Title:
|
Stochastic modelling of particle formation in turbulent flows via transported population balance-PDF method
|
Particle formation in turbulent flows arises in a large range of industrial and environmental
processes. Examples include the formation of soot particles in combustion
engines, the production of nanoparticles via flame synthesis or liquid-phase reactors,
or the evolution of aerosol particles in urban/non-urban environments. Simulations'
can significantly improve the understanding and design/control of these processes
but several difficulties arise when trying to model the underlying physics. These are
mainly rooted in both the requirements of appropriate description of the particle's
polydispersity and dynamics, as well as of the random fluctuations of the turbulence.
In the preliminary part of this work, the turbulent effects on the particle's dynamics
are discussed, showing how correlations of various orders appear in the main
equation that describes them, the Population Balance Equation (PBE). The thesis
elaborates a new approach for the simulation of particle formation in inhomogeneous
turbulent flows, the PBE-PDF method, whose concept was recently introduced by
[105]. An equation for the joint pdf of the number density and various scalars (such as
the chemical species kinetically involved) is derived from fundamental equations that
describe the class of considered problems. An algorithm for the numerical solution
of the pdf equation is developed, based on Monte Carlo simulations. The method is
discussed both at the level of the general concept of the stochastic simulations and
at the level of the specific requirements of the PBE-PDF approach. The structure of
the implementation is discussed in detail.
Starting from empirical or theoretical kinetics, the PBE-PDF is applied to two
different turbulent processes involving particle nucleation and growth. The first case
considered involves the precipitation of barium sulphate crystals in a tubular reactor,
while the second one involves dibutyl phthalate condensation in a free jet. For both
cases the method allows for clear analysis and understanding of the particulate phase
evolution, emphasis being put on the turbulent effects. Turbulence is found to have
significant impact on the overall processes by spatially redistributing the intensity of
the particles' mechanisms. Both processes were selected because previously studied
experimentally [5][63], which allows the comparison of computed particles' size distribution
with measured data. For the crystallization process the simulations' results
can be directly compared, showing excellent agreements. Identified uncertainties in
the experimental methods are discussed for the aerosol process, even if the computed
distributions still show good agreement with the measured ones, particularly
for intermediate molar fractions values.
|