Michael Stöhr

FLOX® Combustion at High Power Density and High Flame Temperatures

In this contribution, an overview of the progress in the design of an enhanced FLOX ® burner is given. A fuel fiexible burner concept was developed to fulfill the requirements of modern gas turbines: high specific power density, high turbine inlet temperature, and low NO x emissions. The basis for the research work is numerical simulation. With the focus on pollutant emissions, a detailed chemical kinetic mechanism is used in the calculations. A novel mixing control concept, called HiPerMix®, and its application in the FLOX ® burner are presented. In view of the desired operational conditions in a gas turbine combustor, this enhanced FLOX ® burner was manufactured and experimentally investigated at the DLR test facility. In the present work, experimental and computational results are presented for natural gas and natural gas +hydrogen combustion at gas turbine relevant conditions and high adiabatic flame temperatures (up to T ad = 2000 K). The respective power densities are P A = 13.3 MW/m 2 bar (natural gas (NG)) and P A =14.8 MW/m 2 bar (NG + H 2 ), satisfying the demands of a gas turbine combustor. It is demonstrated that the combustion is complete and stable and that the pollutant emissions are very low.

Effects of Flow Structure Dynamics on Thermoacoustic Instabilities in Swirl-Stabilized Combustion

The thermoacoustic coupling caused by dynamic flow/flame interactions was investigated in a gas-turbine model combustor using high-repetition-rate measurements of the three-component velocity field, OH laser-induced fluorescence, and OH* chemiluminescence. Three fuel-lean, swirl-stabilized flames were investigated, each of which underwent self-excited thermoacoustic pulsations. The most energetic flow structure at each condition was a helical vortex core that circumscribed the combustor at a frequency that was independent of the acoustics. Resolving the measurement sequence with respect to both the phase in the thermoacoustic cycle and the azimuthal position of the helix allowed quantification of the oscillatory flow and flame dynamics. Periodic vortex/flame interactions caused by deformation of the helices generated local heat-release oscillations having spatially complex phase distributions relative to the acoustics. The local thermoacoustic coupling, determined by statistically solving the Rayleigh integral, showed intertwined regions of positive and negative coupling due to these vortices. In the quietest flame, the helical vortex created a large region of negative coupling that helped damp the oscillations. In the louder flames, the shapes of the oscillating vortices and flames were such that large regions of positive coupling were generated, driving the instability. From these observations, flame/vortex configurations that promote stability are identified.

Flow Field and Combustion Characterization of Premixed Gas Turbine Flames by Planar Laser Techniques

Lean premixed natural gas/air flames produced by an industrial gas turbine burner were analyzed using laser diagnostic methods. For this purpose, the burner was equipped with an optical combustion chamber and operated with preheated air at various thermal powers P, equivalence ratios Φ, and pressures up to p=6 bars. For the visualization of the flame emissions OH∗ chemiluminescence imaging was applied. Absolute flow velocities were measured using particle image velocimetry (PIV), and the reaction zones as well as regions of burnt gas were characterized by planar laser-induced fluorescence (PLIF) of OH. Using these techniques, the combustion behavior was characterized in detail. The mean flow field could be divided into different regimes: the inflow, a central and an outer recirculation zone, and the outgoing exhaust flow. Single-shot PIV images demonstrated that the instantaneous flow field was composed of small and medium sized vortices, mainly located along the shear layers. The chemiluminescence images reflected the regions of heat release. From the PLIF images it was seen that the primary reactions are located in the shear layers between the inflow and the recirculation zones and that the appearance of the reaction zones changed with flame parameters.

Flow field and combustion characterization of premixed gas turbine flames by planar laser techniques

Lean premixed natural gas/air flames produced by an industrial gas turbine burner were analyzed using laser diagnostic methods. For this purpose, the burner was equipped with an optical combustion chamber and operated with preheated air at various thermal powers P, equivalence ratios Φ , and pressures up to p = 6 bar. For the visualization of the flame emissions OH* chemiluminescence imaging was applied. Absolute flow velocities were measured using particle image velocimetry (PIV), and the reaction zones as well as regions of burnt gas were characterized by planar laser induced fluorescence (PLIF) of OH. Using these techniques, the combustion behavior was characterized in detail. The mean flow field could be divided into different regimes: the inflow, a central and an outer recirculation zone, and the outgoing exhaust flow. Single-shot PIV images demonstrated that the instantaneous flow field was composed of small and medium sized vortices, mainly located along the shear layers. The chemiluminescence images reflected the regions of heat release. From the PLIF images it was seen that the primary reactions are located in the shear layers between the inflow and the recirculation zones and that the appearance of the reaction zones changed with flame parameters.Copyright

Experimental Study of Turbulence-Chemistry Interactions in Perfectly and Partially Premixed Confined Swirl Flames

Abstract A gas turbine model combustor (Turbomeca Burner) for premixed methane/air flames has been operated at atmospheric pressure in two different modes of premixing. In the partially premixed mode, fuel was injected into the air flow within the swirl generator shortly upstream of the combustion chamber while in the perfectly premixed mode fuel and air were mixed far upstream. The main objective of this work is the study of the influence of the mode of premixing on the combustion behavior. Stereoscopic particle image velocimetry has been applied for the measurement of the flow field, OH chemiluminescence imaging for the visualization of the flame shapes and single-shot laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature. The mixing and reaction progress and effects of turbulence-chemistry interactions are characterized by scatterplots showing the correlations between different quantities. To isolate effects of mixing from combustion instabilities that were frequently observed in this combustor, operating conditions without thermo-acoustic oscillations or coherent flow structures were chosen. While the mode of premixing had no major influence on the general flame behavior characteristic differences were observed with respect to flame anchoring, the flow field in the inner recirculation zone and the CO concentration level. The results further extend the data base of previous experimental and numerical investigations with this burner.

Investigation of Flame Stabilization in a High-Pressure Multi-Jet Combustor by Laser Measurement Techniques

In this contribution, comprehensive optical and laser based measurements in a generic multi-jet combustor at gas turbine relevant conditions are presented. The flame position and shape, flow field, temperatures and species concentrations of turbulent premixed natural gas and hydrogen flames were investigated in a high-pressure test rig with optical access.The needs of modern highly efficient gas turbine combustion systems, i.e., fuel flexibility, load flexibility with increased part load capability, and high turbine inlet temperatures, have to be addressed by novel or improved burner concepts. One promising design is the enhanced FLOX® burner, which can achieve low pollutant emissions in a very wide range of operating conditions. In principle, this kind of gas turbine combustor consists of several nozzles without swirl, which discharge axial high momentum jets through orifices arranged on a circle. The geometry provides a pronounced inner recirculation zone in the combustion chamber. Flame stabilization takes place in a shear layer around the jet flow, where fresh gas is mixed with hot exhaust gas. Flashback resistance is obtained through the absence of low velocity zones, which favors this concept for multi-fuel applications, e.g. fuels with medium to high hydrogen content.The understanding of flame stabilization mechanisms of jet flames for different fuels is the key to identify and control the main parameters in the design process of combustors based on an enhanced FLOX® burner concept. Both experimental analysis and numerical simulations can contribute and complement each other in this task. They need a detailed and relevant data base, with well-known boundary conditions. For this purpose, a high-pressure burner assembly was designed with a generic 3-nozzle combustor in a rectangular combustion chamber with optical access. The nozzles are linearly arranged in z direction to allow for jet-jet interaction of the middle jet. This line is off-centered in y direction to develop a distinct recirculation zone. This arrangement approximates a sector of a full FLOX® gas turbine burner. The experiments were conducted at a pressure of 8 bar with preheated and premixed natural gas/air and hydrogen/air flows and jet velocities of 120 m/s.For the visualization of the flame, OH* chemiluminescence imaging was performed. 1D laser Raman scattering was applied and evaluated on an average and single shot basis in order to simultaneously and quantitatively determine the major species concentrations, the mixture fraction and the temperature. Flow velocities were measured using particle image velocimetry at different section planes through the combustion chamber.Copyright

Numerical Investigation of a Laboratory Combustor Applying Hybrid RANS-LES Methods

In this paper the three-dimensional non-reacting turbulent flow field of a swirl-stabilized gas turbine model combustor is analysed with compressible CFD. For the flow analysis URANS and Hybrid RANS/LES (DES, SAS) turbulence models were applied. The governing equations and the numerical method are described. The simulations were performed using the commercial CFD software package ANSYS CFX-10.0. The numerically achieved velocity components show a good agreement with the experimental values obtained by Particle Image Velocimetry (PIV). Furthermore, a precessing vortex core (PVC) could be found in the combustion chamber.

Numerical Analysis of Probe Microphones Used for Thermoacoustic Measurements

Acoustic measurements within combustion chambers are expensive due to high thermal loads applied on the measurement devices at operating conditions. As a more feasible substitute, probe microphones can be used to lead acoustic waves from combustion chambers to externally mounted microphones. Since these probe microphones are purged by nitrogen at atmospheric temperature, high thermal loads are avoided. However, the acoustic signal measured by the probe microphone is altered compared to the signal within the combustion chamber. This change in the acoustic signal can be characterised by means of the acoustic transfer function of the probe microphone, which mainly depends on the probe microphone geometry and the combustion chamber temperature distribution. Both impacts are studied in the present paper. The main subject is to analyse the inuence of the combustion chamber temperature distribution on the acoustic transfer function of probe microphones. In addition, the eect of changes in the probe microphone diameter is discussed. For this scope, experiments at standard conditions and transient CFD simulations for dierent temperature distributions have been carried out.

Influence of flow-structure dynamics on thermo-acoustic instabilities in oscillating swirl flames

The thermo-acoustic coupling caused by dynamic owame interactions was investigated in a gas turbine model combustor through analysis of high-repetition-rate laser measurements. Planar three-component velocity elds and OH radical distributions, as well as the line-of-sight integrated chemiluminescence from OH*, were measured at a sustained repetition rate of 5 kHz. Three fuel-lean, swirl-stabilized ames were investigated, each of which underwent thermo-acoustic pulsations. The most energetic ow structure at each condition was a helical vortex core that spiraled around the burner axis and circumscribed the combustor at a rate that was independent of the acoustics. By resolving the measurement sequence with respect to both the phase in the thermo-acoustic cycle and the azimuthal position of the helical vortex core, the repeatable oscillatory processes could be reconstructed in three dimensions. Periodic deformations in the helices at the thermoacoustic frequency were found to cause oscillations in the ame surface area. The local ame area oscillated either inor out-of-phase with the acoustic pulsations depending on the relative shapes of the ame and helices. To investigate this further, the local thermoacoustic coupling was determined by statistically solving the Rayleigh integral. In all cases, intertwined regions of positive and negative coupling occurred near the burner nozzle due to the helical vortices. In the quietest ame, the helical vortex created a large region of negative coupling that helped damp the thermo-acoustic oscillations. In the moderately louder ame, the shapes of the helix and ame were such that there was a large helical region of positive thermo-acoustic coupling that contributed energy to the thermo-acoustic pulsations. In the loudest ame, positive thermo-acoustic coupling occurred in both a large helical region and in the outer recirculation zone.

Analysis of flow-flame interactions in a gas turbine model combustor under thermo-acoustically stable and unstable conditions

Flow-flame interactions in a swirl-stabilized gas turbine model combustor are investigated using doubly-phase-resolved analysis of high-repetition-rate laser and optical measurements. Three flames were studied, each of which exhibited self-excited thermo-acoustic oscillations of different amplitude ranging from fairly stable to highly unstable combustion. High-repetition-rate stereoscopic particle image velocimetry, OH planar laser induced fluorescence, and OH* chemiluminescence measurements were performed at a sustained repetition rate of 5 kHz, which was sufficient to resolve the thermo-acoustic combustor dynamics. Using spatio-temporal proper orthogonal decomposition, it was found that the flow-field contained several simultaneous periodic motions: the reactant flux into the combustion chamber periodically oscillated at the thermo-acoustic frequency, a helical precessing vortex core (PVC) circumscribed the burner nozzle at a frequency set by the global flow rate, and the PVC underwent axial contraction and extension at the thermo-acoustic frequency. The amplitude of the axial PVC dynamics increased with increasing thermo-acoustic oscillation amplitude. The global heat release rate fluctuated at the thermo-acoustic frequency, while the heat release centroid circumscribed the combustor at the difference between the acoustic and PVC frequencies. This latter motion was caused by the axial PVC dynamics. Hence, the three-dimensional location of the heat release and heat release fluctuations depended on the interaction of the PVC with the flame surface. This motivated the compilation of doubly-phase-resolved statistics based on the phase of both the acoustic and PVC cycles, which showed highly repeatable periodic flow-flame configurations. These doublyphase-resolved statistics were used to reconstruct the dynamics of the three-dimensional periodic flow structures and flame surface oscillations over the thermo-acoustic cycle. It was found that the majority of the heat release oscillations at the thermo-acoustic frequency were caused by oscillations in the reaction layer length. By filtering the instantaneous reaction layers at different scales, the importance of the various flow-flame interactions affecting the flame length was determined. The greatest contributor was large-scale elongation of the reaction layers associated with the fluctuating reactant flow rate, which accounted for approximately 50% of the fluctuations. The remaining 50% was distributed between fine scale stochastic corrugation and large-scale corrugation due to the PVC.