In order to develop better design tools, it is of prime importance to acquire a basic comprehension of flame flow interaction phenomena under the widely variable operative conditions used in practical devices ranging from power generation burners to aeronautical or rocket combustion chambers. Computational Fluid Dynamics (CFD) is a fundamental tool in research by its extreme flexibility and the availability of a wide range of existing models and the ever increasing computational resources with respect to the ex- periments which are by far more expensive. The numerical simulation of a compressible reactive flow, occurring in such devices at typical operating conditions, is still an open problem in the scientific community. The main reason lies in the fact that diffusion, advection, acoustics and chemical reactions introduce a broad range of spatial and temporal scales. In the context of multi-scale phenomena a relatively unexplored subject is the interplay of the hydrodynamic instability, or Darrieus-Landau (DL), in large turbulent pre- mixed combustion. In fact, the intrinsic instability can be modify the flame propagation and morphology. The DL mechanism appears to play a substantial role in premixed turbulent combustion with high pressure condition. The DL mechanism causing modifications of the flame surface area modify the turbulent burning velocity and mean flame brush thickness. Therefore in this project a study about large scale effects on the methane air premixed flames in turbulent regime and for different pressure values are proposed, considering a low Mach approximation (i.e. neglecting the acoustic interaction phenomena) with one step and finite chemistry.
The role of the intrinsic instability in turbulent premixed combustion is an unexplored field [37, 13]. DL instability plays an important role in the propagation and morphology of the flame in laminar and turbulent flow field. Many experimental data [2] are often claimed to show this role in premixed turbulent combustion. The important point is that the exact domain of influence of these instabilities in turbulent premixed combustion remains unknown [13] and is certainly a matter of investigation. Recently, Creta et al. [27, 28] performed 2D simulations of the slot Bunsen in weakly turbulent flow field. In a mildly turbulent setting, a completely different morphology between stable and unstable flames is observed, which suggests that the statistical properties of curvature can use as an unambiguous marker for DL effects. In particular, the skewness of curvature probability density function, measuring its asymmetry about the mean, is an observable expected to act as such marker. These findings [27] suggest that DL effects on the turbulent propagation of a premixed flame are clearly evident though confined to a low turbulence intensity regime. The study of the exact domain of influence of the hydrodynamic instability on the flame propagation can be used as a important new results to calculate the turbulent flame speed. The novelties of this research project are the development of the DNS database with both simplified and detailed chemistry in 2D and 3D. The database can be used to generate a closure a-priori models, inclusive of intrinsic instability effects, to filtered simulations as RANS and LES which are a practical tools for a variety of combustion problems of technical interest. The closure models, in the state of the art, for RANS and LES don't considered the flame instability effects.
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