Carbon dioxide diluted methane/oxygen combustion in a rapidly mixed tubular flame burner

An inherently safe technique of rapidly mixed tubular flame combustion, in which fuel and oxidizer are separately injected from their own slits, has been extended to CO2 diluted methane/oxygen combustion. Two different cases are investigated. When CO2 is added only to the oxidizer slit, a stable tubular flame is obtained for a wide range of equivalence ratios with the oxidizers of oxygen mole fraction between 0.21 and 0.50. However, once the oxygen mole fraction exceeds 0.60, the stable flame is obtained within a small equivalence ratio range near the lean extinction limit, and the stable range becomes narrower and narrower with increasing the oxygen mole fraction. When CO2 is added also to the fuel slit so as to maintain the oxidizer/fuel injection velocity ratio near unity, and the flow rates of fuel and oxidizer are increased, the stable range is dramatically widened and a stable flame at stoichiometry can be obtained up to the oxygen mole fraction of 0.86, which yields adiabatic flame temperature around 2950 K. The Damköhler number, defined as the mixing to reaction time ratio, is examined to discuss the criterion for the establishment of stable tubular flame. It is found that when CO2 is added only to the oxidizer slit, stable flame can be obtained for Da < 1, whereas not for Da > 1 due to formation of diffusion flames near the fuel slits. When CO2 is added also to the fuel slit, however, stable flame can be established even for Da ⩾ 1 as well as Da < 1. Detailed observations indicate that when Da is adequately smaller than unity, the flame is uniform in luminosity, whereas the flame is non-uniform when Da is near or larger than unity. It is considered that the diffusion flame formation at the fuel slits is prohibited due to the high injection velocities, which gives a chance of fuel/oxidizer mixing, however, the tubular flame obtained is of non-uniform structure due to insufficient mixing time when Da ⩾ 1.