Effects of preferential diffusion on spherical flame initiation and minimum ignition energy

Spherical flame initiation from an ignition kernel is studied theoretically and numerically for different fuel (hydrogen, methane, and propane)/oxygen/helium/argon mixtures. The objectives are to find the controlling length scale for spherical flame initiation and to reveal its relationship with the minimum ignition energy. The results show that it is the critical flame radius, rather than the flame thickness or flame ball radius, that controls spherical flame initiation. The minimum ignition energy for successful spherical flame initiation is found to be proportional to the cube of the critical flame radius. Furthermore, the preferential diffusion between heat and mass (Lewis number effect) is found to play an important role in the spherical flame initiation. It is shown that the critical flame radius and the minimum ignition energy increase significantly with the Lewis number. Therefore, for transportation fuels with large Lewis numbers, blending of small molecule fuels will significantly reduces ignition energy.