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.