Modeling of Moderately Swirling Turbulent Non-premixed Flames

Even if the goal of reducing NOx emissions pushes towards the use of premixed combustion, moderately swirling turbulent non-premixed flames are still widely employed in both industrial furnaces and gas turbines, because of their stable and safe behaviour in a wide range of fuels and operating conditions. Computational Fluid Dynamics (CFD) has become in recent years a key tool for the design of combustion systems, providing valuable information on aero-thermal characteristics of the combustion chamber, emissions, and allowing preliminary screening of different design options. Moreover, CFD results are often used as boundary conditions for subsequent thermo-structural Finite Element Analysis (FEA) of combustion components. It is clear that in an industrial framework it is important to obtain the most accurate results with the simplest and most robust models; several different approaches have been developed in the past decades to optimally integrate chemistry and fluid dynamics in turbulent reacting flows such as turbulent flames. In this work, the description of turbulence-chemistry interaction is based on the fluid probability density function (PDF) approach. The proposed model belongs to the so-called presumed PDF methods, and is usually referred to as the Finite-Mode PDF or MultiEnvironment model, which has been already successfully employed for modelling precipitation and crystallization in turbulent systems. The distribution is represented by a finite sum of Dirac delta functions, centered in some particular values of the independent variable (nodes) and weighted by some weight functions, representing the related probabilities. The Finite-Mode PDF model has been applied to the study of lab-scale turbulent nonpremixed flames, and compared with other standard models for turbulence-chemistry interaction, already available in commercial CFD codes. The proposed model has been implemented as User-Defined Functions within the commercial CFD code FLUENT 6.1.