Winfried Stricker

Periodic combustion instabilities in a swirl burner studied by phase-locked planar laser-induced fluorescence

Quasi-simultaneous phase-correlated measurements of different species in a turbulent swirl flame with a self-excited instability are presented for the first time. Phase-resolved OH* chemiluminescence and planar laser-induced fluorescence (PLIF) spectroscopy of OH, CH, and H 2 CO were used to follow the temporal evolution of flame structures in a pulsating swirl-stabilized model injector for gas-turbine applications. H 2 CO is a suitable indicator for chemical heat release in combination with OH; CH LIF and OH* emission were shown to be suitable indicators for the average shape and location of flame fronts, while OH LIF marked regions with high temperature, both in the flame front and the burned gas regions. The combustor was operated on methane fuel at atmospheric pressure. Lasers and detectors were locked to the phase angle of the self-excited pressure oscillation using a trigger signal derived from a microphone. Measurements at different phase angles were performed by variable delays with respect to the trigger pulse. Due to a high degree of turbulence, a large number of single-pulse measurements at each phase angle had to be performed in order to retrieve phase-sensitive effects from the dominating turbulent fluctuations. Noticeable changes of the flame structure with phase angle, particularly near the injector exit, are indicative of a strong coupling between the flame and a periodically fluctuating flow field.

Laser-induced-fluorescence detection of nitric oxide in high-pressure flames with A–X(0, 2) excitation

Laser-induced fluorescence techniques have been used successfully for quantitative two-dimensional measurements of nitric oxide. The commonly applied D-X(0, 1) or A-X(0, 0) schemes are restricted to atmospheric-pressure flames and engines driven with gaseous fuels because of strong attenuation of the exciting laser beam by combustion intermediates. The properties of a detection scheme for which excitation in the nitric oxide A-X(0, 2) band was used were investigated. We discuss the advantages of the A-X(0, 2) system (excited at 247.95 nm) based on measurements in laminar premixed methane/air flames at 1-40 bars.

SIMULTANEOUS RAMAN/LIF MEASUREMENTS OF MAJOR SPECIES AND NO IN TURBULENT H2/AIR DIFFUSION FLAMES

A single-pulse spontaneous Raman scattering apparatus, based on a flashlamp-pumped dye laser, was used to determine the concentrations of the major species and the temperature in turbulent H2/N2/air jet diffusion flames. The concentrations of nitric oxide were simultaneously measured by Laser-Induced Fluorescence (LIF) after excitation of theA2Σ+−X2Π transition with a Nd: YAG-pumped dye laser. Some fundamentals of the employed methods, including the calibration procedure, quenching corrections, and accuracy are discussed. Besides a detailed study of the experimental technique, a main goal of the presented investigations was the generation of comprehensive data sets of high accuracy from well-defined turbulent flames which allow for a quantitative comparison with model calculations. Two flames with different fuel dilution and Reynolds numbers were investigated in a pattern of typically 100 measuring locations each comprising 300 single shots. In addition, four flames with different flow velocities but same fuel composition were compared with respect to their temperature and NO concentration profiles. The results show that differential diffusion plays an important role in these flames, especially near the flame base, where the temperature is increased above the adiabatic flame temperature and deviations from adiabatic equilibrium are large. The correlations between NO and mixture fraction and NO and temperature reveal characteristic features of the different flames.

LIF imaging and 2D temperature mapping in a model combustor at elevated pressure

Planar laser-induced fluorescence (PLIF) has been used to measure time-resolved spatial distributions of the fuel, the OH radical, and the temperature field in a jet engine model combustor segment. For temperature measurements, a two-line PLIF scheme was used: two different spectral lines of the OH radical, which served as indicator molecule, were excited successively within a short time delay using nanosecond pulses from two UV laser systems operating on different wavelengths. The ratio of the two fluorescence signals depends on the temperature; this forms the basis of the temperature measurement. To our knowledge, this temperature mapping technique has been applied for the first time in the high pressure combustion of kerosene. The fluorescence signal resulting from excitation by one of the two lasers is proportional to the OH density and provides thus information on the OH radical distribution and on flame structure. The same technique can be utilized to excite fluorescence from the fuel, thus providing qualitative information on kerosene distributions. These measurements yield information on flame structure, heat release and mixing properties, which can serve as design aids, as well as for CFD code validation purposes.

LASER-BASED INVESTIGATION OF SOOT FORMATION IN LAMINAR PREMIXED FLAMES AT ATMOSPHERIC AND ELEVATED PRESSURES

ABSTRACT An experimental investigation into soot formation in laminar premixed flames at atmospheric and elevated pressures (1–5 bar) has been conducted. The flames were produced in a dual-flame burner enclosed in a pressure housing. Quantitative soot volume fraction measurements were obtained using laser-induced incandescence coupled with a quasi-simultaneous absorption measurement for calibration; the data were corrected for signal trapping using an “onion peeling” algorithm. Temperature measurements were obtained using shifted vibrational coherent anti-Stokes Raman scattering, which yields well-resolved, accurate temperature measurements in sooting and nonsooting environments. Results are presented for stable homogeneous flames using air as oxidizer and ethylene, propylene, and toluene as fuels. The variation of soot volume fraction and temperature with height above burner and as a function of fuel, equivalence ratio, and pressure are presented and discussed. The present soot data are well represented by a first-order growth rate law. The data identify trends and features useful for the validation of numerical models of soot formation.

Quantification of NO A-X (0, 2) Laser-Induced Fluorescence: Investigation of Calibration and Collisional Influences in High-Pressure Flames

Laser-induced-fluorescence techniques have been used successfully for quantitative two-dimensional measurements of nitric oxide. NO A-X(0, 2) excitation at 248 nm recently found applications in internal-combustion engines. We assess the collisional processes that influence quantification of signal intensities in terms of saturation, rotational energy transfer, and line broadening, using laminar high-pressure methane/air and n-heptane/air flames at pressures as high as 80 bars (8 x 10(6) Pa). A calibration method that is applicable in technical combustion systems based on addition of NO to the burning flame is investigated for various air/fuel ratios and pressures and yields information about the influence of NO reburn processes.

Quantitative Raman imaging investigations of mixing phenomena in high-pressure cryogenic jets

A two-dimensional Raman technique was used to investigate mixing phenomena of cryogenic jets under both supercritical and transcritical conditions. The aim of this study was to enlarge the experimental data basis for modeling purposes and to provide quantitative information to help to improve the design of injectors for high-pressure rocket engine combustion chambers. Cryogenic nitrogen, which served as substitute for liquid O(2), was injected into N(2) at room temperature at pressures up to 6.0 MPa. The liquid N(2) jet could be atomized by a coaxial H(2) flow. Raman scattering was generated with a XeF excimer laser. The resulting signal images were discriminated against background by spectral filtering and preferential detection of light with a polarization corresponding to the polarization of the laser, thus making use of the conserved polarization of the Raman-scattered light. The Raman images were converted into density distributions of N(2) and H(2), respectively, as well as into temperature distributions for a variety of experimental conditions.

Comparison of laser-induced incandescence method with scanning mobility particle sizer technique: the influence of probe sampling and laser heating on soot particle size distribution

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.

Laser-induced fluorescence thermometry and concentration measurements on NOA–X (0-0) transitions in the exhaust gas of high pressure CH4/air flames

Laser-Induced Fluorescence (LIF) excitation spectra in the NOA–X (0-0) band were used for temperature measurements in the postflame region of high-pressure CH4/air flames. To improve the quality of the measured spectra and to perform reliable line-shape measurements, the initial mixture was doped with approximately 400 ppm NO. At pressures up to 18 bar, excellent agreement was obtained between NO LIF temperatures and NARS/rotational Raman temperatures. Effective broadening coefficients were also determined in these flames. Problems with quantitative concentration measurements of NO and single-pulse temperature measurements at high pressures are discussed.

Laser-induced fluorescence determination of flame temperatures in comparison with CARS measurements

Temperature profiles in several premixed low pressure H2/O2/N2 flames and in an atmospheric pressure CH4/air flame were determined by laser-induced fluorescence (LIF) and by CARS experiments. In the LIF study, temperatures were derived from OH excitation spectra, CARS temperatures were deduced from N2Q-branch spectra. The present study is the first quantitative comparison of these two methods for temperature determination in flames burning at pressures up to 1 bar. The resulting temperatures showed good agreement.