Stagnation-flow burner

Figure 4.1.2 is a photograph of a coimterflow burner assembly. The experimental particle paths in this cold, nonreacting, counterflow stagnation flow can be visualized by the illumination of a laser sheet. The flow is seeded by submicron droplets of a silicone fluid (poly-dimethylsiloxane) with a viscosity of 50 centistokes and density of 970 kg/m, produced by a nebulizer. The well-defined stagnation-point flow is quite evident. A direct photograph of the coimterflow, premixed, twin flames established in this burner system is shown in Figure 4.1.3. It can be observed that despite the edge effects.

A further consequence of the upstream diffusion to the burner face could be heterogeneous reaction at the burner. Such reaction is likely on metal faces that may have catalytic activity. In this case the mass balance as stated in Eq. 16.99 must be altered by the incorporation of the surface reaction rate. In addition to the burner face in a flame configuration, an analogous situation is encountered in a stagnation-flow chemical-vapor-deposition reactor (as illustrated in Fig. 17.1). Here again, as flow rates are decreased or pressure is lowered, the enhanced diffusion tends to promote species to diffuse upstream toward the inlet manifold. 

The flow configuration is oriented upward so that the buoyancy effect on the stagnation-flow field is diminished. This configuration provides a stagnation-flow field with a radially uniform velocity profile at the inlet. Gas fines are also heated to prevent the condensation of liquids. The gases are exhausted through an annular pipe and burned in a Bunsen burner which is also housed in the reactor. 

Beginning with the innovative work of Tsuji and Yamaoka [409,411], various counter-flow diffusion flames have been used experimentally both to determine extinction limits and flame structure [409]. In the Tsuji burner (see Fig. 17.5) fuel issues from a porous cylinder into an oncoming air stream. Along the stagnation streamline the flow may be modeled as a one-dimensional boundary-value problem with the strain rate specified as a parameter [104], In this formulation complex chemistry and transport is easily incorporated into the model. The chemistry largely takes place within a thin flame zone around the location of the stoichiometric mixture, within the boundary layer that forms around the cylinder. 

Fig. 13.12 Opposed-flow diffusion flame between parallel, axisymmetric, burner faces that are fabricated as honeycomb monoliths. As illustrated, the flame is positioned on the oxidizer side of the stagnation plane. However, depending on the flow conditions as well as the fuel and oxidizer composition, the flame may form on the fuel side of the stagnation plane.

In contrast to the combustion in stabilized flames, flameless oxidation is mixture and temperature controlled and is achieved by specific flow and temperature conditions. A prerequisite for a stable flame front is a balance between flow and flame velocity. This is true in premixed and in diffusion flames and stability depends on species concentrations, flow velocity, flow field, temperature, pressure, and other parameters. Creating flow conditions for flame stabilization is an essential burner design criterion. Swirl or bluff body are most often used to create stagnation points or areas of low velocity for stabilization. The species concentration also plays an important role. Air, with an oxygen content of 21% can create a flammable mixture with... 

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