Performance of a Turbine Rim Seal Subject to Rotationally-Driven and Pressure-Driven Ingestion

For decades, the main strategy of gas turbine designers has targeted improvements in efficiency derived from an increase in the turbine entry temperature incurring extra cooling cost and increased risk of hot gas ingestion. Overestimating the amount of secondary air required to seal the volume between the stator-rotor discs existing below the hub results in an efficiency penalty whilst the undersupply of this critical sealing flow may challenge the integrity of the components and detriment the operating life of the engine. Accurate prediction is therefore critical and a deep understanding of the underlying physics of the governing phenomenon of paramount importance to develop adequate design tools. This paper describes the second part of an experimental investigation carried out at the Oxford Rotor Facility that aims to improve the understanding of the complex factors influencing the mechanism of hot gas ingestion.

The first part of this study [1, 2] looked at the sole effect of the rotor disc pumping in absence of external pressure asymmetries (no blading in the annulus gas path). This second part introduces circumferential pressure variations in the main gas path with the addition of nozzle guide vanes (but no rotor blades) to systematically increase the complexity of the system. This approach aims to investigate the synergetic effect of the rotationally induced and externally imposed pressure and velocity fields on the sealing performance of the chute seal previously studied. This research covers a wide span of experimental conditions in which the main variables were the seal clearance, sealing flow rate and rotational speed. The main gas path flow was scaled to match engine representative conditions with the same axial Mach number and axial Reynolds number throughout the entire test matrix.

The sealing effectiveness data from this research reveal an inversion of the effect of the rotor disc rotation when the external pressure asymmetries are included if compared to the trends found with an axisymmetric annulus flow. Furthermore, the experimental data obtained during this investigation suggest that the pressure asymmetries due to the presence of the NGVs in the main gas path dominate the mechanism of ingestion at low rotor disc speeds. However, at high rotational speeds (closer to engine design conditions) the rotational effects are such that, despite the existing annulus pressure asymmetries, the distribution of the sealing effectiveness against seal-to-disc velocity ratio is in good agreement with results without the vanes fitted. These results challenge the assumptions on the relative contribution of external asymmetries to the hot ingestion mechanism, and raises questions for further research in the field.

[1] Beard PF, Chew J, Gao F, Chana KS. (2016) 'UNSTEADY FLOW PHENOMENA IN TURBINE RIM SEALS'. Proceedings of ASME Turbo Expo 2016, Seoul, South Korea: ASME Turbo Expo 2016. (Also ASME J. Engng Gas Turbines and Power).

[2]Bru-Revert, A., Chew J, Beard PF, Abstract submitted to Turbo Expo 2020.