Measurements of Instantaneous Velocity Field of a Non-Reacting Pulsating Buoyant Plume by Particle Image Velocimetry

abstract

Digital particle image velocimetry (DPIVj was employed to measure velocity field of a naturally pulsating plume of helium-air mixture. Velocity measurements were made across a diametric plane of an axisymmetric plume originating from a lOcm. diameter circular nozzle at a Richardson number of 283. The plume oscillated at a frequency between 3.5 and 4.0 Hz. Measurements were performed in a phase resolved manner to characterize the different phases of the plume velocity field during its oscillation period. Because of the large field of view, velocity field was assembled from DPIV data in sub regions at the same phase of the oscillation. For the periodic flow studied, the repeatability and matching of velocities at sub region boundaries were found to be very good. The obtained velocity fields show a strong acceleration along the plume axis within one diameter height above the nozzle. The strong upward acceleration is accompanied by a radial inflow which fixes the entrainment rate of ambient fluid into the plume. Formation of a toroidal vortex structure occurs at around a height of one half nozzle diameter which coincides with the region where the streamwise acceleration goes through zero. Subsequently, the toroidal vortex convects downstream significantly influencing the flow field in its vicinity. Entrainment is predominantly a result of periodic engulfment of ambient air into the plume.

keywords:

• Buoyant plumes
• Particle Image Velccinnetry