This issue attempts to give a feeling of the state-of-the-art of the application
of computational fluid dynamics (CFD) in chemical engineering. It is, however,
not limited to a snap-shot but is aimed at providing a perspective: how did we
arrive at the present status and where do we go from here? To do so, contributions
from five complementary contributions are brought together. From the
definition of CFD as the ensemble ‘‘of all computational approaches that solve
for the spatial distribution of the velocity, concentration, and temperature
fields’’ recalled by Fox, it is clear that a selection had to be made as to the topics
covered. In the wake of volume 30 on ‘‘Multiscale Analysis’’ the present volume
is organized from ‘‘small’’ to ‘‘large’’: from ‘‘bubbles and droplets’’ in the first
contribution, to a ‘‘fixed catalyst bed’’ in the last one. The application of direct
numerical simulations (DNS) clearly is still limited to the small scale. Today
subgrid-scale (SGS) models are required to cover the full spectrum.
The reader will be confronted with some redundancy but this allows each
contribution to stand on its own. Also, a good balance is maintained between
the style of a tutorial and that of a research paper. Those who will read the
complete volume will realize that opinions can vary from looking at CFD as an
alternative for experimentation to emphasizing the need of experimental validation.
Some contributions are entirely limited to velocity and temperature
fields. Others, on the contrary, emphasize the difficulties associated with the
combination of transport and reaction. The latter can introduce stiffness even
for laminar flow. Averaging (e.g. Reynolds-averaged Navier–Stokes, RANS) or
filtering (e.g. large eddy simulations, LES), performed to model velocity fields,
does not alleviate this difficulty. Clearly, this is still quite a challenge.
The contribution from the Ohio State University by Ge and Fan is dealing
with the simulation of gas–liquid bubble columns and gas–liquid–solid fluidized
beds. A scientist of a major engineering company told me a few years ago that
when he wanted to know how serious an academic group was about CFD, he
would ask whether they could simulate bubble columns. He would only engage
into further conversation if the answer was negative. The group from Columbus
is wise enough to focus on a single air bubble rising in water, and bubble
formation from a single nozzle. In a second part the hydrodynamics and heat
transfer phenomena of a liquid droplet in motion and during the impact process
with a hot flat surface, as well as with a particle are studied. The applied
numerical techniques, such as the level set and immersed boundary method, are
outlined and important contributions are highlighted. Next, detailed implementations
for particular problems are presented. Finally, numerous simulation
results are shown and compared with experimental data. and many more
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