Methane/oxygen combustion in a rapidly mixed type tubular flame burner

An inherently safe technique of rapidly mixed type tubular flame combustion has been applied to pure oxygen combustion. The flame characteristics under various oxygen mole fractions are experimentally investigated with use of burners of different slit widths, and in addition, with optically accessible quartz burners. To fundamentally understand the requirements for rapidly mixed pure oxygen tubular flame combustion, the characteristic reaction times were calculated with Chemkin code, while the characteristic mixing times were estimated based on the width of mixing layer determined. Results show that the rapidly mixed type tubular flame combustion has been obtained under oxygen-enriched and even pure oxygen conditions. At high oxygen mole fraction, diffusion flames are anchored at the exits of the fuel slits, which restrains mixing between fuel and oxidizer, resulting in a failure of tubular flame combustion. By reducing the slit width, however, the diffusion flame is inhibited and a uniform tubular flame can be obtained from 0.11 to 0.18 in overall equivalence ratio. To quantify the mixing process, the flow field was visualized, and it has been found that there exist two types of flows, a boundary layer type flow near the exits of the slits close to the wall and an axisymmetric potential flow around the axis of rotation. Based on the mixing layer width of the boundary layer type flow, the Damkohler numbers are obtained and it is clearly shown that when the Damkohler number is less than unity, the mixing is completed before the onset of reactions, resulting in establishment of tubular flame combustion.