Structural characteristics of lifted turbulent-jet flames

Abstract

Instantaneous planar images of the CH4 concentration in a lifted turbulent-jet flame were obtained by collecting Raman-scattered light perpendicular to a thin sheet of laser light passing through the axis of the flame. The instantaneous images were averaged to obtain mean and fluctuating statistics and probability distributions of the CH4 concentration. The timeresolved planar images reveal flow and mixing structure information that is lost when timeaveraged point measurement techniques are used. Spatial variations in the time-averaged statistics and the effects of combustion on these quantities are interpreted in terms of structural characteristics of these flows. Based on the instantaneous images, two sources of turbulent fluctuations are identified: 1) large-scale movement of the interface separating unmixed fuel and air; and 2) smaller scale turbulent fluctuations that are superimposed on the largescale jet structure. Combustion heat release is found to have a significant effect on both the turbulence and mixing characteristics of these flows. The primary differences between a lifted flame and a nonreacting jet are a lower centerline decay rate of CH4, higher instantaneous gradients, and higher concentration fluctuations in the lifted flame. It is shown that the lower centerline decay rate of the CH4 concentration is due to the reduced mass entrainment rate of the central-fuel jet in the lifted flame. The higher rms fluctuations of CH4 in the flame are due to the combined effects of: 1) the generally higher CH4 concentrations along the centerline; 2) the steeper instantaneous CH4 gradients along the central jet boundaries; and 3) the larger turbulence scales in the mixing region.