Abstract: Based on a dedicated test facility (designated TB03B) for rim seal investigations in high-efficiency low-carbon gas turbines, experimental studies focused on a 1.5-stage turbine forward disc cavity with fish-mouth seal configuration using CO₂ tracing technique for gas ingestion monitoring. The rig enabled systematic investigations of sealing efficiency, gas intrusion, cavity flow, and turbine aerodynamic comprehensive efficiency. Results show that 1.5-stage turbine testing capability with maximum rotational speed of 3 000 r/min and mainsteam mass flow rate of 10 kg/s, and performance parameters achieved and mostly exceeded design specifications. Sealing efficiency increases with secondary flow pressure ratio while also improving circumferential uniformity. Sealing efficiency at high radius in the cavity exhibits significant negative correlation with pressure gradient at stator blade root, and the correlation is weakened at low radius due to secondary flow mixing effects.