Statistics of Scalar Dissipation and Strain/Vorticity/Scalar Gradient Alignment in Turbulent Nonpremixed Jet Flames

摘要

Turbulence and mixing statistics are investigated in a set of flames at a jet Reynolds number of 15000 achieving a Taylor's scale Reynolds number in the range 100 <= Re-lambda <= 150. In particular, the impact on small scale turbulence statistics of different levels of flame extinction, induced imposing different Damkohler numbers in the three simulated cases, is investigated. It is found that the non-dimensional scalar dissipation depends on the Damkohler number slightly. This deviation from self-similarity manifests itself as a decrease of the non-dimensional scalar dissipation with increasing occurrence of localized extinction events. This is caused by the decrease of molecular diffusion due to the lower flame temperatures in the low Damkohler number cases. Probability density functions of the scalar dissipation chi show important deviations from the log-normal distribution. The left tail of the pdf scales as chi(1/2) while the right tail scales as e(-c chi alpha), as shown for incompressible turbulence. In all flames, the vorticity vector displays a pronounced tendency to align with the direction of the intermediate strain and the gradient of mixture fraction aligns with the most compressive strain. Conditioning on the local values of mixture fraction and heat release does not affect the statistics. The alignment statistics of vorticity are in agreement with those in homogeneous isotropic turbulence while they show some difference compared to previous results in non premixed flames. The alignment between strain and mixture fraction gradient differs slightly from the homogeneous isotropic turbulent case but agree remarkably well with previous results observed in homogeneous shear incompressible flows.