Enhancement of flame blowout limits by the use of swirl

Abstract

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

Experiments and simulations of NOx formation in the combustion of hydroxylated fuels

“…Previous work with this burner has investigated the emission from the combustion of glycerol and crude glycerol from biodiesel production and is reported in the work of Bohon et al [2,3]. This setup utilizes a high swirl burner, functionally similar to that of Chen et al [13][14][15], in which two independently controlled combustion air streams create swirling flow. A schematic illustration Table 1 Thermophysical properties of fuels from DIPPR database [12].…”

“…The calculation of the geometric swirl number is based on the analysis of Claypole and Syred [16]. Analysis of the swirl number in this similar geometry was conducted by Feikema [15] using laser velocimetry. Their work has shown that the swirl number was proportional to the geometric swirl number times a constant with the geometric swirl number calculated using Eq.…”

Unsteady Aspects of Lean Premixed Prevaporized Gas Turbine Combustors: Flame-Flame Interactions

“…Swirl is introduced by two coannular sets of pilot swirl vanes, and an outer annulus of main swirl vanes. Swirl improves the flame stability for reasons that are discussed in [33].…”

Scaling and development of low-swirl burners for low-emission furnaces and boilers

“…The swirl number, S, of jet-LSBs can be defined by the mass flow rate of the reactants, ṁ ‫ס‬ ṁ air ‫ם‬ ṁ fuel , the swirl air, ṁ h , and the total swirl injector area, A h [12,13]. 2 …”