The effect of mass loading and inter-particle collisions on the development of the polydispersed two-phase flow downstream of a confined bluff body
Authors: BORÉE J. , ISHIMA T. , FLOUR I. 
Published: July 2002

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.