No abstract

The current study utilizes digital image sequences of flames to better understand the blowout phenomenon. Methane flames are studied near blowout conditions to determine if the disappearance of the diffusion flame prior to extinguishment signifies the leading edge of the reaction zone reaching the leanlimit. Various concentrations of nitrogen are used to dilute methane flames. The axial position of the flames is compared with the calculated position of the lean flammability limit to determine the role of the diffusion flame. The blowout limits of these flames are established and a blowout parameter is empirically determined from the data. Results from flames in co-flow show agreement with the blowout parameter previously published; however, the analysis shows that, the disappearance of the bulk diffusive reaction zone occurs at the lean flammability limit and is an accurate predictor of blowout for diluted and non-diluted methane flames.

The effects that various charged electrodes, and associated eiectric fields, have on lifted propane flames have been investigated. Two electrodes were used to provide an electric field with potentials ranging from 0 to 11,000 V. The primaiy electrode was around the flame and the secondary electrode was the fuel nozzle. Electrode polarity and primary electrode location with various flame field locations (near, mid, far) were varied, resulting in a variety of flame behavior. Results show that the body force resultant from the bulk flow of formed ions, from a positively charged fuel nozzle, and grounded ring electrode, will increase flame liftoff height and, eventually, cause blowout. However, for the opposite polarity (positively charged ring electrode and grounded fuel nozzle), the flame progresses toward reattachment with increasing potentiais. Observing the narrow window of flame blowout or reattachment (varying with polarity), it was observed that the lifted flame height fluctuations were increased with the presence of the grounded ring electrode, but reduced when the polarity was shifted to positive configuration (positively charged primary electrode). Flame hysteresis was obsen'ed when the ring electrode was positively charged and it was found that the hysteresis regime increased when the potential of the ring electrode was increased to 1500 V but had little changes at lower potentials. While the ring electrode was positively charged, a distinct hole was observed in the center of the flame. Several images are presented that show these flame holes that are present when the electrodes are charged.

The purpose of this study is to observe the effects of hydrogen enrichment on the stability of lifted, partially premixed, methane flames. Due to the relatively large burning velocity of hydrogen-air flames when compared to that of typical hydrocarbon-air flames, hydrogen enriched hydrocarbon flames are able to create stable lifted flames at higher velocities. In order to assess the impact of hydrogen enrichment, a selection of studies in lifted and attached flames were initiated. Experiments were performed that focused on the amount of hydrogen needed to reattach a stable, lifted methane jet flame above the nozzle. Although high fuel velocities strain the flame and cause it to stabilize away from the nozzle, the high burning velocity of hydrogen is clearly a dominant factor, where as the lifted position of the flame increased, the amount of hydrogen needed to reattach the flame increased at the same rate. In addition, it was observed that as the amount of hydrogen in the central jet increased, the change in flame liftoff height increased and hysteresis became more pronounced. It was found that the hysteresis regime, where the flame could either be stabilized at the nozzle or in air, shifted considerably due to the presence of a small amount of hydrogen in the fuel stream. The effects of the hydrogen enrichment, however small the amount of hydrogen compared to the overall jet velocity, was the major factor in the flame stabilization, even showing discernible effects on the flame structure.

The stability limits of a jet flame can play an important role in the design of burners and combustors. This study details an experiment conducted to determine the liftoff and blowout velocities of oblique-angle methane jet flames under various air coflow velocities. A nozzle was mounted on a telescoping boom to allow for an adjustable burner angle relative to a vertical coflow. Twentyfour flow configurations were established using six burner nozzle angles and four coflow velocities. Measurements of the fuel supply velocity during liftoff and blowout were compared against two parameters: nozzle angle and coflow velocity. The resulting correlations indicated that flames at more oblique angles have a greater upper stability limit and were more resistant to changes in coflow velocity. This behavior occurs due to a lower effective coflow velocity at angles more oblique to the coflow direction. Additionally, stability limits were determined for flames in crossflow and mild counterflow configurations, and a relationship between the liftoff and blowout velocities was observed. For flames in crossflow and counterflow, the stability limits are higher. Further studies may include more angle and coflow combinations, as well as the effect of diluents or different fuel types.