Felix Guethe

A DETAILED ANALYSIS OF THERMOACOUSTIC INTERACTION MECHANISMS IN A TURBULENT PREMIXED FLAME

A combined theoretical and experimental analysis of thermoacoustic interaction mechanisms of a lean pre-mixed swirl-stabilized gas turbine burner is presented. A full-scale gas turbine burner has been tested in an atmospheric test rig. The test facility was equipped with loudspeakers to excite the acoustic field and with arrays of microphones to measure the response of the acoustic field to the forcing signal. With this set-up transfer matrices relating the acoustic pressure and velocity on both sides of the flame front have been measured. A laser absorption measurement technique allowed for measurement of the fluctuations of fuel concentration in the mixture. Heat release fluctuations were monitored using a photo-multiplier. The measurement of the acoustic field, heat release and equivalence ratio fluctuations have been measured simultaneously. Special attention has been given to the role of fuel concentration fluctuations in the thermoacoustic interaction mechanism. In order to be able to clearly separate this mechanism from other possible mechanisms, all the experiments have been performed in pre-premixing mode as well. In pre-premixing mode the fuel is injected far upstream of the burner in order to avoid fuel concentration fluctuations at the burner location. This is in contrast with premixing mode where fuel and air are mixed in the burner. An acoustic flame model has been derived. The model includes the well-known interaction mechanism of equivalence ratio fluctuations but also includes a novel mechanism that is caused by fluctuations of vorticity. This latter mechanism relates the turbulent flame speed via turbulence intensity fluctuations to the acoustic field. The idea is that periodic acoustic fluctuations cause periodic changes of the turbulence intensity. The turbulence intensity strongly affects the turbulence flame speed. The fluctuations of the turbulent flame speed result in fluctuations of the heat release. This turbulence intensity fluctuation model has been compared with the measured pre-premix transfer functions and shows an excellent agreement. The measured transfer functions in premix mode have been compared with the model that includes fluctuations of fuel concentration and turbulence intensity. Also in this case a very good agreement is found. Moreover, it has been demonstrated that the phase relation between measured equivalence ratio fluctuation and heat release corresponds to the model.Copyright

Thermoacoustic Modeling of a Gas Turbine Using Transfer Functions Measured Under Full Engine Pressure

Thermoacoustic transfer functions have been measured of a full-scale gas turbine burner operating at full engine pressure. Excitation of the high-pressure test facility was done using a siren that modulated part of the combustion airow.

Thermoacoustic Modeling of a Gas Turbine Using Transfer Functions Measured at Full Engine Pressure

Thermoacoustic transfer functions have been measured of a full-scale gas turbine burner operating at full engine pressure. Excitation of the high-pressure test facility was done using a siren that modulated part of the combustion airflow. Pulsation probes have been used to record the acoustic response of the system to this excitation. In addition, the flame’s luminescence response was measured by multiple photomultiplier tubes and a light spectrometer. Three techniques to obtain the thermoacoustic transfer function are proposed and employed: two combined acoustical-optical technique and a purely acoustic technique. The first acoustical-optical technique uses one single optical signal capturing the chemiluminescence intensity of the flame as a measure for the heat release in the flame. It only works, if heat release fluctuations in the flame have only one contribution, e.g. equivalence ratio or mass flow fluctuations. The second acoustic-optical acoustic-optical technique makes use of the different response of the flame’s luminescence at different optical wavelengths bands to acoustic excitation. It also works, if the heat release fluctuations have two contributions, e.g. equivalence ratio and mass flow fluctuation. For the purely acoustic technique, a new method was developed in order to obtain the flame transfer function, burner transfer function and flame source term from only three pressure transducer signals. The purely acoustic method could be validated by the results obtained from the acoustic-optical techniques. The acoustic and acoustic-optical methods have been compared and a discussion on the benefits and limitations of the methods is given. The measured transfer functions have been implemented into a non-linear, three-dimensional, time domain network model of a gas turbine with an annular combustion chamber. The predicted pulsation behavior shows a good agreement with pulsation measurements on a field gas turbine.Copyright

Flame imaging on the ALSTOM EV-burner: thermo acoustic pulsations and CFD-validation

The ALSTOM low emission swirl-induced premix EV-burner is investigated by OH-planar laser induced fluorescence (PLIF) and OH*-chemiluminescence (CL) imaging on a full-scale industrial burner test rig. Three different burner variants have been compared by their flame shape and position as well as emission and pulsation behavior. The flame images have been used to enable comparison and validation of thermo acoustic and computational fluid dynamics (CFD) models. The flame movement upstream inside the burner can be related to emissions and pulsation. Depending on the burner two different mechanisms dominate the acoustic pulsations: One is based on equivalence ratio fluctuations coupled to a sudden displacement of flame anchoring point into the burner. Another mechanism seems to be related to turbulence intensity fluctuations. The experimental images were compared with the results of Reynolds-averaged Navier-Stokes (RANS) CFD simulations for varying parameters for validation. The turbulence treatment in time-averaged RANS models is not sufficient to describe the flame movement properly and encourages to use a more sophisticated treatment.

Flue Gas Recirculation in Gas Turbine: Investigation of Combustion Reactivity and NOX Emission

Flue gas recirculation (FGR) is a promising technology for the optimization of post-combustion CO2 capture in natural gas combined cycle (NGCC) plants. In this work, the impact of FGR on lean gas turbine premix combustion is predicted by analytical and numerical investigations as well as comparison to experiments. In particular the impact of vitiated air condition and moderate increase of CO2 concentration into combustion reactivity and NOx emission is studied. The influence of inlet pressure, temperature and recirculated NOx are taken as parameters of this study. Two different kinetic schemes are used to predict the impact that FGR has on the combustion process: the GRI3.0 and the RDO6_NO, which is a newly compiled mechanism from the DLR Stuttgart. The effects of the FGR on the NOx emissions are predicted using a chemical reactor network including unmixedness as presumed probability density function (PDF) to account for real effects. The magnitude and ratio of prompt to post-flame thermal NOx changes with the FGR-ratio producing less post flame NOx at reduced O2 content. For technical mixtures (i. e. an industrial fuel injector), NOx emission can be expected to be lower with the vitiation of the oxidizer. This is due to several effects: at low O2 concentration, the highest possible adiabatic flame temperatures for stoichiometric conditions decreases resulting in lower NOx when averaged over all mixing fractions. Further effects result from lower post flame NOx production and the role of “reburn” chemistry, actually reducing NOx (recirculated from the exhaust), which might become relevant for the high recirculation ratios, where parts of the flame would operate at rich stoichiometry at given unmixedness. Therefore in general for each combustor technical mixing could decrease NOx with respect to perfect mixing at high FGR-ratio assuming the engine can still be operated. Although the findings are quite general for gas turbines the advantage that reheat engines have in terms of operation are highlighted. For reheat engines this can be understood as an extension of the “reheat concept” and used as the next step in the goal to achieve minimal emissions at increasing power. In addition, NOx emission obtained in FGR combustion reduces even further when the engine pressure ratio increases, making the concept particularly well suited for reheat engines.

FLUE GAS RECIRCULATION OF THE ALSTOM SEQUENTIAL GAS TURBINE COMBUSTOR TESTED AT HIGH PRESSURE

Concerning the efforts in reducing the impact of fossil fuel combustion on climate change for power production utilizing gas turbine engines Flue Gas Recirculation (FGR) in combination with post combustion carbon capture and storage (CCS) is one promising approach. In this technique part of the flue gas is recirculated and introduced back into the compressor inlet reducing the flue gas flow (to the CCS) and increasing CO2 concentrations. Therefore FGR has a direct impact on the efficiency and size of the CO2 capture plant, with significant impact on the total cost. However, operating a GT under depleted O2 and increased CO2 conditions extends the range of normal combustor experience into a new regime. High pressure combustion tests were performed on a full scale single burner reheat combustor high-pressure test rig. The impact of FGR on NOx and CO emissions is analyzed and discussed in this paper. While NOx emissions are reduced by FGR, CO emissions increase due to decreasing O2 content although the SEV reheat combustor could be operated without problem over a wide range of operating conditions and FGR. A mechanism uncommon for GTs is identified whereby CO emissions increase at very high FGR ratios as stoichiometric conditions are approached. The feasibility to operate Alstom’s reheat engine (GT24/GT26) under FGR conditions up to high FGR ratios is demonstrated. FGR can be seen as continuation of the sequential combustion system which already uses a combustor operating in vitiated air conditions. Particularly promising is the increased flexibility of the sequential combustion system allowing to address the limiting factors for FGR operation (stability and CO emissions) through separated combustion chambers. Nomenclature FGR Flue gas recirculation FGR-ratio exhaust ion recirculat

CH*/OH* Chemiluminescence Response of an Atmospheric Premixed Flame Under Varying Operating Conditions

In this work the relationship between the ratio of the global CH* and OH* flame chemiluminescece and the global equivalence ratio of a technically premixed swirl-stabilized flame is investigated. The burner allows for a modification of the premix fuel injection pattern. The global flame chemiluminescence is monitored by a high-sensitivity light spectrometer and multiple photo-multipliers. The photo-multipliers were equipped with narrow optical band-pass filters and recorded the flame’s OH*, CH* and CO2* chemiluminescence intensity. To ensure an approximately uniform equivalence ratio distribution in the combustion zone, the spatial OH* and CH* flame chemiluminescence was recorded simultaneously with one ICCD camera using a special optical setup, which incorporated among other things one fully reflective and one semi-reflective mirror and appropriate optical filters. The flame chemiluminescence intensity was mapped for a range of equivalence ratios and air mass flows. The mapping shows that (as stated for perfectly premixed flames in the literature) the OH*, CH* and CO2* intensity of the investigated flame depends linearly on the air mass flow and exponentially on the equivalence ratio (i.e., I = km * φβ ). Hence for the investigated operating conditions (i.e., quasi premix conditions) the global CH*/OH* intensity can be employed as a measure of the global equivalence ratio for the operating conditions investigated in this work. However, the contribution of broadband CO2* chemiluminescence in the wave length range of CH* chemiluminescence has to be accounted for.Copyright

Optical Transfer Function Measurements for Technically Premixed Flames

This paper deals with a novel approach for measuring thermo-acoustic transfer functions. These transfer functions are essential to predict the acoustic behavior of gas turbine combustion systems. Thermoacoustic prediction has become an essential step in the development process of low-NOx combustion systems. The proposed method is particularly useful in harsh environments. It makes use of simultaneous measurement of the chemiluminescence of different species in order to obtain the heat release fluctuations via an inverse method. Generally, the heat release fluctuation has two contributions: one due to equivalence ratio fluctuations, the other due to modulations of mass flow of mixture entering the reaction zone. Because the chemiluminescence of one single species depends differently on the two contributions, it is not possible to quantitatively estimate the heat based on this information. Measurement of the transfer matrix based on a purely acoustic method provides quantitative results, independent of the nature of the interaction mechanism. However, this method is difficult to apply in industrial full-scale experiments. The method developed in this work uses the chemiluminescence time traces of several species. After calibration, an over-determined inverse method is used to calculate the two heat release contributions from the time traces. The optical method proposed here has the advantage that it does not only provide quantitative heat release fluctuations, but it also quantifies the underlying physical mechanisms that cause the heat release fluctuations: it shows what part of the heat release is caused by equivalence ratio fluctuations and what part by flame front dynamics. The method has been tested on a full scale, swirl stabilized gas turbine burner. Comparison with a purely acoustic method validated the concept

HYDROGEN COMBUSTION WITHIN A GAS TURBINE REHEAT COMBUSTOR

This paper describes a novel lean premixed reheat burner technology suitable for Hydrogen-rich fuels. The inlet temperature for such a combustor is very high and reaction of the fuel/oxidant mixture is initiated through auto-ignition, the delay time for which reduces significantly for Hydrogen-rich fuels in comparison to natural gases. Therefore the residence time available for premixing within the burner is reduced. The new reheat burner concept has been optimized to allow rapid fuel/oxidant mixing, to have a high flashback margin and to limit the pressure drop penalty. The performance of the burner is described, initially in terms of its fluid dynamic properties and then its combustion characteristics. The latter are based upon full-scale highpressure tests, where results are shown for two variants of the concept, one with a pressure drop comparable to today’s natural gas burners, and the other with a two-fold increase in pressure drop. Both burners indicated that Low NOx emissions, comparable to today’s natural gas burners, were feasible at reheat engine conditions (ca. 20 Bars and ca. 1000C inlet temperature). The higher pressure drop variant allowed a wider operating window. However the achievement of the lower pressure drop burner shows that the targeted Hydrogen-rich fuel (70/30 H2/N2 by volume) can be used within a reheat combustor wit hout any penalty on gas turbine performance.

Plasma assisted GT combustion

A study reported in the present note was focused mainly on possibilities to control the auto-ignition delay in the GT combustion chamber using the plasma discharge. An extensive activity on the plasma assisted combustion is underway, which concerns the boiler ignition and the gas turbine combustion stability. Three approaches to the plasma assisted GT combustion have been theoretically analyzed using the plasma-chemical and chemical kinetic modelling: Methane pyrolysis; Plasma ignition of the lean CH4/air mixture; Methane pre-reforming (partial oxidation) in a rich CH4/Air mixture.