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

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 iso thermal conditions by using a two-component phase-Doppler anemometer allowi ng 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 en sure that the mass flow rate of particles per size class from data integrat ion is correct. We show that the development of the continuous phase is very sensitive to i nitial mass loading of the inner jet. An increase in mass loading correspon ds to an increase in momentum flux ratio between the central jet and annula r flow. In the present situation, this implies a complete reorganization of the recirculation zone and the turbulent field. The importance of direct m odulation of turbulence induced by particles is demonstrated in the inner j et. Moreover, our data confirm that the prediction of fluid/particle veloci ty correlation is essential to take these effects into account for partly r esponsive 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 t he 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 i nitial jet is detected far downstream at the highest loading and the disper sion of particles is reduced. Axial or radial profiles of mean and r.m.s. v elocity of the dispersed phase are displayed and analysed. The role of larg e-scale intermittency is discussed. Relevant Stokes numbers are introduced to account for different driving mechanisms in the turbulent field. Non-Sto kesian effects are particularly important. We show that the anisotropy of t he particle fluctuating motion is large and associated with production mech anisms via interaction with mean particle velocity gradients. A focus on th e 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 i s highly probable that inter-particle collisions occur in the jet stagnatio n region at low mass loading and all along the inner jet flow at the highes t mass loading. Redistribution of mean momentum and fluctuating kinetic ene rgy between all colliding classes is therefore expected, which implies a fu lly coupled fluid and particle system. The data and analysis presented provide a severe test case for the recent d evelopment in two-phase flow modelling and offer further challenges both to experimentation and model development. The validated data set has been sel ected for benchmarking and is, available on the internet.