Knowledge Bank

Laser probing of practical combustion processed offers the potential for the remote, spatially precise, in-situ measurement of gas temperature and species concentrations. With pulsed lasers of sufficient energy, ""instantaneous"" measurements of medium properties are possible permitting probability density functions to be obtained from which parameter averages and turbulent fluctuation magnitudes can be ascertained. For the point probing of practical flames, i.e. highly luminous, soot laden, turbulent, two techniques, namely, coherent anti-Stokes Raman spectroscopy (CARS) and saturated laser-excited fluorescence appear quite $promising.1$ The two approaches provide complementary information. CARS is most suited for thermometry and measurements of major species concentrations, while saturated fluorescence is capable of providing flame radical profiles at trace (ppm) levels. CARS spectra of $N2$, $H2$, $O2$, CO, $H2O$, $CO2$ and $CH4$ in a variety of flames will be presented. For the first five, numerical routines to synthesize CARS spectra have been developed which display excellent agreement with experimental signatures. CARS measurements will be presented for a variety of situations including highly sooting flames and jet fuel fired combustors. Practical implementation of CARS diagnostics and extension to high pressures will also be discussed. Saturation of laser-excited fluorescence at high laser intensities permits flame radicals to be measured without the need for uncertain analytic quenching corrections to the data. In addition, operation in the saturation regime maximizes detection sensitivity. Saturated laser fluorescence has been employed to measure CH and CN in atmospheric pressure acetylene flames. Measurement accuracy and uncertainties in saturated fluorescence will be discussed.