The present work investigates a model combustor that approximates a full-scale segment of a high momentum jet stabilised combustion concept with the objective to broaden fuel flexibility limits. The rig features optical access for detailed laser based diagnostics. The current experiments are conducted at a pressure of 8 bar, constant jet velocity and adiabatic flame temperature. Three different injection systems are used to inject oil and oil / water blends. Droplet size distributions and turbulent droplet transport are quantified by means of phase Doppler interferometry. The injection system has a noticeable impact on the droplet size distribution. Water addition affects the fuel placement in radial direction significantly with distinct droplet transport into the reaction zone. The jet core and droplet transport is maintained as far as 7 x/d axial downstream from the nozzle exit. The shear layer shows a turbulent intensity of 12% of the main jet bulk velocity that drives the radial droplet transport into the recirculation zone. Mass flow and number density estimations illustrate the spatial distribution of the liquid loading in the combustion chamber. The present findings are complementary to the work in part A–D, support the development of fuel and load flexible high momentum jets based combustion concepts and the development and validation of numerical models.

This work investigates the influence of coaxial air flow on droplet distribution, velocity, and size generated by a pressure-swirl atomizer. The experiments were performed inside a generic test section with large optical access at atmospheric conditions. The flow conditions replicate the mixing duct sections of high momentum jet stabilized combustors for gas turbines, e.g. high axial air velocities without swirl generation and high preheat temperatures. High momentum jet stabilized combustors based on the FLOX ® burner concept are used successfully in gas turbines due to its fuel and load flexibility and very low pollutant emissions. In previous and ongoing studies, different model combustors have been under investigation mainly with the focus of broadening fuel flexibility and operational limits. Operation with different liquid fuel injection systems in high pressure experiments showed a significant impact from the injector shape and injection strategy on the fuel air mixing behavior, the flame position and stability, and thus NOx emissions. This experiment will give a more detailed understanding of the turbulent mixing and interaction of primary and secondary atomization with the surrounding air in such burners. The setup will also allow for the testing of different injection systems for various burner configurations by the variation of injection type, location, fuel, and air flow properties. In the present experiments a pressure-swirl atomizer was set to a constant pressure drop and mass flow. Liquid fuel was replaced by deionized water due to safety concerns. The coaxial air mass flow was preheated up to 473 K and set to bulk velocities of 20 m/s, 50 m/s, and 80 m/s. Particle Image Velocimetry (PIV) was used to characterize the flow field downstream of the point of injection. The droplet size and velocity distributions were quantified by double frame shadow imaging combined with a long-distance microscope with a resolution below 1 μm per pixel. Moreover, the formation of ligaments as well as primary spray break-up was visualized. The results show a significant change of the spatial droplet distribution with increasing co-flow velocity for a given atomizer geometry. The spray cone angle widens at high co-flow velocities due to the formation of a pronounced recirculation zone behind the backward facing step of the injector near the nozzle orifice. This also leads to a change in the initial droplet momentum and the spatial distribution of large droplets. Smaller droplets are concentrated to the center of the spray due to turbulent transport. These findings assist the ongoing developments of liquid fuel injection systems for high momentum jet based combustors and provide validation data for numerical simulations of primary and secondary atomization.

A detailed investigation on flame structures and stabilization mechanisms of confined high momentum jet flames by 1D-laser Raman measurements is presented. The flames were operated with natural gas (NG) at gas turbine relevant conditions in an optically accessible high pressure test rig. The generic burner represents a full scale single nozzle of a high temperature FLOX ® gas turbine combustor including a pilot stage. 1D-laser Raman measurements were performed on both an unpiloted and a piloted flame and evaluated on a single shot basis revealing the thermochemical states from unburned inflow conditions to burned hot gas in terms of average and statistical values of the major species concentrations, the mixture fraction and the temperature. The results are supported by complementary measurement techniques that have been previously conducted and presented in the connected papers part A and B [1,2], such as OH*-chemiluminescence, planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV), that combine to a big picture of the flame structures and help to interpret the results. The results show a distinct difference in the flame stabilization mechanism between the unpiloted and the piloted case. The former is apparently driven by strong mixing of fresh unburned gas and recirculated hot burned gas that eventually causes autoignition. The piloted flame is stabilized by the pilot stage followed by turbulent flame propagation. The findings help to understand the underlying combustion mechanisms and to further develop gas turbine burners following the FLOX ® concept. The combined results of all measurement techniques that have been applied to these two flames thus form a unique and comprehensive data set for the validation of numerical simulation models.

A promising alternative to modern swirl combustors for gas turbines are high momentum jet stabilized combustors. This gas turbine burner concept consists of circular arranged jet nozzles through which partially premixed high momentum jets enter the combustion chamber in axial direction. Furthermore, it features fuel flexibility, load flexibility and low pollutant emissions. The investigated generic combustor consists of an eccentric single nozzle in a square chamber. This nozzle represents a full-scale segment of a concentrically arranged multi-nozzle configuration. All measurements were carried out at the high pressure combustion test rig (HBK-S) at the German Aerospace Center (DLR) in Stuttgart. The generic single nozzle model combustor has been operated in a high-pressure test rig with large optical access in order to gain a detailed understanding of fuel distribution, droplet distribution, fuel air mixing and high temperature regions through various sections of the combustion chamber. For this purpose, different laser based measurement techniques have been applied simultaneously under gas turbine relevant conditions on liquid fuels (oil and oil/water). Other measurements in this combustor on gaseous fuels were presented in preceding (parts A and B) and current publications (part C). Mie scattering was used to visualize the liquid phase of oil and water downstream of the nozzle. In order to gain knowledge about the droplet velocity, a Nd:YAG double pulse laser at 532 nm was used for Particle Image Velocimetry (PIV). Additionally the gaseous and liquid phases of oil have been visualized through Planar Laser Induced Fluorescence (PLIF) by excitation of poly-cyclic aromatic hydrocarbons (PAHs) with a laser wavelength of 266 nm. To observe high temperature regions, OH and PAH PLIF was also performed with a low bandwidth at 283 nm from a Nd:YAG pumped dye laser. It was possible to separate the low-intensity OH signal of the hot gas regions from the PAH signal by collecting the different LIF signals simultaneously through a dual camera setup. Instantaneous PAH LIF images of the liquid and gaseous phase were compared with Mie scattering images for a qualitative impression of the evaporation. For this a structural comparison between the liquid phases of both images has been carried out. Results indicate, that the evaporation of most of the liquid fuel takes place near the hot gas region, as a large proportion of droplets are carried far downstream of the nozzle by the high momentum jet.

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 time resolution. To study the feasibility of this technique for full scale gas turbine applications a high momentum confined jet combustor at 8 bar was used. Successful measurements up to 1700 K on a ceramic surface are shown with good accuracy. In the same combustor, temperatures on the combustor quartz walls were measured, which can be used as boundary conditions for numerical simulations. An atmospheric swirl-stabilized flame was used to study transient temperature changes on the bluff body. For this purpose, a high-speed setup (1 kHz) was used to measure the wall temperatures at an operating condition where the flame switches between being attached (M-flame) and being lifted (V-flame) (bistable). The influence of a precessing vortex core (PVC) present during M-flame periods is identified on the bluff body tip, but not at positions further inside the nozzle.

A method for selective, frequency-resolved analysis of spatially distributed, time-coherent data is introduced. It relies on filtering of Fourier-processed signals with periodic structures in frequency-domain. Therefrom extracted information can be analyzed in both, frequency- and time-domain using an inverse transformation ansatz. In the presented paper, the approach is applied to a laboratory scale, twelve nozzle FLOX ® -GT-burner for the investigation of high-frequency thermoacoustic pressure oscillations and limit-cycle mechanisms. The burner is operated at elevated pressure for partially premixed combustion of a hydrogen and natural gas mixture with air. At a certain amount of hydrogen addition to fuel injection, the burner exhibits self-sustained high-frequency thermoacoustic oscillation. This unstable operation is simulated with the fractional step approach SICS (Semi Implicit Characteristic Splitting), a pressure based solver extension of the Finite Volume based research code THETA (Turbulent Heat Release Extension for the TAU Code) for the treatment of weakly compressible flows with combustion. A hybrid LES/URANS simulation delivers time-resolved simulation data of the thermoacoustically unstable operation condition, which is analyzed with the presented SFFFA (Selective Fast Fourier Filtering Approach). Acoustic pressure distribution in the combustion chamber is explicitly resolved and assigned to different characteristic modes by signal decomposition. Furthermore, the SFFFA is used for the analysis of acoustic feedback mechanism by investigating filtered transient heat release, acoustic pressure, velocity and mixture fraction. Coherent structures in flow field and combustion as well as periodic convective processes are resolved and linked to transient acoustic pressure, extensively describing the acoustic feedback of the examined burner configuration.

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.

In this work, results of comprehensive high-pressure tests and numerical simulations of high momentum jet flames in an optically accessible combustion chamber are presented. A generic single nozzle burner was designed as a full-scale representation of one duct of a high temperature FLOX ® gas turbine combustor with a model pilot burner supporting the main nozzle. As an advanced step of the FLOX ® gas turbine combustor development process, tests and simulations of the entire burner system (consisting of a multi nozzle main stage plus a pilot stage) are complemented with this work on an unscaled single nozzle combustor, thus supporting the development and testing of sub concepts and components like the mixing section and dual-fuel injectors. These injectors incorporate a gaseous fuel stage and a spray atomizer for liquid fuels, both separately exchangeable for testing of different fuel placement concepts. The combustor was successfully operated at gas turbine relevant conditions with natural gas including a variation of the Wobbe index, and with light heating oil with and without water admixture. The presented work is the first of two contributions and covers the description of the experimental setup, an overview of the numerical methods, high-pressure test results for different fuels and variations of the operating conditions including exhaust gas measurements and basic optical diagnostic methods, together with CFD results for several cases. The other part will present detailed and focused investigations of few conditions by complex and extensive optical and laser combustion diagnostics.

A liquid fuel combustor based on the FLOX ® burner concept has been developed for application in a Micro Gas Turbine (MGT) Range Extender (REX) for next generation cars. The characterization of this combustor was performed at the High Pressure Optical Test rig (HIPOT) at DLR Stuttgart. The operability limits of the burner were mapped out for full load conditions at 3.5 bars by varying global lambda (λ G ) from 1.25–2.00 and bulk jet velocity (v Bulk ) from 80–140 m/s. Exhaust gas measurements show NO x and CO levels below 5 and 10 ppm respectively (corrected for reference 15% O 2 ) at λ G = 1.89. Optical and laser diagnostic measurement techniques have been employed to characterize the spray flames. The flames at stable burner operation points (BOPs) show a predominantly jet like flame shape irrespective of λ G and v Bulk . Droplets in the size range 2–40 μm have been measured close to the nozzle exit plane. Velocities conditioned on the droplet size show large droplets d > 15 μm transitioning from negative slip velocity at the exit plane to positive slip velocity at downstream location. The positive slip velocities and slow evaporation of large droplets lead to droplets travelling further into the combustion chamber and hence resulting in long flames. A comprehensive data set for the spray characteristic of the new liquid FLOX ® burner is made available.

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 NO x 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.

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.

A large operational envelope is a key requirement for modern gas turbines. Fuel staging is used here to improve the part load performance of an enhanced FLOX ® type combustor. A swirl-stabilized pilot stage is integrated in the FLOX ® burner and the results of high pressure lab-scale experiments at system relevant conditions are presented. The operational envelope of the piloted system could be extended by approximately 10%. Pressure scaling and variations of air preheat temperature and jet velocity describe fundamental characteristics of the piloted system. OH* chemiluminescence imaging is used to investigate flame shapes and the effect of the interacting flames. Emissions and pressure pulsations define limits, and optimum operation conditions of the combustor and show the influence of part load relevant parameters.

An experimental analysis of confined premixed turbulent methane/air and hydrogen/air jet flames is presented. A generic lab scale burner for high-velocity preheated jets equipped with an optical combustion chamber was designed and set up. The size and operating conditions were configured to enable flame stabilization by recirculation of hot combustion products. The geometry of the rectangular confinement and an off-center positioning of the jet nozzle were chosen to resemble one burner nozzle of a FLOX ® -based combustor. The off-center jet arrangement caused the formation of a pronounced lateral recirculation zone similar to the one in previously investigated FLOX ® -combustors [1, 2]. The analysis was accomplished by different laser measurement techniques. Flame structures were visualized by OH* chemiluminescence imaging and planar laser-induced fluorescence of the OH radical. Laser Raman scattering was used to determine concentrations of the major species and the temperature. Velocity fields were measured with particle image velocimetry. Results of measurements in two confined jet flames are shown. The mixing of fresh gas with recirculating combustion products and the stabilization of the methane flame are discussed in detail. The presented findings deliver important information for the understanding of confined jet flames operated with different fuels. The obtained data sets can be used for the validation of numerical simulations as well.

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 (NO x 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.

In this contribution, an overview of the progress in the design of an enhanced FLOX ® burner is given. A fuel flexible 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 is 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 (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.