Christoph M. Arndt’s research while affiliated with German Aerospace Center (DLR) and other places

In this work, the thermo-acoustic instabilities of a gas turbine model combustor, the so-called SFB606 combustor, are numerically investigated using Large Eddy Simulation (LES) combined with tabulated chemistry and Artificial Thickened Flame (ATF) approach. The main focus is a detailed analysis of the thermo-acoustic cycle and the accompanied equivalence ratio oscillations and their associated convective time delay. In particular, the variations of the thermo-chemical state and flame characteristics over the thermo-acoustic cycle are investigated. For the operating point flame B ($P_{th}=25\,$kW), the burner exhibits thermo-acoustic instabilities with a dominant frequency of 392Hz, the acoustic eigenmode of the inner air inlet duct. These oscillations are accompanied by an equivalence ratio oscillation, which exhibits a convective time delay between the injection in the inner swirler and the flame zone. Two LES, one adiabatic and one accounting for heat losses at the walls by prescribing the wall temperatures from experimental data and Conjugated Heat Transfer (CHT) simulations, are conducted. Results with the enthalpy-dependent table are found to predict the time-averaged flow field in terms of velocity, major species, and temperature with higher accuracy than in the adiabatic case. Further, they indicate, that heat losses should be accounted for to correctly predict the flame position. Subsequently, the thermo-chemical state variations over the thermo-acoustic cycle for the enthalpy-dependant case are analyzed in detail and compared with experimental data in terms of phase-conditioned averaged profiles and conditional averages. An overall good prediction is observed. The results provide a detailed quantitative analysis of the thermo-acoustic feedback mechanism of this burner.

The eighteenth topical meeting on Laser Applications to Chemical, Security, and Environmental Analysis (LACSEA) was held in Vancouver, Canada from 11–15 July 2022, as part of the Optica Optical Sensors and Sensing Congress in a hybrid format allowing on-site and online attendance. The meeting featured a broad range of distinguished papers focusing on recent advances in laser and optical spectroscopy. A total of 52 contributed and invited papers were presented during the meeting, including topics such as photo-acoustic spectroscopy, imaging, non-linear technologies, frequency combs, remote sensing, environmental monitoring, aerosols, combustion diagnostics, hypersonic flow diagnostics, nuclear diagnostics, fs/ps applications, and machine learning and computational sensing.

In this work, the thermo-acoustic instabilities of a gas turbine model combustor, the so-called SFB606 combustor, are numerically investigated using Large Eddy Simulation (LES) combined with tabulated chemistry and Artificial Thickened Flame (ATF) approach. The main focus is a detailed analysis of the thermo-acoustic cycle and the accompanied equivalence ratio oscillations and their associated convective time delay. In particular, the variations of the thermo-chemical state and flame characteristics over the thermo-acoustic cycle are investigated. For the operating point flame B (Pth = 25kW, Φ = 0.7), the burner exhibits thermo-acoustic instabilities with a dominant frequency of 392 Hz, the acoustic eigenmode of the inner air inlet duct. These oscillations are accompanied by an equivalence ratio oscillation, which exhibits a convective time delay between the injection in the inner swirler and the flame zone. Two LES, one adiabatic and one accounting for heat losses at the walls by prescribing the wall temperatures from experimental data and Conjugated Heat Transfer (CHT) simulations, are conducted. Results with the enthalpy-dependent table are found to predict the time-averaged flow field in terms of velocity, major species, and temperature with higher accuracy than in the adiabatic case. Further, they indicate, that heat losses should be accounted for to correctly predict the flame position. Subsequently, the thermo-chemical state variations over the thermo-acoustic cycle for the enthalpy-dependant case are analyzed in detail and compared with experimental data in terms of phase-conditioned averaged profiles and conditional averages. An overall good prediction is observed, although an overestimation of the oscillation amplitude yields a slight over-prediction of the velocity field in the low-pressure phases. The results provide a detailed quantitative analysis of the thermo-acoustic feedback mechanism of this burner.

In several technical combustion systems for lean, premixed combustion, mixture stratification plays an important role, such as in stationary and aero gas turbines. The current paper focuses on a detailed characterization of a dual-swirl gas turbine model combustor operated in a stratified regime. The influence of mixture stratification on flame stabilization and self-induced thermo-acoustic oscillations was studied using laser and optical diagnostics in combination with microphone probes to measure pressure oscillations in the plenums and combustion chamber. The overall flame-shape was imaged using OH* chemiluminescence. Laser Raman scattering was applied to study the thermo-chemical state of the flame, the flow-field was measured using Particle Image Velocimetry and pressure oscillations in the combustion chamber and in the air plenums were determined by using calibrated microphone probes. OH planar laser-induced fluorescence was used to determine fluctuations of the hot gas with high spatial resolution. Significant mixture stratification within the combustion chamber was found to only occur at or upstream of the flame root. Varying the stratification level did not significantly influence the mean flame shape and flow field, however, a clear influence on thermo-acoustic oscillations was observed.

Wall temperature measurements with fiber coupled online phosphor thermometry were, for the first time, successfully performed in a full scale H-class Siemens gas turbine combustor. Online wall temperatures were obtained during high-pressure combustion tests up to 8 bar at the Siemens CEC test facility. Since optical access to the combustion chamber with fibers being able to provide high laser energies is extremely challenging, we developed a custom-built measurement system, consisting of a water-cooled fiber optic probe and a mobile measurement container. A suitable combination of chemical binder and thermographic phosphor was identified for temperatures up to 1800 K on combustor walls coated with a thermal barrier coating (TBC). To our knowledge these are the first measurements reported with fiber coupled online phosphor thermometry in a full scale highpressure gas turbine combustor. Details of the setup and the measurement procedures will be presented. The measured signals were influenced by strong background emissions, probably from CO*2 chemiluminescence. Strategies for correcting backgroundemissions and data evaluation procedures are discussed. The presented measurement technique enables detailed study of combustor wall temperatures and using this information an optimization of the gas turbine cooling design.

Wall temperature measurements with fiber coupled online phosphor thermometry were, for the first time, successfully performed in a full scale H-class Siemens gas turbine combustor. Online wall temperatures were obtained during high-pressure combustion tests up to 8 bar at the Siemens CEC test facility. Since optical access to the combustion chamber with fibers being able to provide high laser energies is extremely challenging, we developed a custom-built measurement system, consisting of a water-cooled fiber optic probe and a mobile measurement container. A suitable combination of chemical binder and thermographic phosphor was identified for temperatures up to 1800 K on combustor walls coated with a thermal barrier coating (TBC). To our knowledge these are the first measurements reported with fiber coupled online phosphor thermometry in a full scale high-pressure gas turbine combustor. Details of the setup and the measurement procedures will be presented. The measured signals were influenced by strong background emissions, probably from CO2* chemiluminescence. Strategies for correcting background-emissions and data evaluation procedures are discussed. The presented measurement technique enables detailed study of combustor wall temperatures and using this information an optimization of the gas turbine cooling design.

Instantaneous 2D phosphor thermometry was performed on the windows and combustion chamber posts of an optically accessible dual-swirl gas turbine model combustor operated with CH4 at atmospheric pressure. Two phosphors were used in order to measure different surface temperature ranges: YAP:Eu (temperature range 850 K – 1150 K) and YAG:Eu (temperature range 1100 K – 1300 K). Phosphor coatings were applied to the combustion chamber post and to three vertically oriented stripes on one combustion chamber window. Using interpolation, this allowed the determination of the surface temperature of the complete combustion chamber window. Heat losses across the combustion chamber window were determined by measuring the surface temperature on the inner and outer surface of the window. The phosphor coating was illuminated using the fourth harmonic of a Nd:YAG laser, which was formed into a broad light sheet. The spatially resolved, temperature-dependent decay rate of the phosphorescence was measured with a high speed CMOS camera. Three flames with similar flow fields and thermal powers between 22.5 kW and 30 kW and equivalence ratios between 0.63 and 0.83 were studied. Because the flame characteristics including flow field, heat release and temperature distributions were known from previous measurements the interaction between the flame and the surface temperature could be examined. In this way, the different wall temperatures of the flames could be explained.