Characterization of a fast gas analyzer based on Raman scattering for the analysis of synthesis gas.

A sensor system for fast analysis of synthesis gas (mixtures of CO and H(2)) is proposed and characterized. The system is based on spontaneous Raman scattering, which enables simultaneous concentration measurements of all relevant species. For typical synthesis gas applications, this system has to face large variations of temperature and pressure. In addition, strong fluctuations in mixture composition may occur, which lead to rather inconvenient signal intensities. In this paper, we describe a low resolution spectrometer designed to function as a synthesis gas sensor and characterize pressure and temperature effects on concentration measurements. In addition, the use of different spectral ranges and calibration strategies is investigated in view of measurement accuracy and precision.

Simultaneous Two-Dimensional Flow Velocity and Gas Temperature Measurements by use of a Combined Particle Image Velocimetry and Filtered Rayleigh Scattering Technique.

For the first time, to the best of our knowledge, two-dimensional instantaneous measurements of the flow velocity and the gas temperature have been performed in a turbulent flame with simultaneous use of particle image velocimetry and planar filtered Rayleigh scattering. These single-shot measurements provide simultaneous information on the local flame structure (curvature and temperature gradients) and on the local flow conditions (vortices, flow divergences, and strain rates). The applicability of the technique is demonstrated in a turbulent lean CH(4)-air V flame.

Simultaneous temperature and relative O2-N2 concentration measurements by single-shot pure rotational coherent anti-stokes Raman scattering for pressures as great as 5 MPa.

Dual-broadband pure rotational coherent anti-Stokes Raman scattering is a valuable nonintrusive tool for gas diagnosis that provides simultaneous and time-resolved information about temperature and relative species concentration. A systematic investigation of single-shot precision and accuracy of simultaneous measurement of temperature and O(2)/N(2) concentration is presented. Various O(2) concentrations (1.0-15.6%) in binary mixtures with N(2) have been investigated in a temperature range from 300 to 773 K and for pressures of 1-50 bars (0.1-5 MPa). A comparison of two least-sum-squared differences fit evaluation procedures for the spectral shape, weighted constantly or inversely with respect to the relative signal intensity, is given. The results yielded good accuracy and precision for measuring temperature as well as concentration. The influence of temperature, O(2) concentration, pressure, and evaluation techniques on both accuracy and precision is discussed.

Dynamic Range Enhancement for Pure Rotational Coherent Anti-Stokes Raman Scattering Thermometry by use of an Optical Arrangement with Two Dye Lasers.

A disadvantage of pure rotational coherent anti-Stokes Raman scattering (CARS) compared with vibrational CARS is the limited dynamic range for temperature measurements. Here an optical configuration is described that overcomes this limitation by the use of two different center-frequency dye lasers in a dual-broadband CARS approach. Its performance is demonstrated for simultaneous pure rotational CARS temperature and relative N(2)-O(2)concentration measurements up to 1950 K at ambient pressure.

One-Dimensional, Time-Resolved Raman Measurements in a Sooting Flame made with 355-nm Excitation.

Single-shot vibrational Raman measurements were performed along an 11-mm-long line crossing the reaction zone in a premixed, fuel-rich (phi = 10), laminar methane-air flame by use of a frequency-tripled Nd:YAG laser with a 355-nm emission wavelength. This laser source seems to have advantages relative to KrF excimer lasers as well as to Nd:YAG lasers at 532 nm for hydrocarbon combustion diagnostics. The Raman emissions of all major species (N(2), O(2), CH(4), H(2), CO(2), H(2)O) were detected simultaneously with a spatial resolution of 0.4 mm. By integration over selected spectral intervals, the mole fractions of all species and subsequently the local gas temperatures have been obtained. A comparison of the temperatures that were found with results from filtered Rayleigh experiments showed good agreement, indicating the success of what are to the best of our knowledge the first one-dimensional single-shot Raman measurements in a sooting hydrocarbon flame.

Performance characteristics of soot primary particle size measurements by time-resolved laser-induced incandescence.

A detailed analysis of various factors that influence the accuracy of time-resolved laser-induced incandescence for the determination of primary soot particles is given. As the technique relies on the measurement of the signal ratio at two detection times of the enhanced thermal radiation after an intense laser pulse, guidelines are presented for a suitable choice of detection times to minimize statistical uncertainty. An error analysis is presented for the issues of laser energy absorption, vaporization, heat conduction, and signal detection. Results are shown for a laminar ethene diffusion flame that demonstrate that concurring results are obtained for various laser irradiances, detection characteristics, and times of observation.

Pure rotational coherent anti-stokes Raman scattering: comparison of evaluation techniques for determining single-shot simultaneous temperature and relative n(2)-o(2) concentration.

The accuracy and precision of time-resolved simultaneous temperature and O(2)-concentration measurements in binary N(2)-O(2) mixtures by single-pulse dual-broadband pure rotational coherent anti-Stokes Raman scattering (CARS) have been investigated. We present a detailed comparison of the applicability of six evaluation procedures to measurements of air in a temperature range 300-2050 K. Special emphasis is put on the dependence of the results on experimental restrictions and distortions. This comparison includes the least-sum-of-squared-differences fit (LSF) in the frequency space obtained by use of three different kinds of weighting with respect to signal intensity and in Fourier space by use of the complex or the cosine Fourier transformation, both of which permit a great reduction in the number of data points necessary for multidimensional evaluation. Additionally, a cross-correlation technique is tested that, to the best of our knowledge, was not previously applied to pure rotational CARS. We also present the results of measurements directed to the determination of low O(2)-concentration levels that were performed for various binary mixtures (1.0-15.6% O(2)) and for natural air within a temperature range of 300-773 K. A comparison is given for the three evaluation techniques that have proved most promising for the high-temperature investigations, i.e., the constant and the inverse weighted LSF in frequency space and the Fourier analysis technique.

Accuracy and precision of single-pulse one-dimensional vibrational coherent anti-Stokes Raman-scattering temperature measurements.

The accuracy and precision of time-resolved one-dimensional temperature measurements using single-pulse one-dimensional N(2) vibrational coherent anti-Stokes Raman scattering along a line have been investigated in air in the temperature range from 300 to 1500 K. For this, the experimental spectra were taken in a high-temperature oven at atmospheric pressure. A planar BOXCARS phase-matching geometry was employed to generate the signal along a 6.16-mm line directed perpendicular to the beam propagation. With the used imaging optics, in this direction a spatial resolution of 86 mum was achieved. Depending on the set temperature, the agreement between the thermocouple readings and the mean values of the evalutated coherent anti-Stokes Raman-scattering temperatures is better than 40 K. The applicability of this new technique for the time-resolved measurement of temperature gradients is demonstrated along a line that crosses the flame front in a premixed laminar CH(4)-air flame.

Simultaneous temperature and relative nitrogen-oxygen concentration measurements in air with pure rotational coherent anti-Stokes Raman scattering for temperatures to as high as 2050 K.

The accuracy of temperature and simultaneous relative N(2) -O(2) concentration measurements of accumulated as well as of single-pulse rotational coherent anti-Stokes Raman spectra has been investigated in air in the temperature range from 300 to 2050 K. The experimental spectra were taken in a high-temperature oven at atmospheric pressure for a constant oxygen concentration of 20.9% (air). The evaluation procedure is based on the energy-corrected sudden-power scaling law. The agreement of the thermocouple readings with the mean values of the evaluated coherent anti-Stokes Raman spectroscopy temperatures is higher than 50 K and independent of the temperature. The evaluated oxygen concentration is found to be in the range from 20.0 to 21.7% and is also independent of the temperature.

Two-dimensional temperature determination in the exhaust region of a laminar flat-flame burner with linear Raman scattering.

For planar temperature measurements in combusting flows, the well-established laser Raman technique has been further developed to provide two-dimensional local resolution. After excitation with a frequency-doubled Nd:YAG laser, the anti-Stokes and the Stokes Raman signals of the vibrational Q branch of molecular N(2) were detected at 473.3 and 607.3 nm, respectively. From the ratio of the two images, two-dimensional temperature distributions have been obtained by application of an analytical function, which was determined from theoretically calculated Raman spectra. Time-averaged measurements have been performed in the exhaust region of an atmospheric-pressure laminar CH(4)/air flat-flame burner for different equivalence ratios. The accuracy and precision of the results are discussed in combination with the prospects for time-resolved single-pulse measurements.

Light scattering by surface waves on a vertical layer of liquid toluene.

It is demonstrated that light scattering from surface waves on a vertical liquid layer can be used for the determination of surface tension and kinematic viscosity of the liquid under investigation. In contrast to usual approaches of surface light scattering, a setup is described that enables measurements with the same setup as that with experiments for the determination of other thermophysical properties by light scattering from bulk fluids and without an imposed grating or seed particles. The experiments rely on a heterodyne detection scheme and a signal analysis by photon correlation spectroscopy. First results are presented for toluene over a temperature range from 323 to 483 K at saturation conditions.

Two-dimensional temperature determination in sooting flames by filtered Rayleigh scattering.

We present what to our knowledge are the first filtered Rayleigh scattering temperature measurements and use them in sooting f lame. This new technique for two-dimensional thermography in gas combustion overcomes some of the major disadvantages of the standard Rayleigh technique. It suppresses scattered background light from walls or windows and permits detection of two-dimensional Rayleigh intensity distributions of the gas phase in the presence of small particles by spectral filtering of the scattered light.

One-dimensional vibrational coherent anti-Stokes Raman-scattering thermometry.

Quantitative one-dimensional single-pulse temperature measurements by broadband N(2) vibrational coherent anti-Stokes Raman scattering (CARS) are presented. A planar BOXCARS phase-matching geometry was used to generate the CARS signal along a 6.16-mm path positioned perpendicularly to the beam propagation. Depending on the imaging optics used, a spatial resolution of 86 mm was achieved. The applicability of this technique for measuring temperature gradients is demonstrated in a premixed laminar CH(4) -air f lame.

Experimental comparison of single-shot broadband vibrational and dual-broadband pure rotational coherent anti-Stokes Raman scattering in hot air.

Broadband vibrational and dual-broadband pure rotational coherent anti-Stokes Raman scattering (CARS) have been compared in a high-temperature oven, in which the accuracy and single-shot precision of gas temperature and relative O(2)- and N(2)-concentration measurements in hot air were probed over a temperature range that is typical for many combustion processes. To ensure a realistic comparison, we used nearly the same experimental setup for both CARS techniques. Besides temperature information, dual-broadband pure rotational CARS offers the possibility of achieving simultaneous single-shot concentration measurements. The comparison shows that this technique also has significant advantages in temperature evaluation over a large temperature range in comparison with vibrational CARS.

Evaluation of two different gas temperatures and their volumetric fraction from broadband N(2) coherent anti-Stokes Raman spectroscopy spectra.

Technical flames often contain regions of high temperature gradients at a length scale that is smaller than or comparable with the size of the probe volume used. In these situations the coherent anti-Stokes Raman spectroscopy (CARS) signal is composed of contributions of gases of different temperatures. Here a fitting routine is presented that allows the evaluation of the different temperatures that occur in the CARS spectra by splitting the temperature mixed spectra into contributions of two main temperatures. Additionally, the volumetric fraction of both gas components with those two different temperatures can be determined. The evaluation procedure has been tested by the use of measured temperature mixed CARS spectra that have been taken in two gas samples with different gas temperatures.

Simultaneous coherent anti-Stokes Raman scattering and two-dimensional laser Rayleigh thermometry in a contained technical swirl combustor.

The simultaneous application of vibrational coherent anti-Stokes Raman scattering (CARS) and the two-dimensional (2D) UV laser Rayleigh technique is reported for the investigation of a highly turbulent swirl frame inside a contained technical combustor. The CARS technique has been used to determine accurate temperature values at one point within the 2D Rayleigh-probed combustion field. These values were necessary to normalize the Rayleigh data to overcome influences of absorption effects along the detection path of the Rayleigh-scattered light through the exhaust gas volume and by the sealing window of the combustion chamber. At several different downstream positions, 500 simultaneous measurements with the point and with the 2D technique were performed to cover the whole combustion field. These data can be used for both the evaluation of 2D temperature structures in single frames and for the calculation of temperature probability density functions from the Rayleigh data at one single camera pixel over 500 frames. With this information, characterization of a highly turbulent flame is possible.

Determination of the dynamic viscosity of transparent fluids by using dynamic light scattering.

The application of dynamic light scattering to the determination of the dynamic viscosity of transparent liquids is shown by the use of low-power (10-mW) laser sources and a multiple-tau correlator. A novel data evaluation scheme is demonstrated by the application of a nonlinear fit to various intervals of lag times, which can detect relevant systematic errors. An appropriate experimental procedure permits results with good accuracy and precision for selected liquids.

Sound velocity measurements by the use of dynamic light scattering: data reduction by the application of a Fourier transformation.

By the use of photon correlation spectroscopy in a heterodyne technique, the ultrasonic velocity of transparent fluids can be determined. The quantity measured is the intensity correlation function that forms a damped oscillation. The oscillation frequency value can be found easily by the application of a numerical Fourier transformation. However, a dependence on the sample time of the applied correlator and the oscillation decay time must be taken into account. An estimation for the accuracy of the deduced frequency is given on the basis of a Fourier transformation of noisy synthetic data. For example, we find that a sampling interval of 100 ns, the sound frequency from scattering 514.5-nm light at an angle of 3° would be determined to ±0.3% for a typical near-critical fluid with a sound velocity of 100 m/s. For larger sound velocities, and for fluids far from the critical point, the uncertainty would decrease rapidly.

Two-dimensional temperature measurements in a technical combustor with laser Rayleigh scattering.

Application of the two-dimensional laser Rayleigh technique to the investigation of a large-scale industrial combustor is reported for the first time to our knowledge. Two-dimensional laser Rayleigh scattering was used to perform quantitative measurements of the temperature fields in different downstream positions of a 150-kW industrial, premixed, turbulent low-emission swirl combustor. Because of the possible interferences of the Rayleigh signal with Mie scattering and laser reflections of the burner components, some minor modifications of the design of the combustor and its gas supply were necessary. This was done without changing the basic characteristics of the burner. The quantitative and instantaneous character of the collected data allows calculation of ensemble-averaged temperature distributions and analysis of the flame structure in the turbulent combustion field. The measured temperature distribution confirms that the flame is stabilized by a central recirculation zone.