Laser Doppler anemometry measurements in an index of refraction matched column in the presence of dispersed beads: Part I

Laser Doppler anemometry was used to measure axial and radial velocity components in a liquid flow field in the presence of dispersed beads. Index of refraction matching between the fluid (para-cymene) and large, spherical beads, made of polymethyl methacrylate, was essential to extract the turbulence data from the fluid phase because of the high concentration of beads. The match allowed measurements without generating noise from the surface of the beads. The measurements were obtained under transient flow conditions to avoided need for an external solids circulation loop. The resulting data were statistical and ensembled averaged for short periods during the transient experiments where the concentration could be taken as approximately constant. The concentration levels were determined from separate particle tracking velocimetry measurements, to be described in Part II. The bead density was greater than the fluid and thus wakes were created. These local areas of fluid experienced a lowering of the velocity. The turbulent fluctuation results showed that the presence of the beads extracted energy from the mean flow and transferred that energy to the turbulence. The increase of turbulent intensity was impressive along the centerline of the column where the measurements were made. The axial turbulent intensity of the fluid without beads was of the order of 10% and with beads, the level was increased to nearly 70%.