2. Methods
Computational fluid dynamics is an approach to solving fluid flow
problems by solving models that include numerical methods and algorithms
used to represent fluids. The methods and algorithms are generally
solved by computers [49, 50]. The solution of the models can provide
a simulation of an interaction of fluids with a structure defined by
surfaces, each of which can be defined by boundary conditions within the
model. The results, or solution, can be used to validate and improve
designs, for example. Computational fluid dynamics is the analysis and
prediction of fluid flows and heat transfer using a computer model.
Computational fluid dynamics uses mathematical methods to solve problems
that include fluid flow [51, 52]. It may be used to predict the
flows of fluids through a heat exchanger, or through a valve or a mixing
vessel for example [53, 54]. The first stage involves constructing a
numerical model of the structure around or through which the flows are
occurring, this being similar to the process of computer aided design.
It is also necessary to provide to the numerical model the nature of the
fluid flow as it enters the structure. The second stage is to a perform
the computational fluid dynamics modeling for that structure and that
input flow, this typically being performed in an iterative manner. The
final stage is to convert the resulting flow information into an output
form, for example a graphical representation showing the flow paths
[55, 56]. Highly sophisticated software is now available for
performing these activities, which enables a skilled user to model fluid
flows and heat transfer in or around any conceivable structure [57,
58]. However, the very sophistication that enables the software to
cope with any fluid flow problem, makes it expensive and also complex.
The gas turbine combustor configured to combust propane is represented
physically in Figure 1 for the reduction of nitrogen oxides emissions by
heterogeneous catalysis. The gas turbine combustor comprises a
concentric annular channel, wherein the concentric annular channel
further comprises an inner annular channel and an outer annular channel.
A platinum catalyst is deposited only upon the interior surface of the
inner channel, and the wall of the outer channel is chemically inert and
catalytically inactive. The reactant stream flows through the
catalytically-coated inner channel and the product stream flows out of
the outer non-catalytic channel. Fuel is present for combustion in both
the catalytic and non-catalytic channels. The concentrically arranged
annular channel is 5.0 millimeters in inner channel length, 5.6
millimeters in outer channel length, 0.8 millimeters in innermost
diameter, 2.6 millimeters in outermost diameter, 0.1 millimeters in
catalyst layer thickness, and 0.2 millimeters in wall thickness, unless
otherwise stated. The gas turbine combustor can have any dimension
unless restricted by design requirements. All the walls have the same
thickness. The spacing between the inner channel and the outer channel
is 0.4 millimeters and remains constant. One of the potential problems
associated with the gas turbine combustor, as with all micro-scale
combustion systems, continues to be combustion stability. The scale of
the gas turbine combustor is on the order of sub-millimeters, which is
much smaller than the quenching distance of the combustible mixture in
the absence of a catalyst. The quenching distance defines a critical
dimension under which propagation of the propane flame is not possible.
The quenching distance is approximately 2.5 millimeters, at which
combustion cannot be sustained within the gas turbine combustor in the
absence of a catalyst.