Stretched Diffusion Flames in von Karman Swirling Flows

Flame stretch plays a critical role in determining the fundamental characteristics of a diffusion flame, including its extinction and burning efficiencies. The concept of 'flame stretch' to characterize the effects of straining flows on flames was first introduced by Karlovitz (1953). Following his work, this concept was further exploited to characterize flame dynamics by a number of authors including Lewis and von Elbe (1961), and Markstein (1964). The mathematical definition of flame stretch and its physical meanings can be found in the review article by Law (1988). Most of the experimental studies on stretched diffusion flames thus far have been in the high velocity, high stretch regime. Under normal gravity conditions high temperature differences encountered in a flame causes buoyancy induced flows making it difficult to obtain low flow, low stretch flames in practice. Even the experiments in the high-stretch regime employ flow configurations that are essentially two-dimensional. The three dimensionality of the swirling flow field observed in many common combustion devices are missing in these configurations. The most commonly used experimental configurations are the counter flow diffusion flames, stagnation-point boundary layers, spherically symmetric combustion such as droplet combustion or porous sphere combustion with injected fuel, cylindrical flames and flat flame burners. The nature of the influences of the swirl on the structure and extinction of diffusion flames have not been addressed in any great detail thus far in the literature.