Investigating Molecular Decomposition via High-Speed Laser-Induced Rayleigh Scattering 2023-32-0118

Molecular decomposition is a key chemical process in combustion systems. Particularly, the spatio-temporal information related to a fuel’s molecular breakdown is of high-importance regarding the development of combustion models and more specifically about chemical kinetic mechanisms. Most experiments rely on a variety of ultraviolet or infrared techniques to monitor the fuel breakdown process in 0-D type experiments such as those performed in shock-tubes or rapid compression machines. While the information provided by these experiments is necessary to develop and adjust kinetic mechanisms, they fail to provide the necessary data for applied combustion models to be predictive regarding the fuel’s molecular breakdown.

In this work, we investigated the molecular decomposition of a fuel by applying high-speed planar laser Rayleigh scattering (PLRS). The experiments were performed in the vaporized region of initially liquid sprays of n-dodecane injected in an optically- accessible constant-volume vessel at temperature and pressure conditions relevant to compression ignition engines. The sensitivity of Rayleigh scattering to the molecular cross-section enables the diagnostic to track the mixing of the fuel and oxidizer, and also detect the time and location where the injected fuel decomposes into smaller species. We also explored the ability for PLRS to detect other molecular processes such as ignition or other species growth as a result of hydrocarbon combustion. The results indicate that molecular breakdown occurs first in leaner regions, on the jet periphery, and that it shortly precedes the first appearance of low temperature reactions, as measured via high-speed laser-induced fluorescence of formaldehyde. It was demonstrated that PLRS is able to detect heat release, providing information about ignition characteristics. Similarly, and under certain conditions, a strong rise in Rayleigh scattered signal was attributed to the formation and growth of soot particles. This work developed new ways to detect important combustion processes by applying high- speed PLRS.