Effect of Dimensionless Inner Secondary-Air Cone Length on Near-Field Mixing and Gas-Solid Transport in a Centrally Fuel-Rich Swirl Burner Under Deep Air-Staging

Near-nozzle scalar mixing and gas-solid transport strongly affect ignition stability, burnout, CO formation, and NOx control in centrally fuel-rich swirl burners under deep air-staging conditions. Cold-flow experiments were performed on a centrally fuel-rich swirl burner under an over-fire air ratio of 25% using heated inner-secondary-air temperature scalar tracing and phase Doppler anemometry (PDA) measurements of gas-solid two-phase flow. The dimensionless inner secondary-air cone length is defined as η = Li/Lo, where Li and Lo are the inner and outer secondary-air cone lengths, respectively. The temperature-tracing tests covered η = 0.25–1.00, and the PDA tests covered η = 0–1.00. Increasing η shifted the scalar-mixing window downstream and reduced the near-field reverse-flow region and outward particle transport. For the temperature-tracing tests, the maximum normalized residual temperature increased by 34.1% at x/d = 0.1 and by 25.3% at x/d = 0.5 when η increased from 0.25 to 1.00, whereas the difference at x/d = 1.5 was within the estimated uncertainty. In the PDA tests, at x/d = 0.1, the negative axial-velocity region contracted from r = 20–76 mm for η = 0 to r = 40–68 mm for η = 0.50; for η = 1.00, only a shallow minimum remained near r = 48 mm. At x/d = 0.1, the negative particle-volume-flux region narrowed from r = 28–80 mm for η = 0 to r = 60–64 mm for η = 1.00. These results provide quantitative cold-flow evidence of how η modifies near-field mixing, recirculation, and particle transport; implications for emissions and burnout require validation under reacting conditions.