| Triple flames are a special type of partially premixed flames (PPFs) that contain three reaction zones, namely a nonpremixed zone (NPZ) that is sandwiched between rich premixed (RPZ) and lean premixed reaction zones (LPZ). A comprehensive numerical model that uses detailed descriptions of transport and chemistry is used for the simulations of axisymmetric coflow and counterflow flames. The first part of the thesis examines the NO x emissions in triple flames. Most of the NOx is formed through the thermal and prompt NO mechanisms in the NPZ. An optimum level of partial premixing is identified corresponding to the lowest amount of NOx emission in triple flames with superior NOx characteristics compared to premixed and double flames. The second part of the thesis examines the effectiveness of fuel and air stream dilution using CO2 in extinguishing lifted triple flames at various levels of partial premixing. As CO2 is added in the fuel or air stream, undiluted flames established in these jets are lifted, with their stabilization governed by a balance between reaction rate and scalar dissipation rate. With increasing dilution, the liftoff height increases, and the flames exhibits a triple flame structure, with the flame stabilization also characterized by a balance between the triple flame speed and the local flow velocity at the flame base. With further increase in dilution, the liftoff height increases rather rapidly until the flame blows out. The effectiveness of air versus fuel stream dilution in extinguishing both coflow and counterflow PPFs is strongly influenced by the level of partial premixing. The third part of the thesis focuses on triple flames propagating in a jet mixing layer containing CH4-H2 fuel blend and air. By igniting the fuel-air mixing layer in the far jet region, a well-defined triple flame is formed that propagates upstream. As the flame approaches the burner, it transitions to a double flame and subsequently to a nonpremixed flame, stabilizing at the burner rim. Irrespective of the amount of H2 in the blend, propagating triple flames are positively stretched at the base, and there is a positive correlation between the flame speed and the stretch. The effect of H2 addition is to increase the flame speed, extend the flammability limits, and enhances the preferential mass diffusion. In addition, the flame sensitivity to stretch is reduced, enhancing the flame's tendency towards diffusive-thermal instability. |
