Visualization of soot formation and oxidation from evaporating fuel films

• Applicant: Professor Dr. Sebastian Kaiser
• Subject Area: Energy Process Engineering ; Hydraulic and Turbo Engines and Piston Engines
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 464457035

The formation of solid soot particles from gaseous polycyclic aromatic hydrocarbons (PAH) - both important carcinogenic pollutants in combustion - is still not well understood. PAH fluoresce after excitation in the visible and ultraviolet (UV) spectral ranges. The excitation and emission spectra shift to longer wavelengths as the molecule size increases. This can be exploited to optically image the stages of PAH growth by laser-induced fluorescence (LIF). Soot can be detected by laser-induced incandescence (LII) excited in the infrared. In principle, this allows for optical tracking of the soot formation process. So far, however, corresponding work in the laboratory has concentrated on laminar flames, and in technical combustion on Diesel engines, whose "soot problem" is well known. But significant amounts of PAH and soot can also be formed in modern gasoline engines. This occurs mainly when the gasoline injection wets the wall with fuel, which then evaporates poorly. In this project, optical imaging techniques are developed and applied to investigate and better understand the formation and oxidation of PAH and soot from liquid fuel films. In particular, a goal is to clarify whether soot formation in gasoline engines is more of a high-temperature pyrolysis in an oxygen-depleted environment or more like a mixing-controlled pool fire. It will be analyzed where and when soot oxidation takes place in the combustion chamber, to what extent PAH of different size classes are exposed to oxidation by hydroxyl radicals (OH), and what is the influence of engine conditions such as temperature, injection quantity, and fuel type on the spatio-temporal structure of the transition from PAH to soot and the oxidation of PAH and soot. Beyond the project, the experiments will provide data for the development of simulation tools, in particular in the context of the international cooperation "Engine Combustion Network".For this purpose, investigations are mainly performed in an optically accessible research engine. The formation and evaporation of the fuel films is quantitatively imaged by LIF. Parameter variations include the global air/fuel ratio, the fuel composition, and the wall temperature. In a well-characterized laminar diffusion flame, the excitation wavelengths and detection filter-bands are systematically varied to find combinations that allow for size-class specific detection of PAH, and strong signals. Suitable combinations are then used in the engine. To investigate the spatiotemporal sequence of soot formation and oxidation, suitable combinations of PAH LIF, soot LII, and OH LIF are used simultaneously. These stroboscopic light-sheet techniques are complemented by line-of-sight-integrating "high-speed" visualizations of the combustion process.

• DFG Programme: Research Grants