The bluff body stabilized premixed flame in an acoustically resonating tube: combustion CFD and measured pressure field

The resulting limit cycle amplitude and frequency spectrum of a flame placed in a combustor of rectangular cross section is investigated. The partially premixed flame is stabilized on a bluff body placed in the upstream half of the combustor. The bluff body is an equilateral triangular wedge with one of the edges pointing in upstream direction. Acoustically there is an open downstream end and theer are variable acoustic conditions at the upstream end.\ud In order to assess the properties of the flame in this combustor, steady state flame simulations have been performed of the flame in the enclosure. These provided the fields of the mixing of gases, temperature and the velocity.\ud A test rig was manufactured for this burner at the University of Twente. In a first set of experiments, gas temperature, pressure field and flame chemiluminescence in the combustor were measured as a function of power and acoustic inlet condition. It was observed that the combustor exhibited strong natural pressure oscillations. The measured pressure, temperature and chemiluminescence data are compared to the CFD simulations and to numerical calculations of the acoustics presented in a companion paper by M.Heckl

A Transfer Function Approach to Structural Vibrations Induced by Thermoacoustic Sources

To decrease NOx emissions from a combustion system, lean premixed combustion in combination with an annular combustor is used. One of the disadvantages is an increase in sound pressure levels in the combustion system, resulting in an increased excitation of the surrounding structure, the liner. This causes fatigue, which limits the life time of the combustor. To model the interaction between flame, acoustics and structure, a transfer function approach is used. In this approach, the components are represented by the frequency dependent linear transfer between their inputs and outputs. For the flame a low pass filter with convective time delay is used as transfer function between velocity perturbations at the burner outlet and the flame as acoustic volume source. The acoustic transfer from volume source to velocity perturbation at the burner outlet is obtained from a harmonic finite element analysis, in which a temperature field from CFD calculations is used. The calculated response is subsequently curve-fitted using a pole-zero model to allow for fast calculations. The finite element model includes the two way coupling between structural vibrations and acoustics, which allows extraction of the vibration levels. The different transfers are finally coupled in one model. Results show frequencies of high acoustic response which are susceptible to thermoacoustic instability. Damping mechanisms and the phase relation between the different components determine stable or unstable behavior and the amplitude of the resulting perturbations. Furthermore there are also frequencies of high structural response. Especially when the two coincide, the risk of structural damage is high, whereas when they move away from each other, the risk decreases

Vibration of the liner in an industrial combustion system due to an acoustic field

The subject of this paper is a numerical study of the properties of the liner of a test rig to be built in the future. The test-rig consists of a flexible tube of square crosssection surrounded by a pressure vessel, also with a square cross-section. At first instance, a two dimensional structural analytical model of the cross-section is made. The influence of the air between liner and pressure vessel and that within the liner on the vibration of the liner is studied using a coupled 2D finite element model. Furthermore the influence of the vibration of the liner on the acoustics of the setup is studied. After this the problem is extended to three dimensions and again the influence of the cavity surrounding the liner is analyzed. Both 2D and 3D results are compared. The cavities are found to substantially influence the structural behavior and therefore they cannot be neglected in predicting the behavior of the liner

Numerical prediction of combustion induced vibro-acoustical instabilities in a gas turbine combustor

Introduction of lean premixed combustion to gas turbine technology reduced the emission of harmful exhaust gas species, but due to the high sensitivity of lean flames to acoustic perturbations, the average life time of gas turbine engines was decreased significantly. Very dangerous to the integrity of the gas turbine structure is the mutual interaction between combustion, acoustics and wall vibration. This phenomenon can lead to a closed loop feedback system, with as a result fatigue failure of the combustor liner and fatal damage to the gas turbine rotor. \ud In this paper the use of numerical tools for CFD and CSD analysis is described to predict the hazardous frequencies at which the instabilities can occur. The two way interaction of the combustible compressible flow and structural walls is investigated with the application of the partitioning fluid-structure interaction approach. In this technique the fluid and structural model are considered as individual but coupled dynamic systems. Information of conditions at the fluid-structure interface is exchanged at given time steps through the interface connection created between the numerical domains. Therefore, the partitioned approach can take the full advantage of existing, well developed and tested codes for both, fluid and structure analysis. Next to the fluid-structure interaction analysis, acousto-elastic and modal models are applied to get insight into the acoustic and vibration pattern during the instability process. The calculations use elements devoted to the solution of the acoustic and structural fields. This approach has the advantage of high resolution of the acoustics, but takes into account only one way combustion dynamics (taken from the CFD results). All numerical solutions are compared to experimental results obtained on a laboratory test rig. The data is evaluated for both, pressure and velocity fields

Experimental validation of the interaction between combustion and structural vibration

To decrease NOx emissions from combustion systems, lean premixed combustion is used. A disadvantage is the increase in sound pressure levels in the combustor, resulting in an increased excitation of the surrounding structure: the liner. This causes fatigue, which limits the life time of the combustor. To study this problem experimentally, a test setup has been built consisting of a single burner, 500kW, 5 bar combustion system. The thin structure (liner) is contained in a thick pressure vessel with optical access for a traversing laser vibrometer system to measure the vibration levels and mode shapes of the liner. The acoustic excitation of the liner is measured using pressure sensors measuring the acoustic pressures inside the combustion chamber and in the cooling passage between the liner and the pressure vessel. To validate models, measurements were performed in steps of increasing complexity. Firstly, the structural properties, obtained by modal analysis of the liner outside the pressure vessel, have been compared with a finite element model. Subsequently, results of an acoustic finite element model of the setup have been compared to acoustic measurements on the test rig to validate the acoustic properties of the model, which are made by mounting a well defined acoustic source to the rig. Finally, measured pressures and vibration levels in the presence of combustion are shown

Study of the Wall Thermal Condition Effect in a Lean-Premixed Downscaled Can Combustor Using Large-Eddy Simulation

The primary purpose of this study is to evaluate the ability of LES, with a turbulent combustion model based on steady flamelets, to predict the flame stabilization mechanisms in an industrial can combustor at full load conditions. The test case corresponds to the downscaled Siemens can combustor tested in the high pressure rig at the DLR. The effects of the wall temperature on the prediction capabilities of the codes is investigated by imposing several heat transfer conditions at the pilot and chamber walls. The codes used for this work are Alya and OpenFOAM, which are well established CFD codes in the fluid mechanics community. Prior to the simulation, results for 1-D laminar flames at the operating conditions of the combustor are compared with the detailed solutions. Subsequently, results from both codes at the mid-plane are compared against the experimental data available. Acceptable results are obtained for the axial velocity, while discrepancies are more evident for the mixture fraction and the temperature, particularly with Alya. However, both codes showed that the heat losses influence the size and length of the pilot and main flame.The research leading to these results has received funding through the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7, 2007-2013) under the grant agreement No. FP7-290042 for the project COPA-GT and the European Union’s Horizon 2020 Programme (2014-2020) and from Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project, grant agreement No. 689772. The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the Red Española de Supercomputación (RES).Peer ReviewedPostprint (author's final draft

Numerical simulation of sound propagation through the can-annular combustor exit

Thermo-acoustic instabilities in high power density gas turbine engines have to be predicted in order to avoid unexpected shutdown events. To predict these instabilities, the acoustics behavior of the combustion system needs to be analyzed. The work presented in this paper on combustor-turbine interaction is focused on reflection coefficient analysis. The study is based on a simplified two-dimensional (2D) geometry representing the vane section and another geometry corresponding to a real engine alike combustor/turbine design. Compressible Large Eddy Simulation (LES) is applied based on the open source Computational Fluid Dynamics package OpenFOAM. A forced response approach is used imposing a sound wave excitation at the inlet of the combustion chamber. The applied Non-Reflecting Boundary Conditions (NRBC) are verified for correct behavior and plausibility of the acoustic set up. Multi-harmonic excitation with small amplitudes is used to preserve linearity. The numerical results are compared to analytical formulae in order to test the validity of both approaches for the chosen geometries

Characterisation of Interaction between Combustion Dynamics and Equivalence Ratio oscillations in a pressurised combustor

In regular operation, all gas turbine combustors have a significant spontaneous noise level induced by the turbulent high power flame. This noise is characteristic for the operation as it is the result of the interaction between turbulence and combustion. Pressure fluctuations may also be generated by thermoacoustic instabilities induced by amplification by the flame of the acoustic field in the combustor. This paper focuses on the characterisation of the latter process, the combustion dynamics, in a pressurized premixed natural gas combustor. In order to predict the thermo-acoustically unstable operating ranges of modern gas-turbines with the use of an acoustic network model, it is essential to determine accurately the flame transfer function. This transfer function gives the relationship between a perturbation upstream of the flame and its combustion response, leading to acoustic forcing. In this paper, the flame transfer function is obtained by experimental means in a combustor test rig. This test rig was built in the framework of the European DESIRE project, and has the ability to perform thermo-acoustic measurements up to an absolute pressure of 5 bars. The maximum power of the setup is 500 kW. The paper presents a method to determine the flame transfer function by factorizing it in six subfunctions. Systematically these subfunctions are determined. With the method presented, acoustic measurements on the steady, unperturbed flame and on the unsteady, actively perturbed flame are performed. The effect of pressure is investigated. The steady measurements are used to provide an acousto-combustion finger print of the combustor. In the unsteady measurements, the flame transfer function is reconstructed from the measured acoustic pressures. These flame transfer functions are compared to transfer functions obtained from a numerical experiment in CFD. Good agreement is obtained

Transient combustion modeling of an oscillating lean premixed methane/air flame

The main objective of the present study is to demonstrate accurate low frequency transient turbulent combustion modeling. For accurate flame dynamics some improvements were made to the standard TFC combustion model for lean premixed combustion. With use of a 1D laminar flamelet code, predictions have been made for the laminar flame speed and the critical strain rate to improve the TFC (Turbulent Flame Speed Closure) combustion model. The computational fluid dynamics program CFX is used to perform transient simulations. These results were compared with experimental data of Weigand et al [1]. Two different\ud turbulence models have been used for predictions of the turbulent flow