Laminar Spherical Flame Kernel Investigation of Very Rich Premixed Hydrocarbon-Air-Mixtures in a Closed Vessel under Microgravity Conditions

In this work very rich premixed laminar spherical flame kernels of hydrocarbon-air combustible mixtures were experimentally and numerically investigated under microgravity conditions. These microgravity combustion experiments were carried out in the Drop Tower of Bremen University. The Closed-Vessel-Bomb-Method (CVBM) was applied for all experimental investigations combined with a monochromatic Helium-Neon-Schlieren Measurement Technique. Images of the propagating spherical flames were tracked with a High-Speed-Camera. The pressure vessel enables optical access and contains a volume of approx. half a litre. Combustible Mixtures were investigated at initial pressures up to 30 bar and initial temperatures were 420 K for all experiments, whereas the equivalence ratio for investigated N-Pentane-Air-Mixtures was Φ=3.0, N-Hexane-Air-Mixtures was Φ=3.3, N-Heptane-Air-Mixtures was Φ=3.5 and the equivalence ratio for investigated Isooctane-Air-Mixtures was Φ=3.9 for all experiments. All investigated flames appeared to be smooth. Mainly no wrinkles due to flame instabilities could be observed on the flame surface. The propagating spherical radii over time of the spherical flames were tracked with an image processing code. Furthermore, linear extrapolation of flame propagating velocity with respect to the burned mixture to the zero stretch value was done. Interestingly, at these “very” rich equivalence ratios the laminar flame velocities increase with increase in initial pressure. This diametric effect, due to e.g. stoichiometric or lean flames, could be shown in the numerical results as well. One dimensional laminar flame calculations [1] were done with a modified skeletal mechanism [2] for N-Heptane- and Isooctane-Air-Mixtures.