Numerical Study of Combustion in a Methane–Hydrogen Co-Fired W-Shaped Radiant Tube Burner

Three-dimensional computational fluid dynamics (CFD) simulation was performed using the eddy-dissipation concept coupled with detailed hydrogen oxidation kinetics and a reduced two-step methane mechanism for a newly proposed W-shaped radiant tube burner (RTB). The effects of the hydrogen volume fraction (0–100%) and excess air ratio (0%, 10%, 20%) on the flame morphology, temperature distribution, and NOX emissions are systematically analyzed. The results deliver three main points. First, a flame-shape transformation was identified in which the near-injector flame changes from a triangular attached mode to a splitting mode as the mixture reactivity increases with the transition occurring at a characteristic laminar flame speed window of about 0.33 to 0.36 m/s. Second, NOX shows non-monotonic behavior with dilution, and 10% excess air can produce higher NOX than 0% or 20% because OH radical enhancement locally promotes thermal NO pathways despite partial cooling. Third, a multi-parameter coupling strategy was established showing that hydrogen enrichment raises the maximum gas temperature by roughly 100 to 200 K from 0% to 100% H2, while higher excess air improves axial temperature uniformity and can suppress NOX if over-dilution is avoided. These findings provide a quantitative operating map for balancing stability, uniform heating, and NOX–CO trade-offs in hydrogen-enriched industrial RTBs.