a b s t r a c tDirect numerical simulations of premixed n-heptane/air flames at different Karlovitz numbers are performed using detailed chemistry. Differential diffusion effects are systematically isolated by performing simulations with both non-unity and unity Lewis numbers. Different unburnt temperatures and turbulence intensities are used and their effects on the flame structure and chemical source terms are investigated. As the unburnt gases are preheated, the viscosity ratio across the flame is reduced and the Karlovitz number at the reaction zone is increased. The increase in turbulence intensity suppresses differential diffusion effects on the flame structure (i.e. species dependence on temperature). However, differential diffusion effects on the chemical source terms are still noticeable even at the highest Karlovitz number simulated. Simulations with differential diffusion effects exhibit lower mean fuel consumption and heat release rates than their unity Lewis number counterparts. However, the difference is reduced as the reaction zone Karlovitz number is increased. Transition to distributed burning is characterized by a broadening of the reaction zone resulting from enhanced turbulent mixing. Local extinctions in the burning rate are observed only in non-unity Lewis number simulations and their probability decreases at high Karlovitz numbers. These results highlight the importance of using the reaction zone Karlovitz number to investigate the effect of turbulence on the chemical source terms and to compare flames at different unburnt temperatures.

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Fig. 3: Two-dimensional slices of a 5L × L region centered around the flame showing temperature for the different equivalence ratios non-unity Lewis number cases. The temperature range is [298, 2200].

Assumptions behind closure models for the filtered source term are studied a priori using results from DNS of turbulent n-heptane/air premixed flames at varying Karlovitz numbers. Simulations with both detailed chemistry and tabulated chemistry, as well as unity and non-unity Lewis numbers, are used to determine if finite-rate chemistry and differential diffusion effects affect the filtered chemical source terms. While the unfiltered source term shows large fluctuations, the filtered source terms from detailed chemistry and tabulated chemistry are in good agreement at sufficiently large filter widths (∆ l F). Using the concept of optimal estimators, it is shown that a tabulation approach using the filtered progress variable and its variance can predict accurately the filtered chemical source terms. Finally, the filtered source terms from the DNS are compared to predictions from two commonly assumed sub-filter probability density function models. Both models show deviations from the filtered DNS source terms but predict accurately the mean turbulent flame speed. The results illustrate the potential of using simple tabulated chemistry approaches based on presumed PDFs for LES of premixed flames in the thin and distributed reaction zones regimes.

Accurate computation of molecular diffusion coefficients in chemically reacting flows can be an expensive procedure, and the use of constant non-unity Lewis numbers has been adopted often as a cheaper alternative. The goal of the current work is to explore the validity and the limitations of the constant non-unity Lewis number approach in the description of molecular mixing in laminar and turbulent flames. To carry out this analysis, three test cases have been selected, including a lean, highly unstable, premixed hydrogen/air flame, a lean turbulent premixed n-heptane/air flame, and a laminar ethylene/air coflow diffusion flame. For the hydrogen flame, both a laminar and a turbulent configuration have been considered. The three flames are characterised by Lewis numbers which are less than unity, greater than unity, and close to unity, respectively. For each flame, mixture-averaged transport simulations are carried out and used as reference data. The current analysis suggests that, for numerous combustion configurations, the constant non-unity Lewis number approximation leads to small errors when the set of Lewis numbers is chosen properly. For the selected test cases and our numerical framework, the reduction of computational cost is found to be minimal.

Preliminary results of unsteady numerical simulations of disk cavity flow in interaction with the main gaspath flow in an axial turbine are presented in this article. A large periodic sector including vanes, blades and disk cavity of approximately 74° has been used in order to allow for the formation of large scale flow structures within the cavity. Three purge flow rates have been tested, namely no purge, low purge and high purge flow rates. Energetic large scale flow structures are detected through flow visualizations for the two lowest purge flow rates. They are found to rotate at an angular velocity slightly less than the rotor speed. The presence of the large scale structures involves important pressure perturbations inside the cavity that may lead to deep mass flow ingress, whereas the unsteady vane-blade interaction seems to cause only shallow ingress. Increasing purge flow rate appears to have a stabilizing effect on the pressure fluctuations inside the cavity and to reduce the intensity of the large scale flow structures.

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