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20.1.2 Governing Equations for NOx Transport

FLUENT solves the mass transport equation for the NO species, taking into account convection, diffusion, production and consumption of NO and related species. This approach is completely general, being derived from the fundamental principle of mass conservation. The effect of residence time in NOx mechanisms, a Lagrangian reference frame concept, is included through the convection terms in the governing equations written in the Eulerian reference frame. For thermal and prompt NOx mechanisms, only the NO species transport equation is needed:

 \frac{\partial}{\partial t}(\rho Y_{\rm NO}) + \nabla \cdot ... ... = \nabla \cdot (\rho {\cal D} \nabla Y_{\rm NO}) + S_{\rm NO} (20.1-1)

As discussed in Section  20.1.5, the fuel NOx mechanisms are more involved. The tracking of nitrogen-containing intermediate species is important. FLUENT solves a transport equation for the HCN, NH $_3$ or N $_2$O species in addition to the NO species:

 \frac{\partial}{\partial t} (\rho Y_{\rm HCN})+ \nabla \cdot... ...HCN}) = \nabla \cdot (\rho {\cal D} Y_{\rm HCN}) + S_{\rm HCN} (20.1-2)

 \frac{\partial}{\partial t} (\rho Y_{\rm NH_3})+ \nabla \cdo... ...3}) = \nabla \cdot (\rho {\cal D} Y_{\rm NH_3}) + S_{\rm NH_3} (20.1-3)

 \frac{\partial}{\partial t} (\rho Y_{\rm N{_2}O})+ \nabla \c... ...= \nabla \cdot (\rho {\cal D} Y_{\rm N{_2}O}) + S_{\rm N{_2}O} (20.1-4)

where $Y_{\rm HCN}$, $Y_{\rm NH_3}$, $Y_{\rm N{_2}O}$, and $Y_{\rm NO}$ are mass fractions of HCN, NH $_3$, N $_2$O, and NO in the gas phase, and ${\cal D}$ is the effective diffusion coefficient. The source terms $S_{\rm HCN}$, $S_{\rm NH_3}$, $S_{\rm N{_2}O}$, and $S_{\rm NO}$ are to be determined next for different NOx mechanisms.

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