Development of Laser-Spectroscopic Techniques for New Detection Schemes in Combustion Diagnostics

Abstract: The thesis work, aimed at the development of laser-spectroscopic detection schemes, consists of three parts, concerned with the development and application of different techniques. The development of UV filtered Rayleigh scattering (FRS) for thermometry and fuel/air ratio measurements, using a frequency tripled, single-longitudinal-mode alexandrite laser at ~254 nm together with an isotopically enhanced mercury filter, is examined first. Mercury has almost ideal features for serving as an atomic filter, having sharp cut-off slopes and a high extinction ratio, making it possible to effectively block unbroadened scattered light from particles, for example, and still let Doppler broadened light from the gas-phase pass through. The S6-model was utilized to model the Rayleigh-Brillouin scattering lineprofile. The FRS was employed in several applications, the thermometry measurements agreeing well with earlier measurements and calculations. Fuel/air ratio measurements were also obtained, various preliminary results being discussed.
A second area considered was use of polarization spectroscopy (PS) to probe species relevant to combustion. The CH radical was studied in a low-pressure methane/oxygen flame and compared with laser-induced fluorescence (LIF) measurements acquired simultaneously. Atomic hydrogen was imaged at ambient pressure in a hydrogen/oxygen flame, utilizing a two-photon-pump polarization spectroscopy probe (TPP-PSP) approach in which a 243 nm laser beam was used to excite the atoms through a two-photon process to the 2s state. The 2s state was then probed by use of the 2s-4p transition through a PS scheme employing 486 nm. PS was also extended into the mid-infrared spectral regime and used to probe ro-vibrational transitions of species not accessible in the UV and visible spectral range. Use of infrared PS (IRPS) for detection of minor species such as OH and C2H2 produced in flames, as well as of stable species such as CO2, CH4 and H2O was demonstrated. The signal-to-noise ratio obtained was higher than for IRLIF.
Finally, the development of planar LIF (PLIF) for CH involving improved detection sensitivity for a single-shot investigation of lean and turbulent premixed flames was reported. An alexandrite laser characterized by a long pulse duration (~150 ns) and the possibility to operate both in a single-mode manner with a narrow bandwidth (~100 MHz) and in a multimode manner with a broad bandwidth (~8 cm-1), was utilized to excite the B-X (0,0) band. A sharp and thin CH layer was visible from Φ = 0.6 to Φ = 1.5 in methane/air flames. Finally, use of the technique in a lean and partially premixed methane/air flame in a co-axial jet flame burner was demonstrated.