Section Fourteen
POLLUTION PREVENTION
52    MODELING ATOMIZATION PROCESSES
S. D. Heister, Associate Professor
School of Aeronautics and Astronautics
Purdue University
West Lafayette, Indiana. 47907
ABSTRACT
Recent nonlinear simulations of various atomization processes are have been generated using
computational schemes based on Boundary Element Methods. Both single fluid and two-phase
computational models are discussed. Example simulations of finite-length liquid jets and
droplets under forced excitation are provided to highlight current capabilities. A brief summary
is included for future prospects using these tools, and the application to problems of interest in
waste management.
INTRODUCTION
The stringency of current and near-term federal regulations regarding industrial waste has
prompted a renewed emphasis on increasing our understanding of basic processes germane to
this field. In particular, injection and atomization of liquids is a process which has relevance to
many industrial waste problems. While we currently cannot provide complete physical description of a complex spray emanating at high-speed from a nozzle, great strides have been made in
recent years to successfully model lower speed flows. In this paper, we shall inform the reader as
to the current state-of-the-art in atomization modeling, and provide some specific examples from
this author's particular research program.
There are two primary obstacles to successfully modeling the nonlinear surface deformations
present in any atomization problem. The presence of surface tension as an active force in these
flows implies that local surface pressures depend on surface shape. However, the instantaneous
surface shape cannot be determined without a knowledge of the pressure field. This situation implies a basic nonlinearity in the free-surface boundary conditions which must be addressed to
successfully model flows of this nature. The second obstacle involves topological changes in the
computational mesh which occur during "atomization events," i.e., when droplets are pinched
from the parent surface.
Classical works due to Rayleigh.1 Weber.2 and others,"<' ignore both obstacles by performing
linear analyses under the assumption of infinitesimal deformations. While this body of knowledge is still drawn upon for many of today's practical injection problems, none of these analyses
address surface shapes under large deformations. While there have been a variety of "weakly
nonlinear" analyses performed,7,8 a complete description of the atomization process has eluded
us until the past couple of decades. Computational methods based on finite element methods
(FEMs) and Boundary Element Methods (BEMs), and the Volume of Fluid (VOF) methodology
have been developed to provide high-resolution simulations of nonlinear free-surface problems.
Most FEM simulations9 include the effect of viscosity; the main difficulty in using these methods for atomization lies in the grid distortion which occurs near pinch points. The VOF
schemes10 interpolate surface location from a fixed computational mesh (thereby eliminating the
grid distortion issue). However, the interpolation process leads to poor resolution of the surface
tension force since this force depends on surface curvature. Most recently, two new schemes
52nd Purdue Industrial Waste Conference Proceedings, 1997. Ann Arbor Press, Chelsea. Michigan 48118. Printed in
U.S.A.
511