Impact of fuel supply impedance and fuel staging on gas turbine ...

7 Acoustic analysis
Once the acoustic characteristics <strong>of</strong> the flame dynamics <strong>and</strong> the axial swirler
are determined with the CFD/SI method, an acoustic network model <strong>of</strong> the
combustion configuration can be set up. The setup <strong>of</strong> the model is part <strong>of</strong> the
next section. With the network model the impact <strong>of</strong> the acoustic <strong>impedance</strong>
on the thermo-acoustic stability is analyzed. Furthermore, it is shown how
the location <strong>of</strong> the <strong>fuel</strong> injection nozzles influences the acoustics <strong>of</strong> the entire
system, followed by the investigation <strong>of</strong> the impact <strong>of</strong> multiple <strong>fuel</strong> injection
stages. From the results obtained, design guidelines are derived.
7.1 Setup <strong>of</strong> the acoustic network model
The acoustic network model consists <strong>of</strong> the plenum, the mixing section, the
<strong>fuel</strong> <strong>supply</strong>, the axial swirler <strong>and</strong> the combustion chamber. The acoustic characteristics
<strong>of</strong> the combustor parts, like plenum or <strong>fuel</strong> <strong>supply</strong>, are represented
by acoustic elements, e.g. acoustic boundary conditions, simple ducts, area
changes <strong>and</strong> ”T-junctions”. The analytic relations <strong>of</strong> the acoustic elements
were derived in chapter 3.3. The flame dynamics obtained by CFD/SI is implemented
using the practical premixed flame element (chapter 3.3). The acoustic
characteristics <strong>of</strong> the axial swirler are represented by a transfer matrix,
which was also determined by CFD/SI as shown in chapter 6.4. As in the numerical
analysis, two basic configurations are considered with two <strong>and</strong> three
<strong>fuel</strong> injection stages, respectively. They are denoted again as case A <strong>and</strong> B. The
structure <strong>of</strong> the acoustic network model <strong>of</strong> the combustion test rig with two
<strong>fuel</strong> injection stages is presented in Fig. 7.1. Here, the names, e.g. ”Plenum”,
”AC I”, ”Mixing section I” are the names <strong>of</strong> the individual elements <strong>of</strong> the combustor
network model, whereas their colors (see legend in Fig. 7.1) define the
types <strong>of</strong> the acoustic elements, e.g. simple duct, area change,etc. The network
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