Peter Griebel

Experimental Investigation of an Atmospheric Rectangular Rich Quench Lean Combustor Sector for Aeroengines

In this research work the potential of rich quench lean combustion for low emission aeroengines is investigated in a rectangular atmospheric sector, representing a segment of an annular combustor. For a constant design point (cruise) the mixing process and the NOx formation are studied in detail by concentration, temperature and velocity measurements using intrusive and non-intrusive measuring techniques.Measurements at the exit of the homogeneous primary zone show relatively high levels of non-thermal NO. The NOx formation in the quench zone is very low due to the quick mixing of the secondary air achieved by an adequate penetration of the secondary air jets and a high turbulence level. The NOx and CO emissions at the combustor exit are low and the pattern factor of the temperature distribution is sufficient.© 1997 ASME

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.

Experimental investigation of a generic, fuel flexible reheat combustor at gas turbine relevant operating conditions

Fuel flexibility in stationary gas turbines (GT) is becoming increasingly important due to the use of a broader spectrum of primary energy sources, particularly H2 -rich fuels derived from the gasification of coal or biomass. GTs also must be able to operate at extremely low emission levels, which is currently achieved with lean-premixed burner designs. To investigate the performance of highly reactive fuels in the reheat combustion concept, mainly with respect to autoignition and flashback limits, a generic reheat combustor with excellent optical access has been developed. The first objective of this work was to carefully characterize the mixing section in order to derive well-defined boundary conditions for the subsequent autoignition studies. Initial autoignition results at T > 1000 K and p = 15 bar are presented for natural gas (NG) and H2 -rich fuels. No autoignition was detected for NG at the investigated operating conditions. For H2 /NG/N2 blends with a constant volumetric N2 concentration of 20% and H2 concentrations higher than 76%, autoignition in the mixing section was detected.Copyright

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 Simulations of Turbulent Mixing and Autoignition of Hydrogen Fuel at Reheat Combustor Operating Conditions

Turbulent mixing and autoignition of H2 -rich fuels at relevant reheat combustor operating conditions are investigated in the present numerical study. The flow configuration under consideration is a fuel jet perpendicularly injected into a crossflow of hot flue gas (T > 1000 K, p = 15 bar). Based on the results of the experimental study for the same flow configuration and operating conditions two different fuel blends are chosen for the numerical simulations. The first fuel blend is a H2 /natural gas/N2 mixture at which no autoignition events were observed in the experiments. The second fuel blend is a H2 /N2 mixture at which autoignition in the mixing section occurred. First, the non-reacting flow simulations are performed for the H2 /natural gas/N2 mixture in order to compare the accuracy of different turbulence modeling methods. Here the steady-state Reynolds-averaged Navier-Stokes (RANS) as well as the unsteady scale-adaptive simulation (SAS) turbulence modeling methods are applied. The velocity fields obtained in both simulations are directly validated against experimental data. The SAS method shows better agreement with the experimental results. In the second part of the present work the autoignition of the H2 /N2 mixture is numerically studied using the 9-species 21-steps reaction mechanism of O’Conaire et al. [1]. As in the reference experiments, autoignition can be observed in the simulations. Influences of the turbulence modeling as well as of the hot flue gas temperature are investigated. The onset and the propagation of the ignition kernels are studied based on the SAS modeling results. The obtained numerical results are discussed and compared with data from experimental autoignition studies.Copyright

Investigation of a Rectangular Rich Quench Lean Combustor Sector

The chemically reacting flow in the mixing and secondary zone of the improved RQL combustor sector of the DLR has been numerically investigated. The combustion model is adapted to meet experimental conditions and proves to underpredict temperature fluctuations. The pressure dependence of the NOx emission index for the quench and lean zone of the combustor can be described with an exponential factor of 0.66. Although the level of agreement between measured and calculated results is not consistently good, the code can be used to study the effect of various operating conditions as well as the effect of geometrical changes on the flow and the emissions.

Numerical and Experimental Investigation of a Rich Quench Lean Combustor Sector.

The flow in a three sector model, representing a segment of an annular rich quench lean combustor for an aeroengine is investigated. Detailed knowledge of flow, temperature and species concentration distributions is of decisive importance to control the NOx formation, essential to the RQL concept. Velocities, temperatures and species concentrations are measured. They are partly used to aquire data on the inlet boundaries in the numerical calculation and partly used to compare with the numerical results. The calculation reveals many details which are not accesable in the experiment. It also shows the effects of the specific inlet dataset. Experimental data and numerical results furnish complementary information.Copyright

Auto-Ignition of In-Line Injected Hydrogen/Nitrogen Fuel Mixtures at Reheat Combustor Operating Conditions

De-carbonization of the power generation sector becomes increasingly important in order to achieve the European climate targets. Coal or biomass gasification together with a pre-combustion carbon capture process might be a solution resulting in hydrogen-rich gas turbine (GT) fuels. However, the high reactivity of these fuels poses challenges to the operability of lean premixed gas turbine combustion systems because of a higher auto-ignition and flashback risk. Investigation of these phenomena at GT relevant operating conditions is needed to gain knowledge and to derive design guidelines for a safe and reliable operation.The present investigation focusses on the influence of the fuel injector configuration on auto-ignition and kernel development at reheat combustor relevant operating conditions. Auto-ignition of H2-rich fuels was investigated in the optically accessible mixing section of a generic reheat combustor. Two different geometrical in-line configurations were investigated. In the premixed configuration, the fuel mixture (H2 / N2) and the carrier medium nitrogen (N2) were homogeneously premixed before injection, whereas in the co-flow configuration the fuel (H2 / N2) jet was embedded in a carrier medium (N2 or air) co-flow. High-speed imaging was used to detect auto-ignition and to record the temporal and spatial development of auto-ignition kernels in the mixing section.A high temperature sensitivity of the auto-ignition limits were observed for all configurations investigated. The lowest auto-ignition limits are measured for the premixed in-line injection. Significantly higher auto-ignition limits were determined in the co-flow in-line configuration. The analysis of auto-ignition kernels clearly showed the inhibiting influence of fuel dilution for all configurations.© 2015 ASME

Autoignition of Hydrogen / Natural Gas / Nitrogen Fuel Mixtures at Reheat Combustor Operating Conditions

Previous autoignition studies at conditions relevant to reheat combustor operation have indicated that the presence of relatively small amounts of natural gas (NG) in H2/N2 fuel significantly changes the autoignition behavior. The present study further elucidates the influence of NG on autoignition, kernel propagation, and subsequent flame stabilization at conditions that are relevant for the practical operation of gas turbine reheat combustors (p = 15 bar, Tinlet > 1000 K, hot flue gas, appropriate residence times). The experimental investigation was carried out in a generic, optically accessible reheat combustor. Autoignition events in the mixing zone were recorded by a high-speed camera at frame rates of up to 30,000 fps. This paper describes the autoignition behavior as the H2 volume fraction is increased (decreasing NG) in a H2/NG/N2 fuel mixture for two different jet penetration depths. Additionally, the subsequent flame stabilization phenomena and the structure of the stabilized flame are discussed. The results reveal that autoignition kernels occurred even for the lowest H2 fuel fraction, but they did not initiate a stable flame in the mixing zone. Increasing the H2 volume fraction decreased the distance between the initial position of the autoignition kernels and the fuel injector, finally leading to flame stabilization. The occurrence of autoignition kernels at lower H2 volume fractions (H2/(H2+NG) < 85%) was not found to be significantly influenced by the fluid dynamic and mixing field differences related to the different jet penetration depths. In contrast, autoignition leading to flame stabilization was found to depend on jet penetration; flame stabilization occurred at lower H2 fractions for the higher jet penetration depth (H2/(H2+NG) ≈ 89 compared to H2/(H2+NG) ≈ 95 vol. %).© 2012 ASME