Nonpremixed bluff-body burner flow and flame imaging study

Abstract

A nonpremixed bluff-body burner flow and flame have been studied using planar flow visualization and species concentration imaging techniques. The burner consists of a central jet of CH 4 in a cylindrical bluff-body and an outer coflowing-air stream. Planar flow visualization, using Mie scattering from seed particles added to the fuel jet, Raman scattering from CH 4 and laser-induced fluorescence of CH combined with Raman scattering of CH 4 provided information on turbulent flow, mixing and combustion. The CH 4 imaging system utilized two cameras, which enhanced the dynamic range of the diagnostic system by a factor of l0 over a single-camera system. It was observed that the fuel jet stagnated on the axis due to interaction with the high velocity air flow. The flow and mixing were found to have significant coherent and noncoherent, large-scale, time-varying structures. The detailed CH 4 Raman and CH fluorescence measurements of an air-dominated bluff-body flame revealed that the stagnation zone governs mixing and flame stability. Through large-scale mixing, the stagnated jet feeds the recirculation zone and also creates a favorable condition to stabilize the flame detached from the bluff-body. The instantaneous flame zone, as defined by CH, was found to be narrow and concentrated in an envelope around the stagnation zone. This narrow flame characteristic is consistent with that observed for jet flames. Although the internal structure of the flame envelops have not yet been defined, these results suggest that this bluff-body flame can be modeled by a "flame sheet" type approach, where the reaction front is captured by the large-scale structures. This should simplify the development of modeling approaches for these flows since molecular mixing and chemical reaction, which occur within the "flame sheet", can be separated from the large-scale mixing process.

Spark ignition of turbulent nonpremixed bluff-body flames

“…The structure of these recirculation zones is affected by different parameters, such as the blockage ratio (BR), the cone angle, the confinement of the flow, and the incoming velocity and turbulence intensity. The majority of the previous investigations of turbulent nonpremixed bluff-body flames used axial fuel injection [17][18][19][20][21][22], while only a few examined annular or radial fuel injection [23][24][25][26]. In addition, to avoid penetrating the CRZ, the bluff-body radial fuel injection has a noticeable improvement on the symmetry and stability of the nonpremixed flames [26].…”

Hysteresis and transition in swirling nonpremixed flames

“…The thickness of the instantaneous reaction zones in the lifted, blue flame is somewhat larger, but comparable to that in the attached, yellow flame. However, it appears that the reaction zones in the lifted flame are less well connected, which is likely caused by the much higher turbulence and strain levels in the lifted flame [20].…”

Effect of Confinement on Bluff-Body Burner Recirculation Zone Characteristics and Flame Stability

“…Estimated uncertainties in the mean concentration data presented below are ± I %. Based on calibration data, the shot noise of the system was estimated as 3% of the signal from pure CH 4 • Further details of the imaging system can be found in Namazian et al (1989).…”

“…The laser beam was formed into a sheet by a multipass cell (Namazian et al, 1989). Two anti-reflection-coated narrow (5 mm) flat quartz windows were used to pass the sheet of light through the center of the confinement.…”