M. Aigner - Academia.edu (original) (raw)

Papers by M. Aigner

$Research paper thumbnail of Low pressure premixed CH4/air flames with forced periodic mixture fraction oscillations: experimental approach$

Applied Physics B, 2009

An experimental setup for the generation and investigation of periodic equivalence ratio oscillat... more An experimental setup for the generation and investigation of periodic equivalence ratio oscillations in laminar premixed flames is presented. A special low pressure burner was developed which generates stable flames in a wide pressure range down to 20 mbar and provides the possibility of rapid mixture fraction variations. The technical realization of the mixture fraction variations and the characteristics of the burner are described. 1D laser Raman scattering was applied to determine the temperature and concentration profiles of the major species through the flame front in correlation to the phase-angle of the periodic oscillation. OH* chemiluminescence was detected to qualitatively analyze the response of the flame to mixture fraction variations by changing shape and position. Exemplary results from a flame at p = 69 mbar, forced at a frequency of 10 Hz, are shown and discussed. The experiments are part of a cooperative research project including the development of kinetic models and numerical simulation tools with the aim of a better understanding and prediction of periodic combustion instabilities in gas turbines. The focus of the current paper lies on the presentation of the experimental realization and the measuring techniques.

Volume 2: Combustion, Fuels and Emissions, Parts A and B, 2010

Laser-based and optical measurements of a gas turbine (GT) model combustor undergoing transitions... more Laser-based and optical measurements of a gas turbine (GT) model combustor undergoing transitions between a thermo-acoustically stable and unstable state are presented. Planar laser-induced fluorescence of the OH radical, OH chemiluminescence and the planar three-component velocity field were simultaneously measured at a sustained repetition rate of 5 kHz. The combustor was operated with a lean, technically premixed CH4/air flame at ambient pressure that transitioned unpredictably between a thermo-acoustically unstable (‘noisy’) state and a state without pulsations (‘quiet’ state). The transition from the noisy to the quiet state was correlated with the lift-off of the flame from the burner nozzle and a subsequent stabilization of the flame above the nozzle. During the transition from the quiet to the noisy state, the flame reattached to the nozzle. It was observed that the transitions occurred consistently at a particular phase of the thermo-acoustic cycle. The axial velocity field...

Combustion Science and Technology, 2003

... techniques have been applied in a phase-coupled mode for the investigation of periodic ... de... more ... techniques have been applied in a phase-coupled mode for the investigation of periodic ... detailed understanding of the underlying mechanisms can be achieved only by numerical simu-lation ... RJ (2000) An experimental estimation of mean reaction rate and flame structure during ...

Combustion and Flame, 2006

A gas turbine model combustor for swirling CH 4 /air diffusion flames at atmospheric pressure wit... more A gas turbine model combustor for swirling CH 4 /air diffusion flames at atmospheric pressure with good optical access for detailed laser measurements is discussed. Three flames with thermal powers between 7.6 and 34.9 kW and overall equivalence ratios between 0.55 and 0.75 were investigated. These behave differently with respect to combustion instabilities: Flame A burned stably, flame B exhibited pronounced thermoacoustic oscillations, and flame C, operated near the lean extinction limit, was subject to sudden liftoff with partial extinction and reanchoring. One aim of the studies was a detailed experimental characterization of flame behavior to better understand the underlying physical and chemical processes leading to instabilities. The second goal of the work was the establishment of a comprehensive database that can be used for validation and improvement of numerical combustion models. The flow field was measured by laser Doppler velocimetry, the flame structures were visualized by planar laser-induced fluorescence (PLIF) of OH and CH radicals, and the major species concentrations, temperature, and mixture fraction were determined by laser Raman scattering. The flow fields of the three flames were quite similar, with high velocities in the region of the injected gases, a pronounced inner recirculation zone, and an outer recirculation zone with low velocities. The flames were not attached to the fuel nozzle and thus were partially premixed before ignition. The near field of the flames was characterized by fast mixing and considerable finite-rate chemistry effects. CH PLIF images revealed that the reaction zones were thin (0.5 mm) and strongly corrugated and that the flame zones were short (h 50 mm). Despite the similar flow fields of the three flames, the oscillating flame B was flatter and opened more widely than the others. In the current article, the flow field, structures, and mean and rms values of the temperature, mixture fraction, and species concentrations are discussed. Turbulence intensities, mixing, heat release, and reaction progress are addressed. In a second article, the turbulence-chemistry interactions in the three flames are treated.

Applied Physics B, 2012

The auto-ignition of a pulsed methane jet issuing into a laminar coflow of hot exhaust products o... more The auto-ignition of a pulsed methane jet issuing into a laminar coflow of hot exhaust products of a lean premixed hydrogen/air flat flame was examined using high-speed laser and optical measurement techniques with frame rates of 5 kHz or more. OH* chemiluminescence was used to determine the downstream location of the first auto-ignition kernel as well as the stabilization height of the steady-state lifted jet flame. OH planar laser-induced fluorescence (PLIF) was used to determine further details of the auto-ignition with a higher spatial resolution. Simultaneous imaging of broadband luminosity from a viewing angle perpendicular to the OH* chemiluminescence was applied to three-dimensionally reconstruct the ignition kernel location in space and to determine whether the first occurrence of the kernel was within or beyond the PLIF laser sheet. The development and expansion of the jet was characterized by high-speed Schlieren imaging. Statistics have been compiled for both the ignition time as well as the downstream location of the first auto-ignition kernel and the stabilization height of the steady-state lifted jet flame. From the PLIF images it was found that auto-ignition tended to occur at the interface between bulges of the inflowing jet and the coflow. For steady-state conditions, auto-ignition kernels were observed frequently below the flame base, emphasizing that the lifted jet flame is stabilized by auto-ignition.

Journal of Engineering for Gas Turbines and Power, 1992

Based on fundamental research concerning swirling flows, including the vortex breakdown phenomeno... more Based on fundamental research concerning swirling flows, including the vortex breakdown phenomenon, as well as on stability considerations of premixed flames, a second generation of low-emission burners has been developed. The lean premixing technique provides NOx emissions below 25 ppmv for natural gas. For liquid fuels the oxides of nitrogen are limited to 42 ppmv (oil No. 2). The novel burner technology will be applied to the well-known ABB silo combustor. As a first step the Conical Premix Burner will be used to retrofit the ABB type 11N. For the ABB gas turbine type 8 the design of a novel fully annular combustor is in progress. Most of the conceptual work concerning burner aerodynamics and burner-burner interaction has been carried out on scaled-down burner and combustor models. For a second step a sector of the combustor in 1:1 scale has been tested at atmospheric pressure. Additional high-pressure tests provide information about the combustor performance at engine conditions...

Proceedings of the Combustion Institute, 2009

Journal of Engineering for Gas Turbines and Power, 2012

ABSTRACT The use of highly reactive fuels in the lean premixed combustion systems employed in sta... more ABSTRACT The use of highly reactive fuels in the lean premixed combustion systems employed in stationary gas turbines can lead to many practical problems, such as unwanted autoignition in regions not designed for combustion. In the present study, autoignition characteristics for hydrogen, diluted with up to 30 vol. % nitrogen, were investigated at conditions relevant to reheat combustor operation (p = 15 bar, T > 1000 K, hot flue gas, relevant residence times). The experiments were performed in a generic, optically accessible reheat combustor, by applying high-speed imaging and Particle Image Velocimetry (PIV). Autoignition limits for different mixing section (temperature, velocity) and fuel jet (N2 dilution) parameters are described. The dominant factor influencing autoignition was the temperature, with an increase of around 2% leading to a reduction of the highest possible H2 concentration without “flame-stabilizing autoignition kernels” of approximately 16 vol. %. Furthermore, the onset and propagation of the ignition kernels were elucidated using the high-speed measurements. It was found that the ability of individual autoignition kernels to develop into stable flames depends on the initial position of the kernel and the corresponding axial velocity at that position. While unwanted autoignition occurred prior to reaching the desired operating point for most investigated conditions, for certain conditions the reheat combustor could be operated stably with up to 80 vol. % H2 in the fuel.

Flow, Turbulence and Combustion, 2005

The phenomenon of periodic combustion instabilities has been investigated in a gas turbine model ... more The phenomenon of periodic combustion instabilities has been investigated in a gas turbine model combustor by application of two-line planar laser induced fluorescence (PLIF) of OH for the determination of temperature distributions. The measurement technique has been evaluated using laser Raman scattering for comparison. The results showed that even with a lower accuracy compared to single-point techniques like CARS or Raman, valuable information concerning the stabilization mechanism can be drawn from the phase-locked mean temperature. The fact that the 2D technique is less time consuming compared to single-point techniques makes it attractive for phase-resolved measurements. The investigation showed that the two-line OH-PLIF thermometry technique can very well contribute to the understanding of combustion instabilities phenomena and assist the validation and the improvement of CFD models.

48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010

SIAM Journal on Discrete Mathematics, 1990

ABSTRACT Let G be a graph on n vertices. An irregular assignment of G is a weighting w:E(G)→{1,..... more ABSTRACT Let G be a graph on n vertices. An irregular assignment of G is a weighting w:E(G)→{1,...,m} of the edge-set of G such that all weighted degrees w(v)=∑ v∈e w(e) are distinct. The minimal number m for which this is possible is called the irregularity strength s(G) of G. Lehel and others have shown that s(G)<∞ implies s(G)≦n-1 for connected graphs on n≥4 vertices, and s(G)≤2n-3 for arbitrary graphs. By using decompositions for the additive group ℤ r (integers modr), these results are strengthened. Main Theorem: s(G)≤n+1 for any graph with s(G)<∞.

Measurement Science and Technology, 2005

We present a simple method for comparing particle size measurements, obtained with laser-induced ... more We present a simple method for comparing particle size measurements, obtained with laser-induced incandescence (LII) and a scanning mobility particle sizer (SMPS) in a premixed laminar sooting flame. A quartz cell was installed in line with the SMPS probe to allow LII measurements within the SMPS sample line. In this configuration, the LII and SMPS measurements gave similar results in terms of mean particle size. After the probe, the soot particles appear to be made of tight compact particles. In addition, with this experimental configuration, the influence of the probe in the flame is studied for different particle size ranges by applying LII before and after the probe. Application of SMPS with and without LII in the quartz cell shows that laser heating during LII measurements has an influence on the soot particle size distribution. The method could be used to improve probe sampling of particulate matter in reactive fields as well as to validate the interpretation of relevant physical mechanisms involved in the LII process.

Journal of Combinatorial Theory, Series B, 1989

Discrete Mathematics, 1995

We want to find an unknown acyclic orientation ~* of an (undirected) graph G by testing for certa... more We want to find an unknown acyclic orientation ~* of an (undirected) graph G by testing for certain edges how they are oriented according to ~*. How many tests do we need in the worst case? We give upper and lower bounds for this number c(G) in terms of the independence number of G and study the class of exhaustive graphs, i.e~ graphs satisfying c(G) --]E(G)]. It is shown that there exist nonexhaustive graphs with arbitrarily large girth. The extremal exhaustive graphs are determined for n/> 7.