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 these flame edges provide information on the mean flame position, and the mean and standard deviations of local flamelet orientation and curvature. Emphasis is given to how these parameters vary through the flame brush as the flames are pushed toward extinction. The flames are essentially planar in the mean and the probability density function (pdf) of flamelet orientation is symmetric about this mean orientation. The standard deviation of flame angle 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 and negative at its trailing edge. Similar to the flame angle, the standard deviation of flame curvature is also constant throughout most of the central portion of the flame brush. As the mean nozzle exit velocity and the turbulence intensity are increased to bring the flame nearer to extinction, the individual flame brushes thicken as much as 50%. The standard deviations of 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 consistently and significantly more wrinkled than the upper flame, resulting in the lower brush being thicker by as much as 25% and having larger standard deviations of flame angle and curvature than the upper flame. (C) 1999 by The Combustion Institute.