Mathematical Modelling of Thermal NOX Emissions in Combustion Chambers
A mathematical model of combustion in typical burners is presented. The model differs from other similar ones presented by other investigators, in that it features a two-step approach, which allows the calculation of the main exothermic reactions of fuel in air, and of those responsible for the formation of pollutants, specifically nitrogen oxides (NOx) to be performed in tandem. The model uses CFD techniques to compute the flowfield in the burner and transport equations are solved for all relevant reacting species. To ensure industrial relevance all factors affecting combustion performance and heat transfer have been included, i.e. three- dimensionality, an exact representation of geometry, the air swirler, turbulence and radiation.
In the first step, the calculation deals with the combustion of methane in air for which a global, single step reaction is assumed. The limiting effect of turbulence on reaction rate is taken into account using an eddy-break-up model. Radiation between the burner walls, the flame and intervening gas is accounted for using a flux model.
In the second step, the product stream resulting from the first step calculation is analysed to determine the concentrations of O, OH and H radicals, subsequently used in a Zeldovich type scheme to form oxides of nitrogen. The convective and diffusive transport terms of the first step are used to solve conservation equations for N and NO.
The paper illustrates the technique in an application to an axial recirculating burner.
KeywordsFlame Structure Furnace Wall Burner Design Turbulent Exchange Coefficient Furnace Size
Empirical expression in combustion model
Turbulent kinetic energy per unit mass, J/kg
Ideal gas constant
Reaction rate per unit volume, kgs/m3
Stoichiometric oxidant to fuel ratio
Source term in finite difference equations
Activation temperature, K
- \(\vec v\)
Turbulent exchange coefficient for variable ϕ
Turbulence energy dissipation rate per unit mass, W/kg
Dynamic viscosity, Ns/m2
General dependant variable
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