Optical properties and reaction kinetics of shock-heated gas-phase tracers for quantitative laser-induced fluorescence
Reliable and quantitative application of tracer-based laser-induced fluorescence (LIF) for im-aging measurements of fuel concentration, equivalence ratio, and temperature in combustion processes requires knowledge about photophysical properties and thermal stability of the re-spective tracers at high temperature.</br> In this work, ultraviolet absorption and LIF emission spectra of toluene, anisole, p-xylene (p-XL), 1,2,4-trimethylbenzene (1,2,4-TMB), and acety-lene were studied at high temperatures and different reaction times behind reflected shock waves. Relative fluorescence quantum yields of these tracers were determined to extend existing data to higher temperatures. After the onset of pyrolysis of these tracers, effective absorp-tion cross-sections and the corresponding absorption and LIF spectra are reported.</br> Pyrolysis products were found to be the major contributors to the absorption and LIF signals at higher temperatures. The temporal behavior of the absorption at 266 nm during the pyrolysis of trac-ers was compared to simulations based on literature kinetics models and available high-temperature absorption cross-sections. To aid the interpretation of absorption and LIF experi-ments of anisole at high temperatures, the products of anisole pyrolysis were investigated us-ing a shock tube coupled to a high-repetition-rate time-of-flight mass spectrometer (HRR-TOF-MS) for time-resolved multispecies measurements. Concentration-time profiles for ani-sole and products such as benzene, C2H4, and CO were measured and compared to simula-tions using two kinetics models from literature.</br> Moreover, a novel experimental concept is presented that allows reaction-time-resolved LIF (RTR-LIF) measurements with one single laser pulse using a new shock-tube test section that has several optical ports. After the passage of the shock wave, the reactive mixture is excited along the center of the tube with a 266-nm laser beam directed through a window in the end-wall of the shock tube. The emitted LIF signal is collected through elongated sidewall win-dows and focused onto the entrance slit of an imaging spectrometer coupled to an intensified charge-coupled device (ICCD) camera.</br> The one-dimensional spatial resolution of the meas-urement translates into a reaction-time-resolved measurement according to the characteristics of the shock wave, while species information can be gained from the spectral axis of the de-tected two-dimensional image.