, incomplete = 0, incomplete_parallel = 0
Parallel particle tracking concurrency: 12%

  1632 2.2688e-03 1.8252e-04 2.2522e-04 1.8308e-04 1.0133e-04 1.8126e-04 5.7111e-04 8.9342e-05 3.4547e-04 4.8546e-05 6.3124e+02 6.1534e+02 5.9411e+02  2:19:15  545

 turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 97101 cells 
  1633 2.8380e-03 9.6682e-05 1.3500e-04 9.7134e-05 4.6253e-05 1.2884e-04 4.1273e-04 5.0446e-05 1.8599e-04 4.3897e-05 6.3113e+02 6.1526e+02 5.9406e+02  2:27:28  544

 turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 93080 cells 
  1634 4.1602e-03 5.3542e-04 6.1055e-04 5.2838e-04 3.5126e-05 1.4833e-04 3.7456e-04 4.7603e-05 1.4393e-04 4.2551e-05 6.3155e+02 6.1568e+02 5.9448e+02  2:21:17  543

 turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 94129 cells 
  1635 4.0336e-03 3.2497e-04 3.8269e-04 3.2224e-04 7.5555e-05 1.7263e-04 5.2619e-04 6.7395e-05 2.8646e-04 4.7405e-05 6.3145e+02 6.1560e+02 5.9444e+02  2:23:32  542

DPM Iteration ....

 turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 95817 cells 
number tracked = 100, escaped = 0, aborted = 0, trapped = 0, evaporated = 100, incomplete = 0, incomplete_parallel = 0
Parallel particle tracking concurrency: 11%
It was still saying that "turbulent viscosity limited to viscosity ratio of 1.000000e+05 in xxxxxx cells"...
I changed turbulent viscosity ratio limit from 1e5 to 2.5e5, and decreased turbulent relaxation factors. Also increased turbulent intensity in boundary conditions from 5 to 10%. Information "turbulent viscosity limited to..." disappeared, but now I have information that
Minimum PDF table temperature in xxxx cells on zone 2...

I think that it also might be problem with viscosity. In mixture properties viscosity is set to be temperature dependent. But on the plots, I get constant value of mentioned property ( about 1.51 e-5 )

I'm trying many variants, but all of them takes a lot of time... I'd be really grateful for any help
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Old   February 5, 2012, 11:36
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You did not create correct PDF table.
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Old   February 6, 2012, 11:02
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Hey

xrs333 thank you for answer

PDF was ok. Problem was with combustion model. Not partially premixed, but non-premixed.

I have changed, and now it's ok.
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