A Large Eddy Simulation (LES) using the Conditional Moment Closure (CMC) as a sub-grid turbulence-chemistry model has been applied to piloted jet diffusion flames (Sandia D & F). A 3D CMC grid was used which allowed different CMC boundary conditions to be applied in the jet and pilot streams. The code was found to give very good agreement with experiment in the low extinction case of Flame D. For Flame F transient extinction and reignition events were observed with LES-CMC which lead to reductions in averaged unconditional and conditional temperature consistent with experimental observations. Further analysis revealed that the CMC extinction/ignition events were the result of a combination of high conditional scalar dissipation rate and transport in the CMC grid.

The Large Eddy Simulation (LES)/three-dimensional Conditional Moment Closure (3D-CMC) model with detailed chemistry and finite-volume formulation is employed to simulate a swirl-stabilized non-premixed flame with local extinction. The results demonstrate generally good agreement with the measurements concerning velocity, flame shape, and statistics of flame lift-off, but the penetration of fuel jet into the recirculation zone is under-predicted possibly due to the over-predicted swirl velocities in the chamber. Localized extinctions are seen in the LES, in agreement with experiment. The local extinction event is shown by very low heat release rate and hydroxyl mass fraction and reduced temperature, and is accompanied by relatively high scalar dissipation. In mixture fraction space, CMC cells with strong turbulence-chemistry interaction and local extinction show relatively large fluctuations between fully burning and intermediate distributions. The probability density functions of conditional reactedness, which shows how far the conditionally-filtered scalars are from reference fully burning profiles, indicate that for CMC cells with local extinction, some reactive scalars demonstrate pronounced bimodality while for those cells with strong reactivity the PDFs are very narrow.

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a b s t r a c tThe response of orifices to incident acoustic waves, which is important for many engineering applications, is investigated with an approach combining both experimental measurements and numerical simulations. This paper presents experimental data on acoustic impedance of orifices, which is subsequently used for validation of a numerical technique developed for the purpose of predicting the acoustic response of a range of geometries with moderate computational cost. Measurements are conducted for orifices with length to diameter ratios, L/D, of 0.5, 5 and 10. The experimental data is obtained for a range of frequencies using a configuration in which a mean (or bias) flow passes from a duct through the test orifices before issuing into a plenum. Acoustic waves are provided by a sound generator on the upstream side of the orifices. Computational fluid dynamics (CFD) calculations of the same configuration have also been performed. These have been undertaken using an unsteady Reynolds averaged Navier-Stokes (URANS) approach with a pressure based compressible formulation with appropriate characteristic based boundary conditions to simulate the correct acoustic behaviour at the boundaries. The CFD predictions are in very good agreement with the experimental data, predicting the correct trend with both frequency and orifice L/D in a way not seen with analytical models. The CFD was also able to successfully predict a negative resistance, and hence a reflection coefficient greater than unity for the L=D ¼ 0:5 case.