Coaxial Jet Flame Research Articles

Soot Volume Fraction Measurements in the Soot-forming Regions of Ethylene-AirTurbulent Partially-premixed Flames

Soot formation characteristics of ethylene-air turbulent partially premixed flames have been experimentally determined by measuring the soot volume Traction, via laser-induced incandescence. Measurements were made in the near-burner soot-forming region up to axial distance of 25 burner diameters. The amount of soot formed in these flames varies non-monotonically as the equivalence ratio is lowered from that of non-premixed flames. Initial premixing with air results in a substantial increase in the integrated soot volume fraction, up to a factor of nearly 2 in comparison to non-premixed flames. Only at overall equivalence ratio near 10, the amount of soot is brought below the level of non-premixed flames in the region. Further premixing with air results in a continuous decrease in the soot volume fraction. The radial profiles of soot volume fraction show more evenly distributed soot particles in partially-premixed flames, indicating that soot-forming regions are significantly broadened in these flames. The injection location of premixing air has a significant effect on the sooting characteristics of partially-premixed flames. In the current co-axial jet flame burner configuration, when the premixing air is added co-axially at the burner exit plane, the partial premixing always resulted in a decrease in the integrated soot volume fraction. Moving the injection point upstream caused the amount of soot to increase rapidly. The pdf's of the soot volume fraction are consistent with the above observations in that there is a shift in the pdf toward larger soot volume fraction when there is an increase in the integrated soot volume fraction. Except, the effect of the air injection location is manifest through small but finite probabilities of large soot volume fractions, while the peak of the pdf remains at small soot volume fractions. These results point out some important trends in the sooting characteristics as a function of partial premixing parameters.

Interaction of a vortex with a flat flame formed between opposing jets of hydrogen and air

Studies on individual vortex-flame interactions constitute important elements for the understanding of the turbulent-flame structure. Vortices having sufficiently high normal velocity can pass through the flame by extinguishing it locally. In several circumstances they deform the flame surface significantly before attaining extinction conditions. The development of curvature on the flame surface, especially in hydrogen flames, could lead to different quenching patterns. An experimental/numerical investigation is performed to explore possible quenching patterns in opposing-jet diffusion flames. A diluted hydrogen-nitrogen mixture is used as the fuel. Vortices are driven toward the flame surface with different velocities from the air side. The changes in the structure of the flame during its interaction with the incoming vortex are recorded by measuring instantaneous OH-concentration field using the laser-induced fluorescence (LIF) technique. A time-dependent CFDC code that incorporates 13 species and 74 reactions is used for the simulation of these vortex-flame interactions. Both the experiments and calculations have identified two types of quenching patterns: namely, point and annular. It is found that when an air-side vortex is forced toward the flame at a relatively high speed, then the flame at the stagnation line quenches, resulting in a well-known point-quenching pattern. On the other hand, when the vortex is forced at a moderate speed, the flame surface deforms significantly, and quenching develops in an annular ring away from the stagnation line, resulting in an unusual annular-quenching pattern. Detailed analyses performed just before the development of annular quenching and 1 ms later suggest that this unusual annular quenching did not result from the strain rate. Based on the understanding gained from previous investigations on curvature effects in coaxial hydrogen jet flames and the findings made in the present study, it is argued that such quenching develops as a result of the combined effect of preferential diffusion and flame curvature.

Structure of Lean Turbulent Partially-Premixed Flames Stabilized in a Co-Axial Jet Flame Burner

Lean premixed flames at very low equivalence ratios and high mean flow velocities can besustained in a co-axial jet flame burner. The flame shape involves a thin, highly turbulent flame region where most of the lean combustion occurs, and a base flame region which burns at the initial equivalence ratio. Due to the entrainment velocity induced by the high-speed inner jet flow, the flame also has a thin "pinched" region near the flame base. The entrainment of the hot product gas and mixing with the reactant, leading to preheating, along with the fact that the turbulence intensity increases gradually in the presence of the inner jet, are believed to be contributing factors in the stabilization of the flames in the current burner geometry. Addition of the inner jet of always lowers the overall equivalence ratio and enhances the stability limits. The minimum overall equivalence ratio achieved is 0.56 with a mean velocity of 60 m/s for the inner jet. This capability to generate lean premixed flames at high turbulence intensities and mean flow velocities is expected to have useful practical applications. Temperature measurements have been obtained using a point Rayleigh scattering technique, and the temperature profiles show a significant reduction in the flame temperature as the overall equivalence ratio is reduced using the inner jet air. The temperature pdf's show a shift of the cold reactant temperature to a higher value, indicating preheating of the reactant due to the entrainment of hot product gas that surrounds the lean flame. Also, there can be significant departure from flamelet behavior, associated with a bimodal temperature pdf, when the inner jet velocity becomes high. The temperature pdf's are no longer bi-modal at high turbulence intensities caused by the strong inner jet, with temperature pdf's showing a broadened temperature range in the flame zone.

Highly turbulent premixed flames stabilied in a co-axial jet flame burner

A co-axial jet flame configuration is found to provide a stable turbulent premixed flame for a wide range of turbulence conditions up to u′/SL of 20 and higher, for an outer to inner diameter ratio of 8 in the current burner. This flame configuration supports premixed flames in flows with initial mean flow velocity in xcess of 150 m/s. Thus, this arrangement can be used for premixed flame stabilization for large flow velocities and for investigations of turbulent premixed flames over a wide range of turbulence properties, with u′/SL exceeding 10 in a steady jet flame. An increase in inner jet velocity increases the turbulence intensity, and a gradual increase in the turbulence intensity from the outer flame to the inner flame acts to stabilize the flames. Flame surface properties such as the flame perimeter ratio and flame curvature statistics are also obtained, where the flame curvature exhibits a nearly symmetric PDF at high turbulence levels in spite of the jet flame geometry.