Premixed flames, stagnation point flow

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 number of theoretical (5), (19-23). experimental (24-28) and computational (2), (23), (29-32). studies of premixed flames in a stagnation point flow have appeared recently in the literature. In many of these papers it was found that the Lewis number of the deficient reactant played an important role in the behavior of the flames near extinction. In particular, in the absence of downstream heat loss, it was shown that extinction of strained premixed laminar flames can be accomplished via one of the following two mechanisms. If the Lewis number (the ratio of the thermal diffusivity to the mass diffusivity) of the deficient reactant is greater than a critical value, Lee > 1 then extinction can be achieved by flame stretch alone. In such flames (e.g., rich methane-air and lean propane-air flames) extinction occurs at a finite distance from the plane of symmetry. However, if the Lewis number of the deficient reactant is less than this value (e.g., lean hydrogen-air and lean methane-air flames), then extinction occurs from a combination of flame stretch and incomplete chemical reaction. Based upon these results we anticipate that the Lewis number of hydrogen will play an important role in the extinction process. 

V. Giovangigli and M.D. Smooke. Calculation of Extinction Limits for Premixed Laminar Flames in a Stagnation Point Flow. J. Comp. Phys., 68 327-345,1987. 

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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