Numerical study on dispersed fuel injection strategies for thermal performance enhancement in a self-flue-recirculating radiant tube

This study proposes and evaluates a dual-stage dispersed fuel injection strategy for a novel U-type radiant tube with internal/external flue gas self-recirculation. A comparative analysis was conducted on the flow structure, flame position, heat transfer characteristics, and overall performance of single-stage and dual-stage dispersed fuel injection modes utilizing a validated three-dimensional CFD combustion model. The influence mechanism of the secondary dispersed fuel fraction (χsec-disp, 5%–30%) on tube wall temperature uniformity and NOx emissions was specifically investigated. Results demonstrate that dual-stage dispersed combustion effectively compensates for the axial heat loss along the flow path, significantly elevating the temperature and expanding the high-temperature zone in the second straight tube, despite causing a slight increase in NOx. Increasing the χsec-disp from 5% to 30%, markedly enhanced tube wall temperature uniformity (maximum temperature difference reduced by 21.6%) and suppressed NOx generation and emission (outlet concentration as low as 49.6 mg/m3 @ 8% O2). These improvements stem primarily from the increased proportion of dispersed fuel, which promotes global fuel dispersion, mitigates localized flame concentration, and fosters more uniform combustion within the tube. Therefore, a moderate increase in χsec-disp proves effective in enhancing heating uniformity and reducing NOx emissions, offering pivotal guidance for the design optimization of next-generation radiant tubes.