The simulation of soot evolution is a problem of relevance for the development of low-emission aero-engine combustors. Apart from detailed CFD approaches, it is important to also develop models with modest computational cost so a large number of geometries can be explored, especially in view of the need to predict engine-out soot particle size distributions (PSDs) to meet future regulations. This paper presents an approach based on Incompletely Stirred Reactor Network (ISRN) modeling that simplifies calculations, allowing for the use of very complex chemistry and soot models. The method relies on a network of Incompletely Stirred Reactors (ISRs), which are inhomogeneous in terms of mixture fraction but characterized by homogeneous conditional averages, with the conditioning performed on the mixture fraction. The ISRN approach is demonstrated here for a single sector lean-burn model combustor operating on Jet-A1 fuel in pilot-only mode, for which detailed CFD and experimental data are available. Results show that reasonable accuracy is obtained at a significantly reduced computational cost. Real fuel chemistry and a detailed physicochemical sectional soot model are consequently employed to investigate the sensitivity of ISRN predictions to the chemical mechanism chosen and to provide an estimate of the soot particle size distribution at the combustor exit.

The understanding of the processes involved in soot formation and oxidation is a critical factor for a reliable prediction of emissions in aero-engines, particularly as legislation becomes increasingly stringent. This work studies the flame structure and soot formation in a lab-scale burner, which reproduces the main features of a Rich-Quench-Lean (RQL) combustor, using high-fidelity numerical simulations. The investigated burner, developed at the University of Cambridge, is based on a bluff-body swirl-stabilised ethylene flame, with air provided in the primary region through two concentric swirling flows and quenching enabled by means of four dilution jets at variable distance downstream. Measurements for different air split between the two inlet swirling flows and dilution ports, and different height of the dilution jets, indicate noticeable differences in the soot tendency. Numerical simulations have been performed using Large-Eddy Simulation with the Conditional Moment Closure combustion model and a two-equation model for soot, allowing a detailed resolution of the mixing field and to directly take into account the effect of turbulent transport on the flame structure, which has been shown to have an important effect on the soot formation and evolution. The main objective of this work is to study the flow field and mixing characteristics in the burner’s primary region, in order to improve the understanding of the mechanisms leading to the soot behaviour observed in the experiment at different operating conditions. Results show the key role of mixing in determining the level of soot in the burner, with the soot production mainly related to the extension of the flame zone characterized by a rich mixture, with pyrolysis products and soot precursors. The presence of additional dilution air seems to improve the oxidation and leads to a leaner mixture in the primary combustion region whereas the air added through the outer swirl stream seems to have less impact on the mixture formation in the primary region. Analysis of the solution in mixture fraction space shows the importance of residence time for the soot formation and highlights the existence of a range of values of mixture fraction, between 0.1 and 0.2, where the soot production terms are maximum. High residence times and local air-to-fuel ratio in the range of high soot production should be avoided to decrease the level of soot mass fraction in the burner.