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 ( U F d F ) ( S L 2 α F ) . The fuel velocity at blowout ( U F ) was measured while four parameters were systematically varied: the fuel tube diameter ( d F ), the fuel type and thus reaction rate, which is related to the maximum laminar burning velocity ( S L ), the coaxial air velocity ( U A ), and the swirl number. Results show that for zero swirl, the blowout curves agree with curves predicted by previous analysis. However, as swirl 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 swirl number is not the appropriate governing parameter. Instead, a Damkohler number based on swirl 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.