Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
1Development of Acoustic Damper andShort-Term Prediction Technology forStable Operation of High Efficiency Gas Turbine

TAKAYA KODA*1

TOMOHITO NAKAMORI*1

KEISUKE MATSUYAMA*2 ATSUSHI KOYAMA*3YOSHIKAZU MATSUMURA*4 SOSUKE NAKAMURA*5

A lean premix combustion method, which is used to raise the inlet temperature of gas
turbines to increase the efficiency and output thereof and to achieve low emissions, has the riskthat combustion becomes unstable and pressure fluctuation (combustion instability) of thecombustor increases. In order to realize stable gas turbine operation, we have developed an
acoustic damper, which is a device that stabilizes combustion instability by applying acoustic
damping and a technology for the short-term prediction of combustion instability that sharply
increases in amplitude in a short period of time. We verified these developments in the gas turbine
combined cycle (GTCC) power plant validation facility (hereinafter, power plant validation
facility) at Takasago Works of Mitsubishi Power, Ltd. to contribute to the realization of stable gasturbine operation.

|1. Introduction
A common issue for products related to combustion is pressure fluctuation (combustioninstability) caused by unsteadiness of flame. Combustion instability is self-excited vibration
generated by the coupling of the unsteady heat release of the flame and the acoustic field of thecombustor. Large pressure fluctuation of combustion instability causes fatigue fracturing of thestructure or flashback and in the worst case, the combustor is damaged in a short period of time as aresult.
In the case of gas turbine combustors, combustion instability is increased by a higher turbine
inlet temperature and lean premixed combustion and there are problems such as an increase in thecombustion instability amplitude and the generation of suddenly-changing combustion instability
such as vibration growth in an extremely short time.
This report presents these combustion instability suppression and short-term prediction
technologies.

|2. New acoustic damper that achieves reduction of both NOx
emission and combustion instability
2.1

Conventional acoustic damper

In order to realize stable gas turbine operation, an acoustic damper, which is a device thatstabilizes combustion instability by applying acoustic damping, is used(1). The acoustic damper is aresonator composed of a perforated plate at the inlet and a back cavity. Pressure fluctuation of thecombustor is guided into the damper by a resonance phenomenon. As the fluid moves in and out of*1

Vibration Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Manager, Vibration Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*3

Large Frame Gas Turbine Engineering Department, Gas Turbine Technology & Products Integration Division, Mitsubishi*4

Chief Staff Manager, Large Frame Gas Turbine Engineering Department, Gas Turbine Technology & Products Integration

Power. Ltd.
Division, Mitsubishi Power. Ltd.
*5Manager, Large Frame Gas Turbine Engineering Department, Gas Turbine Technology & Products Integration Division,Mitsubishi Power. Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)2

the perforated plate, a vortex is generated and the pressure fluctuation energy is dissipated(Figure 1).
Conventional acoustic dampers require cooling air because they are installed in a combustion
region. Since part of the combustion air is used for cooling, the flame temperature rises, whichleads to an increase in NOx emissions. In order to improve the sound absorbing performance, it iseffective to increase the opening area, but this requires more cooling air and lowers the NOxperformance. In addition, increasing the opening area also requires the volume of the back cavity
behind it to increase, but a further increase in the size is difficult due to the limitation of theinstallation space. For these reasons, conventional acoustic dampers have a problem in that it isdifficult to improve the sound absorbing performance.

Figure 1 Principle of acoustic damper

2.2

Development of new acoustic damper

Conventional acoustic dampers are installed in a combustion region, which is an antinode of
the acoustic mode and a position where the heat release of the flame occurs. However, as a result ofanalyzing the acoustic mode of actual combustor, it was found that an unburnt region on theupstream side also has a mode amplitude. Therefore, we developed a new acoustic damper thateliminates the need for cooling air since it is installed in an unburnt region at the inlet of thecombustor and reduces combustion instability without increasing NOx emissions (Figure 2).
Since the boundary conditions such as sound pressure, grazing and bias flow are different
between the combustion region and the unburnt region, it was expected that the damping in the
acoustic holes would decrease and the damper resonance magnification would increase. We
clarified the changes in acoustic damping due to these condition changes through elemental
experiments and large-scale unsteady CFD (Computational Fluid Dynamics). By optimizing theacoustic damping of the perforated plate for the unburnt region, the damping of the acousticdamper was maximized.

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Figure 2

2.3

Structure of new acoustic damper

Actual combustor verification

We conducted actual combustor verification of the new acoustic damper using the power
plant validation facility. This verification included the measurement of the transfer function at thedamper inlet and end for confirmation of the acoustic performance of the damper alone. As a result,it was confirmed that the analysis and the actual measurement were in good agreement with eachother in terms of both the gain and the phase of transfer function (Figure 3).
As a result of trial runs with and without the new acoustic damper, it was shown that the new
acoustic damper reduces combustion instability by 25%, increases the operating margin under rated
operating conditions and enables stable operation (Figure 4). It was also confirmed that NOxemissions do not increase as a side effect. Currently, the new acoustic damper is being applied tolong-term power supply operation and verification of the acoustic performance and structuralreliability is also being carried out continuously for long periods.

Figure 4 Actual combustor verification
results of new acoustic damper
Figure 3 Transfer function of a new acoustic
damper measured in an actual combustor

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
4|3. Short-term prediction technology of combustion instability thatsuddenly increases in amplitude
3.1

Combustion instability that suddenly increases in amplitude

The operating conditions of gas turbines are becoming stricter as the efficiency and output
increase, and the operating state is likely to change suddenly or become unstable. Figure 5 shows
the pressure fluctuation time history waveform, sonargraph and combustor circumferential acoustic
mode during intentional unstable-region gas turbine operation to grasp the stability margin ofcombustion instability at a turbine inlet temperature of 1650°C in the power plant validation
facility. It is shown that the pressure fluctuation amplitude increases sharply around 0 sec and thepressure fluctuation is increased to nearly 10 times larger than its normal state. Fluctuation withstrong randomness in the 100-300 Hz band during normal operation changes to fluctuation inwhich a sine vibration consisting of 100 Hz basic frequency and harmonics is predominant after asudden change in state. In the process of sudden state change, the pressure fluctuation mode shapechanges from a distorted mode shape in which the phases of the combustors are not aligned to amode shape with the same phase and same amplitude. Once an abnormal state occurs even for ashort time, the combustor can be damaged, so the output of the gas turbine must be reduced
immediately and rapidly or the turbine must be stopped in order to recover the normal state. For
stable operation, a technology that can predictively detect a sudden state change in order to detectand avoid a high-risk abnormal state during operation, in addition to the robust hardware design towhich damping is added using an acoustic damper, etc., is required.

Figure 5 Example of combustion instability that suddenly changes in state(gas turbine combustor)

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
53.2

Development of short-term prediction technology of abnormal vibration
applying complex system science

In recent years, the application of complex system science, which expresses complexnonlinear phenomena with mathematical equations and solves the structure of the solution, tononlinear problems in the field of combustion has been advancing. For example, there are several
reports of research that was incorporated into the advance detection of combustion instability (blowoff) of gas turbine combustors(2).
The combustion instability of a gas turbine combustor that suddenly changes in state as
shown in Figure 5 is a self-excited vibration phenomenon due to the coupling between the acoustic
mode and the combustion in which sixteen combustors are coupled in the circumferential direction.
From the viewpoint of complex system science, the combustion instability can be regarded as acollective synchronization phenomenon of a group consisting of a large number of vibrators.
Therefore, we extended the concept of phase synchronization parameter, which is a parameter
indicating the instantaneous phase synchronization state between two points proposed by R.
Gutierrez et al.(2), to define the frequency at which the sixteen combustors synchronize per unit
time as the synchronization probability and we developed a technology to detect the transition fromthe normal state to an abnormal state based on the time change of the synchronization probability(Figure 6).

Figure 6 Short-term prediction technology of abnormal vibration applying
complex system science

3.3

Actual combustor verification

Figure 7 shows the result of applying the short-term prediction technology to the data ofactual combustor verification in the power plant validation facility. The conventional monitoring
method could not avoid a sudden state change of the pressure fluctuation of combustion instability.
However, the developed method can avoid an abnormal state during operation by monitoring thesynchronization probability of the pressure fluctuations calculated from multiple sensors to detect atransition to the abnormal state 40 seconds before the sudden state change. We will continue toverify the usefulness of this technology over the long term.

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Figure 7 Actual combustor verification results of short-term prediction
technology|4. Conclusion
In order to realize stable gas turbine operation, we developed a device that stabilizes
combustion instability and a short-term prediction technology for advance detection of a sudden
state change and avoiding vibration risks. Then we verified these developed technologies in thepower plant validation facility to contribute to the realization of stable operation.
The development of the new acoustic damper that was described in this report was carriedout as a part of the "High-efficiency Gas Turbine Technology Demonstration Project (thedevelopment of ultra-high efficiency 1700°C-class gas turbines)," which is underway as a projectwith a subsidy from the New Energy and Industrial Technology Development Organization(NEDO).

References
(1)
(2)

(3)

Ikeda, K. et al., Robust Gas Turbine Combustor with Acoustic Liner, Journal of Thermal Science andTechnology, Vol.7 No.1 (2012) p.199-210
S. Murayama. et. Al, Characterization of dynamic behavior of combustion noise and detection of blowout
in a laboratory-scale gas-turbine model combustor, Proceedings of the Combustion Institute Volume 37,Issue 4, 2019, Pages 5271-5278
R. Gutiérrez et al., Paths to Synchronization on Complex Networks, Physical Review Letter, vol. 98,034101, 2007