LES of Jets and Sprays Injected into Crossflow

The objective of this thesis is to numerically simulate a fluid jet injected
into a crossflow of the same or another fluid, respectively. Such flows are
encountered in many engineering applications in which cooling or mixing
plays an important role, e.g. gas turbine combustors. The jet in crossflow
(JICF) is used both for cooling and for injecting liquid fuel into the air
stream prior to combustion. The numerical simulations regard three space
dimensions and track also the flow dynamics by integrating the governing
equations in time. The spatial and the temporal resolution are such that the
large-scale flow structures are resolved. Such an approach is referred to as
large eddy simulations (LES). The motion of the fuel droplets is treated by
Lagrangian particle tracking (LPT) with the stochastic parcel method, along
with submodels for evaporation, collision, breakup, and a novel submodel for
aerodynamic four-way coupling: The particle drag is corrected depending on
relative positions of the particles. Mixture fraction and temperature transport
equations are solved to enable the modeling of droplet evaporation and
the mixing of the gaseous fuel with ambient air.
In the simulations of multiphase JICF, several computed results are shown
to be inconsistent with the underlying assumptions of the LPT approach:
The magnitude of the Weber numbers indicates that droplets are not spherical
in large portions of the flow field in wide ranges of parameters which
are relevant for gas turbine operation. The magnitude of the droplet spacing
suggests that aerodynamic interaction (indirect four-way coupling) among
droplets may be important. The LES with aerodynamic four-way coupling
reveals significant effects compared to two-way coupling for monodisperse
particles in a dense multiphase flow.
For single-phase JICF, the impact of nozzle shape on the large-scale coherent
structures and the mixing is studied. Effects of circular, square, and
elliptic nozzles and their orientation are considered. It is demonstrated that
square and elliptic nozzles with blunt orientation raise turbulence levels significantly. The scalar distribution in a cross-sectional plane is found to be
single-peaked for these nozzles whereas circular and the nozzles with pointed orientation show double-peaked scalar distribution. It is the nozzles with a
single-peaked distribution which are the better mixers.
The differences and similarities of single- and multiphase JICF are compared,
and it is demonstrated that the flow field solution for multiphase flow
approaches the flow field solution of single-phase flow in the limit of small
Stokes numbers.