a1 Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA
Previous studies have shown that a dilute dispersion of fine particles can either augment or attenuate the gas-phase turbulent kinetic energy (TKE). However, such turbulence modification is not accurately captured by numerical simulation models. A critical reason is that the models do not incorporate flow distortion occurring at small scales on the order of the particle diameter or the Kolmogorov scale. These scales are too small to be resolved by most experiments and simulations, so the small-scale effects remain poorly understood. The main objective of this study is to investigate experimentally the small-scale turbulence structures that affect the overall turbulence modification to improve understanding and prediction of the macroscopic turbulence modification. A high resolution particle image velocimetry (PIV) system was developed that provided two-dimensional velocity field measurements with a sub-Kolmogorov vector spacing of 60 μm. Measurements of gas-phase isotropic turbulence were performed in the facility developed by Hwang & Eaton (Exp. Fluids, vol. 36 (3), 2004a, p. 444) in the presence of dispersed 500 μm glass, 250 μm glass or 250 μm polystyrene particles at mass loading ratio up to 0.45. The Reynolds number based on the Taylor microscale was 130 for the unladen case. The TKE was attenuated by up to 25 % in the presence of particles. The high-resolution measurements of the dissipation rate show that changes in the dissipation rate are smaller than changes to the TKE, in contrast to previous underresolved experiments. An analysis of a large set of PIV images allowed calculation of the average turbulence distortion around particles. The measurements also showed strong damping of the TKE and strong augmentation of the dissipation rate in a roughly spherical region surrounding the particles.
(Received May 13 2008)
(Revised September 05 2009)
(Accepted September 07 2009)
(Online publication January 05 2010)