Oliver Lammel

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.

PLIF Thermometry Based on Measurements of Absolute Concentrations of the OH Radical

Abstract A method for measurements of planar temperature distributions based on planar laser-induced fluorescence (PLIF) of the OH radiacal is described. The technique was developed specifically for the application in lean combustion systems, where OH equilibrium concentrations are largely independent on equivalence ratio and a function of temperature only. It is thus possible to derive a temperature information from measurements of absolute OH concentration, which can be obtained from a combined PLIF/absorption measurement. This paper discusses the basics of the method, and describes validation experiments in high pressure laminar premixed flames which were performed to asses its applicability and accuracy. Therefore, we compared our LIF based results with CARS measurements performed in the same flames. Finally, an example for the application in a lean gas turbine model combustor is discussed.

Investigation of Soot Formation and Oxidation in a High-Pressure Gas Turbine Model Combustor by Laser Techniques

Swirl-stabilized, non-premixed C2 H4 /air flames were investigated at pressures up to 9 bar in order to contribute to a better understanding of soot formation and oxidation under gas turbine like conditions. The flames had a thermal power of up to 45 kW and were confined by a squared combustion chamber with quartz windows. Secondary air could be injected into the post-flame zone to study the influence of cooling air on the oxidation of soot. The good optical access to the flame enabled the application of optical and laser measuring techniques. Temperatures were measured by coherent anti-Stokes Raman scattering (CARS) and soot concentrations by 2D laser induced incandescence (LII). The influence of the flow field characteristics, known from previous measurements in similar configurations, on the soot distributions is discussed. Furthermore, the effects of pressure, equivalence ratio and secondary oxidation air on the instantaneous and mean soot distributions were studied. All measurements were performed under well-defined and documented conditions, so that the data sets are suitable for the validation of numerical simulations.© 2007 ASME

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

Development of a Jet-Stabilized Low-Emission Combustor for Liquid Fuels

In this work the ongoing development of a jet-stabilized FLOX®(Flameless Oxidation)-type low-emission combustor for liquid fuels is described. The desired application of this concept is a micro gas turbine range extender for next generation car concepts. Diesel DIN EN 590 was used to operate the combustor, which is very similar to other fuels like bio-diesel, light heating oil and kerosene and therefore provides a link to other gas turbine applications in power generation. The investigation of flame stabilization of jet flames as well as fuel atomization, spray dispersion and evaporation is essential for the design of an effective and reliable combustor for liquid fuels. An axisymmetric single-nozzle combustion chamber was chosen for the initial setup. A variety of burner configurations was tested in order to investigate the influence of different design parameters on the flame shape, the flame stability and emissions. Two pressure atomizers and one air-blast atomizer were combined with two types of air nozzles and two different burner front plates (axisymmetric and off-centered jet nozzle). Finally, a twelve nozzle FLOX® combustor with pre-evaporator was designed and characterized. The combustor was operated at atmospheric pressure with preheated air (300° C) and in a range of equivalence ratios φ between 0.5 and 0.95 (λ = 1.05–2). The maximum thermal power was 40 kW. For each combustor configuration and operating condition the flame shape was imaged by OH*-chemiluminescence, together with an analysis of the exhaust gas emissions. Laser sheet imaging was used to identify the spray geometry for all axisymmetric combustors. Wall temperatures were measured for two configurations using temperature sensitive paints, which will be utilized in future CFD modeling. The results show a dependence of NOx emissions on the flame’s lift-off height, which in turn is defined by the spray properties and evaporation conditions. The tendency to soot formation was strongly dependent on the correlation of the recirculation zone to the spray cone geometry. In particular, strong soot formation was observed when unevaporated droplets entered the recirculation zone.Copyright

Experimental Investigations of Flame Stabilization of a Gas Turbine Combustor

While today’s gas turbine (GT) combustion systems are designed for specific fuels there is an urgent demand for fuel-flexible stationary GT combustors capable of burning natural gas as well as hydrogen-rich fuels in future. For the development of a fuel flexible, low-emission, and reliable combustion system a better understanding of the flow field – flame interaction and the flame stabilization mechanism is necessary. For this purpose, a down-scaled staged can combustion system provided with an optical combustion chamber was investigated in a high pressure test rig. Different optical diagnostic methods were used to analyze the combustion behavior with a focus on flame stabilization and to generate a comprehensive set of data for validation of numerical simulation methods (CFD) employed in the industrial design process. For different operating conditions the size and position of the flame zone were visualized by OH* chemiluminescence measurements. Additionally, the exhaust gas emissions (NOx and CO) and the acoustic flame oscillations were monitored. Besides many different operating conditions with natural gas different fuel mixtures of natural gas and hydrogen were investigated in order to characterize the flashback behavior monitored with OH* chemiluminescence. For selected operating conditions detailed laser diagnostic experiments were performed. The main flow field with the inner recirculation zone was measured with two-dimensional particle image velocimetry (PIV) in different measuring planes. One-dimensional laser Raman spectroscopy was successfully applied for the measurement of the major species concentration and the temperature. These results show the variation of the local mixture fraction allowing conclusions to be drawn about the good premix quality. Furthermore, mixing effects of unburnt fuel/air and fully reacted combustion products are studied giving insights into the process of the turbulence-chemistry interaction and reaction progress.Copyright

Wall temperature measurements in gas turbine combustors with thermographic phosphors

Phosphor thermometry has been developed for wall temperature measurements in gas turbines and gas turbine model combustors. An array of phosphors has been examined in detail for spatially and temporally resolved surface temperature measurements. Two examples are provided, one at high pressure (8 bar) and high temperature and one at atmospheric pressure with high

High Momentum Jet Flames at Elevated Pressure: B — Detailed Investigation of Flame Stabilization With Simultaneous PIV and OH-LIF

A model FLOX® combustor, featuring a single high momentum premixed jet flame, has been investigated using laser diagnostics in an optically accessible combustion chamber at a pressure of 8 bar. The model combustor was designed as a large single eccentric nozzle main burner (Ø 40 mm) together with an adjoining pilot burner and was operated with natural gas. To gain insight into the flame stabilization mechanisms with and without piloting, simultaneous Particle Image Velocimetry (PIV) and OH Laser Induced Fluorescence (LIF) measurements have been performed at numerous two-dimensional sections of the flame. Additional OH-LIF measurements without PIV-particles were analyzed quantitatively resulting in absolute OH concentrations and temperature fields. The flow field looks rather similar for both the unpiloted and the piloted case, featuring a large recirculation zone next to the high momentum jet. However, flame shape and position change drastically. For the unpiloted case, the flame is lifted, widely distributed and isolated flame kernels are found at the flame root in the vicinity of small scale vortices. For the piloted flame, on the other hand, both pilot and main flame are attached to the burner base plate, and flame stabilization seems to take place on much smaller spatial scales with a connected flame front and no isolated flame kernels. The single shot analysis gives rise to the assumption that for the unpiloted case small scale vortices act like the pilot burner flow in the opposed case and constantly impinge and ignite the high momentum jet at its root.

Investigation of confined turbulent jet flames using kHz-rates

An atmospheric, single-jet combustor with rectangular confinement was previously developed based on the FLOX concept. In this study, premixed, preheated H2-air and CH4-air turbulent jet flames were stabilized in this combustor. Self-sustained jet oscillation was observed in both non-reacting and reacting cases. A 3-D and periodic jet flapping is identified via proper orthogonal decomposition (POD) of various 2-D views of the flow field, measured by particle imaging velocimetry (PIV). The frequency of the oscillations and couplings between different Eigenmodes are determined based on results from PIV at 5 kHz repetition rate. The influence of jet flapping on combustion stability is examined in detail using simultaneous PIV/OH chemiluminescence imaging and PIV/planar laserinduced fluorescence of OH radicals (OH PLIF) at 5 kHz repetition rate. The up/down and expand/contract motions of the lateral recirculation zone (LRZ) due to jet flapping is found to modify the flame shape and flame lift-off height. Such variations are traced to the transformation of low-speed regions in the junction of the back flow and the jet. These regions contain counter flows and vortices with relatively long flow residence time and can therefore enhance the mixing between recirculated hot burned gas and fresh fuel-air mixtures. In some cases, the jet flapping is also found to cause temporary local extinction of the flame, due to reduced entrainment of burned gas. However, the flame is able to recover as the jet flaps away from combustor chamber and reopens the back flow channel.

Investigation of Confined Turbulent Jet Flames Using kHz-Rate Diagnostics

An atmospheric, single-jet combustor with rectangular confinement was previously developed based on the FLOX concept. In this study, premixed, preheated H2-air and CH4-air turbulent jet flames were stabilized in this combustor. Self-sustained jet oscillation was observed in both non-reacting and reacting cases. A 3-D and periodic jet flapping is identified via proper orthogonal decomposition (POD) of various 2-D views of the flow field, measured by particle imaging velocimetry (PIV). The frequency of the oscillations and couplings between different Eigenmodes are determined based on results from PIV at 5 kHz repetition rate. The influence of jet flapping on combustion stability is examined in detail using simultaneous PIV/OH chemiluminescence imaging and PIV/planar laserinduced fluorescence of OH radicals (OH PLIF) at 5 kHz repetition rate. The up/down and expand/contract motions of the lateral recirculation zone (LRZ) due to jet flapping is found to modify the flame shape and flame lift-off height. Such variations are traced to the transformation of low-speed regions in the junction of the back flow and the jet. These regions contain counter flows and vortices with relatively long flow residence time and can therefore enhance the mixing between recirculated hot burned gas and fresh fuel-air mixtures. In some cases, the jet flapping is also found to cause temporary local extinction of the flame, due to reduced entrainment of burned gas. However, the flame is able to recover as the jet flaps away from combustor chamber and reopens the back flow channel.