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Annular Instabilities and Transient Phenomena in Gas Turbine Combustors

Periodic Reporting for period 1 - ANNULIGhT (Annular Instabilities and Transient Phenomena in Gas Turbine Combustors)

Reporting period: 2017-10-01 to 2019-09-30

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. Efforts are now being made to move towards zero-carbon operation of gas turbines for power generation using hydrogen as fuel. This has the potential to significantly reduce global CO2 emissions whilst at the same time accelerating the energy transition by acting as a back-up to intermittent renewable energy sources.

The development of next generation low-emission or even zero-emission gas turbines is hindered by unsteady combustion problems that are often only discovered late into development because our scientific understanding is currently insufficient to predict them at the design stage. For reasons of cost and simplicity, we have been trying to solve these problems by studying them in simpler single flame configurations when, in reality, many gas turbines have annular combustor chambers with multiple flames. These annular chambers are known to have different stability characteristics than single flame/sector combustors.

The research focus of ANNULIGhT is to make progress in our scientific understanding to solve these problems by studying them in annular combustor geometries. The main challenges for modern and future low-emission concepts are: i) the occurrence of self-excited combustion instabilities for both aeroengines and power generation gas turbines which take the form of azimuthal modes in annular geometries resulting 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 each with experimental, numerical and theoretical components.

Work Package 1 - Thermoacoustics of annular combustion chambers
Work done and results so far:
EM2C: Experiments in the MICCA annular combustor with spray flames showed that self-excited azimuthal instabilities can be excited when the inner and outer walls of MICCA are of equal length and fuel type has a significant effect on the instabilities.
CERFACS; LES of SICCA-spray and the annular MICCA-spray at EM2C as well as the pressurized rig at NTNU to investigate the different flames dynamics during the oscillation also to be compared with the experimental observations.
TUM: Improvements to an inhouse acoustic network code taX have been done including, Bloch element to reduce computational costs, FTF representations in astate space framework and Model Order Reduction techniques for predicting instabilities in annular combustors.
NTNU: Experiments on the effect of H2, equilibrium wall temperatures, and operating conditions on modal dynamics. Preliminary results show that the stochasticity of model dynamics is reduced but still occur but over longer time-scales.
ETHZ: A theoretical model was successfully developed to qualitatively capture the complex nonlinear dynamics that are observed in annular combustors and provides analytical insight into the influence of different parameters on modal dynamics.

Work package 2 - Passive control methods and tools
Work done and results so far:
TUB: A novel annular combustion chamber model has been built which consists of 12 ducts connected to an annular chamber. Each tube is equipped with an electroacoustic feedback system that produces acoustic excitation in the tubes. Experiments to assess the effect of mean azimuthal swirl have been performed including the effect of asymmetric heat release.
Ansaldo: A non-linear analytic model had been developed describing how temperature fluctuations in a reheat combustor are convected to the flame and cause heat release fluctuations and will be compared with experiments at ETH Zurich.
ETHZ: The behavior of aeroacoustic azimuthal modes presents some similarities with thermoacoustic modes, a theoretical study is planned to identify the formal shape of the aeroacoustic source terms.
UCAM: A 2D adjoint Helmholtz solver was developed and implemented into the Finite Element package Fenics and validated by calculating the sensitivity of a thermoacoustic mode to deformations in the combustor geometry.
CERFACS: A new acoustic network tool STORM has been developed, based on concepts of modal expansion on an overcomplete set of acoustic eigenmodes instead of on an orthogonal set of modes and State-Space methods. The tool has been validated for simple configurations quasi-1D elements.

Work package 3 - Ignition and extinction dynamics in annular combustion chambers
Work done and results so far:
UCAM: Experiments on ignition, light around and blow-off have been conducted. A strong relationship between ignition time and laminar flame speed was found and that turbulent flame propagation is more dominant than thermal expansion.
NTNU: Design work was carried out to develop a simplified laboratory scale of the spinning combustor technology developed by SAFRAN. Preliminary tests on the ignition dynamics of the rig have been carried out to investigate flame- flame interactions and flame-cooling film interactions.
EM2C: Ignition dynamics have been studied numerically using LES in a single-injector combustor called SICCA-spray to validate models. Two different flame configurations have been observed upon relight (i.e. re-ignition with heated combustor walls).
SAFRAN: LES simulations of simplified injection systems have been performed in non-reactive conditions with the software YALES2. Routines have been developed for tracking and processing the spray formed after the resolved atomization process. A tool for postprocessing the spray obtained in the LES simulations has been developed in Python.
The work performed as described above has advanced the state-of-the-art in combustion dynamics with aim of the following longer term potential impacts:

Potential impacts:
• New technical innovations to accelerate flexible, low-carbon, power generation in accordance with the SET-plan. 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.