T. Lehre

Size distributions of nanoscaled particles and gas temperatures from time-resolved laser-induced-incandescence measurements

Laser-induced-incandescence (LII) signal decays are measured in sooting premixed atmospheric and low-pressure flames. Soot particle temperatures are obtained from LII signals measured at two wavelengths. Soot particle size distributions P(r) and flame temperatures T are measured spatially resolved by independent techniques. Heat and mass transfer kinetics of the LII process are determined from measured soot particle temperatures, flame temperatures, and particle sizes. Uncertainties of current LII models are attributed to processes during the absorption of the laser pulse. Implications for LII experiments are made in order to obtain primary soot particle sizes. Soot particle size distributions and flame temperatures are assessed from measured particle temperature decays by use of multi-D nonlinear regression.

Development of a measuring technique for simultaneous in situ detection of nanoscaled particle size distributions and gas temperatures

Point measurements of time-resolved LII signals have been performed in sooting premixed low pressure flames. Soot particle size distribution and gas temperature in these flames are known from independent measurements. This data is used to validate parameters of an improved LII model, where special emphasis is taken on the accurate modelling of mass and heat transfer rates. Using this model particle size distributions and gas temperatures can be estimated from time-resolved LII signals using non-linear regression. Standard numerical methods are applied. An experimental setup is presented, which allows measuring one-dimensional maps of particle size distribution and gas temperature. The technique is based on the one-dimensional and time-resolved detection of LII signals using a Streak camera.

Two-dimensional imaging of sizes and number densities of nanoscaled particles

In this work, an optical measuring technique for nonintrusive, in-situ, two dimensional mapping of volume fractions, number densities and median radii of nanoscaled particles is presented. The method is based on the simultaneous two-dimensional detection of RAYleigh-scattering and Laser Induced Incandescence (LII) combined with the measurement of the integral eXtinction from one single Nd-YAG laser pulse and is called RAYLIX. The experimental setup of this technique utilizes a standard Nd-YAG laser, which requires an optical delay line or a modified double pulse PIV-laser. The data evaluation based on the Mie theory of scattering in the Rayleigh regime and the linear dependence of the LII-signal on the volume fraction is discussed as well as an error analysis of the technique is given. Main error sources in the case of measuring soot arise from assumptions for the standard deviation of the lognormal particle size distribution and the refractive index of the soot particles. Applications of the RAYLIX technique are presented including investigations of sooting laminar and turbulent acetylene/nitrogen diffusion flames burning in air. From this, fundamental conclusions concerning soot formation and oxidation can be drawn and consequences for modelling of soot formation processes are discussed. Furthermore, laminar diffusion flames are investigated under conditions that are comparable to exhaust gas recirculation. In these flames increasing inert gas concentration in the coflow decreases rates of soot formation. Nevertheless, increasing inert gas concentration decreases oxidation rates leading to higher soot emission levels. Promising future applications of the RAYLIX method are discussed such as online monitoring of soot particles in carbon black production and the investigation of other nanoscaled particles. Time resolved point measurements of LII signal decays in a laminar low pressure premixed flat flame are compared with model predictions. It is shown, that information about particle size distributions can be extracted from the decay of the LII signal. By providing a method to invert LII signal decays measurement of particle size distributions is possible.