The global residence time and the deviations from chemical equilibrium (i.e., the Damkohler number) were varied for a number of jet diffusion flames. The resulting effects on the nitric oxide emission index were measured and were compared with existing analysis. The global residence time is defined as L//U F, where L/ is the flame length and Up is the fuel jet velocity. Flame length is varied by increasing the jet diameter, by adding either premixed air or inerts to the fuel jet, or by adding a coaxial air stream. In particular, a unique jet flame was studied that is composed of helium-diluted hydrogen fuel; this flame is free of the complicating effects of flame radiation, buoyancy, and prompt NO and provides a useful baseline comparison to theory. It is found that NO x levels for three types of fuels were consistently less than levels predicted by thermal theory, which suggests that one or both of the two mechanisms that suppress NOx, namely strain and radiative cooling, are important. The use of a Damkohler number was found to successfully correlate the NO x data for the hydrogen/helium-air flames that have simple chemistry. As the helium concentration is increased in order to reduce the Damkohler number, the measured NO x emission index exceeds that of the equilibrium theory by as much as a factor of 24, which is further indication that it is important to add the correct nonequilibrium oxygen atom chemistry to current models.
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/dA), 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. NOMENCLATURE AF C 1 , C 2 dF, dA Lf S SL UA, UF stoichiometric air-to-fuel mass ratio constants that depend on fuel type diameter of fuel tube and air tube, respectively flame length swirl number maximum laminar burning velocity for a given fuel type initial axial velocity of air and fuel, respectively Greek Symbols a thermal diffusivity ~, 7, ~ parameters defined in Eq. 2 * To whom all correspondence should be sent.
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