Characteristics of Flame Quenching and Blowout Mechanisms

Abstract

The present study employs detailed simulati ons of coflow and counterflow flames in order to examine the underlying differences between the flame quenching and flame blowout phenomena. Both nonpremixed and partially premixed flames are investigated. A time accurate, implicit algorithm that uses a de tailed description of the chemistry and includes radiation effects is used for the simulations of coflow flames. Because of its flame suppression characteristics, CO 2 is used as a diluent. Results indicate that in counterflow flames, as diluent is added to the fuel or air stream, it decreases the flame temperature due to thermal and dilution effects. This in turn reduces the radical pool since the chain branching reactions, which are temperature sensitive, become less dominant than the chain terminating rea ctions. When the flame temperature is reduced sufficiently, further increase in dilution turns off the chemical kinetics, extinguishing the flame through quenching. Hence, the effect of dilution is to reduce the Damkohler number (Da) until the flame exting uishes. On the other hand, in jet flames, as diluent is added to either the fuel or air stream, it increases the scalar dissipation rate ahead of the flame ( �) and the specific heat capacity. These two effects reduce the flame temperature and chemical reactivity, causing local quenching. This local quenching in the jet flame is due to reduced Damkohler number (Da) similar to that occurring in counterflow flame s. The flame consequently moves downstream along the stoichiometric mixture fraction line (f S) to a location of lower scalar dissipation rate ( �) (or higher Da), and the flame liftoff height (L f) increases. The flame at this location exhibits a double flam e structure, with the stabilization mechanism depending on a balance between the local scalar dissipation rate ( �tri ) and the chemical reactivity. Further increase in dilution moves the flame further downstream where it exhibits a triple flame structure. H ere the flame stabilization also depends on a balance between the triple flame speed (S tri ) and the local flow velocity, which in addition involves the balance between the global flame speed (U F) and the incoming flow velocity. In contrast with the flame s peed associated with counterflow flames, the triple flame speed increases with dilution due to the reduced scalar dissipation rate near the triple point ( �tri ) (i.e. increased Da) and enhanced diffusive -thermal instability (Ma �0), both of which counteract the thermal -dilution suppression effect of the diluent. The increase in triple flame speed (S tri ) due to dilution in turn enhances the global (or far -fiel d) flame speed (U F) through the flow redirection effect (i.e. U F/S tri =�(�b/�u)). When the global flame speed (U F) exceeds the incoming the flow velocity, the triple flame cannot longer be stabilized in the flow field and extinguishes through blowout.