Stabilization of a diluted H2 lifted jet flame: numerical study and control
Christophe Duwig* - Haldor Topsøe A/S – DK-2800 Lyngby
*also with Dept. Energy Sciences – Lund University (Sweden)

Presentation layout
Background: reforming within ammonia technology
Presentation of the idealized problem & methodology
– Cabra’s jet flame
– Reactive LES simulation tool with tabulated chemistry
Presentation of the results
– comparisons with experimental data
– Analysis of vortex dynamics: identification of the coherent structures
– Effect of small perturbations upon these structures: vortex control
Conclusion

Overview of a Topsøe ammonia plant http://www.topsoe.com/Business_areas/Ammonia/Processes.aspx
(1) CH
4
+H
2
O=CO+6H
2
(2) CH
4
+0.5O
2
=>CO+2H
2
Flame!
CO+H
2
O=CO
2
+H
2
3H
2
+N
2
=>2NH
3

In real life, it looks like …
Burner/ flame location
CH
4
+H
2
O=CO+6H
2
CH
4
+0.5O
2
=>CO+2H
2

Secondary reformer burner
Process gas is a hot syngas: mixture CO, CO2, H20, H2, CH4
=
We deal with a hot but very diluted fuel
Process gas
Process gas air http://www.topsoe.com/Business_areas/Ammonia/Processes.aspx
Air nozzles: high-speed jets

How does a flame front really look like?
what we see and what we hear
Even simple a Bunsen flame is complex as shown beside!
There is a need of advanced investigation techniques (experimental and numerical), here laser based OH-PLIF
Does it happen only in the lab. ??? … NO!
OH PLIF from Buschmann et al., 26th Symp.
On Combustion, pp. 437-445, 1996

Emulation of a secondary reformer flame
Diluted H
2
/N
2
/O
2 flame in a hot coflow
X
X
IG http://www.me.berkeley.edu/cal/VCB/Data/
T~1045 K
H
2
O/O
2
/N
2
T~1045 K
H
2
O/O
2
/N
2
T~305 K
H
2
/N
2

Idealized scenario of a fuel fluid parcel:
The parcel:
Exits the nozzle
Is convected downtream
Mixes with vitiated gases - dilution
T , Z and Y
F
WITHOUT reaction Y
OH
~0
Mixes with vitiated gases and auto-ignites: reaction starts after a delay t~X
IG
/U
BULK
>
AI
(Z,T)
Burns and mixes further

Generation of kinetic lookup tables
X
System of equations: dY i i dt dT i
T dt
Initial conditions:
( t 0 )
Y
1
...
Y i
...
Y n
T
Z X
Fuel
•Two reaction coordintates
( , t) including detailed chemical kinetics
•PSR computations performed using Cantera
1 Z X
Ox
•Allow to store the chemical information in look-up talbes & mapping in the (Z, T)-space

Tabulated chemistry or Z or Z

Constitutive equations for reactive LES
Filtered continuity and Navier-Stokes u 0
SGS term: here FSF t u u u p u u uu u t
2 additional scalars: mixture fraction and temperature
~
Z u
~
Z (( D D
~
Z ).
t
~
T t u T
~
( D
T
D
~
T ) w
T
( Z , T )
Filtered density function: top-hat w
T
( Z , T ) w
T tabulation
( Z , T ) P (
~
Z , T
~
, Z , T ,...) dZdT

Numerical methods for reactive LES
• Numerical methods
• Finite Difference Cartesian code
• Spatial discretization: 4th order centred except the convective terms in the T & Z-eq. ->WENO5
• Temporal discretization: 2nd order implicit with multi-grid acceleration & CFL < 0.3
•Numerical grid
• Computational box: 22 d ·11 d ·11 d
• average cell size at the inlet h=d/20
• number of cells: 2 millions
•Vortices are visualized like
2
-isosurfaces (gray)

Mean quantities – comparisons with exp.
OH mass fraction & temperature
Direct light visualization
OH-field
Temperature [K]

Animation of the OH and temperature fields
T [K]
OH

Fuel jet
3D snapshot: flame & vortex dynamics
Jet core
Small vortex tubes
Flame envelope
Vortex rings
Vortex rings merge giving helices
Macro-mixing micro-mixing
Sequence: rings ->helices->small tubes->flame

Control by inflow perturbations A(t, ) u
JET
( X , t ) A ( t , ) u
Average
( X )
Fluctuation (Klein et al., JCP 2005) u ' ( X ) F ( t )
Promote Case base
A(t, )
1
Axi-sym.
1+0.05 2cos(2 ·St p
·t/
VariFlap 1+0.05 2cos(2 ·St p
·t/ +
0.05 2cos( ·St p
·t/ ·(2y/d)
Star 1+0.2 cos(5 ) · (2r/d) rings helices
Small scale vortex tubes
Coflow
Jet daSilva & Métais, PoF, 2002

Axisymmetric pulsation:
3D vortices and 2D OH field
Vortex ring
Tubes
Flame leading edges

VariFlap perturbation:
3D vortices and 2D temperature field
Sequence from daSilva & Métais, PoF 2002

VafiFlap perturbation:
3D vortices and 2D OH field

Star:
3D vortices and temperature field

Star:
3D vortices and OH field

Conclusion and prospects
Turbulent combustion is still a fascinating and mysterious thing
A small amplitude perturbation enables to control the flame dynamics (jet core length, leading edge position, …)
The star perturbation attaches the flame -> dramatic reduction of the flame fluctuation -> attractive stabilization tool!!
– Already used for noise reduction
Chevron on nozzles for jet noise reduction
Callender et al. AIAA J., 2005

Modeling: ideal PSR interpretation
Vitiated gas
Fuel mixture
( ,t) are the reaction coordinates t( )