Development and application of coherent anti-Stokes Raman scattering systems in reacting flows

Aman Satija, Purdue University


The objective of this research is to develop and apply coherent anti-Stokes Raman scattering (CARS) systems in reacting flows. CARS is a well established laser diagnostic technique for point measurements in reacting flows. Its accuracy has been extended to a larger temperature range (300-3000K) along with the added diagnostic potential for simultaneous concentration measurements of relevant flame species, without a significant increase in the CARS system complexity. This advance has been made possible by the advent of two-beam pure rotational CARS (PRCARS), first demonstrated in this thesis research. Two-beam PRCARS allows the use of a third laser beam, also a frequency doubled Nd:YAG output at 532 nm to simultaneously generate a vibrational CARS (VCARS) signal. The VCARS and PRCARS signals are separated from the other input beams in the detection channel, into two separate detection paths. Both signals are detected using one spectrometer. A 532-nm mirror was mounted inside the spectrometer to reflect the PRCARS signal on to a second CCD camera. A single-bandpass filter was introduced in the VCARS signal's path in order to prevent any stray 532.2 nm radiation from interfering with the PRCARS signal. The bandpass filter served a secondary purpose of reducing the flame-chemiluminescence entering the spectrometer and thus, enhanced the signal to noise ratio of the CARS signals. This new, combined CARS system has been used for measurements in a Hencken burner calibration flame. Temperature measurements obtained in counter-flow diffusion flames (CFDF) helps combustion modelers validate chemical, transport and thermodynamic properties used in combustion modeling. Specifically, an understanding of combustion dynamics (via combustion modeling) in CFDF's can lead to improved design of systems using either H2 or H2-based fuels. Two examples of such systems are power gas turbines and high-speed propulsion engines. VCARS thermometry was performed in laboratory scale H2-air CFDF. Mixtures of H2 and N2 flowed from the bottom nozzle as fuel, and air flowed from the top nozzle as the oxidizer. Temperature profiles were obtained along the center line of the nozzles of a novel counterflow burner by systematically varying dilution of N2 in the fuel stream and by varying the global strain rate of the non-premixed flames. Global strain rate is defined as the ratio between the relative flow velocity (of the fuel and the oxidizer stream) and the nozzle separation distance. The counterflow burner has a modular design which allows various nozzle geometries to be used with variable separation between the bottom and the top nozzle. At a fixed global strain rate, the peak temperature and "width" of the temperature profiles were found to decrease with an increase in N2 dilution in the fuel stream. Also a shift of the flame front towards the fuel nozzle was observed. At constant fuel composition, with a reduction in the strain rate, broadening in the temperature profile along with a reduction in the flame peak temperature was observed. Furthermore, the flame front shifted towards the oxidizer nozzle with decreasing strain rates. The experimentally obtained temperature measurements were found to be in good agreement with one-dimensional numerical solutions obtained using OPPDIFF software. Beyond a strain rate of approximately 450 s-1, unsteady flow oscillations were observed in the flame which permitted only qualitative imaging of the flame structure. The extinction limits for H2-Air non-premixed flames, with low percentage of H2 in the fuel stream, were obtained qualitatively. The extinction limits were obtained by carefully increasing the global strain rate until the flames were extinguished. Extinction limits obtained using one-dimensional numerical solutions from OPPDIFF are in rough agreement with experiments. This suggests the need for more rigorous computational tools to predict extinction with more accuracy. Flame structure for one particular flame, containing 17% H2 in the fuel stream, was investigated quantitatively just below its extinction limit. Nozzle-centerline temperature measurements were obtained with varying flow rates. The temperature profiles were similar with a small drop in peak temperature just prior to flame extinction. (Abstract shortened by UMI.)




Lucht, Purdue University.

Subject Area

Mechanical engineering

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