Markus C. Weikl

Design and characterization of a Raman-scattering-based sensor system for temporally resolved gas analysis and its application in a gas turbine power plant

A sensor system for fast gas composition analysis is presented. Using linear Raman scattering the simultaneous detection of virtually all components of fuel gas mixtures such as natural gas and biogas can be achieved. The system consists of commercially available hardware components, in detail a frequency doubled continuous wave laser at 532 nm and a compact spectrometer with an embedded charge coupled device chip. For the evaluation of the Raman spectra a fast software module based on a contour fit algorithm is developed. Moreover, modules for controlling the hardware components are implemented in the sensor software ensuring simple operability of the entire system. In this paper the sensor is characterized in terms of, e.g., accuracy, reproducibility, detection limits and temporal performance. Finally its application for natural gas analysis in a gas turbine power plant is demonstrated, and the results obtained are compared to gas chromatography results.

Simultaneous temperature and exhaust-gas recirculation-measurements in a homogeneous charge-compression ignition engine by use of pure rotational coherent anti-Stokes Raman spectroscopy

Pure rotational coherent anti-Stokes Raman spectroscopy was used for the simultaneous determination of temperature and exhaust-gas recirculation in a homogeneous charge-compression ignition engine. Measurements were performed in a production-line four-cylinder gasoline engine operated with standard gasoline fuel through small optical line-of-sight accesses. The homogenization process of fresh intake air with recirculated exhaust gas was observed during the compression stroke, and the effect of charge temperature on combustion timing is shown. Single-pulse coherent anti-Stokes Raman spectroscopy spectra could not only be taken in the compression stroke but also during the gas-exchange cycle and after combustion. Consequently, the used method has been shown to be suitable for the investigation of two of the key parameters for self-ignition, namely temperature and charge composition.

Combined coherent anti-Stokes Raman spectroscopy and linear Raman spectroscopy for simultaneous temperature and multiple species measurements

The simultaneous application of pure rotational coherent anti-Stokes Raman spectroscopy (CARS) and vibrational linear Raman spectroscopy (LRS) for the measurement of temperature and species concentrations in combustion systems is demonstrated. In addition to the standard rotational CARS experimental setup, only one detection system (spectrometer and intensified CCD camera) for the collection of the LRS signals was applied. The emission of the broadband dye laser used for CARS was shifted to the deep red to avoid interferences with the LRS signals located in the visible region. First experimental results from a vaporizing propane spray using an engine injection system are shown.

Application of an optical pulse stretcher to coherent anti-Stokes Raman spectroscopy

An external optical cavity pulse stretcher for nanosecond-long laser pulses has been applied to coherent anti-Stokes Raman spectroscopy (CARS). An increased signal-to-noise ratio was achieved for both vibrational and pure rotational CARS, while the power density of the laser beams remained constant. Moreover, it was demonstrated that the use of the pulse stretcher also leads to improved precision of the determined temperatures and concentrations as a result of repeated excitation of the dye laser.

Development of a simplified dual-pump dual-broadband coherent anti-Stokes Raman scattering system.

A dual-pump dual-broadband coherent anti-Stokes Raman scattering (CARS) setup for simultaneous pure rotational and vibrational CARS is further developed by reducing the experimental efforts due to the use of only one detection system. With this system dual-pump CARS for probing ro-vibrational transitions of N(2)/CO/H(2) and dual-broadband CARS for pure rotational transitions of N(2)/O(2)/CO/CO(2) are applicable simultaneously. In this work the improvements to the setup and the data evaluation are described. First simultaneous temperature and concentration results from cell measurements and from a partially premixed propane flame are presented. Interference effects due to smeared vibrational CARS are investigated. It is shown that with similar experimental effort to a conventional dual-pump CARS setup considerably more information can be gained.

Time-resolved measurement of the local equivalence ratio in a gaseous propane injection process using laser-induced gratings

For the first time laser-induced gratings (LIGs) have been used for the investigation of a non-stationary pulse-repetitive injection process of gaseous propane, C(3)H(8), into air. By recording and evaluating single-shot LIG signals it was possible to determine, on a cycle-averaged basis, the temporal evolution of the local (within a probe volume 300 microm in diameter and 10 mm in length) equivalence ratio and by this the fuel-air ratio. Two different data treatment strategies, subject to C(3)H(8) concentration range, were first tested at stationary conditions and then used to evaluate the LIG signals obtained during the injection process. The relative standard deviation of single-shot measurements were estimated to be 0.14 and 0.32 at 0.8 % and 10 % of propane concentration, respectively.

Determination of probe volume dimensions in coherent measurement techniques

When investigating combustion phenomena with pump-probe techniques, the spatial resolution is given by the overlapping region of the laser beams and thus defines the probe volume size. The size of this probe volume becomes important when the length scales of interest are on the same order or smaller. We present a new approach to measure the probe volume in three dimensions, which can be used to determine the probe volume length, diameter, and shape. The optical arrangement and data evaluation are demonstrated for a dual-pump dual-broadband coherent anti-Stokes Raman scattering setup that is used for combustion diagnostics. This new approach offers a simple, quick alternative with more capabilities than formerly used probe volume measurement methods.

Characterization of a Combined CARS and Interferometric Rayleigh Scattering System

This paper describes the characterization of a combined Coherent anti-Stokes Raman Spectroscopy and Interferometric Rayleigh Scattering (CARS-IRS) system by reporting the accuracy and precision of the measurements of temperature, species mole fraction of N2, O2, and H2, and two-components of velocity. A near-adiabatic H2-air Hencken burner flame was used to provide known properties for measurements made with the system. The measurement system is also demonstrated in a small-scale Mach 1.6 H2-air combustion- heated supersonic jet with a co-flow of H2. The system is found to have a precision that is sufficient to resolve fluctuations of flow properties in the mixing layer of the jet.

Two-photon stimulated Raman excitation of thermal laser-induced gratings in molecular gases using broadband radiation of a single laser

For the first time to our knowledge, thermal laser-induced gratings (LIGs), generated via two-photon stimulated Raman excitation of pure rotational (and low-lying vibrational) transitions in molecules employing broadband radiation of a single pump laser, are observed. The efficiency of LIGs excitation using a few ns pulse duration dye laser with the spectral width of about 400 cm(-1), which covers the frequency range of the characteristic rotational transitions, is experimentally investigated in a number of molecular gases (N(2), CO(2), C(3)H(8)) at room temperature and pressures of 0.1-5 bar. The physical mechanisms of LIG formation are discussed and comparison of the shapes of the LIG signals obtained in different gases is presented in relation to the spectra of rotational and vibrational energies of the molecules under study, as well as to the selection rules for Raman transitions.

Enhancement of laser-induced gratings diffraction efficiency by forcing molecular rotation

For the first time to our knowledge, thermal laser-induced gratings (LIGs) are generated via two-photon stimulated Raman excitation of pure rotational transitions in molecules employing a single broadband pump laser.