The Passive Scalar Concentration and Velocity Fields of Isolated Turbulent Puffs

• Main Author: Ghaem-Maghami, Elham
• Other Authors: Hamid Johari, Advisor
• Format: Others
• Published: Digital WPI 2006
• Subjects: Turbulnet Puffs; passive scalar; Velocity field
• Online Access: https://digitalcommons.wpi.edu/etd-dissertations/335; https://digitalcommons.wpi.edu/cgi/viewcontent.cgi?article=1334&context=etd-dissertations

"Passive scalar concentration and velocity fields of isolated turbulent puffs were examined experimentally using the planar laser Mie scattering and PIV techniques, respectively. Work in WPI laboratories on reacting, fully-modulated jets has indicated significantly reduced flame lengths for compact puffs in comparison with steady and pulsed jets. Of particular interest is the entrainment and mixing of isolated turbulent puffs away from the nozzle. The present experiments were carried out in order to enhance fundamental understanding of the velocity fields associated with isolated, turbulent puffs. Puffs were generated by injecting air through a 5 mm diameter nozzle into a flow chamber with a weak co-flow. The injection time was varied by the use of a fast-response solenoid valve from 20 ms to 161 ms. Puffs with a Reynolds number of 5,000 were examined in the range of 25 - 75 diameters downstream of the nozzle. The results indicate that as the injection volume increases, puffs evolve from a spherical geometry to one with a tail. The distribution of a passive scalar within the examined turbulent puffs is unlike that in turbulent vortex rings. The half-width of radial concentration profile through the puff center decreases as the injection volume increases. On the other hand, the puff length in the axial direction increases with the injection volume. The results from phase-locked PIV measurements indicate that the largest axial mean velocities and the radial velocity fluctuation are within the central portion of the puff and the largest axial velocity fluctuation are typically present above the puff center. The turbulent shear stress profiles within puffs are antisymmetric about the centerline and the maximum magnitude for the smallest injection volume is 2.5 times the steady jet value. The vorticity fields calculated from phase-locked velocity field data indicate the presence of vorticity throughout the puff volume. The ratio of puff volume flow rate to steady jet at the puff center location was largest for the smallest injection volume. The majority of entrainment into the puff occurs below the puff center while the puff cap pushes out into surrounding fluid. In general, the puff characteristics did not reveal an internal structure analogous to that in the turbulent vortex ring."