EXPERIMENTAL IDENTIFICATION OF MIXING REGIMES IN THE ANALYSIS OF TURBULENT-DIFFUSION FLAMES

A statistical criterion for the classification of gaseous mixing regim es is presented. It is based on the evaluation of two quantities: the mixing layer thickness and the separation distance between two neighbo ring interface segments, which are preliminarily defined in a specific section along with other quantities (stretch ratio and stretch rate) relevant to the mixing analysis. A unique experimental method for a la grangian measure of the aforementioned quantities as a function of the residence time is described. Measurements use a two-dimensional laser light scattering technique in a two-dimensional, transitional, isothe rmal how which can be considered a prototypical condition of mixing. T he probability density function of the stretch rate as well as average Values of stretch ratio, mixing layer thickness and interface separat ion distance are also reported. The last two quantities are used in th e determination of a single parameter named ''saturation factor'' (C-s at), that allows the identification of three regimes: (a) ''Isolated m ixing layer regime'' (C-sat = 1) in which both the mirdng and oxidativ e reactions occur in the neighborhood of the interface. This is a well recognized regime, to which many studies on the characterization of o ne-dimensional time-dependent stretched diffusion flames refer. (b) '' Interacting mixing layer regime'' (10(-2) < C-sat < 1) in which the di ffusion still occurs in the whole flow held, but more sophisticated mo dels than those relative to the previous regime are needed for its cha racterization. (c) ''Saturated mixing layer regime'' (C-sat < 10(-2)) in which the mixing process is no longer related to the intermaterial surface in its wholeness, but only to that part for which it is still far from other segments of the interface.