An experimental investigation of the flow field of a 12 burner annular combustor and a single burner combustor with the same burner was performed. It has revealed the aerodynamic effect, which causes the discrepancies in the flame transfer function behavior measured at the same operating conditions in the single and the annular combustion chambers. The results have shown significant differences in the flow field. In particular, it is seen that for the investigated system in the annular combustor a free swirling jet flow forms, while in the single burner configuration, a swirling wall jet flow regime exists. In this paper, we discuss the physical mechanism and show how to generalize an earlier finding, which identified a critical confinement value for a given swirler. We propose a new correlation for coswirling burners, which explains the changes found for the investigated system. It compares also well with the experimental data from other burner geometries. The correlation should allow to design single burner tests as to match the annular combustor flow regime.

In this paper alternative ways to obtain the thermo-acoustical characteristics of perfectly premixed flames given by their flame transfer matrix (FTM) is investigated. In particular a model based data reduction procedure which greatly reduces the experimental effort and therefore enables to provide this flame data for many more operation points than previously possible is proposed and validated. It is shown how the acoustic pressure field measured from two forcing states using the multi microphone method leads to the determination of the direct experimental FTM. The next relatively simpler method shown is the hybrid method which is based on Rankine-Hugoniot relations and the experimental flame transfer function (FTF) from OH*-chemiluminescence measurements for heat release fluctuations. Later to obtain the FTM using a network model based on Rankine-Hugoniot relations and an n-τ-σ FTF model representing the flame by regression analysis of the acoustical measurements is presented. Experimental results for the direct experimental FTM and the hybrid FTM are compared with the model based result. The results indicate very good consistency between the direct, hybrid and model based techniques providing a global check of the methods/tools used for analysing the thermoacoustic mechanisms of flames.

In this paper we apply a new method to obtain the dynamical characteristics of premixed flames in acoustically complex systems like an annular combustor. For this a novel model based reduction method was applied which describes the acoustic field as a superposition of a discrete number of eigenmodes. The measured dynamic signals are decomposed into modal components which are then used to fit the model parameters. With this approach the complete characterisation of the annular test rig requires significantly less experimental effort, i.e. simultaneous pressure measurements, than before. After describing the new procedure, we show the validation of the acoustical model. Using this model, we were able to obtain the characteristics of the flame in the annular combustor with a much smaller number of pressure sensors as the model provides physical constraints that would otherwise have to be measured. The comparison of the characteristic flame parameters between the single burner and the annular combustor configurations for the same operating conditions shows that in the annular combustor slightly longer convective time delays are found which are consistent with the static flame characterisation showing longer flames in the annular combustor than in the single burner test rig.

The paper investigates the determination and the scaling of thermo acoustical characteristics of lean premixed flames as used in gas turbine combustion systems. In the first part, alternative methods to characterize experimentally the flame dynamics are outlined and are compared on the example of a scaled model of an industrial gas turbine burner. Transfer matrix results from the most general direct method are contrasted with data obtained from the hybrid method, which is based on Rankine-Hugoniot relations and the experimental flame transfer function obtained from OH*-chemiluminescence measurements. Also the new network model based regression method is assessed, which is based on a n − τ − σ dynamic flame model. The results indicate very good consistency between the three techniques, providing a global check of the methods/tools used for analyzing the thermo acoustic mechanisms of flames. In the second part, scaling rules are developed that allow to calculate the dynamic flame characteristics at different operation points. Towards this a geometric flame length model is formulated. Together with the other operational data of the flame it provides the dynamic flame model parameters at these points. The comparison between the measured and modeled flame lengths as well as the n − τ − σ parameters shows an excellent agreement. NOMENCLATURE

An experimental investigation of the flow field of a 12 burner annular combustor and a single burner combustor with the same burner was performed. It has revealed the aerodynamic effect which causes the discrepancies in the flame transfer function behavior measured at the same operating conditions in the single and the annular combustion chambers. The results have shown significant differences in the flow field. In particular it is seen that for the investigated system in the annular combustor a free swirling jet flow forms while in the single burner configuration a swirling wall jet flow regime exists. In this paper we discuss the physical mechanism and show how to generalize an earlier finding which identified a critical confinement value for a given swirler. We propose a new correlation for co-swirling burners which explains the changes found for the investigated system. It compares also well with experimental data from other burner geometries. The correlation should allow to design single burner tests as to match the annular combustor flow regime.

The liner of a gas turbine combustor is a very flexible structure that is exposed to the pressure oscillations that occur in the combustor. These pressure oscillations can be of very high amplitude due to thermoacoustic instability, when the fluctuations of the rate of heat release and the acoustic pressure waves amplify each other. The liner structure is a dynamic mechanical system that vibrates at its eigenfrequencies and at the frequencies by which it is forced by the pressure oscillations to which it is exposed. On the other hand the liner vibrations force a displacement of the flue gas near the wall in the combustor. The displacement is very small but this acts like a distributed acoustic source which is proportional to the liner wall acceleration. Hence liner and combustor are a coupled elasto-acoustic system. When this is exposed to a limit cycle oscillation the liner may fail due to fatigue.In this paper the method and the results will be presented of the partitioned simulation of the coupled acousto-elastic system composed of the liner and the flue gas domain in the combustor. The partitioned simulation uses separate solvers for the flow domain and the structural domain, that operate in a coupled way. In this work 2-way fluid structure interaction is studied for the case of a model combustor for the operating conditions 40-60 kW with equivalence ratio of 0.625. This is done in the framework of the LIMOUSINE project. Computational fluid dynamics analysis is performed to obtain the thermal loading of the combustor liner and finite element analysis renders the temperature, stress distribution and deformation in the liner. The software used is ANSYS workbench V13.0 software, in which the information (pressure and displacement) is also exchanged between fluid and structural domain transiently.

In the design and operational tuning of gas turbine combustors it is important to be able to predict the interaction of the flame stabilization recirculation area with the burner aerodynamics. In the present paper transient computational fluid dynamics analysis is used to study these effects. Vortex interactions with the flame play a key role in many practical combustion systems. The interactions drive a large class of combustion instabilities and are responsible for changing the reaction rates, shape of the flame and the global heat release rate. The evolution of vortex shedding in reactive flows and its effects on the dynamics of the flame are important to be predicted. The present study describes dynamics of bluff body stabilized flames in a partially premixed combustion system. The bluff body is an equilateral wedge that induces the flame recirculation zone. The wedge is positioned at one-third length of the duct, which, is acoustically closed at the bottom end and open at the top. Transient computational modeling of partially premixed combustion is carried out using the commercial ANSYS CFX code and the results show that the vortex shedding has a destabilizing effect on the combustion process. Scale Adaptive Simulation turbulence model is used to compare between non-reacting cases and combustion flows to show the effects of aerodynamics-combustion coupling. The transient data reveals that frequency peaks of pressure and temperature spectra and is consistent with the longitudinal natural frequencies and Kelvin-Helmholtz instability frequency for reactive flow simulations. The same phenomenon is observed at different operating conditions of varying power. It has also been shown that the pressure and heat release are in phase, satisfying the Rayleigh criterion and therefore indicating the presence of aerodynamic-combustion instability. The data are compared to the scarce data on experiments and simulations available in literature.

In this work comprehensive experimental and numerical studies incorporating the most relevant physical mechanisms causing limit cycle pressure and combustion rate oscillations (LCO) in a laboratory scale combustor will be discussed. The strong interaction between the aerodynamics-combustion-acoustic oscillations (ACA), and under specific conditions the aerodynamics-combustion-structural vibrations (ACS), is studied by a careful selection of experiments and numerical simulations performed using commercially available computational models. It is shown predominantly that the convective time scales due to the aerodynamics at the flame stabilizer and the time period related to acoustic propagation have to be of the same order in magnitude to be able to drive the system into LCO. The measurements indicated that the frequency spectrum of the oscillations of the LCO has a distinct peak close to the natural mode of the combustor along with higher order “harmonics” due to non-linear effects. Some non-harmonic higher order peaks are observed that are associated with the structural (liner) natural frequencies of vibration. A numerical simulation has been performed using the commercial code (ANSYS V13.0) that includes the effects of fluid-structure interaction by means of pressure load transfer on to the structure and vice-versa. The fluid domain is modeled using CFX and the structural domain is represented by ANSYS. The information is exchanged between the two domains dynamically at every time step computed. In order to reduce the computational effort and quickly gain insight into the problem only a 2 mm slice of the whole geometry has been considered making it essentially a 2D analysis. The good agreement between the model and measured instability frequencies shows a very promising approach in predicting the limit cycle oscillations in this kind of configurations.