Abstracts - KTH Mechanics

116
Dynamics of droplet deformation and break-up
L. Prahl ∗ , J. Revstedt ∗ ,L.Fuchs ∗
Liquid sprays are modeled usually by assuming spherical non-deforming objects.
The forces on each sphere is determined by the difference between the particle velocity
and the surrounding flow. These forces rely heavily on the sphericity of the droplet.
The individual droplets are tracked by using a Lagrangian Particle Transport (LPT)
approach, assuming non-displacing particles and neglecting inter-particle interactions.
The main purpose with this study is to extend the LPT models with respect to droplet
deformation. The aim is to first account for non-uniformities in the flow around a
droplet, which affects the droplet shape, and secondly, to introduce a more detailed
droplet break-up model. Current models are based on linear models related to Kelvin-
Helmholtz and Rayleigh-Taylor instabilities. The former approach is used in the TAB
model and is controlled by the Weber number, We, whereas the latter approach is
employed in the bag break-up model. Additionally, due to droplet deformation, the
forces acting on the droplet differ also from that acting on a corresponding sphere.
The focus is on studying a single droplet as the Weber number along with the
particle Reynolds number are varied in order to identify the features connected with
droplet deformation and break-up. The Weber and Reynolds numbers are varied in
the range of 0.01 to 100 and 0.01 to 200, respectively. Through these parametric
studies a parametrization of the problem is obtained and thereby, drop deformation
and break-up as well as the effects of droplet deformation on the forces acting on
it can be accounted for. The deforming droplets are modeled using the Volume of
Fluid (VOF) approach. The enclosed figures depict the flow past a droplet for two
different We (0.1 and 10, respectively) and Re=100. It is experimentally established
that droplets do deform for We > 6, thus for We = 10, the droplet shape is no longer
spherical. Also, the drag (and in shear-layers also the lift) differ widely from the
forces acting a corresponding spherical particle. For We = 0.1 the droplets behave
very closely to a solid sphere and therefore current models behave reasonably well.
a) b)
Figure 1: Velocity field at the droplet center plane for (a) We =0.1 andRe = 100.
(b) We =10andRe = 100.
∗ Div. Fluid Mech., LTH, SE-221 00 Lund, Sweden.