The blowout limits of a number of swirl stabilized flames were measured and the trends are explained by applying the concepts proposed in recent flame blowout theories, which previously have been applied only to non-swirling flames. It is shown that swirl flame blowout limits can be compared to wellknown limits for non-swirling simple diffusion flames by using the proper nondimensional parameter, i.e., the inverse DamkohlerUnlike most previous work. four parameters were systematically varied: the fuel tube diameter (dF), the fuel type and thus reaction rate, which is related to the maximum laminar burning velocity (SL), the coaxial air velocity (UA), and the swirl number.Results show that the maximum fuel velocity (UF) and thus the maximum heat release rate for a swirl flame is as much as four times larger than that for a non-swirling flame. Blowout velocity (UF) increases with burner size (dFj and laminar burning velocity squared (SL2); this is similar to non-swirling flames except that a new parameter that includes swirl number must be added. The major reason why swirl increases the stability of a flame is because of a flame-vortex interaction. The toroidal recirculation vortex reduces the centerline velocity below that of a non-swirling flame and the analysis shows that this is strongly stabilizing.
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.
The blowout limits of a number of swirl-stabilized, nonpremixed flames were measured, and the observed trends are successfully explained by applying certain concepts that previously have been applied only to nonswirling flames. It is shown that swirl flame blowout limits can be compared to well-known limits for nonswirling simple diffusion flames by using the proper nondimensional parameter, i.e., the inverse Damkohler number (UF/dF)/(SL 2/ctF). The fuel velocity at blowout (UF) was measured while four parameters were systematically varied: the fuel tube diameter (dE), the fuel type and thus reaction rate, which is related to the maximum laminar burning velocity (St.), the coaxial air velocity (UA), and the swirl number. Results show that for zero swirl, the blowout curves agree with curves predicted by previous analysis. However, as swift is added, the flame becomes five times more stable (based on maximum fuel velocity). To explain the effect of swirl, a simple analysis is presented that is an extension of previous nonswirling flame blowout theory. It shows that the conventional swift number is not the appropriate governing parameter. Instead, a Damkohler number based on swift velocity is suggested by the analysis and is found to help collapse the data at the rich blowout limit to a single, general curve. Swirl causes a jet-vortex interaction; the recirculation vortex reduces the fuel jet velocity on centerline, which strongly stabilizes the lifted flame. As one increases the fuel tube diameter or the reaction rate (by adding hydrogen), the swirl flame becomes more stable, in a manner similar to a nonswirling flame. Another advantage of swirl is that it makes overall fuel-lean operation possible; the present flame is unstable without swirl for fuel-lean conditions. NOMENCLATURE UF dA air tube inner diameter (Fig. 1) Uj dF, dF,o fuel tube inner, outer diameter Dax-1 critical inverse Damkohler number for URZ jet flame blowout, given by Kalghatgi (Eq. 3) Uo DaB-1 critical inverse Damkohler number for jet flame blowout, given by Broadwell Uo et al. (Eq. 5) Dao-1 critical inverse Damkohler number Uo, c measured for UA = 0 S swirl number at throat OfF, l: F Sg geometric swirl number at throat, as given by Eq. 7 as SL, Sr maximum laminar, turbulent burning velocity UA axial air velocity at throat location de-6 fined in Fig. 1 ~bo UCL axial velocity on centerline
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