Flames interact very closely with (cold) walls of the reaction chamber up to the point of extinction. This is caused by heat conduction and radiation to the wall and the associated temperature drop in the reaction zone as well as by the destruction of reactive radical intermediates at the surface. Many pollutants within the exhaust of internal combustion engines, such as unburned hydrocarbons, carbon monoxide (CO) and soot, originate mainly from regions close to cold walls, since the strongly decreasing temperature quenches the combustion process. In the light of more restrictive emission standards, combustion processes and flame extinction phenomena in the vicinity of (cold) walls are becoming increasingly important for the design of modern engine generations.
Soot formed in the combustion process, but also the pyrolytic decomposition of fuel, may lead to the formation of deposits on the combustion chamber walls. In our work we focus on the influence of wall deposits on the flame-wall-interaction of sooting and non-sooting flames. In addition we will investigate the effect of a ceramic wall-coating on the flame-wall interaction and the formation of wall deposits. The results obtained are used to judge, qualitatively and quantitatively, the relevance of soot formation during combustion, as well as those of deposits, coatings and films on the near-wall thermodynamic and chemical processes during the flame-wall interaction.
The experiments are carried out in close cooperation with subproject A04 (Dreizel), in which a test rig based on a pre-mixed, wire stabilized flame is designed and constructed. One branch of the V-flame interacts with a temperature stabilized wall (Figure 1). With an identical setup constructed in Karlsruhe, sidewall quenching (SWQ) can first be investigated under laminar, steady state conditions. In addition to being optically accessible, the generic SWQ-burner configuration has the advantage to mimic the conditions inside a gasoline engine in a simplified manner.
To explore the sidewall quenching in light of the above mentioned background, we use our planar optical emission tomographic technique (POET), which has been specifically developed for the investigation of combustion processes (Figure 2), as well as laser based spectroscopic methods. Where appropriate, a combination of both methods is employed. After suitable modification, the tomographic system allows the simultaneous detection of multiple chemiluminescent species close to the wall at different wavelengths. The simultaneous detection of OH* and / or CH* and C2* will make it possible to visualize the reaction zone close to the wall and its extinction. In the case of sooting combustion, the influence of the wall on the combustion process and the formation and oxidation of soot is studied by simultaneously detecting OH* and soot for walls with and without deposition layers. We complement the (laser based) optical in-situ (sooting) gas phase measurements by simultaneously detecting the wall heat losses through a matrix of thermocouples for measuring the local wall temperatures. In addition, we utilize SEM and TEM methods for the morphological characterization and EDXS to determine the elemental composition of the deposits.