G. Zizak

Determination of the Soot Volume Fraction in an Ethylene Diffusion Flame by Multiwavelength Analysis of Soot Radiation

Abstract The features of the multiwavelength emission technique for the measurement of soot volume fraction and temperature in an ethylene diffusion flame have been investigated. For this purpose we have exploited the emission spectra from 300 nm to 800 nm and a mathematical Abel inversion procedure. To interpret the measurements, the quantity fv/Kabs = λ/(36πF(λ)), here called natural length for absorption, l abs, is modeled as a continuous function according to literature data. Soot temperature profiles were also obtained showing small variations in the investigated region of the flame. Extinction measurements on the same flame were compared, and good agreement was found between the two techniques. With respect to the more-common two-color emission technique, the use of a wide set of spectral data reduces the uncertainties due to the determination of the index of refraction of soot. Different sets of refractive index have been compared and the results are discussed.

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 techniques potential, data about methane and propane diffusion flames are reported.

Two-color laser-induced incandescence (2C-LII) technique for absolute soot volume fraction measurements in flames

A two-color version of the laser-induced incandescence (2C-LII) technique was implemented for measuring absolute soot volume fraction in flames. By using a calibrated tungsten ribbon lamp, soot peak temperatures were measured as a function of fluence at several locations in an ethylene diffusion flame by using a steeply edged laser beam profile. Above a certain fluence threshold, peak temperatures were tightly distributed just above 4000 K independent of the particle size and number density. Radial profiles of soot volume fraction were obtained and compared (not calibrated) with results from the laser extinction technique. Good agreement showed the validity of the 2C-LII technique at a controlled fluence.

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.

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.

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 (PAHs) could be detected. Linear (one-dimensional) visualizations of OH, PAHs, 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.

Absorption correction of two-color laser-induced incandescence signals for soot volume fraction measurements

A numerical iterative procedure is presented for the evaluation of the effect of signal absorption in two-color laser-induced incandescence measurements. The correction process is applied to our experimental data in an axisymmetric flame [Appl. Opt. 44, 7414 (2005)]. The influence of signal trapping on peak soot temperature and on soot volume fraction has been found to be minimal. Some numerical tests were performed to investigate the effects of soot concentration, flame size, and soot refractive index on the magnitude of the signal absorption correction.

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.

A webcam as a light probe beam profiler.

where k(IL, nL, TV) is the nonlinear absorption coefficient (corresponding to the resonant [6S1/2(Fg) → 6P3/2(Fe)] atomic transition), which depends on the laser intensity IL, the laser frequency nL, and the Cs vapor temperature TV; iC is the electrical current associated with the generation of electron–ion Cs2 pairs; and ip is the current due to the laser-induced polarization of the Cs vapor excited at the 6P3/2 level. Since there are two types of charge carriers (electrons and positive ions), we can write

Soot load versus aromatic concentration in diesel oil premixed flames

Abstract Soot production of diesel oils, characterised by a well-defined aromatic content, was investigated in rich premixed flames by laser extinction technique. By analysing the soot plume, a criterion to assign a ranking in sooting tendency was found, based on the connection between the maximum soot volume fraction detected along the flame axis, f v M , and the equivalence ratio. A relationship between the total soot load and f v M was established, which allows to determine a correlation between the soot load and the aromatic composition. The resulting linear dependence is discussed.