## Abstract

Axially staged combustion systems offer both enhanced operability and fuel flexibility for gas turbines, allowing stable operation and low emissions across a wide range of engine loads. The sequential combustion concept, where the first combustion stage is supported by a standard lean premixed flame and the second stage relies on an auto-ignition-dominated flame, forms the focus of the present contribution. Within the present state of the art, the still-oxygen-rich exhaust gases from the first stage are mixed with second stage fuel within a sequential burner. Within the presently investigated concept, the flame is anchored by establishing a positive static temperature gradient within the burner. The advantage of such a concept is that it potentially allows for very small combustor residence times and can be easily incorporated into an integrated combustor–nozzle guide vane. The concept does however present significant challenges, which are investigated within then present contribution. A critical challenge is that, in order to setup the static temperature gradient, the flow has to be accelerated to a relatively high Mach number, ca. 0.7, and then decelerated in a diffusing section where the flame is located. Achieving fuel/air premixing and combustion, while achieving acceptable pressure drops is not trivial at the high velocities. Additionally, the dynamic stability of the concept is not clear and needs to be investigated. Within the present work, compressible computational fluid dynamics (CFD) is used to investigate the pressure drop characteristics within the system. It is demonstrated that for the system a total pressure drop of less than 6% can be achieved. To realize this, the premixing section includes multipoint fuel injection coupled with mixing devices. The arrangement is designed to both limit excessive pressure losses by focusing losses within regions of the flow where they contribute effectively to fuel/air mixing as well as locating the flame where Rayleigh losses are acceptable. The dynamic behavior of the system is studied by way of two-dimensional (2D) fully premixed CFD. Investigation of the flame response to harmonic perturbations in inlet temperature shows that the flame transfer function (FTF) is characterized by amplitude growing, in line with the concept of auto-ignition at low Mach number, linearly with frequency. The rate of growth with frequency of the FTF amplitude is rather high reaching up to 60 times the imposed relative fluctuation of inlet temperature at a frequency of $600 Hz$. This rapid growth is in line with the behavior of auto-ignition at low Mach number. A substantial difference with the low Mach number concept is given by upstream traveling acoustic waves generated by the flame that going through high Mach number locations, can affect, in respect of the conservation of entropy transported by convection, the upstream temperature distribution and therefore auto-ignition itself.

## References

1.
Joos
,
F.
,
Brunner
,
P.
,
Schulte-Werning
,
B.
,
Syed
,
K.
, and
Eroglu
,
A.
,
1996
, “
Development of the Sequential Combustion System for the ABB GT24/GT26 Gas Turbine Family
,”
ASME
Paper No. 96-GT-315. 10.1115/96-GT-315
2.
Düsing
,
M. K.
,
Ciani
,
A.
,
Benz
,
U.
,
Eroglu
,
A.
, and
Knapp
,
K.
,
2013
, “
Development of GT24 and GT26 (Upgrades 2011) Reheat Combustors, Achieving Reduced Emissions and Increased Fuel Flexibility
,”
ASME
Paper No. GT2013-95437.10.1115/GT2013-95437
3.
Pennell
,
D. A.
,
Bothien
,
M. R.
,
Ciani
,
A.
,
Granet
,
V.
,
Singla
,
G.
,
Thorpe
,
S.
,
Wickstroem
,
A.
,
Oumejjoud
,
K.
, and
Yaquinto
,
M.
,
2017
, “
An Introduction to the Ansaldo GT36 Constant Pressure Sequential Combustor
,”
ASME
Paper No. GT2017-64790.10.1115/GT2017-64790
4.
Naik
,
S.
,
Stephan
,
B.
, and
Henze
,
M.
,
2021
, “
GT36 Turbine Development and Full-Scale Validation
,”
ASME
Paper No. GT2021-59470.10.1115/GT2021-59470
5.
Syed
,
K. J.
,
Benim
,
A. C.
,
Pasqualotto
,
E.
, and
Payne
,
R. C.
,
2020
, “
A Novel Approach to the Stabilization of Auto-Igniting Flames Within a Gas Turbine Sequential Combustor, Through the Control of Static Temperature Variation Along the Premixing and Flame Zones
,”
ASME
Paper No. GT2020-14225.10.1115/GT2020-14225
6.
Syed
,
K. J.
,
Poyyapakkam
,
M.
, and
Genin
,
F.
,
2015
, “
Sequential Combustor and Method for Operating the Same
,” EPO, Patent No. EP3115693A1.
7.
OpenFOAM and The OpenFOAM Foundation
, 2022, “OpenFoam User Guide,” OpenFOAM, London, UK, accessed Oct. 18, 2022, https://openfoam.org/
8.
Poinsot
,
T. J.
, and
Lele
,
S. K.
,
1992
, “
Boundary Conditions for Direct Simulations of Compressible Viscous Flows
,”
J. Comput. Phys.
,
101
(
1
), pp.
104
129
.10.1016/0021-9991(92)90046-2
9.
Polifke
,
W.
,
Clifton
,
W.
, and
Moin
,
P.
,
2006
, “
Partially Reflecting and Non-Reflecting Boundary Conditions for Simulation of Compressible Viscous Flow
,”
J. Comput. Phys.
,
213
(
1
), pp.
437
449
.10.1016/j.jcp.2005.08.016
10.
GRI-Mech
, 2022, “GRI-Mech,” accessed Oct. 18, 2022, http://combustion.berkeley.edu/gri-mech/
11.
de Vries
,
J.
, and
Petersen
,
E. L.
,
2007
, “
Autoignition of Methane-Based Fuel Blends Under Gas Turbine Conditions
,”
Proc. Combust. Inst.
,
31
(
2
), pp.
3163
3171
.10.1016/j.proci.2006.07.206
12.
Kulkarni
,
R.
,
Bunkute
,
B.
,
Biagioli
,
F.
,
Duesing
,
M.
, and
Polifke
,
W.
,
2014
, “
Large Eddy Simulation of ALSTOM's Reheat Combustor Using Tabulated Chemistry and Stochastic Fields-Combustion Model
,”
ASME
Paper No. GT2014-26053.10.1115/GT2014-26053
13.
Weller
,
H. G.
,
Tabor
,
G.
,
Gosman
,
A. D.
, and
Fureby
,
C.
,
1998
, “
Application of a Flame-Wrinkling LES Combustion Model to a Turbulent Mixing Layer
,”
27th International Symposium on Combustion
, The Combustion Institute, Pittsburgh, PA, Aug. 2–7, pp.
899
907
.10.1016/S0082-0784(98)80487-6
14.
Bellucci
,
V.
,
Schuermans
,
B.
,
Nowak
,
D.
,
Flohr
,
P.
, and
Paschereit
,
C. O.
,
2005
, “
Thermoacoustic Modeling of a Gas Turbine Combustor Equipped With Acoustic Dampers
,”
ASME J. Turbomach.
,
127
(
2
), pp.
372
379
.10.1115/1.1791284
15.
Gant
,
F.
,
Cuquel
,
A.
, and
Bothien
,
M.
,
2022
, “
Autoignition Flame Transfer Matrix: Analytical Model Versus Large Eddy Simulations
,”
Int. J. of Spray and Combustion Dynamics
, 14(1–2), pp.
72
81
.10.1177/17568277221086261
16.
Polifke
,
W.
,
2014
, “
Black-Box System Identification for Reduced Order Model Construction
,”
Ann. Nucl. Energy
,
67
, pp.
109
128
.10.1016/j.anucene.2013.10.037
17.
Emmert
,
T.
,
Bomberg
,
S.
, and
Polifke
,
W.
,
2015
, “
Intrinsic Thermoacoustic Instability of Premixed Flame
,”
Combust. Flame
,
162
(
1
) pp.
75
85
.10.1016/j.combustflame.2014.06.008
18.
Duran
,
I.
, and
Moreau
,
S.
,
2013
, “
Solution of the Quasi-One-Dimensional Linearized Euler Equations Using Flow Invariants and the Magnus Expansion
,”
J. Fluid Mech.
,
723
, pp.
190
231
.10.1017/jfm.2013.118