Effect of dilution gas on burning velocity of hydrogen-premixed meso-scale spherical laminar flames

Aiming to make clear a method for improving combustion of micro/meso-scale flames, effects of dilution gas and characteristic chemical time τc on the burning velocity of meso-scale flames were studied experimentally using H2-O2-dilution gas mixtures. The meso-scale outwardly propagating spherical laminar flames, in a flame radius rf range of approximately from 1 to 5 mm, were observed by using sequential schlieren images recorded under appropriate ignition condition. The mixtures at each equivalence ratio (ϕ = 0.5-1.0) were diluted with Ar, CO2 or N2 to set different laminar burning velocities (SL0 = 15-90 cm/s) at so-called unstretched flames, because τc can vary depending on SL0. The radius rf and the burning velocity SLl of micro-scale flames were estimated from obtained images. Macro-scale laminar flames with rf >7 mm were also examined for comparison. It was found that the burning velocities of meso-scale flames for the mixtures diluted with CO2 at ϕ = 0.5-1.0 have a tendency to decrease with increasing rf, and approach those of macro-scale flames, whereas such a trend cannot be seen for the mixtures diluted with Ar as well as N2 at ϕ = 0.7-1.0. This shows flame size and Karlovitz number to have optimum values to improve burning velocity. The SLl at the same rf; however, tended to meaningfully decrease with the Lewis number Le and the Markstein number Ma, irrespective of the dilution gas types in addition to ϕ. It also becomes clear that the observed optimal values of each dilution gas are little dependent on τc.