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Annular Instabilities and Transient Phenomena in Gas Turbine Combustors

Periodic Reporting for period 2 - ANNULIGhT (Annular Instabilities and Transient Phenomena in Gas Turbine Combustors)

Berichtszeitraum: 2019-10-01 bis 2022-01-31

The gas turbine industry is a vital driver of innovation, economic growth, jobs, trade and mobility in the EU in both the aviation and power generation sectors. It is a multi-billion Euro high-technology industry whose future competitiveness depends on a new generation of creative engineers with multi-disciplinary skills who can accelerate the development of new innovations needed for flexible, efficient power generation and sustainable aviation. There is a significant and urgent need to move towards zero-carbon operation of gas turbines for power generation using hydrogen as fuel to reduce global CO2 emissions and increase energy security. Unlike renewables, gas turbines burning hydrogen can potentially deliver large-scale power in the short to medium terms and offer an attractive solution to the problem of energy storage. They can potentially also offer zero carbon aviation solutions.

The development of next generation low-emission NOx zero-carbon gas turbines is hindered by unsteady combustion problems that are often only discovered late into development because we cannot yet predict them at the design stage. For reasons of cost and simplicity, we have been trying to solve these problems by studying them in isolation in single flames when, in reality, gas turbines have annular or can-annular combustor chambers with multiple flames. These systems are known to have different stability characteristics that need to be understood.

ANNULIGhT has made significant progress in our scientific understanding of these problems by studying them in annular and can-annular combustor geometries. The main challenges addressed are: i) the occurrence of self-excited combustion instabilities in aeroengines and power generation gas turbines which take the form of azimuthal modes in large pressure fluctuations threatening structural integrity, ii) increased probability of lean Blow-off, which consequently makes iii) ignition and light-around more difficult. These latter two issues are safety critical for aeroengine certification yet very little is understood about the mechanisms which control these processes.
The research work is divided into 3 thematic work packages (WP). WP1 was focused on improving our understanding of the thermoacoustics of annular combustion chambers. At CNRS/EM2C the thermoacoustic response of annular combustors operating with spray flames showed that self-excited azimuthal instabilities are very sensitive to the spray pattern. In turn, it was found that the spray formation is very sensitive to swirl and other small geometric changes to the injector. These effects were captured by new models and implemented into LES by research at CERFACS. At NTNU the effect of acoustic boundary conditions and dynamic operation was investigated in atmospheric and pressurized annular combustors. The effect of hydrogen enrichment on the modal dynamics were studied and found to be different in the pressurized rig due to the change in acoustic boundary condition. Dynamic operating conditions, achieved by ramping up in power or equivalence ratio was shown to be able to alter the thermoacoustic response of the system by moving the bifurcation point. ETH Zurich developed a low order model based on quaternions which showed that the non-linear dynamics observed in the experiments at NTNU were caused by symmetry breaking effects. At TUM improvements to low-order acoustic network models were achieved by implementing, Bloch element to reduce computational costs, FTF representations in a state space framework and Model Order Reduction techniques for predicting instabilities in annular combustors.

The research focus of WP2 was the passive control and exploitation of symmetry breaking methods to develop reduced order tools to control instabilities. An idealied annular combustor with simulated flames was used at TUB to assess the effect bulk azimuthal swirl and boundary conditions demonstrating that the resulting modes depend on the level of asymmetry. At NTNU it was shown experimentally that bulk azimuthal swirl suppresses thermoacoustic modes. A new approach to optimize geometries to suppress instabilities were developed at Cambridge by developing adjoint solvers combined with sensitivity analysis. Together ETH and Ansaldo developed new theoretical models based on quaternions for system identification and showed that they can capture the effects of noise and asymmetries which can ultimately be used to optimize damping strategies. A new low order model called STORM was further developed to reduce the time and computational cost of conducting stability analyses in complex industrially relevant configurations.

WP 3 focused on improving our understanding of ignition and extinction dynamics in annular combustion chambers. These processes are crucial to the safe operation of aeroengines. Experimental work was carried out at CNRS, Cambridge and NTNU with complimentary parallel numerical simulations using LES at CNRS, Cambridge and Safran Helicopters. Experiments on ignition and light around times in premixed flame showed a strong correlation laminar flame speed and to a lesser extent the thermal expansion of the burned gases. In spray flames the opposite was found, thermal expansion was found to be the most important mechanism of flame propagation as it resulted in local modifications to the spray distribution. CNRS also investigated the effect of heated walls on the ignition process and identified the main physics behind the differences in light around times.
Experiments at NTNU showed that azimuthal bulk swirl enhances the ignition and light around process and this process could be well captured by numerical simulations done by SAFRAN.

The findings were disseminated as thoroughly as possible given the Covid-19 pandemic. In an effort to improve the long term impact of ANNULIGhT, an open source community was established here:
The results of the action have gone beyond the state of the art in our understanding of combustion dynamics in annular and can-annular gas turbine combustors for propulsion and power. This is documented in over 40 journal publications which now represent the state of the art in the field.

Potential impacts:
• New technical innovations to accelerate flexible, low-carbon, power generation in accordance with the SET-plan and the recent need for energy security. The potential of operation of gas turbines on pure hydrogen means large-scale CO2 reductions Europe wide could potentially be achieved over a short to mid-term time horizon and accelerate the aim of the goal of being carbon neutral by 2050.
• Develop scientific understanding to ensure the next generation of low-emission aeroengines set out by ACARE
• A new generation of researchers equipped with the skills and expertise needed to address major societal challenges: environmentally sustainable energy and transport.
• Obtain new scientific understanding about the nature of azimuthal modes leading to the development of new, innovative methods and tools to control, characterize and predict them.
• Improve aeroengine safety through an improved scientific and technical understanding of ignition and light-around in annular combustors and the key mechanisms that ensure the highest probability of altitude re-light. This will lead to better physics-based models and improve the design process.
Picture of ANNULIGhT network at the year 1 meeting. ESRs to the left, supervisors to the right.