Influence of pilot-stage swirl direction on spray and near-LBO combustion in a centrally staged aero-engine combustor

This study investigates the combustion characteristics of a centrally staged combustor under near-LBO conditions, focusing on the effects of pilot-stage swirl direction on LBO performance, fuel atomization, combustion behavior, and flame structure. The effects of inlet temperature and fuel-air ratio (FAR) were also examined. The results show that swirl direction significantly affects the LBO limit and flame stability. Compared with the co-swirl, the counter-swirl exhibits smaller droplet Sauter mean diameter (SMD), reduced local liquid accumulation, and better near-LBO combustion performance, which are qualitatively consistent with enhanced near-field shear and fuel-air mixing. Consequently, the counter-swirl achieves a lower FARLBO. Combustion efficiency, CO2 conversion rate (XCO2), and normalized PMT intensity effectively characterize combustion and near-LBO performance. Under all conditions, the counter-swirl exhibits higher combustion efficiency, XCO2, and PMT intensity, together with lower CO and unburned hydrocarbon (UHC) emissions. The normalized PMT intensity and XCO2 both show strong linear correlations with FAR. The PMT signal provides a sensitive indicator of combustion-intensity variation in the primary zone, while the XCO2 offers a useful supplement to conventional combustion-efficiency analysis, especially at low inlet temperatures. In addition, the combined assessment of combustion efficiency and temperature rise indicates that a low FARLBO requires both low temperature rise and high combustion efficiency. As FAR decreases, the counter-swirl flame evolves from a full-bodied structure to a conical and then bifurcated shape, whereas the co-swirl flame remains continuous but weaker. Higher inlet temperature shifts the flame upstream toward the swirler exit.