Implications on flame stabilization in a precessing jet flame from near field velocity measurements

Abstract

The near field of the three-dimensional velocity field in a turbulent precessing jet diffusion flame is investigated. In the present case the precessing jet flame is generated by mechanically rotating a round jet of propane inclined at 45° relative to the axis of rotation. Three-dimensional laser Doppler anemometry enables the fully turbulent, three-dimensional velocity field of a precessing jet flame to be measured. This provides insight into the mechanisms by which the flame is stabilized and supports previous explanations of why NOx emissions are reduced and fuel efficiency is increased in industrial precessing jet gas burners. Velocity data are presented, time-averaged, and phase-averaged at the frequency of precession. The conditional phase-averaging technique enables phase-averaged velocity contours and vectors to be obtained that reveal flow patterns and structures within the flame. Time-averaging of the velocity data also reveals the presence of a reverse flow (recirculation) region between the emerging fuel jet and its spinning axis. The recirculation and high shear is associated with a rapid decay of the mean velocity, which together act to stabilize the flame in the high velocity and high shear region near to the nozzle exit. The characteristics of the velocity field of a precessing jet flame found here are compared with previous cold-flow investigations of the same flow and with other investigations of turbulent jets.