Velocity Measurements in a Turbulent Nonpremixed Bluff-Body Stabilized Flame

Abstract

Velocity measurements have been obtained in a turbulent bluff-body stabilized methane flame. The flow configuration consisted of a 5.4-mm diameter fuel jet separated from an outer, annular-air flow by a 50-mm diameter bluff-body. A two-color laser Doppler velocimetry system was used to measure the mean and fluctuating statistics of all three velocity components and their correlations. Results were obtained under combustion and noncombusting conditions to quantify the effects of combustion on the flow and turbulence properties. The noncombusting flow field can best be characterized as dominated by the reverse flow of the annular air stream and exhibits well-de fined fuel and annular air stagnation points along the centerline. The primary effects of combustion are observed in the centerline region where the fuel-jet penetration increases due to the lower density of the recirculation zone gases. This increased fuel-jet penetration results in a downstream shift in the fuel-jet stagnation point. In contrast, the location of the air stagnation point is relatively insensitive to combustion and is determined primarily by the large-scale flow structure dynamics. Higher turbulence intensities are measured for the combusting flow in the recirculation zone and are related to the laminarization effects of combustion heat release. Maximum departures from isotropy are found in regions of high shear where the axial velocity component is nearly a factor of 2 greater than the radial or azimuthal velocity component. The prob ability distributions of individual velocity components are bimodal in regions of high shear located along the recirculation zone boundaries and on the centerline in the downstream stagnation zone. The two modes of the bimodal probability distribution s are associated with the alternate passage of unmixed fuel and air through the measurement volume and emphasize the importance of large-scale structures and intermittency in these flows.