The detailed flow, temperature and concentration fields of a hydrogen-air Bunsen flame have been examined by a precise numerical study that uses the exact transport properties and the full chemical reaction mechanism. The study has revealed that the chemical reactions along the flame cone portion do not remain uniform; the H2 consumption rate decreases towards downstream. The decrease of H2 concentration of the mixture coming into the cone, caused by the radial outward diffusion of mobile H2 molecules, leads to the slowdown of the main H2 consuming elementary reactions downstream. Circumferential curvature of the axisymmetric flame and thermal diffusion are not important in the observed behavior. It is also revealed that the structure of the flame tip is fundamentally two-dimensional, in the sense that the mixture coming into the tip continues the combustion reaction with a strong aid of heat diffusion from the outer flame cone. The result raises a serious question if the tip can be called a part of the self-sustained premixed flame.
In this study the possibility of ultra-lean combustion of methane-air tubular flames was investigated by numerical calculation with detailed chemistry, and the results were compared with counterflow twin flames. As a result, it was found that tubular flames can realize ultra-lean combustion since the radiative heat loss is much smaller than planar 1-D flames because of the smallness of the residence time of the burned gas, but the equivalence ratios of the leanest limits of tubular flames are larger than those of the counterflow twin flames of the similar stretch rates due to an intrinsic tendency of extinction based on the configuration of tubular flames. In addition, the possibility of ultra-dilute combustion of tubular flames of CO2 diluted methane and air was also investigated numerically. The obtained result showed that tubular flames can also realize ultra-dilution combustion, and the extent of dilutions at the largest dilution limits for tubular flames are smaller than those of counterflow twin flames of the similar stretch rate, due to the same reasons as the ultra-lean combustion case.