Search

N anticipation of increased regulation of soot emissions from aviation engines there has been considerable effort in recent years to better understand, model and predict soot formation in gas turbine combustors. Experimental studies in this effort generally fall into one of two categories: 1) detailed studies of chemical kinetics and mechanisms of specific sub-processes of soot formation, agglomeration and oxidation in simple, well-characterized test flames (e.g., laminar Bunsen or diffusion flames) and 2) studies focusing on system-level parameters such as global soot emissions vs fuel grade and combustor pressure at the expense of detailed understanding of specific subprocesses. Although both categories have specific strengths, the understanding yielded by each is of limited use for the purpose of predictive modelling of gas turbine combustors. The German Aerospace Center (Deutsches Zentrum fur Luftund Raumfahrt, DLR) has led an effort in recent years to bridge the gap between fundamental scientific studies of soot formation and system-level characterization of gas turbine combustors. This effort has focused on acquiring detailed measurements of a series of soot-generating flames in a generic, swirl-stabilized combustor at elevated pressure. These flames are designed to capture much of the complexity of a modern, swirl-stabilized gas turbine combustor, while maintaining excellent optical access for pointand planar laser measurement techniques. As part of this effort, Lammel et al. (2007) applied laser-induced incandescence (LII) and coherent anti-Stokes Raman scattering (CARS) spectroscopy to quantify the effects of pressure, equivalence ratio and secondary oxidation air on mean soot distribution in swirl-stabilized, sooting ethylene-air flames at pressures up to 9 bars and thermal loads up to 45 kW. Their results showed the highest soot concentrations were to be found in the lower part of the inner recirculation zone (IRZ) of the combustor, where residence times and local fluid temperatures are high. The combustor used by Lammel et al. (2007) was based on the well-characterized DLR Dual-swirl Gas Turbine Model Combustor or “DS-burner” [2,3] applied LII, CARS and particle image velocimetry (PIV) in a similar combustor at atmospheric pressure and found that injection of secondary air downstream of the flame zone results in drastic changes to soot distribution in the combustor. Geigle et al. (2014) extended these measurements (in a slightly modified burner geometry) to include flames at pressures up to 5 bars and observed that, in addition to pressure and global equivalence ratio, soot concentration and distribution were sensitive to the ratio of air flow supplied to the innerand outer swirl nozzles. Geigle et al. (2015) applied simultaneous LII and planar laser-induced fluorescence of OH (OH-PLIF) to study the spatial correlation of soot and high temperature combustion products in these flames. Their results showed the addition of air downstream of the main flame zone resulted in secondary combustion zones that consume the soot (previously observed in the IRZ) or even prevent soot formation in the first place. These studies produced a rich database of experimental measurements on sooting, swirl-stabilized flames in a gas turbine model combustor. This database, however, is limited to single-shot measurements which yield only mean and fluctuating quantities of interest. Soot formation, agglomeration and oxidation are highly dynamic processes, dependent upon multiple tightly coupled parameters. It is therefore of considerable interest to acquire time-resolved measurements of quantities such as velocity, soot distribution and reaction zone location. Furthermore, these studies have focused primarily upon globally fuel-rich flame conditions. Although this is certainly of key interest in understanding soot-dynamics, it has been observed that even (globally) lean flames can

Tomographic particle image velocimetry (tomographic-PIV) is a recently developed measurement technique used to acquire volumetric velocity field data in liquid and gaseous flows. The technique relies on line-of-sight reconstruction of the rays between a 3D particle distribution and a multi-camera imaging system. In a turbulent flame, however, index-of-refraction variations resulting from local heat-release may inhibit reconstruction and thereby render the technique infeasible. The objective of this study was to test the efficacy of tomographic-PIV in a turbulent flame. An additional goal was to determine the feasibility of acquiring usable tomographic-PIV measurements in a turbulent flame at multi-kHz acquisition rates with current-generation laser and camera technology. To this end, a setup consisting of four complementary metal oxide semiconductor cameras and a dual-cavity Nd:YAG laser was implemented to test the technique in a lifted turbulent jet flame. While the cameras were capable of kHz-rate image acquisition, the laser operated at a pulse repetition rate of only 10 Hz. However, use of this laser allowed exploration of the required pulse energy and thus power for a kHz-rate system. The imaged region was 29 × 28 × 2.7 mm in size. The tomographic reconstruction of the 3D particle distributions was accomplished using the multiplicative algebraic reconstruction technique. The results indicate that volumetric velocimetry via tomographic-PIV is feasible with pulse energies of 25 mJ, which is within the capability of current-generation kHz-rate diode-pumped solid-state lasers.

The interaction of a helical precessing vortex core (PVC) with turbulent swirl flames in a gas turbine model combustor is studied experimentally. The combustor is operated with air and methane at atmospheric pressure and thermal powers from 10 to 35 kW. The flow field is measured using particle image velocimetry (PIV), and the dominant unsteady vortex structures are determined using proper orthogonal decomposition. For all operating conditions, a PVC is detected in the shear layer of the inner recirculation zone (IRZ). In addition, a co-rotating helical vortex in the outer shear layer (OSL) and a central vortex originating in the exhaust tube are found. OH chemiluminescence (CL) images show that the flames are mainly stabilized in the inner shear layer (ISL), where also the PVC is located. Phase-averaged images of OH-CL show that for all conditions, a major part of heat release takes place in a helical zone that is coupled to the PVC. The mechanisms of the interaction between PVC and flame are then studied for the case P = 10 kW using simultaneous PIV and OH-PLIF measurements with a repetition rate of 5 kHz. The measurements show that the PVC causes a regular sequence of flame roll-up, mixing of burned and unburned gas, and subsequent ignition of the mixture in the ISL. These effects are directly linked to the periodic vortex motions. A phase-averaged analysis of the flow field further shows that the PVC induces an unsteady lower stagnation point that is not present in the average flow field. The motion of the stagnation point is linked to the periodic precession of the PVC. Near this point burned and unburned gas collide frontally and a significant amount of heat release takes place. The flame dynamics near this point is also coupled to the PVC. In this way, a part of the reaction zone is periodically drawn from the stagnation point into the ISL, and thus serves as an ignition source for the reactions in this layer. In total, the effects in the ISL and at the stagnation point showed that the PVC plays an essential role in the stabilization mechanism of the turbulent swirl flames. In contrast to the PVC, the vortices in the OSL and near the exhaust tube have no direct effect on the flame since they are located outside the flame zone.

A detailed analysis of the flow–flame interactions associated with acoustically coupled heat-release rate fluctuations was performed for a 10 kW, CH4/air, swirl stabilized flame in a gas turbine model combustor exhibiting self-excited thermo-acoustic oscillations at 308 Hz. High-speed stereoscopic particle image velocimetry, OH planar laser induced fluorescence, and OH∗ chemiluminescence measurements were performed at a sustained repetition rate of 5 kHz, which was sufficient to resolve the relevant combustor dynamics. Using spatio-temporal proper orthogonal decomposition, it was found that the flow-field contained several simultaneous periodic motions: the reactant flux into the combustion chamber periodically oscillated at the thermo-acoustic frequency (308 Hz), a helical precessing vortex core (PVC) circumscribed the burner nozzle at 515 Hz, and the PVC underwent axial contraction and extension at the thermo-acoustic frequency. The global heat release rate fluctuated at the thermo-acoustic frequency, while the heat release centroid circumscribed the combustor at the difference between the thermo-acoustic and PVC frequencies. Hence, the three-dimensional location of the heat release fluctuations depended on the interaction of the PVC with the flame surface. This motivated the compilation of doubly phase resolved statistics based on the phase of both the acoustic and PVC cycles, which showed highly repeatable periodic flow–flame configurations. These include flames stabilized between the inflow and inner recirculation zone, large-scale flame wrap-up by the PVC, radial deflection of the inflow by the PVC, and combustion in the outer recirculation zones. Large oscillations in the flame surface area were observed at the thermo-accoustic frequency that significantly affected the total heat-release oscillations. By filtering the instantaneous reaction layers at different scales, the importance of the various flow–flame interactions affecting the flame area was determined. The greatest contributor was large-scale elongation of the reaction layers associated with the fluctuating reactant flow rate, which accounted for approximately 50% of the fluctuations. The remaining 50% was distributed between fine scale stochastic corrugation and large-scale corrugation due to the PVC.

Dynamic processes in gas turbine (GT) combustors play a key role in flame stabilization and extinction, combustion instabilities and pollutant formation, and present a challenge for experimental as well as numerical investigations. These phenomena were investigated in two gas turbine model combustors for premixed and partially premixed CH4/air swirl flames at atmospheric pressure. Optical access through large quartz windows enabled the application of laser Raman scattering, planar laser-induced fluorescence (PLIF) of OH, particle image velocimetry (PIV) at repetition rates up to 10 kHz and the simultaneous application of OH PLIF and PIV at a repetition rate of 5 kHz. Effects of unmixedness and reaction progress in lean premixed GT flames were revealed and quantified by Raman scattering. In a thermo-acoustically unstable flame, the cyclic variation in mixture fraction and its role for the feedback mechanism of the instability are addressed. In a partially premixed oscillating swirl flame, the cyclic variations of the heat release and the flow field were characterized by chemiluminescence imaging and PIV, respectively. Using phase-correlated Raman scattering measurements, significant phase-dependent variations of the mixture fraction and fuel distributions were revealed. The flame structures and the shape of the reaction zones were visualized by planar imaging of OH distribution. The simultaneous OH PLIF/PIV high-speed measurements revealed the time history of the flow field–flame interaction and demonstrated the development of a local flame extinction event. Further, the influence of a precessing vortex core on the flame topology and its dynamics is discussed.

Fragestellung: Der unikompartimentale Kniegelenksersatz bietet gegenüber dem bicondylären Ersatz eine raschere Rehabilitation, wobei eine weitgehend normale Aktivität eher erreicht werden kann. Die meisten Typen der UNI-Knie-Gelenke haben ein mobiles oder fixes Polyethyleninlay. Mobile[for full text, please go to the a.m. URL], Deutscher Kongress für Orthopädie und Unfallchirurgie; 73. Jahrestagung der Deutschen Gesellschaft für Unfallchirurgie, 95. Tagung der Deutschen Gesellschaft für Orthopädie und Orthopädische Chirurgie, 50. Tagung des Berufsverbandes der Fachärzte für Orthopädie

Fragestellung: Der unikompartimentale Kniegelenksersatz bietet gegenüber dem bicondylären Ersatz einige Vorteile.Die Operationstechnik ist aber manchmal anspruchsvoller, welches in einer höheren Revisionsrate endet. Es wurde deshalb eine Weichteil balancierende Operationstechnik mit Hilfe[for full text, please go to the a.m. URL], Deutscher Kongress für Orthopädie und Unfallchirurgie; 73. Jahrestagung der Deutschen Gesellschaft für Unfallchirurgie, 95. Tagung der Deutschen Gesellschaft für Orthopädie und Orthopädische Chirurgie, 50. Tagung des Berufsverbandes der Fachärzte für Orthopädie

We describe an approach of imaging the dynamic interaction of the flamefront and flowfield. Here, a diode-pumped Nd:YLF laser operating at 5 kHz is used to pump a dye laser, which is then frequency doubled to 283 nm to probe flamefront OH, while a dual cavity diode-pumped Nd:YAG system produces pulse-pairs for particle image velocimetry (PIV). CMOS digital cameras are used to detect both planar laser-induced fluorescence (PLIF) and particle scattering (in a stereo arrangement) such that a 5 kHz measurement frequency is attained. This diagnostic is demonstrated in lifted-jet and swirl-stabilized flames, wherein the dynamics of the flame stabilization processes are seen. Nonperiodic effects such as local ignition and/or extinction, lift-off and flashback events, and their histories can be captured by this technique. As such, this system has the potential to significantly extend our understanding of nonstationary combustion processes relevant to industrial and technical applications.