Coherent anti-Stokes Raman Spectroscopy Temperature Measurements Research Articles

Measurements of OH mole fraction and temperature up to 20 kHz by using a diode-laser-based UV absorption sensor

Diode-laser-based sum-frequency generation of ultraviolet (UV) radiation at 313.5 nm was utilized for high-speed absorption measurements of OH mole fraction and temperature at rates up to 20 kHz. Sensor performance was characterized over a wide range of operating conditions in a 25.4 mm path-length, steady, C2H4-air diffusion flame through comparisons with coherent anti-Stokes Raman spectroscopy (CARS), planar laser-induced fluorescence (PLIF), and a two-dimensional numerical simulation with detailed chemical kinetics. Experimental uncertainties of 5% and 11% were achieved for measured temperatures and OH mole fractions, respectively, with standard deviations of < 3% at 20 kHz and an OH detection limit of < 1 part per million in a 1 m path length. After validation in a steady flame, high-speed diode-laser-based measurements of OH mole fraction and temperature were demonstrated for the first time in the unsteady exhaust of a liquid-fueled, swirl-stabilized combustor. Typical agreement of approximately 5% was achieved with CARS temperature measurements at various fuel/air ratios, and sensor precision was sufficient to capture oscillations of temperature and OH mole fraction for potential use with multiparameter control strategies in combustors of practical interest.

Comparison of gas temperatures measured by coherent anti-Stokes Raman spectroscopy (CARS) of O_2 and N_2

We investigate the accuracy of temperature measurements by coherent anti-Stokes Raman spectroscopy (CARS) of O(2) and use measurements taken with N(2) CARS and a thermocouple for comparison. Scanning vibrational CARS spectra of O(2) and N(2) were recorded over a broad range of temperatures: between 294 K and 1900 K in air that was heated in a tube furnace and at approximately 2450 K in a fuel-lean CH(4)-O(2)-N(2) flame. Temperatures were derived from least-squares fits of simulated and experimental spectra. Both the fundamental vibrational band and the first hot vibrational band were included in fitting. In the case of the tube furnace, the N(2) and the O(2) CARS temperature measurements agreed to within 3%, and results were similar with the thermocouple; in the flame the agreement was to within 1%. We conclude that, for cases in which O(2) is present in sufficient concentrations ( approximately 10% or greater), the accuracy of O(2) thermometry is comparable with that of N(2).

COMPARISON OF TURBULENT DIFFUSION FLAME CARS TEMPERATURE MEASUREMENTS AND REYNOLDS-STRESS MODEL PREDICTION

Coherent Anti-Stokes Raman spectroscopy (CARS) is a well-established diagnostic technique for probing flames and practical combustion environments. This paper, presents the results of CARS temperature measurements in a turbulent natural gas diffusion flame. The experimental setup and CARS theory are briefly discussed. Experimental single-shot results are shown in the form of mean temperature and standard deviation of temperature versus normalized flame radius and are compared to theoretical values obtained from a Reynolds-stress model with a chemical equilibrium assumption.

Comparison of CARS Temperature Profile Measurements with Flow-Field Model Calculations in an MHD Diffuser

This paper reports on recent gas temperature profile measurements made on a large-scale coal-fired flow facility (CFFF) with a coherent anti-Stokes Raman spectroscopy (CARS) system. These measurements further demonstrate the application of advanced optical diagnostic methods to characterize large-scale processes. Data from the CARS gas temperature measurements taken in the CFFF diffuser during a recent long-duration experiment are discussed and analyzed. Details are given on the CARS system. Comparisons are drawn between the CARS data and flow-field modeling results. These results define the flow field and heat transfer in the diffuser and provide a basis for interpreting the CARS temperature measurements and further evaluating the flow-field model.

Accuracy of simplified CARS temperature measurements.

The accuracy of simplified CARS (Coherent Anti-Stokes Raman Spectroscopy) temperature measurements has been studied. The proposed CARS method provides the instantaneous temperature determination with the quick measurement of the intensity ratio of N2 CARS spectra by the use of a specially prepared monochromator, two photomultipliers and data processor including a circuit for sampling mechanism, an A/D converter, and a personal computer. Various causes and their contributions to the measurement errors introduced in these instruments have been evaluated experimentally. In the static hot air experiment, the standard deviation characteristic of the total error was 55 K for the mean temperature 928 K and 1000-shots. It is recommended that 500 laser-shots be used in order to obtain a reasonable mean temperature.

Coherent anti-stokes raman spectroscopic measurements of temperature and acetylene spectra in a sooting diffusion flame: Colloquium on combustion diagnostics

We report coherent anti-Stokes Raman spectroscopy (CARS) measurements of nitrogen and acetylene Q-branch spectra in a sooting laminar ethylene/air diffusion flame, previously characterized by thermocouple and laser scattering/extinction measurements. Good agreement between CARS and thermocouple temperature measurements was obtained throughout the flame. Effects of the presence of heavy soot-loading on the CARS measurements were minimized by reducing the acceptance angle and spectral bandwidth of the CARS detection system. We also present the first known CARS measurements of flame-generated acetylene. The acetylene spectra are in good agreement with preliminary calculated spectra.

CARS temperature and species measurements in augmented jet engine exhausts

Over the last several years, the development of coherent anti-Stokes Raman spectroscopy (CARS) has progressed through simple flames of increasing complexity to practical application in internal combustion engines, simulations of gas turbine combustors, and a variety of other devices. Here, the first application of CARS to an actual afterburning jet engine is described. The CARS instrument constructed is capable of completely remote operation and permits instantaneous thermometry at a 20-Hz rate. Its design and operation as well as sample exhaust measurements will be presented.

CARS thermometry in practical combustors

The feasibility of coherent anti-Stokes Raman spectroscopy (CARS) for remote, spatially and temporally resolved, measurements in practical combustion systems is demonstrated. N 2 CARS temperature measurements are reported for two different liquid-fueled combustors, a swirl burner, and a JT-12 combustor can, operated at atmospheric pressure and situated in a 50-cm diameter tunnel. Crossed-beam phase-matched CARS was employed to ensure good spatial precision. Delicate instrumentation was housed in a control room adjacent to the burner test cell and the CARS signals were piped out employing 60-μ diameter fiber optic guides. Average temperature measurements and demonstrations of single-pulse thermometry were performed.