Why Do Partially Premixed Flames

Contents
Abstract
Nomenclature

1 Introduction
1.1 Motivation
1.2 Literature review
1.3 NOx formation mechanisms...........................................................................3
1.4 NOx reducing techniques..............................................................................5
1.4.1 Staged combustion..............................................................................5
1.4.2 Partially premixed flames...............................................................6
1.4.3 Flue gas recirculation.......................................................................8
1.4.4 Catalytic combustion...................................................................11

1.5 Computational fluid dynamic.........................................................................15

1.5.1 Governing equations of fluid flow and heat transfer.......................15
1.5.2 Generalized conservation equation...............................................17
1.5.3 Computation of turbulent flow.......................................................18
1.5.4 Modeling of swirl flow.................................................................22
1.5.5 Modeling reacting

This contrary pressure gradient will create a backflow, referred to as central toroidal recirculation zone (CTRZ). The degree of swirl usually is characterized by a swirl number S, given by [9]:

Where Gθ is the axial flux of swirl momentum, x G is the axial flux of the axial momentum and r is a characteristic radius such as the burner radius or the swirler radius. Typical profiles of axial and swirl velocity for a strong swirling flow (S>0.6) are shown in Figure 11

Figure 11: Flow recirculation in a strong swirling flow [3]

Catalytic combustion
The recent years catalytic combustion has proven to be one of the most promising NOx reducing techniques (see Figure 12). In catalytic combustion the fuel energy is converted into heat at a lower temperature than in ordinary combustion, thus reducing production of thermal NOx. In these systems the fuel reacts on the surface of the catalyst by a heterogeneous mechanism, where the catalyst is a ceramic or metal structure coated with noble metals. Because of the durability of the catalyst it is important to combust ultralean fuel-air mixtures with adiabatic combustion temperature below 1500 ˚C [19]. However, the observed reduction in NOx in catalytic combustors is much greater than that expected from the lower combustion temperature. The reaction on the catalytic surface apparently produces no NOx directly, although some NOx may be produced by homogeneous