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Thermo-acoustic and aero-acoustic nonlinearities in green combustors with orifice structures

Final Report Summary - TANGO (Thermo-acoustic and aero-acoustic nonlinearities in green combustors with orifice structures)

Combustion instabilities represent a serious problem for combustion-driven devices, such as gas turbine engines and domestic burners. These instabilities can cause intense pressure oscillations, which in turn causes excessive structural oscillations, fatigue and even catastrophic damage to combustor hardware. In recent years, the development of clean combustion systems with reduced pollution of the environment has become a priority; however, such systems are particularly prone to combustion instabilities. There is an urgent need to understand the physical processes that are responsible so that methods to predict and prevent these instabilities can be developed.

The research in TANGO is intended to address these issues. Fundamental studies will give physical insight into the three-way coupling between sound, combustion and vortices in a combustion chamber. On the applied side, TANGO will develop active and passive control methods to allow safe operation of gas turbines on a variety of fuels and operating conditions.

TANGO is a multi-disciplinary project that provides training for 15 ESRs and 2 ERs in fluid mechanics, thermodynamics, mechanical and control engineering, all from an analytical, numerical and experimental perspective. The consortium has been chosen so as to bring together complementary skills from internationally renowned experts from both academia and industry. The "icing on the cake" of this large engineering project is the number of female scientists involved: 5 out of the 10 scientists in charge are women. It is expected that this will act as a magnet for young women who are considering a career in science or engineering. The network thus addresses the EU policy of increasing the number of female researchers in Europe. In order to promote the public understanding of science, the researchers will engage in various outreach activities.

The scientific and training objectives of TANGO are
SO1: Explore the two-way coupling between sound and hydrodynamics in a combustion instability.
SO2: Explore the three-way coupling between combustion, sound and hydrodynamics in a combustion instability.
SO3: Develop active and passive strategies to prevent combustion instabilities.
TO1: Provide training in a broad range of scientific skills and meet the scientific objectives SO1, SO2 and SO3.
TO2: Produce young researchers with transferable lifetime skills that will be used throughout their subsequent careers.
TO3: Produce young researchers with an interdisciplinary, intersectorial and international outlook.

Our mission:
To develop green combustion technologies and noise control methods in a gender-balanced, multi-disciplinary network with academic and industrial collaboration, while also training highly skilled scientists of the future.

Our values:
Team. Cutting-edge research in a friendly, collaborative and creative atmosphere
Environment. Development of green technologies for combustion and noise control
Portfolio. Scientific discoveries in thermoacoustics and aeroacoustics leading to innovations
Gender equality. Concern for gender issues, promoting positive role models
Publicity. Communication with the general public through dedicated science events and publications
Partnership. Cross-fertilisation in research between academic disciplines and industrial development
People. Educate young researchers to become well-rounded scientists with an analytic and an intuitive side.

The research in the TANGO project is divided into three work packages, each of which has produced significant results.

Work package 1: Aeroacoustics
- Developed framework for precise measurement of acoustic properties (scattering matrix) at the area jump between tubes of different cross-sectional area.
- Extended a linearized Navier-Stokes equations solver to include a perforated plate, where viscous-thermal effects occur.
- Described the sound absorbing properties of micro-perforated plates in the interim regime between small amplitudes (linear) and large amplitudes (strongly nonlinear).

Work package 2: fundamental thermoacoustics
- Developed a robust method to calculate numerically the impulse response function (time-domain) corresponding to a given flame transfer function (frequency domain).
- Documented the range of dynamic states (stable, low-amplitude limit cycle, transitional regime, high-amplitude limit cycle) and hysteresis of a ducted laminar flame, when the axial position of the flame in the duct is taken as control parameter.
- Obtained quantitative measurements for the velocity field and rate of heat release in a turbulent combustor, as it undergoes the transition from a stable to an unstable state.
- Identified time-scales affecting the heat release fluctuations of a laminar flame due the propagation of equivalence rate fluctuations and subsequent modulations of flame speed, heat of reaction and flame shape.
- Developed an analytical tool to predict rapidly the nonlinear stability behaviour of a combustion system (based on the tailored Green's function of the combustion chamber, and an extended heat release law).

Work package 3: Applied thermoacoustics
- Developed an early warning system to predict imminent thermoacoustic instabilities in annular combustors before dangerously high oscillation amplitudes are reached.
- Clarified the mechanism for entropy wave generation in premixed flames by considering the flame front as a moving discontinuity described by the Rankine-Hugoniot jump conditions.
- Modelled combustion system with heat exchanger (hex), demonstrated that the hex can stabilise a combustion instability, and developed a tool to predict the optimal hex parameters (position in the combustion chamber, velocity of mean flow passing between the hex tubes).
- Developed individual models for the burner and heat exchanger, and a consolidated model for the combination of these two elements.
- Extended analytical model for nonlinear sound-combustion interaction and calculated limit cycle oscillations associated with thermoacoustic instabilities.
- Developed a hybrid approach (detailed numerical simulations combined with analytical representation of FTF and network model) to predict thermoacoustic instabilities in gas turbine engines.

Professor Maria Heckl
Faculty of Natural Sciences
Keele University
Staffordshire ST5 5BG