Sooting and spray characteristics of swirl-stabilized Jet A-1 flames with secondary air injection

Modern gas turbine engines have been designed to mitigate high NO X emissions through the implementation of Rich-burn, Quick-quench, Lean-burn (RQL) combustion architecture. This staged-combustion approach incorporates secondary air jets, that maintain an overall low temperature within the combustion chamber and have led to a reduction in thermal NO formation. However, its influence on soot formation remains under-reported. To investigate the effects of secondary air injection on soot formation, a model gas-turbine combustor equipped with secondary air jets was employed. The primary air mass flow rate ( m ̇ p ) and secondary air mass flow rate ( m ̇ s ) were systematically varied while maintaining a constant Jet A-1 fuel mass flow rate. Five cases of secondary air substitution, corresponding to 0%, 15%, 30%, 40% and 45% of m ̇ p , were examined. The size and spatial distribution of spray droplets were characterized using digital inline holography (DIH), and soot volume fraction ( f v ) measurements were conducted using simultaneous two-dimensional, two-color auto-compensating planar laser-induced incandescence (AcPLII). Time-averaged soot volume fraction ( f ̄ v ) results revealed an increasing trend in soot formation at the center plane as the substitution percentage was increased from 0% to 40%. However, subsequent decrease in soot formation was observed when substitution was further increased to 45%. The Sauter mean diameter ( D 32 ) of fuel droplets in reacting flow exhibited an increasing trend with increasing secondary air substitution. At substitution levels of 0% and 15%, D 32 decreased with height, characterized by a linear decrease in droplet diameter. In contrast, substitution levels of 30% to 45% showed an increase in D 32 , implying that high temperatures in the primary zone rapidly evaporate smaller droplets, thereby increasing the relative probability of larger droplets. Additionally, secondary air substitution had negligible effect on droplet clustering; rather, inter-droplet spacing appeared to be governed by the strength of the inner re-circulation zone and the temperature in the primary combustion zone. Novelty and significance statement This study examines the effects of secondary air injection jets on soot formation and spray characteristics in a model gas turbine combustor. A key novelty of this work is the measurement of the three-dimensional spray droplet distribution, including droplet diameter and soot volume fraction, within a reacting flow. The results are expected to enhance numerical modeling of polydisperse sprays in such environments. Furthermore, the observed trends in soot volume fraction provide new insights into how the interaction between spray droplets and degree of secondary air substitution influences soot formation.