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

Temporally resolved two dimensional temperature field of acoustically excited swirling flames measured by mid-infrared direct absorption spectroscopy

The detailed understandings of temperature profiles and flow-flame interaction in unsteady premixed swirling flames are crucial for the development of low emission turbine engines. Here, a phase-locked tomographic reconstruction technique measuring the large absorption cross section of CO₂ at its mid-infrared fundamental band around 4.2 μm is used to acquire the flame temperature and in situ CO₂ volume fraction distribution in a turbulent premixed swirling flame under different levels of external acoustic forcing amplitude. The temporally resolved temperature field variation reveals large temperature fluctuation in unsteady premixed swirling flames produced near the nozzle exit due to vortex-driven mixing of surrounding cold gas. The temperature fluctuation quickly dissipates when moving downstream of the flame with the flow velocity of the burnt gas. The accurate high temporal resolution thermodynamic measurements of the phase-locked tomographic thermometry technique reported in this work can be generally applied to periodic reacting flows.

Three-color vibrational CARS thermometry of fuel-rich ethylene/air flames using a potassium gadolinium tungstate Raman-active crystal as a source of narrowband probe radiation

Three-color broadband vibrational coherent anti-Stokes Raman scattering (CARS) temperature measurements were carried out in laminar fuel-rich sooting ethylene/air flames. Stimulated Raman scattering (SRS) of a picosecond pump laser pulse in a Raman-active potassium gadolinium tungstate [KGd(WO₄)₂] crystal was employed as a source of narrowband probe radiation. In the three-color CARS experiment, this wavelength-shifted radiation enables N₂-based vibrational CARS temperature measurements in sooting flames free of the signal interference with the absorption/emission bands of the flame intermediate radicals C₂. Spatial temperature profiles for different fuel-rich atmospheric pressure ethylene/air flames are presented in comparison with the results of two-color broadband vibrational and dual-broadband pure rotational CARS temperature measurements. The comparison shows the suitability of the three-color CARS measurement technique employing the KGd(WO₄)₂ crystal for accurate, C₂ interference-free, temperature measurements in sooting flames.

Advanced Laser-Based Techniques for Gas-Phase Diagnostics in Combustion and Aerospace Engineering

Gaining information of species, temperature, and velocity distributions in turbulent combustion and high-speed reactive flows is challenging, particularly for conducting measurements without influencing the experimental object itself. The use of optical and spectroscopic techniques, and in particular laser-based diagnostics, has shown outstanding abilities for performing non-intrusive in situ diagnostics. The development of instrumentation, such as robust lasers with high pulse energy, ultra-short pulse duration, and high repetition rate along with digitized cameras exhibiting high sensitivity, large dynamic range, and frame rates on the order of MHz, has opened up for temporally and spatially resolved volumetric measurements of extreme dynamics and complexities. The aim of this article is to present selected important laser-based techniques for gas-phase diagnostics focusing on their applications in combustion and aerospace engineering. Applicable laser-based techniques for investigations of turbulent flows and combustion such as planar laser-induced fluorescence, Raman and Rayleigh scattering, coherent anti-Stokes Raman scattering, laser-induced grating scattering, particle image velocimetry, laser Doppler anemometry, and tomographic imaging are reviewed and described with some background physics. In addition, demands on instrumentation are further discussed to give insight in the possibilities that are offered by laser flow diagnostics.

PLIF Thermometry Based on Measurements of Absolute Concentrations of the OH Radical

A method for measurements of planar temperature distributions based on planar laser-induced fluorescence (PLIF) of the OH radiacal is described. The technique was developed specifically for the application in lean combustion systems, where OH equilibrium concentrations are largely independent on equivalence ratio and a function of temperature only. It is thus possible to derive a temperature information from measurements of absolute OH concentration, which can be obtained from a combined PLIF/absorption measurement.

This paper discusses the basics of the method, and describes validation experiments in high pressure laminar premixed flames which were performed to asses its applicability and accuracy. Therefore, we compared our LIF based results with CARS measurements performed in the same flames. Finally, an example for the application in a lean gas turbine model combustor is discussed.

Two-dimensional temperature measurements in flames using filtered Rayleigh scattering at 254 nm

Two-dimensional temperature measurements using filtered Rayleigh scattering (FRS) have been demonstrated. A tunable single-longitudinal-mode alexandrite laser was employed to provide the tunable narrow-line-width ultraviolet laser beam at 254 nm. Isotopic-enhanced mercury was utilized as an ultraviolet atomic filter. The strong absorption of the filter enabled effective suppression of elastic background. The transmission curve of the mercury filter was characterized by combining experimental measurements and proper modeling. The Rayleigh-Brillouin spectral profiles were calculated employing the S6 model. Quantitative interpretations of two-dimensional FRS signals were performed in both premixed and diffusion flames. The temperature values obtained were in good agreement with adiabatic calculations and earlier measurements.

Phase-locked two-line OH planar laser-induced fluorescence thermometry in a pulsating gas turbine model combustor at atmospheric pressure

Two-line OH planar laser-induced fluorescence (PLIF) thermometry was applied to a swirling CH4/air flame in a gas turbine (GT) model combustor at atmospheric pressure, which exhibited self-excited combustion instability. The potential and limitations of the method are discussed with respect to applications in GT-like flames. A major drawback of using OH as a temperature indicator is that no temperature information can be obtained from regions where OH radicals are missing or present in insufficient concentration. The resulting bias in the average temperature is addressed and quantified for one operating condition by a comparison with results from laser Raman measurements applied in the same flame. Care was taken to minimize saturation effects by decreasing the spectral laser power density to a minimum while keeping an acceptable spatial resolution and signal-to-noise ratio. In order to correct for the influence of laser light attenuation, absorption measurements were performed on a single-shot basis and a correction procedure was applied. The accuracy was determined to 4%-7% depending on the location within the flame and on the temperature level. A GT model combustor with an optical combustion chamber is described, and phase-locked 2D temperature distributions from a pulsating flame are presented. The temperature variations during an oscillation cycle are specified, and the general flame behavior is described. Our main goals are the evaluation of the OH PLIF thermometry and the characterization of a pulsating GT-like flame.

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.

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.

Application of tunable excimer lasers to combustion diagnostics: a review

Tunable excimer lasers are being used to produce species-, space-, and time-resolved images of complex gaseous media. These media may be analyzed for composition, density, temperature, or flow velocities. The techniques are, in general, highly selective, sensitive, and nonintrusive and are being made possible by recent technological developments in these UV lasers and in intensified cameras, imaging spectrographs, and fast digital image processing. We describe the needs for laser diagnostics in combustion, the physical mechanisms, the relevant spectroscopy, typical experimental setups, and equipment considerations. Precision and accuracy are discussed on the basis of some simple, but realistic, calculations intended to guide the experimentalist in design considerations and to reveal potential sources of errors in the often difficult conversion of raw data to values for such quantitative parameters as densities or temperatures. Finally we present an overview of previous results, select some examples that show the power of tunable excimer laser diagnostics in combustion, and present some suggestions for future directions.

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.

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.

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.