Blowout of nonpremixed flames: Maximum coaxial air velocities achievable, with and without swirl

The present study demonstrates how to optimize parameters in order to maximize the amount of coaxial air that can be provided to a nonpremixed jet flame without causing the flame to blow out. Maximizing the coaxial air velocity is important in the effort to reduce the flame length and the oxides of nitrogen emitted from gas turbines and industrial burners, a majority of which use coaxial air. Previous measurements by the latter two authors have shown that a sixfold reduction in the NO x emission index of a jet flame is possible if sufficient coaxial air can be provided without blowing the flame out. The coaxial air shortens the flame and forces the reaction zone to overlap regions of higher gas velocity, which reduces the residence time for NO x formation. The present work concentrates on demonstrating ways to prevent flame blowout when the following two constraints are imposed: (1) the coaxial air velocities must be sufficient to shorten the flame to a specified length (in order to reduce NO x emissions) and (2) the coaxial air flow rate must be sufficient to complete combustion without the need for ambient air, which is a common practical constraint. The zero swirl case is considered first, and the effects of adding swirl are measured and directly compared. The following were systematically varied: fuel velocity, air velocity, fuel tube diameter, air tube diameter, fuel type, and swirl number. Measurements demonstrate that coaxial air alone (with zero swirl) can cause up to a twofold reduction in flame length. However, the flame is stable only if the velocity-to-diameter ratio of the fuel jet does not exceed a critical value. It is found that the addition of swirl improves the maximum-air blowout limits by as much as a factor of 6. The results identify a strain parameter, based on the ratio of air velocity to air tube diameter ( U A d A ), which collapses the blowout curves for ten different conditions (burner size, swirl number) approximately to a single curve. A physical mechanism that explains the swirl flame data is presented. Swirl is believed to be beneficial because it reduces the local velocities, and thus the local strain rates, near the forward stagnation point of the recirculation vortex, where the flame is stabilized.