Masahide Tsujishita

Visualization of CN by the use of planar laser-induced fluorescence in a cross section of an unseeded turbulent CH 4 –air flame

CN is known to be the important species in forming NO(x) in hydrocarbon-air flames, and we describe the qualitative mapping of the CN distribution of the order of less than 1 in 10(6) at atmospheric pressure in a cross section of an unseeded turbulent CH(4)-air flame by the use of a single-shot laser pulse with planar laser-induced fluorescence. We obtained the results by comparing the different excitation-detection schemes to find the most appropriate scheme for visualization. In addition, the images obtained have good spatial resolution of 200 µm. Based on a laminar flame experiment with the same optical systems, it is observed that the CN distribution zone is located outside of the inner cone even at atmospheric pressure.

Two-Dimensional Direct Quenching Measurement of OH in a Cross Section of a Bunsen Flame

In flame analysis, the laser-induced fluorescence (LIF) method is becoming popular for detecting intermediate species and measuring temperature. However, when a typical tunable laser with a pulse duration of around 10 ns is used, the pressure of the target to be measured is often too high to obtain a fluorescent signal having a linear relationship to molecular concentration, due to collisional quenching by which relaxation without fluorescence occurs. In a reactive field such as a flame, especially when two-dimensional measurement is desired, the problem of quenching is more serious because the quenching time constant differs at each measuring point. Accordingly, an effective two-dimensional quenching measurement method is desirable. For this purpose, we attemped to determine the quenching time constant distribution, using an image intensifier and a XeCl excimer-laser-pumped dye laser with a doubling crystal, which is the typical and widely used tunable laser, at the pressure at which the LIF signal is significantly affected by collisional quenching. In this way, we succeeded in determining the quenching time-constant distribution in the cross section of a premixed Bunsen flame at 1/2 atmospheric pressure. In addition, this experiment is the first measurement of two-dimensional fluorescence decay time on the order of a few nanoseconds.

Two-dimensional digital imaging of the CH distribution in a natural gas/oxygen flame at atmospheric pressure and detection of A-state emission by means of C-state excitation.

With laser-induced fluorescence, images of the distribution of intermediate species in flames usually have been taken under the condition that the excitation electronic state and emission electronic state are the same. We have successfully photographed two-dimensional CH distribution in a natural gas/oxygen flame through a 431-nm bandpass filter with A-state emission by means of C-state excitation (312 nm). This method provides a smaller signal than the A-state excitation method does but is free from the scattering of the pumping laser beam surrounding the burner nozzle and Rayleigh scattering. Furthermore, this method gives us the opportunity to observe semiquantitative CH distribution.

Visualization of the CH Molecule by Exciting C2Σ+(v=1) State in Turbulent Flames by Planar Laser-Induced Fluorescence

A novel visualization technique for the CH molecule has been demonstrated. The technique described in this paper is the CH visualization by exciting the C2Σ+ state which predissociates and gives the opportunity of semi-quantitative CH visualization at a significantly high pressure. The visualization methods have been developed by taking practical application into account, and the detection wavelength is made different from that of excitation in order to avoid the stray light problem. In addition, CH can be visualized by a single-shot laser, and this technique has been proven to be applicable for the analysis of nonstationary chemical reactions such as turbulent flames. Furthermore, the visualization results show good spatial resolution, sufficient signal level, and greatly improved image quality.

High Sensitivity Analysis of Dioxin Precursors by Laser Ionization Time-of-Flight Mass Spectroscopy

Recently, a method of analyzing dioxin concentration with high sensitivity is desired. Laser ionization timeof-flight mass spectroscopy (LI TOF-MS) has been developed to satisfy this need. In this study, the detection sensitivity of a few ppt for chlorobenzene, which is a precursor of dioxin, was realized by adjusting the laser beam, the molecular jet and other conditions of the LI TOF-MS system. This sensitivity is 300 times higher than that of gas chromatography mass spectroscopy (GC-MS). Furthermore, selective detection ofthe isomers of dichlorobenzene was performed by making use of the difference of ionization wavelength. LI TOF-MS is expected to be an on-line monitor analyzer of dioxin concentration in exhaust gas by detectingthe precursors of dioxin.

Visualization of Low-Concentration Species in a Turbulent Premixed CH4/Air Flame.

Two-dimensional mapping of CH, CN and NH intermediates in a turbulent flame cross section, which concerns prompt NO formation, has been obtained by the PLIF technique. These results are achieved under low concentration and low quantum yield conditions (that is, a CH4/air flame at atmospheric pressure), by drawing the potential of a high-sensitivity image intensifier capable of photon signal detection by optimizing both the excitation-detection scheme and the light-collecting optics. Thanks to these improvements, semi quantitative CH measurement has been achieved utilizing predissociation. In addition, 2-D imaging of CN (sub ppm) and NH (0.01-0.1 ppm) has been achieved by a single laser pulse in a CH4/air flame, showing that the record of low concentration species detection by PLIF has been improved by an order of 1-2. The authors believe that these visualization techniques will mean a great improvement in the prompt No formation diagnostics in flames.