On the Combination of Large Eddy Simulation and Phenomenological Soot Modelling to Calculate the Smoke Index From Aero-Engines Over a Large Range of Operating Conditions

Abstract

Nowadays, models predicting soot emissions are, neither able to describe correctly fine effects of technological changes on sooting trends nor sufficiently validated at relevant operating conditions to match design office quantification needs. Yet, phenomenological descriptions of soot formation, containing key ingredients for soot modeling exist in the literature, such as the well-known Leung et al. model (Combust Flame 1991). This approach indeed includes contributions of nucleation, surface growth, coagulation, oxidation and thermophoretic transport of soot. When blindly applied to aeronautical combustors for different operating conditions, this model fails to hierarchize operating points compared to experimental measurements. The objective of this work is to propose an extension of the Leung model, including an identification of its constants over a wide range of condition relevant of gas turbines operation. Today, the identification process can hardly be based on laboratory flames since few detailed experimental data are available for heavy-fuels at high pressure. Thus, it is decided to directly target smoke number values measured at the engine exhaust for a variety of combustors and operating conditions from idling to take-off. A Large Eddy Simulation approach is retained for its intrinsic ability to reproduce finely unsteady behavior, mixing and intermittency. In this framework, The Leung model for soot is coupled to the TFLES model for combustion. It is shown that pressure-sensitive laws for the modelling constant of the soot surface chemistry are sufficient to reproduce engine emissions. Grid convergence is carried out to verify the robustness of the proposed approach. Several cases are then computed blindly to assess the prediction capabilities of the extended model. This study paves the way for the systematic use of a high fidelity tool solution in design office constraints for combustion chamber development.