The Role of Recirculation Zones in Soot Formation in Aircraft Combustors

Recirculation zone plays an important role in flame stabilization in combustors and gas turbines. The location, size, and strength of recirculation zones are important features of a combustor. However, the quantitative role of recirculation zones in affecting soot formation from an aero combustor is not fully understood. In a turbulent flow field with swirling flows and high frequency oscillations of the inflow jets, inner recirculation zones and outer recirculation zones have different functions in determining soot evolution. In this study, large-eddy simulation (LES) of soot formation with detailed physical and chemical models is used in order to study the dynamic aspects of soot formation. The soot population is modeled using the hybrid method of moments (HMOM), while the gas phase precursor evolution is modeled using detailed chemical kinetics. A model aero combustor, studied experimentally at DLR, is used as the baseline flow configuration. The simulations are used to understand the transport of soot particles within such complex flows. In particular, the ability of recirculation regions to increase soot formation by increasing residence times is explored. A Lagrangian particle tracking (LPT) analysis is carried out and statistical roles of recirculation zones are determined from these streamlines. Furthermore, source term analysis of these particles are performed to determine the key physical processes that contribute to soot mass in the recirculation zones. From a numerical stand-point, such soot evolution introduces limitations for statistical convergence, which will also be discussed. In particular, a time-scale analysis will be conducted to determine total computational time needed to obtain converged soot statistics.