Dariusz Nowak

Thermoacoustic Modeling of a Gas Turbine Combustor Equipped With Acoustic Dampers

In this work, the TA3 thermoacoustic network is presented and used to simulate acoustic pulsations occurring in a heavy-duty ALSTOM gas turbine. In our approach, the combustion system is represented as a network of acoustic elements corresponding to hood, burners, flames and combustor. The multi-burner arrangement is modeled by describing the hood and combustor as Multiple Input Multiple Output (MIMO) acoustic elements. The MIMO transfer function (linking acoustic pressures and acoustic velocities at burner locations) is obtained by a three-dimensional modal analysis performed with a Finite Element Method. Burner and flame analytical models are fitted to transfer function measurements. In particular, the flame transfer function model is based on the time-lag concept, where the phase shift between heat release and acoustic pressure depends on the time necessary for the mixture fraction (formed at the injector location) to be convected to the flame. By using a state-space approach, the time domain solution of the acoustic field is obtained. The nonlinearity limiting the pulsation amplitude growth is provided by a fuel saturation term. Furthermore, Helmholtz dampers applied to the gas turbine combustor are acoustically modeled and included in the TA3 model. Finally, the predicted noise reduction is compared to that achieved in the engine.

On the Use of Thermoacoustic Analysis for Robust Burner Design

Advanced thermoacoustic analysis is now routinely used in gas turbine combustor development. A thermoacoustic approach based on a combination of numerical analysis (CFD and three-dimensional acoustics), acoustic network models, and dedicated measurements of acoustic flame response is well accepted across the industry. However, its application to specific combustor upgrade or development programs in “prediction mode” as opposed to “analysis mode” remains a challenge. This is mainly due to the large sensitivity of the complex methodology to key inputs, such as flame transfer functions, that can be only predicted in the burner design phase. This paper discusses an example where we made an effort to apply the thermoacoustic approach in predictive mode. The example refers to the upgrade of a first generation diffusion burner with a partially premix burner to achieve low emissions. Thermoacoustic instabilities were predicted as a limiting factor for combustor operation and thus a design parameter was identified to perform the thermoacoustic combustor tuning at engine level. A particular challenge of this development program was that no test rig was available. Therefore, the new premix burner was directly installed into a field engine where it was successfully tested.

Using Thermoacoustic Analysis for Robust Burner Design

Advanced thermoacoustic analysis is now routinely used in gas turbine combustor development. A thermoacoustic approach based on a combination of numerical analysis (CFD and three-dimensional acoustics), acoustic network models and dedicated measurements of acoustic flame response is well accepted across the industry. However, its application to specific combustor upgrade or development programs in “prediction mode” as opposed to “analysis mode” remains a challenge. This is mainly due to the large sensitivity of the complex methodology to key inputs, such as flame transfer functions, that can be only predicted in the burner design phase. This paper discusses an example where we made an effort to apply the thermoacoustic approach in predictive mode. The example refers to the upgrade of a first generation diffusion burner with a partially premix burner to achieve low emissions. Thermoacoustic instabilities were predicted as a limiting factor for combustor operation and thus a design parameter was identified to perform the thermoacoustic combustor tuning at engine level. A particular challenge of this development program was that no test rig was available. Therefore, the new premix burner was directly installed into a field engine where it was successfully tested.Copyright

Numerical Modeling of Thermoacoustic Oscillations in a Gas Turbine Combustion Chamber

The prediction of high-frequency acoustic oscillations in gas turbine combustors is an important issue, related to engine performance, NOx emissions, component lifetime and engine operational flexibility. Different methods with increasing complexity and predictive ability have been discussed in a number of papers. Application of these methods requires large computational capacity and long computational times. Therefore, a limited number of variants of small combustor models or small sectors can be analyzed in a reasonable time. This paper presents an approximate approach, applicable under certain specific conditions. It is based on an understanding that the acoustic pressure oscillations are tied to the oscillation in heat release rate. The interaction is taking place in the heat release zone, independent of the type of the feedback mechanism. For a typical gas turbine combustion chamber, many acoustic modes exist in the frequency range of interest. However, only a few of these modes are excited by the combustion process and thus are relevant. The mode excitation depends both on combustion noise (due to flame excitation contribution independent of the acoustic field) and combustion instability (acoustic mode made unstable by the flame transfer function). With a flame surface obtained from steady state CFD simulation, and with acoustic mode shapes obtained from a Finite Element package, the forced acoustic response of the combustion system to the flame excitation was calculated. In a first validation step, this method has been tested on a single burner atmospheric test facility. In a second step, the method will be applied to an annular SEV combustion chamber of a GT26 ALSTOM gas turbine. The strength of this approach is that large models can be analyzed quickly to show the influence of changes in a flame position and effect of the combustor geometry. The weakness is that combustion instabilities can not be addressed by such a method. Furthermore, the phase relation of the excitation between different parts of the flame is frequency dependant and needs to be given as an input, which requires an experience and expert knowledge.Copyright

Experimental Studies of Crude Oil Combustion in a Top-Mounted Silo Combustor

Crude oil is still an attractive fuel for electricity production due to its low extraction costs in relation to other fuels. However, combustion of crude oil in modern gas turbines must meet certain criteria, which mainly include the reduction of harmful gas emissions, the elimination of harmful dust from the exhaust gas, the improvement of turbine efficiency, the limiting of the power degradation process and elimination of hard deposits. Experimental studies are always needed to meet these requirements because of common complexity in CFD crude oil combustion models. This paper presents experimental investigations of the combustion process of crude oil. Using different sorts of crude oil, all experiments are performed in the atmospheric test rig of a top-mounted combustor, which was scaled down from the baseline system. The test rig was optimized for the typical silo gas turbine boundary conditions. The combustion process is described and quantified with the measured temperature and velocity field distributions in the top-mounted combustion chamber for different injector design’s parameters. Additionally, measured profiles of the molar fraction of CO2 , are discussed and compared with respect to the injector parameters. Finally, based upon the experimental results gathered, the possibility of fuel flexibility in the top-mounted combustor chamber is discussed.Copyright

On Preliminary Experimental Experiences With Crude Oil Combustion in Strong Swirl Flow

The paper presents preliminary experimental analyses of combustion processes for crude oil. The research is started from investigation of combustion of gas in a strong swirl flow as is intended to be an introductory step in studying the mechanism of stability and emission of pollutants in combustion of crude oil occurring in gas turbines. The areas of recirculation and pollutant emission in a strong swirl flow have been studied for the following three cases: - isothermal flow without combustion; - combustion of gas mixture with CH4 and N2 differently composed; - combustion of crude oil. All experiments are performed in the atmospheric test rig of a top-mounted combustor, briefly described in the paper. The velocity field in the combustion chamber is measured by laser doppler anemometry. The measured profiles of temperature and molar fraction of NOx , CO, CO2 , O2 are discussed for natural gas and crude oil. Depending on the degree of the swirl of the flow and on the temperature of entering air, the distribution of molar fraction of most important chemical species has been established. This allows for better understanding the process of combustion in a strong swirl flow. The established characteristics of the flame blow-out make it possible to calculate the limits of capacity power generation available from a given size of a gas burner. For the burner geometry, similar to that with the know characteristic of gas combustion, the parameters for CO and NOx have been established for crude oil. Also, characteristics have been found for a specially designed oil nozzle with a large spray angle — sufficiently large for the optimum supply of fuel into the area of strong swirl flow with combustion established on the basis of the analysis of the burning of gas. It has been found that in cases of combustion crude oil a relatively small increase of the temperature of air supplied for combustion results in a significant drop in CO emission what has an impact on lower NOx emission.Copyright