A Numerical Investigation Of Flame LiftOff And Stabilization

Abstract

*† The effects of both fuel and air stream dilution on the liftoff, stabilization, and blowout characteristics of laminar nonpremixed flames (NPFs) and partially premixed flames (PPFs) are investigated. Lifted methane-air flames were established in axisymmetric coflowing jets. Because of its flame suppression characteristics CO2 is used as a diluent. A time-accurate, implicit algorithm that uses a detailed description of the chemistry and includes radiation effects is used for the simulations. The predictions are validated using measurements of the reaction zone topologies and liftoff heights of both NPF and PPF. While an undiluted PPF is stabilized at the burner rim, characterized by significant radical destruction and heat loss to the burner, the corresponding undiluted NPF is lifted and stabilized in a low-velocity region extending from the wake of the burner. Detailed comparison of diluted NPF with PPF reveals that the base structures of both the flames are similar and exhibit a double flame structure in the near-field region, where the flame stabilization depends on a balance between the reaction rate and the scalar dissipation rate. As dilution is increased, the flames become weaker, move downstream along the stoichiometric mixture fraction line, and stabilize at a location where they can find a local flowfield that has a lower scalar dissipation rate. Further increase in dilution moves the flames further downstream into the far-field region, where the fuel and air stream diluted NPF and PPF exhibit a triple flame structure, and the flame stabilization mechanism also involves a balance between the triple flame speed and local flow velocity. Simulations show that with fuel stream dilution, PPFs stabilize at a higher liftoff height and blowout at a lower CO2 dilution compared to NPFs. In contrast, with air stream dilution, NPFs move to a higher liftoff height and blowout at a lower CO2 dilution compared to PPFs. The observed effects of partial premixing, and fuel stream and air stream dilution on flame liftoff and blowout can be explained using the existing flame stabilization theories.