bifurcations in combustors - combustion energy frontier ... lecture notes/poinsot/10b...resonant...
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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
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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’
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Example: startup sequence in a gas turbine
Time
POWER
IgniFon
WaiFng period‘TransiFon’
Time
FLOW RATE
Time
NOISE
100
100
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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
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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
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Increasing swirl:
S=0 S=0.33 S=0.4
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S=0.56 S=0.69 S=0.9
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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
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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 ?
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• 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
?
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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 ?
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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
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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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