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MECHANISMS OF TURBULENCE MODIFICATION IN DISPERSED TWO-PHASE FLOWS (TIME-RESOLVED PIV MEASUREMENTS OF INTERACTIONS BETWEEN PARTICLES/BUBBLES AND TURBULENCE)

Koichi Hishida
Department of System Design Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8522

Yohei Sato
Department of System Design Engineering, Keio University, 3−14−1 Hiyoshi, Kohoku-ku, Yokohama, 223−8522, Japan

Abstrakt

Turbulence structure of dispersed two-phase flows has been investigated by particle image velocimetry (PIV) with a high time resolution using fluorescent tracer particles for phase velocity discrimination. The present study is confined to energy transport by solid particles in a turbulent channel flow and the effect of bubbles on fluid turbulence in an upward pipe flow.
A filtering technique was applied to the fluid flow amongst particles to extract a characteristic length scale that governs the energy transfer from particles to fluid turbulence. The turbulence intensity in the streamwise direction, which is identical to the gravity direction, was strongly augmented by particles whose size is slightly greater than the Kolmogorov length scale. The directional scale dependency structure was observed, i.e., large eddies were dissipated in front of particles and particle wake generated eddies, which is emphasized when particles aligned perpendicular to the gravity direction. The subgrid scale turbulence energy is increased until Δ/η ~ 10 (Δ : filter width, η : Kolmogorov length scale), which means that particles generate eddies whose size is less than 10 η. The energy backscatter in the presence of particles was observed at Δ/dp ~ 5 (dp : particle diameter), indicating that particles affect the eddy motion whose size is approximately five times particle diameter.
Turbulence modification in the presence of bubbles was investigated by using a PIV/LIF (laser induced fluorescence) system synchronized with a shadow imaging technique to detect the bubble's shape and position simultaneously. Turbulence energy was augmented by both large and small bubbles, which induced an increase in its dissipation rate. On the other hand, the Reynolds stress was strongly reduced in the whole region of pipe, yielding a decrease in the turbulence production. Energy forward/backscatter was significantly enhanced around bubbles, which means that the local vortex structure generated by bubbles may be responsible for the energy transport between large eddies to small ones, and the energy supply from bubbles motion.