CONCENTRATION IMAGING MEASUREMENTS IN TURBULENT CONCENTRIC-JET FLOWS

Raman scattering images were used to determine the CH4 concentration in nonreacting concentric-jet flows. The flow consists of a central CH4 jet and a concentric air jet separated by a bluff body. Three jet velocity ratios were studied under isothermal conditions. At high momentum-flux ratios, the CH4 jet penetrates the recirculation zone formed by the airflow behind the bluff body. At low momentum ratios, the jet is stagnated and fuel is dispersed into the recirculation zone. The large-scale instantaneous boundary between the stagnated jet and the air is time varying and creates a sharp peak in the rms CH4 fluctuations in the stagnation region. Two-point spatial correlations were obtained over the flowfield to quantify the scale of the structures in these flows. Based on the limited data presented, it was found that the CH4 concentration field is similar for equal momentum-flux ratios. The CH4 concentration in the recirculation zone has a maximum at a momentum-flux ratio of unity, corresponding to the nearly penetrated case. In the nonpenetrated case, mixing of the central jet with surrounding air is substantially enhanced. Under these conditions, a considerable portion of the flowfield is mixed to conditions where combustion could occur. This enhanced mixing explains why bluff-body flames are more stable than jet flames into still air.