Eulerian - Lagrangian Description of Stirring and Mixing in Transitional Flow. Interconnections with Autoigniting Structures.

Turbulent non-premixed combustion has a particular relevance since in common devices involved flows are inevitably turbulent and separated before their injection in combustion chambers. In particular, non-premixed combustion can be viewed as a three stage process of stirring, diffusion and ignition spanning the full spectrum of space-time scales of the flow. In particular, an exact description of stirring phenomena is given by the location of the interface between the flows. The related scales of the passive scalar transport has also been the main topic of numerous publications, but there are still open questions related to mixing-relevant flows. The main purpose of this thesis is to show that enucleating the stirring process from the mixing process is one of the most convenient way to face with this problem and that the quantitative assessment of some related stirring characteristics is feasible on one side as well as sufficient to describe the mixing pattern on the other side. The selection of some “critical” quantities to be evaluated on the interface for their exploitation in a mixing characterization should include also significant Lagrangian quantities. Moreover, the ignition processes involved on the interface results strictly interconnected with the advected-surfaces evolution. Therefore the evaluation of the stirring/mixing quantities lays the foundation for a sequentially-structured approach to characterize non-premixed combustion flows. In this context, a description of the autoigniting structures in term of gaseous diffusion flames seems to be useful.