Stretched premixed flames, studies

Ishizuka, S. and Law, C.K., An experimental study on extinction and stability of stretched premixed flames, Proc. Combust. Inst., 19,327,1982. 

One significant result from the studies of stretched premixed flames is that the flame temperature and the consequent burning intensity are critically affected by the combined effects of nonequidiffusion and aerodynamic stretch of the mixture (e.g.. Refs. [1-7]). These influences can be collectively quantified by a lumped parameter S (Le i-l)x, where Le is the mixture Lewis number and K the stretch rate experienced by the flame. Specifically, the flame temperature is increased if S > 0, and decreased otherwise. Since Le can be greater or smaller than unity, while K can be positive or negative, the flame response can reverse its trend when either Le or v crosses its respective critical value. For instance, in the case of the positively stretched, counterflow flame, with k>0, the burning intensity is increased over the corresponding unstretched, planar, one-dimensional flame for Le < 1 mixtures, but is decreased for Le > 1 mixtures. 

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. 

Partial premixing has been proposed as a means of NOj, reduction in gas turbine engines by Jayavant Gore at Purdue University. An experimental and computational study was conducted to observe NO behavior under the circumstances of moderate stretch rate, opposed-flow, partially premixed flames. The results show that the minimum NO emissions at an optimal level of partial premixing result as a consequence of decrease in CH radical concentrations. Partial premixing appears to be a possible practical immediate solution for NO remediation in gas turbines. 

Computational studies of partially premixed flames have also been reported [11-16], Authors have tried to explain the variation of NO emission indices with the level of partial premixing using one or more of the following residence time, flame stretch, radiation heat loss, and chemical mechanism-based arguments. However, a complete explanation of the NO emission has not been offered in the literature. 

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