The effect of mass loading and inter-particle collisions on the development of the polydispersed two-phase flow downstream of a confined bluff body

摘要

The effect of mass loading and inter-particle collisions on the development of the polydispersed two-phase flow downstream of a confined bluff body is discussed. The bluff-body flow configuration, which is one of the simplest turbulent recirculating flows, is relevant for applications and forms the basis of numerous combustion devices. The present data are obtained for isothermal conditions by using a two-component phase-Doppler anemometer allowing size and velocity measurements. Polydispersed glass beads are introduced into the flow. The statistical properties of narrow particle size classes are displayed and analysed in order to allow for the wide range of particle relaxation times. The evolution of mass fluxes and mass concentration per size class is estimated from the PDA data. A correction is introduced to ensure that the mass flow rate of particles per size class from data integration is correct. We show that the development of the continuous phase is very sensitive to initial mass loading of the inner jet. An increase in mass loading corresponds to an increase in momentum flux ratio between the central jet and annular flow. In the present situation, this implies a complete reorganization of the recirculation zone and the turbulent field. The importance of direct modulation of turbulence induced by particles is demonstrated in the inner jet. Moreover, our data confirm that the prediction of fluid/particle velocity correlation is essential to take these effects into account for partly responsive beads. We show that the sensitivity to mass loading greatly affects the dispersion of the glass beads. Particles recirculate at the lowest mass loading and the mass concentration of the dispersed phase in the recirculation zone and in the external shear layer is high. On the other hand, the memory of the initial jet is detected far downstream at the highest loading and the dispersion of particles is reduced. Axial or radial profiles of mean and r.m.s. velocity of the dispersed phase are displayed and analysed. The role of large-scale intermittency is discussed. Relevant Stokes numbers are introduced to account for different driving mechanisms in the turbulent field. Non-Stokesian effects are particularly important. We show that the anisotropy of the particle fluctuating motion is large and associated with production mechanisms via interaction with mean particle velocity gradients. A focus on the jet stagnation region proves that the particulate flow is very sensitive to inertia effects and that no local equilibrium with the fluid turbulence can be assumed when modelling such a configuration. Finally, even at the small volume ratio considered here, we prove that it is highly probable that inter-particle collisions occur in the jet stagnation region at low mass loading and all along the inner jet flow at the highest mass loading. Redistribution of mean momentum and fluctuating kinetic energy between all colliding classes is therefore expected, which implies a fully coupled fluid and particle system. The data and analysis presented provide a severe test case for the recent development in two-phase flow modelling and offer further challenges both to experimentation and model development. The validated data set has been selected for benchmarking and is, available on the internet. [References: 53]