Experimental and reaction kinetic investigations on the formation of NO under elevated pressures using laser diagnostics in a rapid compression machine

 

Background

In many currently used technical combustion processes (e.g. in internal combustion engines or gas turbines) the emission of nitric oxides (NO_x), which are toxic and environmentally harmful pollutants, is an important problem. For the further reduction of nitrogen monoxide (NO) emissions by optimizing the combustion processes of internal combustion engines, accurate kinetic models of the NO formation processes are needed.

While the mechanism of the formation of thermal NO at high temperatures is to a large extend well understood, there is a larger lack of understanding in the models for the NO formation mechanisms which are based on the reactions of CH and CH₂ radicals at low and intermediate temperatures (so-called prompt NO). This leads to discrepancies between predicted and measured NO concentrations. In addition, it is well established that NO, which is already present in the fuel-air mixture before ignition when exhaust gas recirculation is used, alters the low temperature reaction kinetics. However, this effect cannot be sufficiently described by state-of-the-art models.

 

Aim of the project

The aim of this project is to gain an improved understanding of the reaction kinetics of NO at engine relevant pressures, particularly in the low and medium temperature range (500 – 1000 K). The project is conducted in cooperation with the research group “Physico-Chemical Fundamentals of Combustion” (PCFC). In a rapid compression machine (RCM) of the PCFC, different NO-doped fuel-air mixtures relevant for internal combustion engines are ignited by rapid compression. Thereby, particularly the kinetics of the ignition and reaction processes of the homogeneously premixed gas phase can be investigated without the influences of vaporization, mixture formation, and fluid flows. By means of laser optical diagnostics like laser-induced fluorescence (LIF) and spontaneous Raman scattering, quantitative 1-D spatially resolved profiles of the NO concentration and the temperature inside the combustion vessel are measured non-intrusively during the early reaction processes. In particular, this requires an optimization of the Raman scattering technique. Using these measurements, the existing reaction kinetic models will be validated and optimized by the PCFC.