Three-dimensional phenomenology of freely-propagating thermodiffusively-unstable lean premixed hydrogen flames

• Published in: Proceedings of the Combustion Institute 2024, Vol.40 (1-4), p.105634, Article 105634
• Main Authors: Aspden, A.J., Howarth, T.L., Hunt, E.F.
• Format: Article
• Language: English
• Subjects: Detailed chemistry; Direct numerical simulation; Hydrogen; Thermodiffusive instability; Three-dimensional freely-propagating flames
• ISSN: 1540-7489
• DOI: 10.1016/j.proci.2024.105634

Direct numerical simulation of three-dimensional freely-propagating thermodiffusively-unstable lean premixed hydrogen flames have been simulated in a canonical inflow-outflow configuration. Two studies have been considered, which differ mainly in the perturbation to the initial flame surface; specifically, the first used a broadband perturbation, and the second used a single-mode perturbation to constrain the flame evolution to a particular wavelength. The broadband study demonstrated how clusters of leading points interact to form large-scale meta-structures, and confirmed differences between two- and three-dimensional evolution. In two dimensions, two leading points couple to form the typical w-shaped flame finger, but in three dimensions, clusters of typically three-to-five leading points form instead. Nearby clusters can propagate away from each other, leaving a mode 2 structure behind, which undergoes secondary instability, forming a new cluster of leading points. On the larger scale, meta-structures are comprised of many clusters of leading points, and compete to fill the available volume. The domain sizes here are larger than similar previous studies, and confirms that the observations are not a result of computational restrictions. The single-mode study demonstrated that mode 3 structures are longest lived, with mode 4 also relatively stable; increasing the perturbation mode results in increasingly-early onset of symmetry breaking and ultimate breakdown of the cluster. Propagation speeds of leading points were largely independent of perturbation mode, but the separation between leading points and their centroid increased with mode number.