Two-dimensional two-wavelength emission technique for soot diagnostics.

A two-dimensional soot diagnostic technique has been developed as an extension of the well-known two-color pyrometry. Two flame images are simultaneously collected on a CCD at selected wavelengths through suitable optics. By use of the dependence of soot emissivity on the soot volume fraction and by comparison with images from a calibrated light source, both the temperature field and the soot distribution can be determined. Validation was carried out through data obtained with other soot diagnostic methods on ethylene diffusion and Diesel oil-rich premixed flames. The current technique readily allowed us to obtain a large amount of data for a thorough description of the soot distribution within the flame. As an example of the technique's potential, data about methane and propane diffusion flames are reported.

Determination of soot parameters by a two-angle scattering-extinction technique in an ethylene diffusion flame.

We used a two-angle scattering technique to investigate the soot distribution in an ethylene diffusion flame in conjunction with extinction measurements. In the framework of a fractal description, we introduced a modified structure factor to interpret the scattering intensity from polydisperse aggregates. The connection between a mean value of a structural radius of gyration, R(gm1), and the quantities experimentally measured was then established. Soot parameters (volume fraction, particle size, and number densities) were determined along three radial sections of a 8-cm high-diffusion flame. The stability of the results with respect to the parameters of the distribution function was studied.

Time-delayed detection of laser-induced incandescence for the two-dimensional visualization of soot in flames.

The time-delayed detection of soot incandescence is demonstrated to discriminate against other laser-induced signals that have shorter decay times. This technique exhibits high sensitivity and no need for any verification of the spectral content of the signal; it is promising for two-dimensional imaging applications in hostile environments, such as in practical flame and combustion chambers, in which it permits an easy visualization of sooty regions.

Simultaneous one-dimensional visualization of OH, polycyclic aromatic hydrocarbons, and soot in a laminar diffusion flame.

Laser-induced fluorescence measurements were performed in several regions of an ethylene-air diffusion flame. With the use of a 290-nm excitation wavelength, the fluorescence emissions from OH molecules and polycyclic aromatic hydrocarbons (PAH's) could be detected. Linear (one-dimensional) visualizations of OH, PAH's, and soot scattering were performed by utilizing an intensified photodiode array detector. Measurements taken at different heights above the burner permitted a two-dimensional reconstruction of the flame structure. Finally the simultaneous visualization of the three species for a rapid determination of the relative distributions was demonstrated.

Ground state saturated population distribution of OH in an acetylene-air flame measured by two optical double resonance pump-probe approaches.

Two optical double-resonance pump-probe techniques were used to determine the ground-state rotational population distributions of OH in an acetylene-air flame when a saturating laser beam is tuned to the Q(1)4 transition of the (0, 0) Sigma-II band. The saturated absorption technique is based on the detection of absorption by a probe laser under conditions of saturation with a pump laser and no saturation. In the fluorescence technique, a probe laser is scanned through the (1, 0) band, while a saturating pump laser, tuned to the (0, 0) band, is on or off. We found that approximately 15% of the total population of the ground state was transferred to the excited state. Perturbation of the rotational population distribution was greater for rotational levels close to the directly excited laser-coupled level. The rotational energy transfer rate in the ground state was somewhat greater than in the excited state. The assumption of the balanced cross-rate model was verified as a means of determining the absoslute OH number density with adequate accuracy.

Spatially resolved saturated absorption measurements of OH in methane-air flames.

A cross-beam saturated absorption spectroscopy technique, utilizing a single pulsed dye laser, has been developed for local concentration measurements in flames. With a differential detection of the probe and the reference laser beam intensities, a significant improvement of the technique has been achieved. In this work the basic theory of the method is discussed. Its use in combustion studies is demonstrated by presenting OH concentration profiles in two premixed laminar methane-air flames.

Five laser-excited fluorescence methods for measuring spatial flame temperatures. 1: Theoretical basis.

Five methods for measurement of flame temperatures based upon laser-excited fluorescence are discussed with respect to the derived expressions, the assumptions necessary, the advantages, and the disadvantages. The use of each of the five methods for measuring spatial flame volumes (~<1 mm(3)) and temporal flame temperatures (corresponding to ~<1-microsec time period) is given. The five methods consist of one based upon linearity between the fluorescence signal and the laser spectral irradiance, three based upon saturation, and a fifth, which is not critically dependent upon the laser spectral irradiance.

Rate equation solution for the temporal behavior of a three-level system.

The time behavior of a three-level system has been solved with general assumptions using the rate equation approach and under a pulsed spectral irradiance with rectangular shape. Three different specific cases, corresponding to possible conditions for inorganic atom seeds in atmospheric flame, have been investigated making use of literature values for the collisional and radiative rate constants. Results of these calculations are presented for Tl and Ga atoms when excited via the transitions 1 ? 3, 2 ? 3, and 1 ? 2 under different source spectral irradiances, flame conditions, and the most significant collisional coupling constants between the various levels.