Optimization and Computational Fluid Dynamics - Department of ...

2 A Few Illustrative Examples of CFD-based Optimization 21
2.1.3 Optimization Coupled with Chemical Reactions
(Case B)
There is also a great interest at present to optimize complex flows involving
chemical reactions. A detailed description of the chemical kinetics is generally
needed to fully understand the combustion processes in such cases. MOEAs
are often employed for determining and adjusting the reaction parameters
or for reducing the number of reactions [18, 19, 20]. Furthermore, the reduction
of pollutant emission (NOx, CO, soot) is of major practical interest.
EAs have been applied in gas turbines for minimizing NOx emissions and/or
for reducing combustion noise [13, 14, 67]. Mono-objective optimization of
a laminar burner was investigated in [74] where the objective was to obtain
a homogeneous temperature profile at a prescribed distance from the injection
plane. As a whole, optimization of configurations involving the coupled
simulation of flows and combustion processes remains a fairly new field of
research.
Laminar flows involving chemical reactions play an important role in many
practical applications, for example domestic burners. In this section, a twodimensional
simulation dedicated to solving the Navier-Stokes equations in
the low-Mach number limit is presented using accurate models for chemistry,
diffusion and thermodynamics.
A two-dimensional configuration is considered, involving a primary inlet
in the center of the computational domain and a secondary inlet at the periphery.
The optimization problem consists of finding the minimal mass-flow
rate of the pollutant species CO while maintaining a homogeneous temperature
distribution. The corresponding integral value and the variation are
computed at a prescribed distance from the inlet. These two objectives are
in this case the minimal concentration of CO along the corresponding horizontal
cut through the solution and the temperature difference between the
maximum and minimum value. The parameters modified by the optimization
procedure are the fuel and oxidizer mass flows of the primary inlet while the
total amount of fuel and oxidizer injected through both inlets is of course
kept constant. There are, therefore, two parameters that may freely vary between
a lower bound (0: no fuel or no oxidizer injected through this inlet) and
an upper bound (all the available fuel or all the available oxidizer injected
through this inlet).
The investigated configuration is depicted in Fig. 2.2. Due to the symmetry,
only the right half of the domain is considered in the computation.
Depending on the input parameters, methane/air mixtures with different
compositions enter the domain through the primary and secondary inlets
with a fresh gas temperature of 298 K. Atmospheric pressure is imposed at
the outlet. The inlet wall temperature is constant and equal to 298 K.
It will be shown that it is possible using CFD to reach an optimal configuration
for such a complex problem involving coupled fluid flow, heat transfer