Systematic comparisons have been obtained between hydroxyl time series as measured by picosecond time-resolved laser-induced fluorescence (PITLIF) and as predicted by large-eddy simulations (LES) for turbulent non-premixed H2/N2 jet diffusion flames. The measured and predicted autocorrelation functions for OH were found to collapse to the same shape when normalized by their integral time-scales. However, the time-scales predicted by LES were systematically low by a factor of 2-3 as compared to the PITLIF measurements. Time series for hydroxyl and total number density were also obtained using (PITLIF) and laser Rayleigh scattering (LRS), respectively, in non-premixed and partially premixed turbulent H2/CH4/N2-air flames having similar Rayleigh cross-sections for air and the exhaust gases. The measured PSDs for OH and number density were found to collapse to the same shape when normalized by their respective integral time-scales. The density to OH time-scale ratio was typically 2-4, with larger ratios prevailing nearer non-premixed conditions owing to thinner OH flamelets. Finally, a novel PITLIF methodology has been developed which is capable of simultaneous two-point, time-series measurements of OH concentrations in turbulent flames. Two-point time-series data obtained in a standard H2 diffusion flame at Re = 10,000 have been used to compute space-time correlations, spatial autocorrelation functions, and integral length-scales. Preliminary two-point results are discussed with a focus on implications for future measurements in gas-turbine combustors and thrust augmentors.

A two-point method has recently been demonstrated based on picosecond time-resolved laser-induced fluorescence (PITLIF). This technique is capable of simultaneously monitoring OH time series at two locations, hence allowing the characterization of spatial structures in turbulent flames in addition to their temporal behavior. Several refinements have been made to improve upon the previous one-point instrument. Using a specially designed collection-lens system, off-axis optical aberrations have been effectively minimized in the probe volume. Furthermore, additional electronics has been installed which permit absolute species concentration measurements. As a preliminary study, two-point OH time-series measurements were taken in three standard turbulent non-premixed flames (H3, DLR_A and DLR_B). Two-point statistics, including space-time correlations, spatial autocorrelation functions and integral length scales were computed to help elucidate the flame structure and turbulence-chemistry interactions.

A work on turbulent premixed combustion is important because of increased concern about the environmental impact of combustion and the search for new combustion concepts and technologies. An improved understanding of lean fuel turbulent premixed flames must play a central role in the fundamental science of these new concepts. Lean premixed flames have the potential to offer ultra-low emission levels, but they are notoriously susceptible to combustion oscillations. Thus, sophisticated control measures are inevitably required. The editors' intent is to set out the modeling aspects in the field of turbulent premixed combustion. Good progress has been made on this topic, and this cohesive volume contains contributions from international experts on various subtopics of the lean premixed flame problem.

A Rayleigh scattering technique for simultaneous measurements of density at two points in a flame was developed. This technique was used to deduce the spatial and time-space correlation and the two-point joint probability density functions of the density in a premixed turbulent flame in three orthogonal directions. A grid generated turbulent V-shaped flame at a flow velocity of 7.0 m/s and a half angle of 12 degree was used. The analysis show that this flame consists of structures of equal length scale in all three directions which decay as they are convected downstream by the flow. It is also shown that this flame does not consist of flamelets. (Author).