Flame stability mapping and flashback characteristics of oxygen-enriched diffusion-assisted CNG flames in a model gas turbine combustor

This study experimentally investigates the combustion stability, flame macrostructure, and emissions of diffusion-assisted partially premixed Compressed Natural Gas (CNG) flames in a model gas turbine combustor equipped with a perforated plate burner featuring a central diffusion hole. The investigation focused on the effects of fuel diffusion percentage (DP) and oxygen fraction (OF) on flame stability across a range of equivalence ratios (Φ) and oxidizer Reynolds numbers (Φ-ReOxidizer). The first set of experiments established the baseline stability map for CNG-air flames by defining the upper and lower flammability limits in Φ-ReOxidizer domain across DPs of 0%, 5%, 10%, and 15%. The subsequent sets investigated the effects of oxygen enrichments (OF = 22%, 23% and 24%) on the same stability framework, quantifying the shift in operability limits and the onset of flashback phenomena. Increasing DP and OF broadened the stable operating limits, reaching ~30% broader limits. While the lower flammability limit was relatively unaffected by OF, the risk of flashback increased markedly with OF, especially at reduced ReOxidizer. Thermal measurements revealed a rise in peak flame temperature by approximately 4.4% at 10% DP compared to the non-diffusion case. Emission analyses showed that O2 and CO2 concentrations remain largely unaffected by DPs, whereas CO shows a significant increase from 25 to 75 ppm at 0% DP and from 63 to 91 ppm at 10% DP, indicating incomplete combustion. The findings provide operability maps and emission trends for proper design and safe operation of gas turbines.