Particle Dispersion in Turbulent Flow: The effect of particle size distribution

Abstract The measured velocity fluctuation of particles in dispersed twophase flow systems is discussed and an interpretation of particle velocity statistics is given. Besides the turbulent velocity fluctuations resulting from the particle's response to the fluid turbulence, two additional apparent fluctuation components may be distinguished, caused by inertial effects of the particles and by a polydisperse particle phase. Especially for larger particles inertial effects may become important and therefore the particle motion in the flow field is strongly governed by the inlet conditions, namely the instantaneous particle velocity. Therefore, the measured axial particle velocity fluctuation at a given location may be considerably increased as particles from different starting locations with different initial velocities may cross the same measuring point. This dynamic particle behaviour is demonstrated by the numerical simulation of particle trajectories in a given flow field. The results show the importance of including appropriate inlet conditions for the numerical prediction of particulate two‐phase flows. Further, in experimental realizations of particulate two‐phase flows, the particles usually exhibit a certain size distribution. Owing to the different responses of the different sized particles to both the mean flow field and the turbulence characteristics of the flow, the measured particle velocity fluctuations are higher than in the comparable monodisperse case. Therefore, the particle size distribution was taken into account in the present numerical calculations in order to obtain results which are comparable to the corresponding experiments. In this numerical simulation the particle mean velocity and the rms value of the velocity fluctuation were obtained by averaging over the whole particle size distribution and then compared with the measured values. Calculations are presented for two different shaped particle size distributions with a different mean size and compared with calculations with a monosized particle material and experimental results. Further, the behaviour of the different sized particles in the particle size distribution is discussed by considering the changes in the particle size distribution and the particle velocity distribution throughout the flow field.