Bifurcations in combustors

With contributions by M. Falese, S. Hermeth, G. Staffelbach, L. Gicquel

Computers & Fluids 89 (2014) 167–178

Comb. & Flame. 2014, 161, 184-196.

Copyright Dr T. Poinsot 2015

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We hope (expect ?) flames to depend on a limited number of parameters

Fuel:  -­‐  Flow  rate  -­‐  Temperature

Air:  -­‐  Flow  rate  -­‐  Temperature

2

In practice, some hidden parameters exist:

Fuel:  -­‐  Flow  rate  -­‐  Temperature

Air:  -­‐  Flow  rate  -­‐  Temperature

Wall  Temperatures  ?    Time  ?  Humidity  of  air  Leaks  Composi7on  Etc...

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Combustor ‘variability’

Quite  o3en,  combustors  exhibit  variability  phenomena  which  are  difficult  to  master:  -­‐  Most  of  them  are  due  to  ‘uncontrolled’  parameters  which  change  and  which  we  cant  control  -­‐  But  even  when  all  input  parameters  are  well  controlled,  the  flow  itself  may  exhibit  mulFple  ‘states’.  BifurcaFons  are  possible

INPUT  PARAMETERS FLAME

Efficiency  Pollutants  ThermoacousFcs

‘STATE’

4

Example: startup sequence in a gas turbine

Time

POWER

IgniFon

WaiFng  period‘TransiFon’

Time

FLOW  RATE

Time

NOISE

100

100

5

Examples of bifurcating flame

Time

POWER

IgniFonNo  TransiFon

Time

FLOW  RATE

Time

NOISEThe  ‘transi7on’  from  one  state  to  another  is  required  to  reach  full  power  without  excessive  instability

100

6

Swirling flows: make bifurcations more likely

Swirling  flows  exhibit  mulFple  instabiliFes.  

They  also  exhibit  bifurcaFons:  mulFple  states  can  be  obtained  for  the  same  flow  condiFons:  true  with  and  without  combusFon  

The  experiment  of  Vanierschot:

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Experimental Thermal and Fluid Science 31 (2007) 513–524

Increasing swirl:

S=0   S=0.33   S=0.4  

8

S=0.56 S=0.69   S=0.9  

9

S=0.56 S=0.5   S=0.  

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TWO FLOW STATES FOR THE SAME SWIRL:

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S=0.56  when  S  increases

S=0.56  when  S  decreases

PRESSURE  LOSSES  ALSO  SHOW  HYSTERESIS  PATTERN:

1

2

P1-­‐P2

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Bifurcating flame in an industrial gas turbine• Two dynamic phenomena already discussed in swirled confined

flames: the  Precessing Vortex Core (PVC)[1,2]

[1] H. Liang, T. Maxworthy, J. Fluid Mech. 525 (2004) 115-159  [2] Y. Huang, V. Yang, Progress in Energy and Combustion Science 35 (2009) 293-364

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2 - Thermo-acoustic instabilities [3,4,5]  q Resonant feedback between combustion and acoustic waves

[3] T. Lieuwen, V. Yang. Progress in Astronautics and Aeronautics, Vol. 210  [4] S. Candel. International Journal of Aeroacoustics. Vol.8, 2009  [5] T. Poinsot, D. Veynante. Theoretical and Numerical Combustion. R.T. Edwards

Flow and mixture perturbations

Heat release oscillations

Acoustic oscillations

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• PVC and combustion  q The PVC can interact with the flame [14,15]  q Combustion can suppress the PVC [16,17,18]  

• PVC and combustion instabilities  q The PVC can provoke thermo-acoustic instabilities [16]  q Acoustic oscillations can suppress the PVC [14,15]

[14] J. Moeck, J.-F. Bourgouin, D. Durox, T. Schuller, S. Candel, Combust. Flame 159 (2012) 2650-2668.  [15] C. O. Paschereit, E. Gutmark, W. Weisenstein AIAA Journal38 (6) (2000) 1025-1034.  [16] D. Galley, S. Ducruix, F. Lacas, D. Veynante, Combust. Flame 158 (2011) 155-171.  [17] S. Roux, G. Lartigue, T. Poinsot, U. Meier, C. Berat, Combust. Flame 141 (2005) 40-54.  [18] N. Syred, Prog.Energy Comb. Sci. 32 (2) (2006) 93-161.

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Is there a link between PVC and instabilities ?

• The link between PVC and thermoacoustics can also be more complex:  q The swirled flow can be bistable, leading to the existence

of two states for the same regime  q This leads to :  

q Different mean flows  q Different stability behaviors  q The flame can switch from one state to another (with

changing fuel rate or acoustic oscillations) and trigger bifurcations

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Target configuration

• Heavy-duty industrial gas turbine  q Power ~300 MW  q Reynolds number > 1.000.000

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Target configuration

Annular combustion chamber [19] LES of one sector

[19] Ansaldo Energia

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Target configuration

Burner4 air inlets

Diagonal swirler

Axial swirler

Lance cooling

Film cooling

Pilot gas nozzle

Premixing gas  nozzle

2 CH4 inlets

q Lean combustion  q High pressure (p > 15 bar)  q High Reynolds number (Re

> 1.000.000)

Operating conditions

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Mesh & Boundary conditions • Computational grid: 10.5 M tetrahedral elements  • Boundary conditions:  

q All inlets and the outlet: NSCBC formulation [20]  q Chamber side walls: Axi-periodic  q All other walls: logarithmic wall-law [21]

[20] T. Poinsot, S. Lele, J. Comput. Phys. 101 (1) (1992) 104-129.  [21] P. Schmitt, T. Poinsot, B. Schuermans, K. P. Geigle, J. Fluid Mech. 570 (2007) 17-46.

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Initialisation of the LES

Operating point 1 Operating point 2

?

21

Initialisation of the LES

Operating point 2

Attached case Detached case

The flame has two stable positions: it is bistable

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Mean reacting flow fields: temperature

Attached case Detached case

q High temperature at the lance due to pilot injection

q More uniform temperature distribution

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Mean reacting flow fields: velocity

11 1

2

13

14

15

16

17

11 1

2

13

14

15

16

17

Attached case Detached case

q High velocities at the burner exit: the flame reduces the section  

q Two recirculation zones

q Lower velocities at burner exit  q Inner recirculation zone not

closed

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Temperature profiles

0 1 2−1

−0.75

−0.5

−0.25

0

0.25

0.5

0.75

1

y/y

ma

x [−

]

1

0 1 2

2

0 1 2

3

0 1 2

T/Tmean

[−]

4

0 1 2

5

0 1 2

6

0 1 2

7

Attached

Detached

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Axial velocity profiles

−1 0 1−1

−0.75

−0.5

−0.25

0

0.25

0.5

0.75

1

y/y

ma

x [−

]

1

−1 0 1

2

−1 0 1

3

−1 0 1U

ax/U

bulk [−]

4

−1 0 1

5

−1 0 1

6

−1 0 1

7

Attached

Detached

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Turbulent kinetic energy profiles

0 20 40−1

−0.75

−0.5

−0.25

0

0.25

0.5

0.75

1

y/y

max [−

]

1

0 10 20

2

0 10 20

3

0 10 20

k/Ubulk

[−]

4

0 10 20

5

0 10 20

6

0 10 20

7

Attached

Detached

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RMS temperature profiles

0 0.25 0.5−1

−0.75

−0.5

−0.25

0

0.25

0.5

0.75

1

y/y

ma

x [−

]

1

0 0.25 0.5

2

0 0.25 0.5

3

0 0.25 0.5T

RMS/T

mean [−]

4

0 0.25 0.5

5

0 0.25 0.5

6

0 0.25 0.5

7

Attached

Detached

• Detached flame: RMS temperature higher over a wider range  

•  This suggests that NO production would differ.

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Instantaneous reacting flow fields

Attached case Detached case

q No PVC present q Strong PVC turning around a ‘finger-like’ flame structure  

q PVC explains strong values of k

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How is forcing applied ?

Reference point q Sinusoidal modulation of the ingoing acoustic wave at the diagonal swirler [29]  

q Small forcing amplitude  

q Four forcing frequencies f1 < … < f4

[29] A. Kaufmann, F. Nicoud, T. Poinsot. Combust. Flame 131 (2002) 371–385.

F (�) =q(�)/q

u(�)/u

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A flame where equivalence ratio is not constant:

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Pulsating the detached flame at small amplitudes:

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Attached case Detached case

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Flame Transfer Function

0

1

2

f1 f

2

f3

f4

Am

plit

ude [−

]

min max−6.28

−3.14

0

Frequency [−]

Phase

[−

]

Attached

Detached

q Detached flame: higher gain and lower phase response  q Predicting stability will lead to different conclusions

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Bifurcation: going from one state to another ?Attached case Detached case

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Can a change in the pilot flame fuel flow rate change the flame state ?

Can an acoustic fluctuation imposed at the inlet change the flame state ?

Bifurcation: change in fuel flow rateq The pilot flame is known to have a strong influence on

the flame stabilization.  q Is it possible to change the flow state by increasing or

decreasing the fuel flow rate in the pilot flame ?  

q Define pilot fuel ratio pfr:  

q Increase pfr from 1 to 4, starting either from Attached or Detached Flame

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pfr =

Pilot fuel flow rate

Reference value

Attached flame- increasing the pilot flame flow rate:

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Bifurcation: change in fuel flow rate

0 1.0 2.0 3.0 4.0

State A

State D

Pilot fuel ratio [−]

Attached

Detached

Referencecase

q Detached flame remains detached with increasing OR decreasing pilot fuel mass flow rate  

q Attached flame detaches with decreasing pfr and for pfr=4.0 (previous movie)

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Bifurcation: effects of pulsation

q If we ‘shake’ these flames sufficiently (for example during a combustion instability), is it possible for the flame to change state ?  

q Start from Attached or Detached unforced state and force the combustor with increasingly larger acoustic waves (up to 45 % of the mean flow speed)  

q See whether the flame state can change

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Attached flame: pulsation amplitude 45 % at f4

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Bifurcation: change in fuel flow rateDetached flame: pulsation amplitude 45 % at f4

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Detached flame: pulsation amplitude 45 % at f4

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Bifurcation: change of the pulsation amplitude at f4

0 10 20 30 40 50

State A

State D

Forcing amplitude [%]

AttachedDetached

Referencecase

q Detached flame reattaches with increasing forcing amplitude  q Attached flame remains attached

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Conclusion

• In certain chambers, the flame can have different states with very different mean flows, performances and pollution levels  

• The FTFs between the two states also differ suggesting different thermoacoustic effects  

• The transition between the two states observed in the present gas turbine can be triggered by changing:  q The pilot fuel rate  q The forcing amplitude  

• This can explain complex behaviours observed in gas turbines where the flame may trigger different acoustic responses for the same regime.

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