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Changes of Gas Velocities in Passing Through Flamelets in a Premixed Turbulent Bunsen Flame

A laser-Doppler velocimetry (LDV) system was employed to measure three components of velocity at a mid-flame point on the centerline of a propane/air flame of equivalence ratio 1.10 on a 26 mm-diameter Bunsen burner with fully developed turbulent pipe flow upstream at a Reynolds number of 6700. The LDV measurement volume was 1 mm below the leading electrode of a four-element electrostatic probe that provided the velocity and direction of motion of flamelets in the turbulent flame brush. It was found to be possible, during each flamelet passage, to rotate the coordinate system about a vertical axis so that the flamelet and all gas velocity vectors remained very nearly in the same vertical plane, thereby implying locally nearly two-dimensional motion, insofar as the influence of the gas expansion on the gas velocity is concerned. In the local plane of motion, the observed changes in gas velocity caused by flamelet passage exhibited a downward component for upward moving flamelets with the fresh mixture above the burned gas and an upward component for upward moving flamelets with the burned gas above the fresh mixture, consistent with the expected gas expansion through the flamelet. Moreover, within the accuracy of the LDV measurements, profiles of changes of the component of gas velocity normal to the flamelet agreed with velocity profiles calculated for steady, planar, unstretched, premixed laminar flames. These results indicate relatively little distortion of laminar flamelet structures in the turbulent flame brush at these turbulent intensities of about 5%.