Multi-scale effects of outer secondary-air cone length on gas–solid flow, mixing and NOx in a centrally fuel-rich swirl burner under deep air staging

Under deep air staging with overfire air (OFA) = 25%, the reduced secondary-air availability makes near-exit recirculation and primary/secondary-air mixing critical to stable combustion and low-NO x synergy in a centrally fuel-rich (CFR) swirl burner. In this work, the dimensionless outer secondary-air cone length ( L o / L i ) was investigated through a multi-scale framework combining 1:4 cold-flow single-phase experiments, 1:7 cold-flow gas-solid phase Doppler anemometry (PDA) measurements, and 0.5 MWe pilot-scale reacting tests. Increasing L o / L i markedly enlarged the central recirculation zone (CRZ), with D h / d increasing from 0.34 to 0.91, accelerated near-field scalar decay of the primary-air core (31.7% for L o / L i = 2 at x/d = 0.1-0.5), and formed a stronger and wider negative axial-velocity region, indicating enhanced recirculation, earlier mixing, and altered particle transport pathways. The jet spreading behavior also shifted from weak expansion at L o / L i = 0 to stronger guidance under L o / L i = 1-2, reflecting the coupled role of cone geometry in recirculation development and near-field flow organization. Pilot-scale results confirmed earlier ignition, improved flame stability, and synergistic improvement in burnout-related indicators and CO/NO x performance. Overall, L o / L i around 1.5-2.0 is identified as a favorable design window under the present burner configuration and operating conditions, providing a trend-based geometric guideline for stable and low-emission operation of swirl burners under deep staging.