The main focus of this paper is to discover the link between flame macrostructure and thermoacoustic instability in a centrally staged swirl burner. In practical combustors, the flow rate in the pilot stage is much smaller than that in the main stage. However, the modification in the pilot stage could alter the flame macrostructure while maintaining a similar total flow rate. Therefore, the thermoacoustic instability was examined at different flame macrostructures by varying the pilot stage equivalence ratio under identical main stage inlet conditions. High-frequency planar laser measurements and chemiluminescence measurement were conducted to enhance spatial and temporal accuracy, providing a more comprehensive understanding of thermoacoustic instability. Two different flame macrostructures, S-type and I-type flames, were identified based on the preheating zone distribution. They exhibit distinct thermoacoustic instabilities, with the I-type flames demonstrating more intense instability than S-type flames. The results indicate that the variation of flame macrostructure influences the coupling of flame heat release and flow field. Specifically, the preheating zone and heat release of I-type flames exhibit greater sensitivity to flow field fluctuations, resulting in a more intense and complex fluctuation of the flame. This discrepancy leads to variations in thermoacoustic instability intensity, as well as the changes in the phase coupling between heat release and acoustic pressure, which in turn impact the total Rayleigh index. Meanwhile, significant differences exist in the distribution pattern and range of flow field fluctuations between I-type and S-type flames.

This paper examined the influence of adjacent nozzles on the flow and fuel atomization characteristics of the central nozzle in a triple-nozzle model combustor. High-frequency particle imaging velocimetry testing for airflow and phase Doppler particle analyzer testing for fuel particles were employed. Under the central nozzle air intake mode, the momentum exchange between the swirling flow and the surrounding stationary air leads to a seemingly shrinking flow field. While under the triple nozzles air intake mode, the low static pressure at the geometric center between adjacent nozzles attracts the swirling flows to converge, resulting in a larger expansion angle. Confinement has a complex impact on swirling flow. When the angle of the swirling airflow is small, it guides the flow to expand. However, when the angle becomes too large, it restricts further expansion. Observed Sauter mean diameter (SMD) peaks for the tested cases lie within a similar range, suggesting comparable spray angles. Nevertheless, the triple-nozzle air intake mode exhibits consistently higher SMD values and significantly greater spray core penetration depth. Instantaneous flow characteristics deepen the understanding of these differences in atomization results. The large range of airflow oscillation under the central nozzle air intake mode allows the airflow to directly interact with the droplets. While under the triple-nozzle air intake mode, the air flow path is too far away from the spray, resulting in that most droplets do not receive sufficient aerodynamic forces for atomization.

This study introduces a novel predictive model for atomization droplet size, developed using comprehensive data collected under elevated temperature and pressure conditions using a twin swirl airblast nozzle. The model, grounded in flow instability theory, has been meticulously parameterized using the Particle Swarm Optimization (PSO) algorithm. Through rigorous analysis, including analysis of variance (ANOVA), the model has demonstrated robust reliability and precision, with a maximum relative error of 19.3% and an average relative error of 6.8%. Compared to the classical atomization model by Rizkalla and Lefebvre, this model leverages theoretical insights and incorporates a range of interacting variables, enhancing its applicability and accuracy. Spearman correlation analysis reveals that air pressure and the air pressure drop ratio significantly negatively impact droplet size, whereas the fuel–air ratio (FAR) shows a positive correlation. Experimental validation at ambient conditions shows that the model is applicable with a reliability threshold of We₁/Re₁ ≥ 0.13 and highlights the predominance of the pressure swirl mechanism over aerodynamic atomization at higher fuel flow rates (q > 1.25 kg/h). This research effectively bridges theoretical and practical perspectives, offering critical insights for the optimization of airblast nozzle design.

This paper presents an experimental investigation on flame interactions and macrostructures between adjacent domes in a centrally staged triple-dome model combustor under different main-stage equivalence ratio (∅m) and stratification ratio (SR) conditions. This work employs direct photograph and high-speed flame chemiluminescence of CH* and OH* measurements. The degree of interaction is quantitatively evaluated by examining the relative CH* intensity of a few key locations and comparing the intensity on the wall side with that on the adjacent dome side. Flame CH* imaging also reveals three distinct flame macrostructures (single-layer, double-layer, and triple-layer) and two inter-dome flame interaction modes [inverted V-shape (IVS) and inverted funnel-shape (IFS)]. Results demonstrate that flame macrostructures are predominantly influenced by SR variations, while interaction modes were determined solely by ∅m conditions. Compared to the IFS mode, the IVS mode shows an interaction region located closer upstream. Moreover, two distinct interaction-induced phenomena are identified: a stable outer flame in the main-stage jet outer shear layer, and an intermittent pocket flame within the inter-dome triangular recirculation zone. Analysis of local equivalence ratios and high-speed OH* image sequences shows that outer flame stabilization depends primarily on heat feedback from the sub-interaction backflow region, while the intermittent behavior of pocket flame is attributed to the periodic flame fragment detachment in the interaction region.

In the present study, a recently developed improved Self-Adaptive Turbulence Eddy Simulation (SATES) turbulence model (marked as SATES-Mixed), is validated for the simulation of complex multiple swirling turbulent flows. The new SATES-Mixed method aims to enhance both the performances in the wall region as well as the free shear layer region. For comparisons, the Large Eddy Simulation (LES) with WALE and Smagorinsky sub-grid model is also conducted with the same numerical setups. Compared with the original widely used SATES model, the SATES-Mixed model inherits the low grid sensitivity and high accuracy for free turbulence while improving the calculation accuracy in the near-wall regions. Therefore, the prediction ability of the SATES-Mixed model is validated in challenging complex swirling flows encountered in multi-stage swirl combustion chambers. Good overall agreement between SATES-Mixed and experiments is observed with relatively coarse mesh, which is even better than the LES-WALE results. The SATES-Mixed model accurately captures the typical single-vortex tube PVC evolution characteristics in a double swirling combustor (GTMC) and effectively models the complex interactions between single/double vortex tubes in a three-stage swirling combustor, including their intertwined twisting motions and mutually evolving processes.

The pilot flame serves as the primary anchor for global flame stabilization in a centrally staged combustor. In engineering practice, it typically operates in the diffusion mode. The fuel non-uniformity and diffusion kinetics of the pilot flame may have a significant impact on the flow and flames within the combustor. The flame structure and flame–flow interaction in a centrally staged burner featuring a diffusion pilot flame are investigated in the present paper, using high-frequency CH₂O planar laser-induced fluorescence (CH₂O-PLIF), CH^* chemiluminescence, and particle image velocimetry (PIV) measurements. The stratified flame (S-flame) and the lifted flame (L-flame) are identified under two-stage conditions. The S-flame and L-flame correspond to the separated flow and the merged flow of the two stages, respectively. Significant radial oscillation of the pilot stage airflow is also found. Extensive tests demonstrate that the pilot equivalence ratio (Φp) plays an important role in flame mode switching. Silicone droplets with extremely fine sizes are introduced into the pilot fuel to trace its transportation. When the oscillating pilot stage airflow rushes towards the lip in an instant, it can entrain the pilot fuel to reach the inner side of the main stage outlet. With a low pilot fuel supply and relatively low injection velocity, the pilot fuel and the hot radicals are more likely to be entrained and accumulate in larger amounts at the inner side of the main stage outlet. Consequently, the main stage premixed mixture can be ignited at the main stage outlet, forming the S-flame. The flame mode switches from S- to L-flame when the equivalence ratio increases to the point where the corresponding velocity ratio of pilot fuel to air (V_fp/V_ap) approaches 1.0, with a reduced entrainment of the pilot fuel and radicals. Simultaneous CH₂O-PLIF and flow field results show that when the main stage is ignited downstream, hot products cannot recirculate to the pilot stage outlet, causing the extinction of the pilot flame root. This paper reveals that the fuel diffusion characteristics of the pilot stage can dramatically change the flame structure. To achieve the ideal designed flame shape, the interaction between the pilot fuel and pilot air requires very careful treatment in practical centrally staged combustors.