A numerical study has been carried out to address the sooting characteristics of normal and inverse diffusion flames. The simulation was performed on the basis of single-step kinetics with a laminar flow assumption and non-unity Lewis number. Five different coaxial normal and inverse flame combinations with different momentum ratios have been investigated and compared. The results were experimentally validated using an identical burner with concentric cylindrical ports, where flame temperature measurements were employed for flame length comparison while a paper filter was utilized for soot concentration measurement. It was revealed that the momentum diffusion between the parallel streams governs the flame length and size. The results also showed that the single-step kinetics is able to predict the maximum soot formation in the normal flame annular region, provided that the Lewis number is less than unity. In inverse diffusion flames, the region of the soot surface growth is too small and the flow residence time is too short to allow significant soot production. For all combined inverse/normal diffusion flames, the soot emitted by the inner inverse flame had almost no influence on the total soot production. The thermal radiation by soot increased with the increase in both inner and outer flame lengths. The inner flame contributed to a 13 per cent increase in the total thermal radiation with keeping the same exhaust soot concentration level.

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A cylindrical burner accommodating stoichiometric fuel-air mixture combustion via multiple pairs of opposing jets and a cross-flow provided heat intensification and duplication of the stagnation impact for extending the firing limits and maximizing the power density. Six pairs of circumferentially opposing stoichiometric mixture jets sustained bulk injection velocities as high as 21.8 m/s and were associated with NOx emissions of 22 ppm, while emissions of 10 ppm were recorded upon reaching a lean limit equivalence ratio of 0.59. A stoichiometric mixture jet issuing perpendicular to the opposing jets at a momentum flux ratio of 0.3 increased the turbulence production rates to the extent that increased the maximum bulk injection velocity to 28.3 m/s and reduced the NOx emissions to 17 ppm. Since the recirculation zones between the two stagnation centers got compressed by increasing the momentum flux ratio to 0.8, the corresponding residence time reduction decreased the NOx emissions to 12 ppm. As the cross-flow mixture was made fuel-lean, dilution of the stoichiometric mixture by the fuel-lean mixture combustion products made it possible to get NOx emissions of single digit ppm. Emissions of 9 ppm resulted from using the cross-flow fuel-lean mixture jet due to compromising the flame stability limit extension and the temperature reduction in the post flame region. Such emissions, in turn, decreased to 4 ppm as the momentum flux ratio increased to 1.7 at which the stoichiometric mixture flames shrank into their ports. A minimum NOx emission index of 0.27 g/kg fuel was thus obtained at a volumetric heat release of 50.4 MW/m 3. The momentum flux ratio corresponding to merging the two stagnation zones was correlated with Reynolds and Froude numbers, the jets' separation as well as the density and viscosity values pertaining to the lean and stoichiometric mixtures' flame temperatures.