Experimental investigation of spray and combustion behavior in a centrally staged combustor under stable and near-blowout conditions

This study investigates the spray distribution, droplet lifetime, combustion performance, emissions and flame structure of a centrally staged combustor under conditions ranging from stable operation to near lean blowout (LBO) conditions. The results show that SMD and droplet velocity shift from pilot- to main-stage air control, underscoring the growing main-stage influence along the flow direction. As FAR decreases, the SMD increases and droplet lifetime becomes significantly longer, while the chemical reaction times remain short and change very little. Correspondingly, the spray morphology evolves from weakened radial expansion to contraction. At higher inlet temperatures, atomization is enhanced and the fuel distribution becomes more uniform, whereas at lower temperatures, droplet accumulation and limited evaporation result in dispersed, uneven spray structures. Characteristic time analysis confirms that flame stability is dominated by fuel preparation rather than by chemical kinetics, particularly under low inlet temperature conditions. Combustion efficiency and temperature rise jointly determine FAR_LBO: temperature rise sets the lower limit, while efficiency dictates how closely that limit can be approached. An optimal FAR_LBO can be achieved only when the temperature rise is sufficiently low and the combustion efficiency remains high. The CO₂ conversion rate and normalized PMT signal intensity were introduced as new indicators. They exhibit a strong linear correlation with FAR across all inlet temperatures and an inverse relationship with CO formation. Therefore, it is more reliable than combustion efficiency under low inlet temperature. High inlet temperature improves atomization, shortens droplet lifetime, anchors the flame at the swirler exit while maintaining radial uniformity, and thereby delays blowout. In contrast, at lower temperatures, evaporation-limited combustion leads to earlier flame bifurcation at high FAR. POD analysis reveals that as LBO approaches, the flame transitions from a stable recirculation-zone mode to localized fuel-rich modes, with the mode 1 energy fraction continuously decreasing at higher inlet temperatures and the flame progressively fragmenting toward extinction.