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.

A cylindrical burner was developed to combine extensive firing limits, high combustion efficiencies, and low NOx emissions from partially premixed or inverse diffusion flames by setting multiple pairs of circumferentially opposing jets (of concentric fuel and air streams). Normal strain rates as high as 1600 s−1 in addition to transverse strain rates of 640 s−1 were combined with a cross-flow impact and elliptical jet dispersion such that a turbulent kinetic energy peak of 15.4 m2/s2 was produced. The fuel/air mixing was enhanced and the peak temperature was reduced due to the stimulation of cross-flow wake zones and the axis switching of the elliptical jets. Partially premixed methane/air combustion of 12 opposing mixtures at Φ=1.5 led to NOx reduction to 0.36 g/kg fuel via controlling the flow residence time and adjusting the separation between the premixing and diffusion zones of each flame. As elliptical ports are used, the NOx concentrations decreased to 4 ppm with an aspect ratio of 3.0. Increasing the ratio between the diameter of the jets and their axial separation decreased the CO and HC emissions respectively to 489 ppm and 0.019%. Upon having opposing inverse diffusion flames, the blowout stability limit reached a maximum value of 53.2 m/s. With a separation of 4.0 cm, the NOx emissions reached a minimum level of 25 ppm by increasing the momentum flux ratio between the cross-flow air jet and the opposing jets to 1.5. By optimizing the cross-flow/primary air velocity ratio and the primary air/fuel jet diameter ratio, the CO and HC emissions were respectively minimized to 721 ppm and 0.039%. The velocity fluctuations indicated that increasing the opposing jets increases the turbulent kinetic energy and decreases the smallest scales of turbulence.

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The spray comparative tests on diesel/biodiesel–ethanol blends revealed that the spray tip penetration increases by a range from 4.4 to 21.5%, while the spray cone angle decreases by a range between 33.2 and 50.0% upon switching from diesel to biodiesel. Using biodiesel has an impact on the spray angle that is stronger than that on the penetration length. It was found that in order to minimize the relative reduction in the spray angle upon using a 50% petroleum diesel/50% biodiesel blend (B50), injectors of larger spray angles should be used, while no significant changes were found by adding ethanol to the diesel/biodiesel blends. The emission tests on a single cylinder naturally aspirated direct injection diesel engine showed that although the brake-specific fuel consumption (BSFC) increased upon switching from diesel to biodiesel, the unburnt hydrocarbons (HC) and carbon monoxide (CO) emissions decreased. Upon blending ethanol with the diesel/biodiesel mixture, the HC emissions decreased by a relative percent as high as 26.6%, while the percentage of decrease in CO reached 10.8% by increasing the injection pressure from 55 to 95 MPa. Adding biodiesel to diesel increased the nitrogen oxide (NOx) emissions due to the increased oxygen availability. Upon approaching the full load, the correspondingly reduced ignition delay resulted in earlier combustion and higher peak temperatures where an average turbulent kinetic energy of about 320 m2/s2 has been predicted. The NOx emissions were effectively reduced upon adding ethanol to the diesel/biodiesel blend due to the higher latent heat of evaporation of ethanol. Combining the retardation in the injection timing from −25 to −5° crank angle with the exhaust gas recirculation of 15% effectively reduced the NOx emissions to be below 2.6 g/kW.h.