Methane and hydrogen-enriched (25 vol% and 50 vol% H₂-enriched CH₄) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re), showing the dependence on Re and H₂ concentrations. The effects of equivalence ratio (Φ), Re, and H₂ enrichment on flame structure were examined employing OH-PLIF measurement. For CH₄/air cases, the flame was stabilized with an M shape; while for H₂-enriched cases, the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H₂ enrichment, occurring due to the increase in flame speed, higher resistance of the flame to the strain rate, and change in the inner recirculation zone. Flow fields of CH₄/air flames were compared between low and high Re cases employing high-speed PIV. The flashback events, led by two mechanisms (combustion-induced vortex breakdown, CIVB, and boundary-layer flashback, BLF), were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH₄ flames with M shape, whereas the BLF occurs for all H₂-enriched flames with П shape.