Experimental investigation of the flame extinction processes of nonpremixed methane flames inside an air coflow diluted with CO_2, N_2, or Ar

摘要

The flame extinction processes and lifted flame stabilization features of nonpremixed flames issuing in diluted coflowing air were systematically examined under a wide range of aerodynamic conditions in order to reveal the competition between aerodynamics and dilution. Four diluents (CO_2, N_2, Ar, and CO_2 + Ar) were used in preparation for discriminating between the effects, namely, dilution, thermal action, and chemistry, induced by diluent addition. Flame extinction limits, ignition diagrams, liftoff height, and its associated apparent radius were investigated to provide key elements involved in lifted flame stabilization and extinction mechanisms with a diluted air stream. Flame extinction limits have been expressed as a 3D surface E_(extinction) in the physical space (Q_(diluent)/Q_(air), U_(air), U_(CH4). This surface is composed of two parts, a pure extinction surface where flame extinction is necessarily attained from lifted flames, and a common surface between flame lifting and extinction where flame extinction is achieved directly from attached flames. This distinct behavior is due to the presence of the semithick burner rim that protects the flame base. The flame ignition diagrams confirm that the pure extinction surface is independent of U_(CH4) for flames initially lifted without dilution. Furthermore, (Q_(diluent)/Q_(air))/K_(diluent), where K_(diluent) characterizes the capacity of a diluent to act on the flame destabilization process, has proved to be the affine parameter leading to a unified extinction surface for the tested chemically weak diluents. Moreover, by using (Q_(diluent)/Q_(air))/K_(diluent). liftoff height H_L/H_L~o and flame radius R_P/R_P~o reduced by the no-dilution measures merge to unique self-similar curves whatever the diluents and aerodynamic conditions. The key element is the flame leading-edge burning velocity, which was found to be identical for diluted flames when the diluents were added into the air stream in the relative proportions given by K_(diluent). In this way, it is possible to estimate flame behavior based on the results for CO_2, once K_(diluent) is known.