Influence of operating conditions on flow field dynamics and soot formation in an aero-engine model combustor

Abstract

Large-eddy simulations of a sooting aero-engine model combustor are performed for three operating points. They are analyzed to investigate the influence of secondary air injection and primary equivalence ratio on soot formation dynamics. It will be shown how the causes of a strongly intermittent sooting behavior can be investigated quantitatively by mode analysis and correlated statistics. Simulations rely on Arrhenius reaction rate-based finite-rate chemistry, where 79 species transport equations are solved simultaneously to accurately predict the combustion of ethylene: 43 species represent the gas phase, 36 the soot and soot precursors. Soot evolution is described by a well validated sectional soot model including a sectional model for polycyclic aromatic hydrocarbons and their radicals. This approach captures the influence of changing operating conditions on soot formation well. Hence, soot prediction is found to be accurate enough to investigate the formation processes. The presence of coherent and incoherent flow field dynamics in the combustor necessitates the use of multiresolution proper orthogonal decomposition and correlated statistics for the analysis. While the precessing vortex core is found to support soot production continuously, low frequency dynamics are identified to cause the intermittence at all operating conditions. Secondary air injection significantly increases the intensity of the low frequency dynamics. Yet, the soot volume in the combustor varies by the same order of magnitude without secondary air injection. The soot formation intermittence is closely linked to an axially symmetric mixture fraction mode near the primary injector at all operating conditions. Low mode energy required an additional mode analysis of joint statistics which confirmed this conclusion. Increasing the equivalence ratio at the primary injector decreases the influence of the low frequency dynamics on soot formation.