Liftoff and blowout characteristics of laminar syngas nonpremixed flames

Abstract

Liftoff and blowout behavior of nonpremixed syngas flames is investigated using a time-accurate CFD code with a detailed description of transport and chemistry. Lifted flames are established in coflowing laminar jets using N2 dilution in the fuel jet. Results focus on the effects of syngas composition and temperature on the liftoff, stabilization, and the edge (triple) flame structure. For a given syngas mixture, as the N2 dilution exceeds certain value, the flame lifts off from the burner rim and propagates along the stoichiometric mixture fraction line, and its structure changes from diffusion to double flame. With further dilution, the flame liftoff height increases rapidly, the base structure transitions from double to triple flame, and its stabilization involves a balance between the triple flame speed and local flow velocity. The temporal evolution of propagating jet flame also exhibits a similar behavior. The transition from diffusion to double and then to triple flame is examined using state relationships in mixture fraction coordinate. As H2 fraction in syngas and/or temperature is increased, the N2 dilution required for flame liftoff and blowout increases. The ratio of the triple flame speed to the unstretched premixed flame speed also increases with the increase in H2 fraction. For H2 fraction above 30%, the flame liftoff and blowout become less sensitive to syngas composition and temperature.