In order to meet the higher requirements for clean combustion technology in aircraft engine applications and thus reduce harmful emissions, especially nitrogen oxide emissions, the major jet engine manufacturers are developing lean premixed prevaporized (LPP) combustors that operate at very high pressure. In this context, thermoacoustic instabilities may occur within the combustion chamber. The unsteady heat released by the flame generates pressure waves, which are coupled to the inlet air velocity by a feedback loop. This loop amplifies the instabilities of the inlet air velocity, which in turn influences the atomization process. Since the atomization process at the airblast atomizers of most jet engine combustors determines critical operating characteristics such as air-to-fuel ratio (AFR), flame stability, or NO x emissions, predicting the performance of this process under unsteady conditions has a significant value. The present experimental study focuses on the influence of oscillating airflows on the spray characteristics at the airblast atomization process. The experimental setup was based on a two-dimensional prefilmer where a water film flow was introduced on one surface. The airflow was excited by a siren, whereby an excitation frequency near 94 Hz was investigated. The airflow oscillation under this excitation frequency was characterized using a Constant Temperature Anemometer (CTA), while the generated spray was investigated with a Phase Doppler Anemometry (PDA) system. The spray was investigated in a variety of positions along the radial axis, providing spatial information, apart from temporal. The characterization of the spray via PDA includes a two-component droplet velocity detection and diameter measurement, while the spray mass flux for each measured position was also calculated. The acquired data were phase averaged via an in-house developed processing algorithm, while through a statistical analysis the confidence intervals of the calculations were included. The excitation frequency strongly influenced all spray characteristics, namely, the Sauter Mean Diameter (SMD), the droplet velocities, the mass flux, as well as the local air-to-liquid ratio (ALR). Depending on the phase angle, the size distribution of the spray changes, explaining the observed oscillating behavior of the spray characteristics.

The main objective of this research is to assess an innovative, low nitrogen oxides emission combustor concept, which has the potential to achieve the long term European emissions goals for aircraft engines. Lean lifted spray flames and their very low nitrogen oxides emissions are combined with an inclination of burners in annular combustor leading to a more compact combustor with superior stability range. The presented combustor concept was developed in the frame of the European research project CHAIRLIFT (Compact Helical Arranged combustoRs with lean LIFTed flames). CHAIRLIFT combustor concept is based on “low swirl” lean lifted spray flames, which features a high degree of premixing and consequently significantly reduced nitrogen oxides emissions and flashback risk compared to conventional swirl stabilized flames. In the CHAIRLIFT combustor concept, the lifted flames are combined with Short Helical Combustors arrangement to attain stable combustion by tilting the axis of the flames relative to the axis of the turbine to enhance the interaction of adjacent flames in a circumferential direction. A series of experimental tests were conducted at a multi-burner array test rig consisting of up to five modular burners at different burner inclination angles (0° and 45°), equivalence ratios, and relative air pressure drop at ambient conditions. For all investigated configurations, a remarkable high lean blow out for non-piloted burners (ϕ LBO = 0.29–0.37), was measured. The multi-burner configurations were observed having a superior stability range in contrast to the typical decrease in stability from single to high swirl multi-burner. The unwanted flow deflection of highly swirled flames in Short Helical Combustors arrangement, could be avoided with the investigated low swirl lifted flames. Moreover, the flame chemiluminescence (OH*) measurements were used to provide a qualitative characterization of the flame topology. Complementary numerical investigations were carried out using different numbers of burners to evaluate the effect of boundary conditions.

To ensure compliance with stricter regulations on exhaust gas emissions, new industrial burner concepts are being investigated. One of these concepts is the matrix burner, consisting of an array of premixed, non-swirling jet flames. For the design of such burners, the prediction of fundamental burner properties is mandatory. One of these essential quantities is the lean blowout limit (LBO), which has already been investigated experimentally. This study investigates the possibility of numerical LBO prediction using a tabulated chemistry approach in combination with Large-Eddy-Simulation turbulence modeling. In contrast to conventional swirl burners, the numerical description of blowout events of multi jet flames has not yet been studied in detail. Lean blowout simulations have therefore been conducted for multiple nozzle variants, varying in their diameter and global dump ratio for a variety of operating conditions, showing their general applicability. A procedure to induce LBO is introduced where a stepwise increase in total mass flow is applied. LBO is determined based on the temporal progress of the mean reaction rate. A comparison with measurements shows good agreement and demonstrates that the procedure developed here is an efficient way to predict LBO values. Further investigations focused on the flame behavior when approaching LBO. The flame shape shows a drastic change from single jet flames (stable conditions) to a joint conical flame approaching LBO, which increases in length for increasing inlet velocity, showing the importance of jet interaction at LBO.

The characterization of the two-phase kerosene/air flow near the nozzle of an aero engine combustor is important in order to understand the combustion characteristics of the burner. Typically, Particle Image Velocimetry (PIV) or Laser Doppler Velocimetry (LDV) is used to measure velocities inside aero engine combustors. However, these measurement techniques rely on tracer particles to visualize the flow field and are usually only able to measure the velocity field of one phase at a time. In the case of PIV measurements both the flow tracers and the kerosene droplets scatter the laser light and thus, appear on the PIV recordings. Depending on droplet size and flow velocity, these kerosene droplets do not necessarily follow the airflow leading to errors in the derived velocity field. This work presents a method on how to separate kerosene droplets from flow tracers depending on their optical characteristics in the PIV recording. This phase separation enables the independent measurement of the flow fields of both the gaseous and liquid phase at the same time using a standard PIV setup. The method is demonstrated on a laboratory scale aero engine combustor operated at atmospheric conditions. The test rig features liquid kerosene combustion with realistic inlet temperatures and pressure drop as well as good access for optical measurement techniques. The phases are separated by filtering the images with noise reduction filters for suppressing the signal of the flow tracers, and edge detection filters to detect the kerosene droplets. The detected kerosene droplets are removed from the PIV pictures and the pictures are evaluated using standard PIV cross-correlation. Afterwards the liquid phase images are evaluated using Particle Tracking Velocimetry (PTV). This phase separation can lead to errors in the derived velocity fields because of incorrect and incomplete particle detection or due to errors in the cross correlation at the edges of detected particles. These errors in the phase separation are quantified by evaluating artificial two-phase flow PIV pictures with similar optical properties to the actual two-phase PIV pictures, and comparing the derived velocity fields to the results calculated using the original, unaltered pictures. The obtained results show, that in the setup under investigation, gaseous and liquid phase can have significantly different flow fields with kerosene droplets moving in the opposite direction of the recirculating airflow. The influence of essential parameters like seeding and spray density are discussed and at positions with a sufficient data rate, the instantaneous slip velocity between droplets and gaseous flow is calculated. Generally, the presented method appears to be suitable for studying combustion with liquid kerosene injection.

The ability to re-ignite at high altitude after a flameout event is critical for flight safety. One reason that makes the relight process of the engine difficult is the low temperature and pressure, which leads to poor atomization, low degree of evaporation and slow reaction rate of the vaporized fuel. For this research work a rectangular, one sector RQL combustion chamber was utilized for experimental investigations at high altitude conditions. The design of the chamber is modular so that experiments for two configurations, i.e. without and with effusion cooling holes can be conducted. The fuel injection and the ignition system are representative of the ones used in commercial aviation. The investigations were performed in the frame of the European research project SOPRANO at the ISCAR rig. The ISCAR rig is capable of generating low pressure and temperature conditions for flowing kerosene-air mixtures. The investigation focuses on the characterization of the ignition process, in terms of probability, minimum fuel to air ratio (FAR) and ignition timing for a successful ignition event. In addition, the unsteady flame kernel generation and propagation were analyzed by high-speed imaging recording. An in-house image processing code was developed in order to derive quantitative spatial information of the flame and overall trends among ignition sequences for the same or different operating conditions. In order to achieve comparability between the investigated configurations (liners without and with effusion cooling), the pressure drop across the nozzle and the liners was the same depending on the operating condition. Results show that both pressure and temperature affect the ignition process, with the former being the dominant parameter in the investigated conditions. In both configurations, the minimum FAR increased as long as the conditions in the chamber became more adverse, indicating that at high altitude low-pressure situations, the performance of the airblast atomizer deteriorated causing poor ignition. This is overcome by creating a richer fuel-air mixture in the primary zone. Finally, the air injected through the effusion cooling holes near the spark seems to create favorable conditions for the ignition process.

RQL (rich burn, quick quench, lean burn) combustion chambers are common in modern aero engines due to their low NOx emissions and good stability. The rich primary zone leads to lower flame temperatures and in combination with the lack of oxygen, the NOx production is low. The mixing of the secondary air must be quick in order to avoid stoichiometric conditions and at the same time must ensure the oxidation of the soot produced in the fuel rich primary zone to keep soot emissions to a minimum. However, the design of such a combustion chamber is complicated due to the complex interaction between the swirling primary flow and the jets of the secondary airflow. In this paper, we present a new test rig, which was designed to study combustion processes inside RQL combustion chambers at atmospheric conditions. The test rig features liquid kerosene combustion and a realistic quenching zone as well as good access for optical and conventional measurement techniques. For realistic engine like conditions the combustion air is preheated to 600 K and the fuel–air equivalence ratio in the primary combustion zone is set to be between Φ = 1.66 and Φ = 1.25, resulting in an overall thermal power between 80 kW and 110 kW. To get insights into the complex flow field inside the combustion chamber unsteady RANS simulations of both the reacting and the non-reacting case were performed using OpenFOAM. The turbulent flow field was modeled using the k-ω-SST model and the combustion was simulated using the Partially Stirred Reactor model. The experimental investigations showed two stable flame types for the same operating conditions with considerable differences in the visible flame structure and soot radiation. The flow field of both of these flame types were measured using a 1.5 kHz 2D PIV System. The numerical simulations showed good overall agreement with the experimental results but could not represent the change in flame type. In order to understand the underlying effects of the flame change the OH* chemiluminescence was recorded and the two-phase flow near the nozzle exit was investigated. This showed that the change in flame structure might arise due to spray dispersion of the pilot fuel nozzle and the recirculation of the secondary air into the primary zone.

In order to extend the operation regime of existing gas turbine combustion systems to lower the minimum loads, the applicability of matrix burners (arrays of jet flames) as an alternative to conventional swirl stabilized burners has been considered. In comparison to well-studied single jet flame systems, the effects of geometry and thermodynamic parameters on characteristics of matrix burner systems have not been studied in detail. Information, which is essential for design processes e.g. scaling of matrix burners, is not yet available in public domain. This work involves a systematic investigation of a matrix burner system operating at highly turbulent flow conditions (Reynolds Number ≈ 20000) prevailing in gas turbine combustion systems. In order to understand the effects of geometrical scaling, three variants of jet diameter have been investigated. A detailed test campaign including lean blow out limits detection, velocity field measurement and hydroxyl radical (OH*) chemiluminescence recording has been conducted. Influence of variation in stoichiometry and exit velocity of fuel-air mixture has been captured. The results show that it is possible to generalize the scaling of the matrix burner using the well-known Peclet criterion.

The research work of the present study is focused on the numerical simulation of primary breakup of high-viscosity non-Newtonian fluids. For the experimental investigation of fluid properties such as viscosity, surface tension and flow behaviour on the jet breakup an external mixing twin-fluid nozzle is used, as investigated in a previous study [Müller et al., ASME Turbo Expo 2016, GT2016-56371]. To describe the disintegration process of the fluids, characteristic features like liquid jet morphology, breakup length, breakup frequency and spray angle are evaluated. Furthermore, the primary breakup of slurries is simulated without discretizing the particles as a third phase, which heavily reduced the computational effort. Instead, the physical properties (density, viscosity) of the liquid phase take the influence of the particles into account. The primary breakup was investigated using the open source CFD software OpenFOAM. To gather the morphology of the primary breakup and the flow field characteristics compressible large eddy simulations (LES) were performed and the movement of the gas-liquid interface was captured by means of the Volume of Fluid-Method (VOF). The conducted simulations showed good agreement with experimental results with respect to the characteristic features (e.g. breakup length) and the significant influence of viscosity and surface tension on the primary breakup. It is reasonably justified that the used OpenFOAM code and VOF is sufficient to simulate the primary breakup of particle laden liquids without discretizing particles as a third phase. Moreover, those findings contribute to a better understanding of the physics responsible of the breakup of high-viscosity liquid jets and as well to create an experimentally validated CFD based tool for future burner development and optimization.

Detail investigations on the primary breakup of high-viscosity liquids using external-mixing twin-fluid nozzles at increased system pressure are scarce. Therefore, the research work of the present study is focused on the investigation of pressure influence (1 - 11 bar (abs)) on the primary breakup by numerical simulation based on a previously studied nozzle [Müller et al., ASME Turbo Expo 2016, GT2016-56371]. The pressure influence was investigated for two liquids applying a wide range of viscosities (100 mPa s; 400 mPa s) and two atomizing air velocities (58 m/s; 74 m/s). To describe the disintegration process of the fluids, characteristic features like liquid jet morphology, breakup length and breakup frequency were evaluated. The primary breakup was investigated using the open source CFD software OpenFOAM. To gather the morphology of the primary breakup and the flow field characteristics compressible large eddy simulations (LES) were performed and the movement of the gas-liquid interface was captured by means of the Volume of Fluid-Method (VOF). The conducted simulations showed good agreement with experimental results with respect to the characteristic features (e.g. morphology and breakup length) and revealed a decrease of the breakup length with increasing ambient pressure for a constant liquid mass flow and atomizing air velocity. Moreover, those findings will contribute to a better understanding of the physics of the breakup of high-viscosity liquid jets and as well to create an experimentally validated CFD based tool for future burner development and optimization.

The research work of the present study is focused on the influence of design parameters of twin-fluid nozzles used for the atomization of high-viscosity fuels with respect to the primary breakup of the liquid jet. Two external mixing twin-fluid nozzles, which have already been investigated in previous studies [1, 2], were chosen as basic design. Based on the previous findings the web thickness between fuel and oxidizer supply was varied. In addition both designs were extended by a channel for internal mixing of gas and liquid with a length to diameter ratio of one. Moreover one of the basic nozzles was scaled by decrease of the effective areas in a way that momentum flux ratio as well as gas to liquid mass flow ratio was kept constant. The newly designed atomizers were subsequently investigated with regard to the influence of the changes upon the primary jet breakup using CFD simulations. The numerical simulations were conducted by means of the open source package OpenFOAM. The Volume of Fluid method was used for the determination of the gas-liquid interface. These simulations were then compared with experimentally validated simulations of the basic nozzle designs with regard to the breakup morphology of the jet and the mode of the primary surface instability. In addition, the liquid structure was examined by comparison of breakup length and frequency. The results of these simulations showed that small changes in the atomizer design heavily influence the primary breakup, which in turn influences the overall performance of the atomizer (e.g. SMD). Moreover, these findings will contribute to a better understanding of the physics of the breakup of high-viscosity liquid jets and as well to create an experimentally validated CFD based tool for future burner development and optimization.

The present study focuses on the atomization behaviour of liquids in external mixing twin fluid nozzles and investigates a wide range of viscosities as well as different nozzle geometries at a gas to liquid ratio (GLR) typically used in entrained flow gasification. In a first stage experiments were performed using water and water-glycerol-mixtures as Newtonian model fuels with liquid viscosity up to 400 mPa s. Jet breakup was investigated qualitatively using a high speed camera as well as using a PIV and LDA-System for detailed quantitative investigation of the flow field. Two different primary instabilities flapping and pulsating mode were detected which are dependent on operating conditions of the nozzle (e.g. GLR) and rheological properties of the liquid phase (e.g. liquid viscosity) as well as nozzle geometry. For better interpretation of the phenomena occurring during jet breakup a frequency-analysis of the primary instabilities was performed using the pictures of the high speed camera. In addition, compressible large eddy simulations (LES) were preformed to describe the experimental observations and to capture the morphology of the primary breakup as well as the important flow field characteristics. The numerical simulations were conducted by means of the open source CFD software OpenFOAM. A Volume of Fluid (VOF) approach was used to track the unsteady evolution and breakup of the liquid jet. Comparison of experimental and numerical results shows a good agreement concerning breakup frequency, velocity fields and morphology. The breakup frequency varied in a range of 430 to 757 Hz depending on operating condition and nozzle geometry. Based on these results a more detailed understanding of the physics leading to liquid jet breakup and finally atomization process will be available.

The flame speed is a central element not only in theoretical combustion and basic research but also a key parameter for several combustion models. Various methods to define and measure laminar flame speed have been applied. One method that is being relevant to determine flame speed and flame stretch (quantified by Markstein numbers) under high pressure and high temperature conditions is the bomb method, where an unsteady spherical expanding flame is investigated in a closed vessel. Especially for higher pressures, instabilities occur in the flame front of spherically expanding flames, due to the decreased flame thickness and diffusion processes. These so called cellular structures increase the flame surface and therefore the laminar flame speed cannot be determined in the usual manner. As high pressures are common under engine conditions, there is a need to be still able to determine the flame speed within these pressure ranges. By exposing the flame to a turbulent flow field, the vortex interactions are mainly responsible for the deformation of the flame surface. This fact can be used to deduce the laminar flame speed from a turbulent flame. In the present work, a turbulent flame speed model to determine unstretched laminar flame speed and Markstein numbers is introduced and validated. For this purpose the flame is exposed to a well-known turbulent flow field that is nearly homogenous and isotropic. By estimating the flame surface, the laminar flame speed can be calculated, using a turbulent stretch model as well as the flamelet assumption in an implicit approach. A high speed 2D laser imaging technique is used to capture the flame propagation. The most significant issue of this method is to correctly identify the surface and the volume of the flame, because solely a cross section of the flame surface can be visualized in the laser sheet. In case of spherical flames, the radius of the cross section corresponds to the flame surface and volume whereas a single radius is not sufficient to quantify a turbulent flame. The idea is to use the circumference of the cross section in a power law approach to quantify the cellular structures as a mean. The implicit model has been validated for methane and hydrogen flame speed at various equivalence ratios at atmospheric and elevated pressure. The results are in a good agreement with laminar determined flame speed and also the Markstein number is obtained qualitatively correct. Although the flame surface and volume cannot be determined directly by 2D measurement techniques, a model has been developed to determine laminar unstretched flame speed and Markstein numbers by investigating unsteady propagating flames in a turbulent flow field.

One commonly used injection system in gas turbines is the prefilming airblast atomizer which creates a fuel film that should disintegrate at the end of the prefilmer. But under certain conditions droplets are separated from the film before reaching the end of the prefilmer. This phenomenon is called surface stripping. For the investigation of those conditions both an experimental and numerical setup are defined. The experiment measures the film thickness on the prefilmer with the help of the planar laser-induced fluorescence technique. For the numerical investigation the volume of fluid method together with a large eddy simulation of OpenFOAM ® 2.1.1 is used. After the validation of the numerical model with the experimental data, the dependency of the mean film thickness from the relative velocity of air and water is investigated. It is shown that first the momentum flux ratio is not sufficient in describing the behavior of the film on the prefilmer and second a critical relative velocity has to be exceeded so that surface stripping can take place. Three flow regimes are observed by increasing the relative velocity: smooth film surface, wavy film surface and surface stripping. Due to the surface stripping the transferred fuel mass into the gas phase is increased rapidly.

For the prediction of thermoacoustic instabilities in gas turbines, a compressible, unsteady LES approach was validated for 1D, lab-scale and technical-scale test cases. The simulations were performed with a novel combustion model, which relies on a combination of tabulated chemistry and flame thickening. A 1D case was used for fundamental verification and to quantify the mesh resolution dependent error. The verification demonstrated sufficiently accurate predictions. In a second step an elevated pressure lab-scale flame was considered to calibrate the model parameter of the turbulence chemistry interaction at relevant conditions. For this case CO 2 field data measured with the Raman scattering method is available. Finally, a test rig configuration of a can type combustor was investigated. The comparison between experiment and simulation was performed in terms of thermoacoustic pressure amplitudes at stable and unstable operating conditions. The LES combustion model relies on an artificial thickened flame approach to ensure that the flame propagation speed is reproduced with tolerable error. Tabulated chemistry provides the source term for CO 2 (governing species) as a function of the mixture fraction and the CO 2 concentration based on premixed, laminar flamelets. The model distinguishes between inherent thickening due to sub grid scale turbulence and explicit laminar thickening. This novel thickening approach is presented for the first time. The presented approach was able to predict the thermoacoustic stability behavior of a gas turbine combustion system correctly.

This paper presents the evaluation of the turbulence radiation interaction (TRI), which is usually neglected in Reynolds averaged simulations (RAS) of computational fluid dynamics (CFD) problems. As a result of neglecting this interaction, it is obtained that the radiative heat loss in the flame and the radiative thermal load on the combustor is too low. The TRI is evaluated using the optically thin fluctuation approximation (OTFA) in coupled (radiation and CFD) calculations in a highly turbulent lean premixed methane flame. The validity of the OTFA is investigated in terms of the resulting change in temperature and reaction progress. Two common absorption-emission models are used in Monte-Carlo radiation calculations: the weighted-sum-of-grey-gases (WSGG) and the statistical narrow band correlated-absorption coefficient (SNB-CK). Both models show that the radiative thermal load on walls is increased up to 27 % due to TRI. In contrast, the flame shape is not affected since local temperatures changes are less than 25 K.

Due to strict emission legislation, the trend in the development of aero-engine gas turbine combustion is heading towards lean burning approaches. Lean combustion reduces the combustion temperatures and therefore also the nitrogen oxides emissions. Unfortunately, lean combustion suffers from instabilities and the operation close to the point of lean blowout increases the risk of imminent blowoff. Active stability control is therefore inevitable. The objective of this work is to evaluate the signal obtained from an ion current measurement technique to enable combustion control for aircraft propulsion applications in the near future. In the past ion current measurements have been used in several studies as flame turbulence analyzer and to detect the reaction rate. However, investigations in lean burning and swirl stabilized airblast injection combustors for future propulsion concepts are rare. The signal obtained from an ion current detector inside a combustor depends strongly on the measurement position. In this experimental investigation field measurements at atmospheric conditions of the ion concentration in a tubular combustor with a sampling rate of 8 kHz are compared with 4 kHz time resolved temperature and OH* chemiluminescence measurements in order to determine the position of the reaction inside the combustor. Variations were performed of the air to fuel ratio (AFR), the air preheating temperature and the pressure drop across the injection system to clarify the interpretation of the ion current signal. The results indicate a strong dependence of the ion current signal on the AFR and that the technique has distinct advantages compared to OH* chemiluminescence measurements: The measurement equipment is comparable non-expensive and the results reveal that the reaction rate is measured directly and are not interpreted from a 3D image. A transition in flame shape from a compact to a tornado flame can be clearly identified with the applied probe. Furthermore, regions with high temperature fluctuations do not necessarily reveal the reaction zone in a recirculating flow field.

In recent years Avio Aero and the Karlsruhe Institute of Technology (KIT) have significantly enhanced their available experimental technology level in order to consolidate and deepen the investigation of some critical phenomena in aero-engine gas turbine combustion. A key parameter in the development of a lean-burn combustor is the design of the ignition system. The increase in the amount of air that flows through the primary zone to approximately 60% of the overall air proves to be beneficial for emissions reduction. In return it causes shorter residence times in the primary zone and makes the flame kernel generation and propagation a real challenge. With the scope to enhance the fundamental understanding of the ignition process at altitude conditions, the ISCRA rig ( I gnition in S ubatmospheric C onditions – R ig for A ltitude Relight Investigation) has been designed and manufactured at the KIT. The design of the rig allows the generation of altitude conditions at variable flow velocities and turbulence characteristics. An optical access for ignition recording by a CCD camera is also provided. The paper presents the results of a fundamental investigation at the KIT using a generic setup. It consists of a pressure atomizer with known atomization properties mounted in a test rig which allows the variation of several parameters that influence ignition, namely: air pressure; air temperature; velocity; Fuel-Air-Ratio (FAR); Sauter Mean Diameter (SMD); spark energy; turbulence intensity and length scale. Two grey spots that exist in the knowledge map of the ignition process are addressed in this paper. These are data on kerosene spray ignition at temperatures below 293 K, where data is scarce and the relationship between flame kernel generation and propagation, which has not been broadly investigated nor is well understood. The ignition probabilities of kerosene sprays as a function of ignition energy in the temperature range from 253 K to 293 K, pressure of 0.7 bar, flow velocities up to 6 m/s at different turbulence intensities and length scales are measured. Furthermore, the influence of the above mentioned parameters on the kernel generation and propagation is presented and discussed. Results show that the energy required for flame kernel propagation is higher than for flame kernel generation. This difference increases at lower temperature and higher turbulence intensity. The paper gives an insight in magnitude of this difference in energy and compares the tendencies with an earlier model. The absolute values need to and will be corrected by future in situ measurements.

Lifting a flame from the flow generating nozzle to some distance apart has a wide variety of effects on the properties of the resulting combustion phenomenon. The reason of this influence is the generation of a non-reacting flow domain where mixing takes place prior to the combustion reaction. It is obvious that the quality of premixing that can be achieved strongly depends on the time that is given to flow and the intensity of the turbulence that is mixing fuel and air. The most important parameter that is characterizing this time is the size of the premixing zone quantified by the so called lift-off height (LOH). Additionally, when employing liquid fuel the lift-off of a flame provides time to achieve better pre-evaporation of the fuel. As a consequence, better mixing of fuel and air helps to avoid high temperature regions that may be a result from an inhomogeneous equivalence ratio distribution. From safety considerations a major advantage of this method compared to the application of a premixing duct is that the risk of hardware destruction by flame flash back can be eliminated. The current work extents the knowledge on lifted flames by the investigation of flames that are generated with an airblast atomizing nozzle that was designed to resemble systems close to application. Lifting of the flame is achieved applying a combination of swirling and non-swirling inflow ducts. A wide range of operating conditions as well as gaseous and liquid fuels are used to investigate their influence on the lift-off height. The lift-off height and location of the reaction zone was determined by means of chemiluminescence of OH* and it is shown, that the impact of pressure drop and preheating temperature on the LOH is different for gaseous and liquid fuels.

Results of an ongoing collaboration between the engine manufacturer MTU and the German aerospace research center DLR on the NOx reduction potential of conventional combustors are reported. A program comprising optical sector combustor measurements at 1, 6 and 15 bars and CFD calculations is carried out. The aims are to gather information in the combustor at realistic operating conditions, to understand the differences between the sector flow field and data from tubular combustors, to verify the used CFD and to discover the benefits and limitations of the applied optical diagnostics. Selected results of measurement and calculation of the isothermal flow and of measurements at 6 bars and 700 K at a rich-lean and overall lean AFR are reported. The used measurement techniques were LDA, PDA, Mie scattering on kerosene, Quantitative Light Scattering, OH* Chemiluminescence and LIF on OH. The measurements were able to confirm the intended quick and homogeneous mixing of the three staggered rows of secondary air jets.