The effect of entrained air turbulence on dispersion of droplets (with Stokes number
based on Kolmogorov time scale, Stη, of the order of 1) in a polydispersed spray is
experimentally studied through simultaneous and planar measurements of droplet size,
velocity, and gas flow velocity (Hardalupas et al. 2010). The preferential accumulation of
droplets at various measurement locations in the spray was examined by two independent
methods viz. counting droplets on images by dividing the image in to boxes of different
sizes, and by estimating the radial distribution function (RDF). The dimension of droplet
clusters (obtained by both approaches) was of the order of Kolmogorov’s length scale
of the fluid flow implying the significant influence of viscous scales of the fluid flow
on cluster formation. The RDF of different size classes indicated an increase in cluster
dimension for larger droplets (higher Stη). The length scales of droplet clusters increased
towards the outer spray regions, where the gravitational influence on droplets is stronger
compared to the central spray locations. The correlation between fluctuations of droplet
concentration and, droplet and gas velocities were estimated and found to be negative
near the spray edge, while it was close to zero at other locations. The probability density
function of slip between fluctuating droplet velocity and gas velocity ‘seen’ by the droplets
signified presence of considerable instantaneous slip velocity, which is crucial for dropletgas
momentum exchange. In order to investigate different mechanisms of turbulence
modulation of the carrier phase, the three correlation terms in the turbulent kinetic
energy equation for particle-laden flows (Chen & Wood 1985) are evaluated conditional
on droplet size classes. Based on the comparison of the correlation terms, it is recognized
that though the inter-phase energy transfer due to fluctuations of droplet concentration
is low compared to the energy exchange only due to droplet drag (the magnitude of
which is controlled by average droplet mass loading), the former can not be considered
negligible, and should be accounted in two-phase flow modelling.