No abstract

The scalar dissipation rate transport in both the corrugated flamelet and thin reaction zone regimes is studied using three-dimensional direct numerical simulation (DNS) databases for freely propagating statistically planar turbulent premixed flames. Both flames have comparable turbulent Reynolds number but the flame representing the corrugated flamelet combustion regime has a global Damköhler number, Da>1, whereas the second flame representing the thin reaction zone regime has Da<1. It is demonstrated that the turbulent transport of the scalar dissipation rate and its molecular diffusive transport have strong Da dependence. The contributions of dilatation rate, the turbulence-scalar interaction, the reaction-rate–scalar gradient interaction, and the dissipation processes have leading order contributions for both the flames. However, the turbulence-scalar interaction dissipates scalar gradients in the Da>1 flame, while it produces the scalar gradients in the Da<1 flame. Simple algebraic models for the contributions of the various processes noted above to the scalar dissipation rate evolution are proposed and their a priori assessment is carried out with respect to DNS data. These models explicitly have Da and the Karlovitz number Ka dependence, and thus, they can be viewed and used as unified models for both the corrugated flamelet and thin reaction zone regimes. Physical realizability of the modeled scalar dissipation rate transport equation is studied and the model constants obtained by using the DNS data sets are shown to satisfy the realizability condition.

Stereoscopic particle image velocimetry and planar laser induced fluorescence measurements of hydroxyl radical are simultaneously applied to measure, respectively, local turbulence intensities and flame front position in premixed ethylene-air flames stabilized on a bluff body. Three different equivalence ratios, 0.55, 0.63, and 0.7, and three different Reynolds numbers, 14 000, 17 000, and 21 000, are considered. Laser measurements were made for five different flame configurations within the ranges above and in the corresponding cold flows. By comparing the measurements of the cold and the corresponding hot flows, the effect of heat release on the turbulence and its interaction with the flame front is studied. All the flames are in the thin reaction zone regime. Typical flow features forming behind the bluff body are observed in the cold flows, whereas in the reacting flows the mean velocities and thus the shape, size, and characteristics of the recirculating eddy behind the bluff body are strongly influenced by the heat release. The strong acceleration across the mean flame and the radial outward shift of the stagnation plane of the recirculating eddy yield negative radial velocities which are absent in the corresponding cold flow cases. The spatial intermittency of the flame front leads to an increase in the turbulent kinetic energy. Although a decrease in the mean and rms values of the strain rate tensor e ij components is observed for the reacting case as one would expect, the local flow acceleration across the flame front leads to a substantial increase in the skewness and the kurtosis of the probability density functions ͑PDFs͒ of e ij components. The turbulence-scalar interaction is studied by analyzing the orientation of the flame front normal with the eigenvectors of e ij . The PDFs of this orientation clearly show that the normals have an increased tendency to align with the extensive strain rate, which implies that the scalar gradients are destroyed by the turbulence as the scalar isosurfaces are pulled apart. This result questions the validity of passive scalar turbulence physics commonly used for premixed flame modeling. However, the influence of Lewis number on this alignment behavior is not clear at this time.

No abstract

No abstract

The effects of mean flame curvature on reaction progress variable gradient, ∇c, alignment with local turbulent strain rate are studied based on three-dimensional Direct Numerical Simulation (DNS) data of turbulent premixed flame kernels with different initial radii under decaying turbulence. A statistically planar flame is also considered in order to compare the results obtained from the kernels with a flame of zero mean curvature. It is found that the dilatation rate effects diminish with decreasing kernel radius due to defocusing of heat in the positively curved regions. This gives rise to a decrease in the extent of reaction progress variable gradient alignment with most extensive principal strain rate with decreasing kernel radius. The modelling implications of the statistics of the alignment of ∇c with local strain rate have been studied in terms of scalar dissipation rate transport. A new modelling methodology for the contribution of the scalar-turbulence interaction term in the transport equation for the mean scalar dissipation is suggested addressing the reduced effects of dilatation rate for flame kernels and the diminished value of turbulent straining at the small length scales at which turbulence interacts with small flame kernels. The performance of the new models is found to be satisfactory while comparing to DNS results. The existing models for the dilatation contribution and the combined chemical reaction and molecular dissipation contributions to the 26 Flow Turbulence Combust (2010) 85:25-55 transport of mean scalar dissipation, which were originally proposed for statistically planar flames, are found to satisfactorily predict the corresponding quantities for turbulent flame kernels.

No abstract

No abstract