Fundamental burning velocities of meso-scale propagating spherical flames with H2, CH4 and C3H8 mixtures

This study is performed to experimentally examine the fundamental burning velocity characteristics of meso-scale outwardly propagating spherical laminar flames in the range of flame radius rf approximately from 1 to 5 mm for hydrogen, methane and propane mixtures, in order to make clear a method for improving combustion of micro–meso scale flames. Macro-scale laminar flames with rf > 7 mm are also examined for comparison. The mixtures have nearly the same laminar burning velocity (SL0 = 25 cm/s) for unstretched flames and different equivalence ratios ϕ. The radius rf and the burning velocity SLl of meso-scale flames are estimated by using sequential schlieren images recorded under appropriate ignition conditions. It is found that SLl of hydrogen and methane premixed meso-scale flames at the same rf or the Karlovitz number Ka shows a tendency to increase with decreasing ϕ, whereas SLl of propane flames increases with ϕ. However, SLl tends to decrease with the Lewis number Le and the Markstein number Ma, irrespective of the type of fuel and ϕ. It also becomes clear that the optimum flame size and Ka to improve the burning velocity exist for some mixtures depending on Le and fuel types.