Publications

Abstract not provided.

The mixture fraction filtered mass density function (FMDF) used in large eddy simulation (LES) of turbulent combustion is studied experimentally using line images obtained in turbulent partially premixed methane flames (Sandia flames D and E). Cross-stream filtering is employed to obtain the FMDF and other filtered variables. The means of the FMDF conditional on the subgrid-scale (SGS) scalar variance at a given location are found to vary from close to Gaussian to bimodal, indicating well-mixed and non-premixed SGS mixing regimes, respectively. The bimodal SGS scalar has a structure (ramp-cliff) similar to the counter-flow model for laminar flamelets. Therefore, while the burden on mixing models to predict the well-mixed SGS scalar is expected to lessen with decreasing filter scale, the burden to predict the bimodal one is not. These SGS scalar structures can result in fluctuations of the SGS flame structure between distributed reaction zones and laminar flamelets, but for reasons different from the scalar dissipation rate fluctuations associated with the turbulence cascade. Furthermore, the bimodal SGS scalar contributes a significant amount of the scalar dissipation in the reaction zones, highlighting its importance and the need for mixing models to predict the bimodal FMDFs. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

One-dimensional (1-D) line Rayleigh thermometry is used to investigate the effects of spatial resolution and noise on thermal dissipation in turbulent non-premixed CH4/H2/N2 jet flames. The high signal-tonoise ratio and spatial resolution of the measured temperature field enables determination of the cutoff wavenumber in the 1-D temperature dissipation spectrum obtained at each flame location. The local scale inferred from this cutoff is analogous to the Batchelor scale in nonreacting flows. At downstream locations in the flames studied here, it is consistent with estimates of the Batchelor scale based on the scaling laws using local Reynolds numbers. The spectral cutoff information is used to design data analysis schemes for determining mean thermal dissipation. Laminar flame measurements are used to characterize experimental noise and correct for the noise-induced apparent dissipation in the turbulent flame results. These experimentally determined resolution and noise correction techniques are combined to give measurements of the mean thermal dissipation that are essentially fully resolved and noise-free. The prospects of using spectral results from high-resolution 1-D Rayleigh imaging measurements to design filtering schemes for Raman-based measurements of mixture fraction dissipation are also discussed.

Abstract not provided.

Abstract not provided.

Previously unpublished results from multiscalar point measurements in the series of piloted CH4/air jet flames [R.S. Barlow, J.H. Frank, Proc. Combust. Inst. 27 (1998) 1087-1095] are presented and analyzed. The emphasis is on features of the data that reveal the relative importance of molecular diffusion and turbulent transport in these flames. The complete series A-F is considered. This includes laminar, transitional, and turbulent flames spanning a range in Reynolds number from 1100 to 44,800. Results on conditional means of species mass fractions, the differential diffusion parameter, and the state of the water-gas shift reaction all show that there is an evolution in these flames from a scalar structure dominated by molecular diffusion to one dominated by turbulent transport. Long records of 6000 single-point samples at each of several selected locations in flame D are used to quantify the cross-stream (radial) dependence of conditional statistics of measured scalars. The cross-stream dependence of the conditional scalar dissipation is determined from 6000-shot, line-imaging measurements at selected locations. The cross-stream dependence of reactive scalars, which is most significant in the near field of the jet flame, is attributed to radial differences in both convective and local time scales of the flow. Results illustrate some potential limitations of common modeling assumptions when applied to laboratory-scale flames and, thus, provide a more complete context for interpretation of comparisons between experiments and model calculations. © 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.