Sebastian Schimek

An Experimental Investigation of the Nonlinear Response of an Atmospheric Swirl-Stabilized Premixed Flame

Due to stringent emission restrictions, modern gas turbines mostly rely on lean premixed combustion. Since this combustion mode is susceptible to thermoacoustic instabilities, there is a need for modeling tools with predictive capabilities. Linear network models are able to predict the occurrence of thermoacoustic instabilities but yield no information on the oscillation amplitude. The prediction of the pulsation levels and hence an estimation whether a certain operating condition has to be avoided is only possible if information on the nonlinear flame response is available. Typically, the flame response shows saturation at high forcing amplitudes. A newly constructed atmospheric test rig, specifically designed for the realization of high excitation amplitudes over a broad frequency range, is used to generate extremely high acoustic forcing power with velocity fluctuations of up to 100% of the mean flow. The test rig consists of a generic combustor with a premixed swirl-stabilized natural gas flame, where the upstream part has a variable length to generate adaptive resonances of the acoustic field. The OH* chemiluminescence response, with respect to velocity fluctuations at the burner, is measured for various excitation frequencies and amplitudes. From these measurements, an amplitude dependent flame transfer function is obtained. Phase-averaged OH* pictures are used to identify changes in the flame shape related to saturation mechanisms. For different frequency regimes, different saturation mechanisms are identified.

Identification of the Flame Describing Function of a Premixed Swirl Flame from LES

Thermo-acoustic characterization of gas turbine combustion systems is crucial for a successful development of new gas turbine engines to meet emission and efficiency targets. In this context, it becomes more and more necessary to predict the limit cycle amplitudes of thermo-acoustic induced combustion instabilities in order to figure out if they can be tolerated or if they are above the critical design limit and will cause damage to the engine. For the prediction of limit cycle amplitudes, the nonlinear flame response of the combustion system is needed, which is represented in this work by the flame describing function (FDF). In this article, the identification of the FDF from a large eddy simulation (LES) is validated. The test case used was a premixed atmospheric swirl flame, for which experimental data on the FDF were available. First a steady reacting LES solution was obtained and compared to experimental data. The simulation was then excited by superimposing a mono-frequency harmonic wave on the velocity inlet boundary condition. Both the frequency and amplitude of the acoustic wave were varied to obtain the FDF. The calculated FDF was in good agreement with experimental data. At a frequency of 115 Hz, the heat release rate of the flame was found to saturate for larger excitation amplitudes.

Influence of Pressure and Steam Dilution on NOx and CO Emissions in a Premixed Natural Gas Flame

In the current study, the influence of pressure and steam on the emission formation in a premixed natural gas flame is investigated at pressures between 1.5 bar and 9 bar. A premixed, swirl-stabilized combustor is developed that provides a stable flame up to very high steam contents. Combustion tests are conducted at different pressure levels for equivalence ratios from lean blowout to near-stoichiometric conditions and steam-to-air mass ratios from 0% to 25%. A reactor network is developed to model the combustion process. The simulation results match the measured NOx and CO concentrations very well for all operating conditions. The reactor network is used for a detailed investigation of the influence of steam and pressure on the NOx formation pathways. In the experiments, adding 20% steam reduces NOx and CO emissions to below 10 ppm at all tested pressures up to near-stoichiometric conditions. Pressure scaling laws are derived: CO changes with a pressure exponent of approximately −0.5 that is not noticeably affected by the steam. For the NOx emissions, the exponent increases with equivalence ratio from 0.1 to 0.65 at dry conditions. At a steam-to-air mass ratio of 20%, the NOx pressure exponent is reduced to −0.1 to +0.25. The numerical analysis reveals that steam has a strong effect on the combustion chemistry. The reduction in NOx emissions is mainly caused by lower concentrations of atomic oxygen at steam-diluted conditions, constraining the thermal pathway.

Comparison of Nonlinear to Linear Thermoacoustic Stability Analysis of a Gas Turbine Combustion System

Gas turbines offer a high operational flexibility and a good turn down ratio to meet future requirements of power production. In this context, stable operation over a wide range and for different blends of fuel is requested. Thermoacoustic stability assessment is crucial for accelerating the development and implementation of new combustion systems. The results of nonlinear and linear thermoacoustic stability assessments are compared on the basis of recent measurements of flame describing functions and thermoacoustic stability of a model swirl combustor operating in the fully turbulent regime. The different assessment methods are outlined. The linear thermoacoustic stability assessment yields growth rates of the thermoacoustic instability whereas the limit cycle amplitude is predicted by the nonlinear stability method. It could be shown that the predicted limit cycle amplitudes correlate well with the growth rates of excitation obtained from linear modeling. Hence, for screening the thermoacoustic stability of different design approaches a linear assessment may be sufficient while for detailed prediction of the dynamic pressure amplitude more efforts have to be spent on the nonlinear assessment including the analysis of the nonlinear flame response.

Combustion of Natural Gas, Hydrogen and Bio-Fuels at Ultra-Wet Conditions

Humidified Gas Turbines promise a significant increase in efficiency compared to the dry gas turbine cycle. In single cycle applications, efficiencies up to 60% seem possible with humidified turbines. Additionally, the steam effectively inhibits the formation of NOx emissions and also allows for operating the gas turbine on hydrogen-rich fuels. The current study is conducted within the European Advanced Grant Research Project GREENEST. The premixed combustion at ultra wet conditions is investigated for natural gas, hydrogen, and mixtures of both fuels, covering lower heating values between 27 MJ/kg and 120 MJ/kg. In addition to the experiments, the combustion process is also examined numerically. The flow field and the fuel-air mixing of the burner were investigated in a water tunnel using Particle Image Velocimetry and Laser Induced Fluorescence. Gas-fired tests were conducted at atmospheric pressure, inlet temperatures between 200° C and 370° C, and degrees of humidity from 0% to 50%. Steam efficiently inhibits the formation of NOx emissions. For all tested fuels, both NOx and CO emissions of below 10 ppm were measured up to near-stoichiometric gas composition at wet conditions. Operation on pure hydrogen is possible up to very high degrees of humidity, but even a relatively low steam content prevents flame flashback. Increasing hydrogen content leads to a more compact flame, which is anchored closer to the burner outlet, while increasing steam content moves the flame downstream and increases the flame volume. In addition to the experiments, the combustion process was modeled using a reactor network. The predicted NOx and CO emission levels agree well with the experimental results over a wide range of temperatures, steam content, and fuel composition.Copyright

On the Impact of Shear Flow Instabilities on Global Heat Release Rate Fluctuations: Linear Stability Analysis of an Isothermal and a Reacting Swirling Jet

The prediction of large-scale flow structures in combustor flows and their impact on the flame dynamics is of great importance to avoid thermoacoustic instabilities in modern gas turbine design. The streamwise growth of these so-called coherent structures depends on the receptivity of the shear layers, which can be predicted numerically by means of linear stability analysis. We demonstrate this approach on an isothermal swirling jet that is dominated by a self-excited helical mode that features a precessing vortex core, showing that this theoretical concept successfully predicts the frequency, the source, and the shape of this mode. The analysis is further applied to a reacting flow with a swirl-stabilized flame, pointing out important connections between the shear layer receptivity and the measured amplitude dependence of the flame transfer function. The theoretical findings suggest that the saturation of the global heat release rate fluctuations observed at moderate forcing amplitudes is caused by vanishing shear layer receptivity.Copyright

Flame Transfer Function Measurements With CH4 and H2 Fuel Mixtures at Ultra Wet Conditions in a Swirl Stabilized Premixed Combustor

The influence of humidity and fuel composition on thermoacoustic flame characteristics is investigated. This is an important aspect in the development of combustion chambers for humidified gas turbines. Thermoacoustic issues are in the focus of the current investigation. These pressure pulsations result from the interaction of combustor acoustics with an unsteady heat release of the flame and have a negative influence on the combustion process and can even damage components of the engine. A key point concerning combustion dynamics is the role of convective time lags which are determined by the temporal delay between the appearance of a perturbation and the response of the flame. Experiments are conducted with different fuel mixtures containing natural gas, hydrogen, and nitrogen, each with steam contents in the air mass flow of up to 40%. The multi-microphone method and OH* chemiluminescence measurements with a photomultiplier and an intensified CCD camera are used to determine the flame transfer function and flame dynamics. It is observed that the characteristics of the flame response are related to the flame shape and position rather than to the fuel composition or the steam content. Using a Strouhal-number normalization based on the bulk velocity of the annular jet and the distance between fuel injection and the flame, a good agreement between the phases of the flame transfer functions of all flames attached to the burner outlet is found.Copyright

Amplitude-Dependent Flow Field and Flame Response to Axial and Tangential Velocity Fluctuations

The current paper investigates the nonlinear interaction of the flow field and the unsteady heat release rate and the role of swirl fluctuations. The test rig consists of a generic swirl-stabilized combustor fed with natural gas and equipped with a high-amplitude forcing device. The influence of the phase between axial and azimuthal velocity oscillations is assessed on the basis of the amplitude and phase relations between the velocity fluctuations at the inlet and the outlet of the burner. These relations are determined in the experiment with the multimicrophone-method and a two component laser Doppler velocimeter (LDV). Particle image velocimetry (PIV) and OH*-chemiluminescence measurements are conducted to study the interaction between the flow field and the flame. For several frequency regimes, characteristic properties of the forced flow field and flame are identified, and a strong amplitude dependence is observed. It is found that the convective time delay between the swirl generator and the flame has an important influence on swirl-number oscillations and the flame dynamics in the low-frequency regime. For mid and high frequencies, significant changes in the mean flow field and the mean flame position are identified for high forcing amplitudes. These affect the interaction between coherent structures and the flame and are suggested to be responsible for the saturation in the flame response at high forcing amplitudes.

Blue Combustion: Stoichiometric Hydrogen-Oxygen Combustion Under Humidified Conditions

The use of renewable energy sources raises the demand of fast and flexible storage techniques and fast power availability to ensure electrical grid stability. A promising storage approach is the production of hydrogen and oxygen by electrolysis. The possibility of using a completely closed cycle of water, hydrogen and oxygen promises an attractive approach for high efficiency, zero emission energy storage. Since electrolysis can be carried out under high pressure, the compressor part of the gas turbine would be unnecessary, which is beneficial in terms of efficiency. Furthermore, high turbine pressure ratios, compared to typical gas turbine applications, can be reached easily.However, the combustion of hydrogen and oxygen in gas turbines is a challenging task. Hydrogen and oxygen mixtures are extremely reactive and result in very high flame temperatures. In the present study the feasibility of steam-diluted combustion of hydrogen and oxygen at stoichiometric conditions is shown. A suitable combustor is developed and experimentally validated. The degree of humidity is varied systematically for stoichiometric hydrogen oxygen combustion. Flame shapes, temperature estimations and operating limits are compared and discussed.Copyright

Investigation of NOx and CO Formation in a Premixed Swirl-Stabilized Flame at Ultra Wet Conditions

emissions, and also allows operating the gasturbine on hydrogen-rich fuels.The current study is conducted within the European Advanced Grant Research Project GREENEST.The premixed combustion of natural gas at atmospheric pressure is investigated over a wide range ofequivalence ratios, steam content in the air between 0% and 30%, and di erent degrees of nitrogen dilution.Emission formation, local temperature distribution, and ame shape and position were measured in thegas- red tests. A UV probe was used to determine local OH radical concentrations in the ame.A reactor network was designed to model the combustion process and to investigate the inuence ofsteam dilution on important species concentrations and on CO and NO