SUPERLAYER CONTRIBUTIONS TO CONSERVED SCALAR PDFS IN A H//2 TURBULENT JET DIFFUSION FLAME.

The first four moments from experimental conserved scalar probability density functions from a H//2 turbulent jet diffusion flame are analyzed and compared to those found by Pitts and Kashiwagi in a nonreacting CH//4 jet. The measurements are in good agreement, indicating that heat release and combustion have little effect on pdf shapes. However, the measured pdf's are not qualitatively similar to the simple forms often assumed in combustion modeling. When the experimental pdf's are separated into nonturbulent, turbulent and superlayer contributions, the superlayer contributed as much as 90% of the total pdf in both the H//2 flame and the nonreacting CH//4 jet. A physical interpretation based upon large-scale structures is suggested to explain the existence of broad apparent superlayers found in turbulent jet flows.