Large Eddy Simulations of Flame-Wall-Interaction in turbulent reacting flows
Introduction and Motivation
Most combustion processes involving turbulent flows are bounded by solid surfaces. Strong unsteady interaction between the flow field and the surface are known, such as unsteady separation, vortex shedding, flame wrinkling/lifting, near wall reactions and conjugated heat transfer. These phenomena play a major role in the analysis of flame-structure characteristics, flame stabilization and in the formation of pollutants. The accuracy required to predict such reactive flows cannot be achieved using Reynolds-Averaged Navier-Stokes Simulations (RANS). In the case of turbulent combustion, the flow pattern is characterized by large-scale unsteady motions, that strongly interact with solid surfaces. A possible approach to predict turbulent reactive flows is Direct Numerical Simulations (DNS). Within DNS all scales of motion are fully resolved by solving the Navier-Stokes equations without any turbulence model. Since the computational expense of DNS is extremely high and increases rapidly with the Reynolds number, this approach is only feasible for low Reynolds number flows. In Large Eddy Simulations (LES), the large-scale unsteady motions are directly represented, whereas the effects of the smaller-scale motions are modeled using a turbulence model. In computational expense, LES lies between RANS and DNS with comparable accuracy. Consequently, Large Eddy Simulations (LES) seems to be a promising method to capture the behavior of turbulent reacting flows and the flame-wall-interaction for a wide range of Reynolds number flows.
The present work focuses on Large Eddy Simulation of Flame-Wall-Interaction in turbulent combustion. LES of turbulent combustion emerged as a science only in the 1990s and is hence a relatively new field. A lot of research has been carried out over the past years, but there is still a lack on accurate LES combustion models and methods of Flame-Wall-Interaction in turbulent reacting flows.
Methodology and proceeding
In Large Eddy Simulation, the flow variables are decomposed into large-scale three-dimensional unsteady turbulent motions and smaller-scale motions by applying a spatial filtering operation. The filtered conservation equations of the large-scale velocity field are obtained by applying the filtering operation to the Navier-Stokes equations. Similar to the RANS equations, there needs to be closure for the unresolved stresses using turbulence models. In the case of LES, the large-scale motions are resolved directly on a numerical grid, whereas the small-scale motions are represented using a subgrid model.
In combustion LES, filtered transport equations for each individual component in the reaction mechanism has to be solved in addition to the flow field. For complex chemical mechanisms, hundreds of species and thousands of reactions occur, that require an enormous computational effort. Hence, different chemistry reduction methods were developed to reduce the computational effort. In the present work, the chemical reactions will be accounted using the chemistry reduction method FGM (flamlet generated manifold). The effects of the heat transfer will be included in the manifold through the inclusion of the absolute enthalpy as control variable.
The present work involves the following steps:
- Verification and validation of the open source CFD software package OpenFOAM (e.g. solver, models, schemes)
- Implementation and verification of new thermophysical models (e.g. subgrid models, mass and heat transfer, combustion models)
- Testing the applicability of the methodology to realistic burner systems