The Effect of Changing Exhaust Nozzle Geometry on Temperature Distribution and Emissions of Methane Diffusion Flame Under Air/Fuel Swirl Flows

The performance of diffusion flame (DF) burners strongly depends on how effectively combustion gases mix and retain heat, yet the influence of exhaust nozzle geometry on these processes remains insufficiently characterized. This study examines how varying exhaust nozzle angle affects the thermal behavior and emissions of a methane (CH4) diffusion flame under atmospheric conditions. A laboratory-scale burner with interchangeable exhaust nozzles (0°, 25°, and 50°) was operated at 1.8 kW using a fixed methane flow of 3 L/min and co-swirled air and fuel at 30°, across equivalence ratios (Φ) of 1.0, 0.7, and 0.5. Axial temperature measurements and exhaust gas analyses (Carbon dioxide (CO2) and Carbon monoxide (CO)) were conducted to assess mixing, heat retention, and post-flame oxidation. Results show that exhaust nozzle geometry notably influences flame position and heat distribution, producing non-monotonic temperature trends with equivalence ratio. The 25° nozzle angle yielded the highest near-stoichiometric downstream and flue temperatures, reaching about 204 °C at x = 45 cm and 277 °C in the flue, compared with 72 °C and 177 °C for the 0° nozzle. In contrast, the 50° nozzle produced more uniform downstream temperatures (about 150–160 °C) and the lowest CO emissions, approaching zero near Φ ≈ 1.0. These findings demonstrate that coordinated control of swirl and exhaust nozzle angle can enhance thermal response and CO reduction in diffusion flame burners without significantly changing CO2 levels.