**FLUID**

**MECHANICS**

**F. Durst**

University of Erlangen-Nuernberg, Germany

High-Performance Scientific Computing and its Application in Solving Engineering Problems

The discipline called High-Performance
Scientific Computing can provide support to engineering developments through
results obtained by numerical simulations. This is most clearly perceptible
in the field of numerical fluid mechanics, where program developments are
under way to compute fluid flows with complex boundaries and involving
heat and mass transfer. The present paper summarizes the computer program
developments at the Institute of Fluid Mechanics at the University of Erlangen-Nuernberg
and shows that two- and three-dimensional computer programs are available
to compute steady and unsteady fluid flows of compressible and incompressible
media. Multi-grid solution algorithms are incorporated into the programs
and speed up the solution considerably. All programs are parallelized and
can be run on computers of different parallel architectures.

**J. C. F. Pereira**
**J. M. M. Sousa**

Technical University of Lisbon, Portugal

Coherent Sructures in the Flow over a Cavity: Experiments and Computations

This paper presents the results
of experimental and numerical investigations on the formation of organized
motions in the flow over cavity-type geometries. It is shown that stability
analysis can predict the occurrence of coherent, traveling vortices within
the impingement length of the cavity. This is associated to instability
of the separated shear layer. At Re = 3360, Kelvin-Helmholtz vortices can
be experimentally observed to travel and impinge on the trailing-edge of
the cavity. Appropriate numerical simulations have shown to be able of
predicting this behaviour, further disclosing the complex coupling between
shear layer instability and recirculating flow field dynamics. At Re =
63.600, the presence of organized, turbulent structures persists. Thus,
the use of large-eddy numerical simulations for high Reynolds number flow
over a cavity has proved superior performance over classical k-\eps modelling,
from both statistical and topological viewpoints.

**St. Radev**
**M. Kaschiev**

Institute of Mechanics, Bulg.
Ac. Sci., Sofia

Institute of Mathematisc
and Informatisc, Bulg. Ac. Sci., Sofia

Fully Developed Gas-Solid Particles Flow in a Vertical Riser

The aim of the present paper
is to perform a numerical algorithm for the simulation of the flow of gas-particle
mixture in Circulating Fluidized Bed riser in the zone where the flow is
fully developed. In this zone the equations of motion could be reduced
to one-dimensional form. An algorithm for solving the one-dimentional equations
of motion is proposed based on the Continuous Analogue of the classical
Newton Method (CANM). The corresponding results and related comparisons
as well as a discussion are presented.

**P. Stankov**

Technical University of Sofia

Computer Simulation of 3D Complex Turbulent Flows: Real Needs, Possibilities and Perspectives

The paper presents a review
of the state of the art of computer modelling of 3D complex turbulent flows
in heat and mass transfer processes, the real needs for such modelling,
the possibilities for the flows to be predicted numerically as well as
the perspectives for the future development of this technique. The discussion
is based on the particular example of combustion processes in industrial
furnaces as a typical case of a 3D flow process. The real needs for mathematical
modelling of combustion processes aiming to improve the practical predictions
and design are evaluated on the basis of a comparison with other methods
for investigation and modelling of such processes. The discussion and conclusions
are supported by particular numerical predictions of the PTVM 100-type
industrial furnace operating in a power plant in Sofia. On this basis is
drawn the main conclusion: computer simulation of complex 3D turbulent
flows in practical heat and mass transfer processes is the best, if not
the only tool for engineering predictions and design.

**N. Todoriev**
**V. Pavlov**

Energoconsult AD, Bulgaria

Current Problems In Bulgarian
Combustion Technologies

The paper discusses some
of the problems arising in the combustion technologies used in Bulgarian
power industry. The discussion concerns mainly the prevailing organic substances
used as energy resources in Bulgaria and the choice of most appropriate
combustion technology. Technologies both for pulverized - coal burning
of high ballast lignites and for pulverized - coal burning of local coal
mixtures having different characteristics are examined. The problem of
limiting NO_{x} is also discussed.

**J. L. T. Azevedo**
**L. M. R. Coelho**
**M. G. Carvalho**

Technical University of Lisbon, Portugal

Numerical Modelling of In-Fur-
nace NO_{x} Reduction Technologies for Pulverised Coal Combustion

The reduction of NO_{x}
emissions from pulverised coal fired boilers may be performed either by
post-combustion flue gas treatment or by modifications in the combustion
process which is in general more economical. The techniques to reduce NO_{x}
emissions within the boiler furnace consist on the promotion of air and/or
fuel staging either close to the burner (using low NOx burners) or at the
boiler level. For boilers without overfire air ports, air staging may be
performed using burners out of service. Further NOx reduction is achieved
in the reburning technology using fuel staging followed by overfire air
to complete combustion. The paper presents results of the application of
a comprehensive numerical model developed at IST to study the influence
of using different NO_{x} abatement technologies, namely the use
of burners out of service for air staging and the implementation of reburning
technology on the performance of industrial boilers.

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