One of the most promising methods for reducing NOx emissions of jet engines is the lean combustion process. In order to realize this concept the percentage of air flowing through the combustor dome has to be drastically increased. This requirement leads to nozzles with high effective area and to high mean velocities in the primary zone of the combustor chamber. The investigation of the lean blow out limit for those nozzles is of main interest for the design of lean combustor technology. It is reported on investigation of a kerosene-fueled, swirl stabilized flame at atmospheric conditions. Two lean operation conditions are investigated, one in stable regime and the other very close to the weak extinction limit. It has been determined, that the flame shape changes when shifted from the stable regime to the other one close to the weak extinction limit (also referred to further as LBO — lean blowout). Since all field measurement schemes are similar, the gained data can be associated and conclusions regarding the flame stabilization at lean conditions can be drawn. The velocity data yields information about the topology of both isothermal and reacting flow fields in the combustion chamber. The internal recirculation and the corner recirculation zones can be well distinguished, because it can be measured directly in the nozzle exit plane. The comparison of the experimental data at stable and near LBO conditions shows the importance of inner and outer recirculation zones for the stabilization process. Furthermore, a comparison with a gaseous fuel nozzle will exhibit the differences between liquid and gaseous fuel combustion.

One of the most promising methods for reducing NO x emissions of jet engines is the lean combustion process. For realization of this concept the percentage of air flowing through the combustor dome has to be drastically increased, which implies high volume fluxes in the primary zone of the combustion chamber and represents a substantial challenge in regard to the flame stabilization. Swirl motion is thus applied to the air flux by the swirl generator and decisively contributes to the flame stabilization. The current paper reviews an atmospheric investigation of a burner configuration in regard to the weak extinction limit, comprising a confined nonpremixed swirl-stabilized flame. The burner can be supplied with either kerosene or after a small adaption with natural gas (methane). Therefore, a comparison of a kerosene-fuelled flame (spray flame) to a natural gas fuelled one (methane flame) can be performed. Both are realized by almost identical burner configuration and at identical conditions. The main idea of this work is to align the stability characteristics of both flames by means of similarity. However, fundamental differences regarding the flame structures of the flames are detected through in-flame measurements. This determines the limits of the current approach and motivates an appropriate choice of flame modeling. Flow Turbulence Combust (2012) 89:73-95 A Area [m 2 ] AFR Air-Fuel Ratio [-] α Thermal diffusivity C Reaction progress [-] Da Damköhler-number [-] Da t turbulent Damköhler-number [-] D Diameter of comb. chamber [m] D/d Flow expansion ratio [-] d Nozzle throat diameter [m] D Angular momentum [kg·m 2 /s] IRZ Inner Recirculation Zonė I Axial momentum [kg·m/s] k turbulent kinetic energy, mass specific Ka Karlowitz-number [-] L t integral length scale [m] L length of contact zone [m] LBO Lean blowout LDA Laser Doppler Anemometrẏ M Mass flow rate [kg/s] ORZ Outer Recirculation Zone PDA Particle Dynamics Analysis PERM Partially Evaporated, Rapidly Mixed Pe Peclet-number [-] p Pressure in the chamber [bar] r radial position [m] R 0 Nozzle exit radius [m] Re t turbulent Reynolds-number S Swirl Number [-] S t turbulent flame velocity [m/s] S lam laminar flame velocity [m/s] SMD Sauter Mean Diameter [m] T Temperature [K] U 0 Nozzle exit volumetric velocity at referenced condition [m/s] U lim Nozzle exit volumetric velocity at LBO [m/s] UHC Unburned hydrocarbons [-] u mean axial velocity [m/s] u', v', w' velocity fluctuations [m/s] u 2 , v 2 , w 2 normal Reynolds stresses [m 2 /s 2 ] x Axial position [m] p/p normalized pressure loss [%] 3D Three dimensional Flow Turbulence Combust (2012) 89:73-95 75 2D Two dimensional χ, Mass flow rate split [-] φ Equivalence ratio [-] ν kinematic viscosity [m 2 /s] λ 1/φ τ time scale [s]

We report on the experimental and numerical investigation of swirl induced self-excited instabilities in the form of precessing helical structures at the vicinity of an Airblast Atomiser. Within the scope of this work, the increase of the knowledge of the fundamental factors governing the precessing vortex core phenomenon (PVC) (Gupta et al. 1984) by applying dual air-flow Airblast nozzles is aimed. This study concentrates on the experimental investigation of the impact of important parameters of a combustor system on the performance characteristics of this instability. In particular, in terms of this work the properties of the PVC are determined by applying two different Airblast Atomisers, one of them producing an attached swirl flame, and another producing a lifted swirl flame. Measurements are also performed for a confined and a non confined flame, aiming to determine the impact of the confining duct on the performance of the PVC. In order to gain some further knowledge, regarding the impact of this aerodynamic instability on combustion of gaseous fuel, the features of PVC are experimentally identified under reacting conditions, by employing a laser light sheet (LLS) measurement technique. By applying the LLS measurement technique, further investigation on the nature of the PVC is also attempted. The power spectral density (PSD) function of the flow field was determined on the basis of raw data provided by 3D Laser Doppler Anemometry (3D-LDA). In order to validate the measurement technique as well as the nature of the instability, the planar Mie-Scattering of the flow was evaluated by employing a high speed camera performing at 12 kHz. The precessing character of the flow was also confirmed by means of a numerical simulation using the 3D Reynolds Stress Model (3D-RSM). The results of the numerical investigation provided some useful information concerning the onset of the instability within the primary swirler as well as its size and amplitude. According to this analysis, a high 512 Flow Turbulence Combust (2009) 83:511-533 frequency instability was confirmed within a region of about one burner diameter downstream of the burner exit. Finally an evaluation of the (LDA) method in terms of providing accurate (PSD) was performed for the case of a swirl flow field.Keywords Precessing vortex core (PVC) · Swirl flows · Airblast Atomiser · Single helix structure · High frequency instability · Self excited aerodynamic instability Nomenclature D Diameter [m] F Frequency [Hz] k Turbulence kinetic energy [m 2 /s 2 ] L Length [m] · n Mean LDA data rate [Hz] R Radius [m] R Autocorrelation function [-] Re Reynolds-no. [-] S Swirl number [-] Sr Strouhal number [-] St Stokes number [-] T Temperature [K] Tu Turbulence [%] U Axial velocity component [m/s] V Radial velocity component [m/s] · V Volume flow [m 3 /h] W Tangential velocity component [m/s] X Radial coordinate [m] Z Axial coordinate [m]Greek Symbols ρ Density [kg/m 3 ] Equivalence ratio [-]

The objective was to study the effect of equivalence ratio of secondary stage combustible mixture injected into the cross flow stream of vitiated air in a two staged combustion system on the characteristics of the secondary stage combustion zone. The primary cylindrical combustor equipped with low swirl air blast nozzle operating with kerosene generates vitiated air. A methane injector was flush mounted to the inner surface of the secondary combustor. It was used to inject the premixed methane-air mixtures perpendicular into the crossflow of vitiated air. An optical, double shell, secondary combustor with three optical windows on its outer shell was used to image the secondary stage flames. The inner shell was a quadratic fused quartz tube which acts as a thermal barrier and the outer thick quartz windows mounted in the quadratic stainless steel chamber withholds the pressure. Chemiluminescence imaging technique equipped with ICCD camera was used to image the OH* emissions of the secondary stage flame. The vitiated air was generated at 2 bar and 1700 K. The velocity of the vitiated air in the secondary combustor was 57 m/s. A premixed methane air mixture was injected into the cross flow stream of vitiated air. The momentum flux ratio between the jet and the vitiated air was maintained at 1.4. The equivalence ratio of the premixed methane-air mixture was varied from 0.5 to 1.0. As the equivalence ratio of the secondary stage combustible mixture moves towards stoichiometric condition, the secondary stage combustion zone becomes compact and also the distance between the burner and the combustion zone decreases. The turbulent flame stabilized in the secondary combustor exhibited large scale structures and other unsteady phenomena that require time-resolved computational methods. Large eddy simulations (LES) are well suited to the calculation of such complex flows. The flame was embedded in a strong turbulent flow where auto-ignition and quenching are important, which poses a significant challenge for the reaction modeling. The presumed JPDF turbulent reaction model, which has been proven to be a reliable model for these challenging conditions, was successfully coupled with the LES simulation. The qualitative agreement between the results of simulations and measurements was quite satisfactory.