Towards a unified approach for Large Eddy Simulation of turbulent spray flames

The recent limitations imposed by ICAO-CAEP, regulating NOx emissions, are leading to the implementation of lean burn concept in the aero-engine framework. From a design perspective, a depth insight on lean burn combustion is required and Computational Fluid Dynamics (CFD) can be a useful tool for this purpose. Several interacting phenomena are involved and various modelling strategies, with huge differences in terms of computational costs, are available. Nevertheless, up to now few numerical tools are able to account for the effects of liquid fuel preparation inside reactive computations. Spray boundary conditions are normally determined thanks to correlative approaches that are not able to cover the wide range of operating conditions and geometrical characteristics of aero-engine burners. However, as highlighted in the first part of the dissertation, where several literature test cases are analysed through numerical calculations, the impact of liquid preparation can be extremely important. Considerations based on correlative approaches may be therefore unreliable. More trustworthy predictive methods focused on fuel atomization are required. This research activity is therefore aimed at developing a general numerical tool, to be used in an industrial design process, capable of modelling the liquid phase from its injection till the generation of a dispersed spray subject to evaporation. The ELSA (Eulerian Lagrangian Spray Atomization) model, which is based on an Eulerian approach in the dense region and a Lagrangian one in the dilute zone, has been chosen to this end. The solver is able to deal with pure liquid up to the generation of a dispersed phase and to account for the breakup process through the introduction of the liquid-gas interface density. However, several limitations of such method arise considering its application in a highly swirled reactive environment like an aero-engine burner. Therefore, particular attention has been here devoted first to the study of the turbulent liquid flux term, inside the liquid volume fraction equation. This quantity is of paramount importance for a swirled flow-field, with high slip velocities between phases. A completely innovative modelling framework together with a new second order closure for this variable is proposed and validated on a literature jet in crossflow test case. Then, to handle a reactive environment, a novel evaporation model is integrated in the code and assessed against experimental results. Finally, an alternative way to derive the Drop Size Distribution (DSD) in ELSA context for the lagrangian injection is presented and assessed by means of Direct Numerical Simulations. Ultimately, this work introduces an innovative framework towards a unified description of spray combustion in CFD investigations. The proposed approach should lead to a comprehensive description of fuel evolution in the injector region and to a proper characterization of the subsequent reacting flow-field. Several improvable aspects are also highlighted, pointing the way for further enhancements.