An experimental study was conducted on the stability enhancement of conical premixed flames by application of direct current electric fields. Turbulent conical premixed flames were stabilized at the tip of a circular cylindrical bluff-body flame holder. An electric field was set up between a positively charged upper electrode and a grounded flame holder to determine its effects on the lean limit stability characteristics. In these experiments, the flame blowoff equivalence ratios were determined as a function of mixture velocity, electric field strength, and the electrode configuration. It was found that the most pronounced effects were observed at the lowest mixture velocities in this study of about 5.0 m=s with the influence of the electric field virtually disappearing at higher velocities of 10 to 15 m=s. The maximum reduction in blowoff equivalence ratios was 4 to 5% at the low-velocity conditions. These findings are consistent with the estimates of the ionic wind velocities expected 1291 in hydrocarbon=air flames and point to the rather weak electric field effect for applications in high-speed premixed flame stabilization.

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Flow, thermal and emission characteristics of turbulent nonpremixed CH4 flames were investigated for three burner heads of different cone heights. The fuel velocity was kept constant at 15 m/s, while the coflow air speed was varied between 0 – 7.4 m/s. Detailed radial profiles of the velocity and temperature were obtained in the bluff body wake at three vertical locations of 0.5D, 1D, and 1.5D. Emissions of CO2, CO, NOx, and O2 were also measured at the tail end of every flame. Flames were digitally photographed to support the point measurements with the visual observations. Fifteen different stability points were examined, which were the results of three bluff body variants and five coflow velocities. The results show that a blue-colored ring flame is formed, especially at high coflow velocities. The results also illustrate that depending on the mixing at the bluff-body wake, the flames exhibit two modes of combustion regimes, namely jet- and coflow-dominated flames. In the jet-dominated regime, the flames become longer compared to the flames of the coflow-dominated regime. Emissions were largely reduced due to the excess air dilution, surpassing their production.

The thermal characteristics of turbulent non-premixed methane flames were investigated by four burner heads with the same exit diameter but different heights. The fuel flow rate was kept constant with an exit velocity of 15 m/s, while the co-flow air speed was increased from 0 to 7.6 m/s. The radial profiles of the temperature and flame visualizations were obtained to investigate the stability limits. The results evidenced that the air co-flow and the cone angle have essential roles in the stabilization of the flame: An increase in the cone angle and/or the co-flow speed deteriorated the stability of the flame, which eventually tended to blow off. As the cone angle was reduced, the flame was attached to the bluff body. However, when the cone angle is very small, it has no effect on stability. The mixing and entrainment processes were described by the statistical moments of the temperature fluctuations. It appears that the rise in temperature coincides with the intensified mixing, and it becomes constant in the entrainment region.