Premixed and nonpremixed flamelet-generated manifolds have been constructed and applied to large-eddy simulation of the piloted partially premixed turbulent flames Sandia Flame D and F. In both manifolds the chemistry is parameterized as a function of the mixture fraction and a progress variable. Compared to standard nonpremixed flamelets, premixed flamelets cover a much larger part of the reaction domain. Comparison of the results for the two manifolds with experimental data of flame D show that both manifolds yield predictions of comparable accuracy for the mean temperature, mixture fraction, and a number of chemical species, such as CO2. However, the nonpremixed manifold outperforms the premixed manifold for other chemical species, the most notable being CO and H2. If the mixture is rich, CO and H2 in a premixed flamelet are larger than in a nonpremixed flamelet, for a given value of the progress variable. Simulations have been performed for two different grids to address the effect of the large-eddy filter width. The inclusion of modeled subgrid variances of mixture fraction and progress variable as additional entries to the manifold have only small effects on the simulation of either flame. An exception is the prediction of NO, which (through an extra transport equation) was found to be much closer to experimental results when modeled subgrid variances were included. The results obtained for flame D are satisfactory, but despite the unsteadiness of the LES, the extinction measured in flame F is not properly captured. The latter finding suggests that the extinction in flame F mainly occurs on scales smaller than those resolved by the simulation. With the presumed ß-pdf approach, significant extinction does not occur, unless the scalar subgrid variances are overestimated. A thickened flame model, which maps unresolved small-scale dynamics upon resolved scales, is able to predict the experimentally observed extinction to some extent.
|Tijdschrift||Combustion and Flame|
|Nummer van het tijdschrift||3|
|Status||Gepubliceerd - 2008|