Formation of Cellular Flames and Increase in Flame Velocity Generated by Intrinsic Instability

抄録

The formation of cellular flames and the increase in flame velocity generated by intrinsic instability are studied by two-dimensional (2-D) and three-dimensional (3-D) unsteady calculations of reactive flows based on the compressible Navier-Stokes equation. We consider three basic types of phenomena to be responsible for the intrinsic instability of premixed flames, i.e., hydrodynamic, diffusive-thermal, and body-force instabilities. Cellular flames are generated by intrinsic instability, and thus the flame-surface area becomes larger and the flame velocity increases. The increment in flame velocity of 3-D flames is about twice that of 2-D flames, since the increment in flame-surface area of the former is about twice that of the latter. This relationship is due to the difference in the disposition of cells between 2-D and 3-D flames. When the Lewis number is lower than unity, i.e., when diffusive-thermal instability appears, the increment in flame velocity is larger than in the flame-surface area. This is because the increase in the local consumption rate of the unburned gas at a convex flame surface exceeds the decrease at a concave one. When the Lewis number is unity, i.e., when hydrodynamic and body-force instabilities are dominant, the flame velocity is nearly proportional to the flame-surface area, since the local consumption rate is nearly constant.