Experimental study of the lifting characteristics of the leading-edge of an attached non-premixed jet-flame: Air-side or fuel-side dilution

Abstract

The impact of air-side and methane-side dilution (CO2, N2 and Ar) on the lifting process of attached non-premixed methane/air coaxial jet flames is studied over a wide range of aerodynamic conditions. The study of the competition between aerodynamics and dilution has allowed to discriminate and quantify the different phenomena involved in the lifting process. First, two effects, only dependent on the amount of the added diluent, contribute to promoting flame detachment (liftoff): a fluid mechanical effect that causes the bulk velocity of the reactants to increase; a mixing effect that changes the mixture fraction spatial distribution. They are significant for the methane-side dilution but negligible for the air-side dilution. Then, the main mechanism and the dimensionless numbers characterizing flame liftoff are identified. The attached-flame stability is analyzed on the basis of the lifting limits measured by the critical flow rate ratios, (Qd/Qf)lift and (Qd/Qox)lift when the diluent is added either to the methane or the air stream. These limits follow self-similarity relationships based on the fuel and oxidant Peclet numbers of the diluted streams, satisfied whatever the diluents. Results using PIV and CH* measurements are interpreted through a flame-leading-edge approach, where CH4/air/diluent are mixed locally at the flame base. The flame propagation velocity SL which balances the incoming gas velocity, is shown to be described by self-similarity relationships based on the molar fraction at the leading-edge reduced by values at liftoff, Xd/Xliftd. To confirm the leading-edge propagation characteristics, the flame attachment height Ha and radius Ra are investigated at the attached-flame base. Ra is representative of the mixing and mass effects induced by the pure dilution. Contrary to Ra, Ha is piloted by SL, and evolves according to a unique law dictated by Xd/Xliftd. Xliftd is a self-similar parameter highlighting the propagation nature of the leading-edge.