Trabold, Johannes Ludwig (2024)
Flame Topology and Thermochemical States in Laminar and Turbulent Biofuel Flames.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00026491
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

In the pursuit of addressing climate change, biofuels and e-fuels emerge as promising alternatives for powering heavy-duty vehicles, long-distance passenger aircraft, and ships. These sectors account for approximately thirty percent of global carbon dioxide emissions, necessitating effective solutions. This research aims to conduct experiments to capture time and spatially-resolved validation data for these sustainable fuels, while also gaining insights into fuel-specific differences in flames.

This research is dedicated to advancing the understanding of fuel-specific combustion processes by achieving the following three primary objectives. Firstly, it involves the development of vaporized fuel burner systems with well-defined boundary conditions as the foundation for combustion system research. Secondly, it characterizes the flames of the four lowest alcohols stabilized on these burner systems, considering factors such as flame blow-off, size, and topology. Lastly, the research targets ethanol flames through qualitative and quantitative Raman and Rayleigh spectroscopy investigations, addressing the challenge of managing intermediate species associated with complex fuels.

The employed methodologies include OH-Planar laser-induced fluorescence for flame topology characterization, qualitative long-exposure Raman spectroscopy to unveil intermediate species, and single-shot Raman and Rayleigh spectroscopy to provide the first quantitative thermochemical state measurements in laminar and turbulent premixed ethanol flames. Comprehensive comparisons are made between bio- and e-fuels and the established reference fuel methane. One-dimensional numerical flame calculations complement the experimental approaches, providing additional insights.

Key findings include that turbulent alcohol flames exhibit increased wrinkling as the equivalence ratio transitions from lean to rich, accompanied by a decrease in the Lewis number. Qualitative highly-resolved long-exposure Raman spectroscopy reveals methane, formaldehyde, ethylene, and acetaldehyde as major intermediates in ethanol flames, while OME-3 flames predominantly show formaldehyde and methane. Lastly, capturing thermochemical states in ethanol/air flames is made possible through the following two pivotal innovations. Firstly, the traditional calibration role of the flat flame in the Matrix inversion Raman evaluation method is replaced by probing the center of opposed twin flames, overcoming limitations related to vaporized fuels. Secondly, a surrogate signal based on ethanol, carbon monoxide, and temperature is developed, effectively reproducing previously inaccessible intermediates within the ethanol/air flame front. This knowledge enables the quantification of the main flame species and temperatures with significantly enhanced accuracy.%Thermochemical states in ethanol flames are captured through innovations in the Raman matrix inversion calibration and the development of an intermediate surrogate signal based on ethanol, carbon monoxide, and temperature.

In summary, this work advances the apparatuses to study flames of prevaporized fuels, characterizes flame topology, and improves Raman and Rayleigh flame spectroscopy and its data evaluation. These accomplishments contribute to the understanding of biofuel and e-fuel combustion, play a crucial role in numerical model validation, and promote the use of renewable fuels in the transition to clean energy.

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