Spiral waves in expanding hydrogen-air flames: Experiment and theory

We report herein the first experimental observation of spiral waves over propagating flame surfaces in rich hydrogen-air mixtures at elevated pressures up to 40 atm, conducted in a specially designed, optically accessible, constant-pressure combustion chamber. The observed spiral waves are a manifestation of the large Lewis number instability, exhibiting behaviors such as clockwise/counterclockwise rotation, meandering, and fast radial wave speeds that are similar to patterns often observed in other excitable media, for example the Belousov-Zhabotinsky reaction. In addition, these spiral waves also exhibit features that seem to be characteristic of combustion systems, such as the transition criterion for diffusional-thermal pulsating instability, and their confinement within the hydrodynamic cells that also develop over such high-pressure flames of much reduced flame thicknesses. A diffusional-thermal theory was developed that successfully describes the observed spiral patterns.