Experimental study of premixed turbulent combustion in opposed streams. Part III - Spatial structure of flames

The local scalar statistics of premixed flames in turbulent opposed streams has been studied by sheet laser tomography. The statistics collected on th ese flame edges provide information on the mean flame position, and the mea n and standard deviations of local flamelet orientation and curvature. Emph asis is given to how these parameters vary through the flame brush as the f lames are pushed toward extinction. The flames are essentially planar in th e mean and the probability density function (pdf) of flamelet orientation i s symmetric about this mean orientation. The standard deviation of flame an gle is essentially constant throughout the flame brush, but varies strongly at the leading and trailing flame edges. The mean curvature of these flame is positive (i.e., concave to products) at the leading edge of the flame a nd negative at its trailing edge. Similar to the flame angle, the standard deviation of flame curvature is also constant throughout most of the centra l portion of the flame brush. As the mean nozzle exit velocity and the turb ulence intensity are increased to bring the flame nearer to extinction, the individual flame brushes thicken as much as 50%. The standard deviations o f flame angle and curvature also increase but more modestly. An unexpected result of the data collected is the differences between the upper and lower flames, which is probably an effect of buoyancy. The lower flame is consis tently and significantly more wrinkled than the upper flame, resulting in t he lower brush being thicker by as much as 25% and having larger standard d eviations of flame angle and curvature than the upper flame. (C) 1999 by Th e Combustion Institute.