A combined computational fluid dynamics (CFD) and experimental study of interaction of main gas path and rim sealing flow is reported. The experiments were conducted on a two stage axial turbine and included pressure measurements for the cavity formed between the stage 2 rotor disc and the upstream diaphragm for two values of the diaphragm-to-rotor axial clearance. The pressure measurements indicate that ingestion of the highly swirling annulus flow leads to increased vortex strength within the cavity. This effect is particularly strong for the larger axial clearance. Results from a number of steady and unsteady CFD models have been compared to the measured results. Good agreement between measurement and calculation for time-averaged pressures was obtained using unsteady CFD models, which predicted previously unknown unsteady flow features. This led to fast response pressure transducer measurements being made on the rig, and these confirmed the CFD prediction.

A combined computational fluid dynamics (CFD) and experimental study of interaction of main gas path and rim sealing flow is reported. The experiments were conducted on a two stage axial turbine and included pressure measurements for the cavity formed between the stage 2 rotor disc and the upstream diaphragm for two values of the diaphragm-to-rotor axial clearance. The pressure measurements indicate that ingestion of the highly swirling annulus flow leads to increased vortex strength within the cavity. This effect is particularly strong for the larger axial clearance. Results from a number of steady and unsteady CFD models have been compared to the measured results. Good agreement between measurement and calculation for time-averaged pressures was obtained using unsteady CFD models, which predicted previously unknown unsteady flow features. This led to fast response pressure transducer measurements being made on the rig, and these confirmed the CFD prediction.

The aim of this paper is to present advances in the blading design for large steam turbines — ‘Controlled Flow’ technology. The purpose of the design is to improve the turbine efficiency in a cost neutral manner, adding value for the customer. Controlled Flow is a 3D design philosophy which passes more flow through the efficient middle sections of the blade, and less flow through the comparatively inefficient regions near the endwalls. It has been used for the Impulse Technology Blading ITB guide blades. The current improvement builds on the previous successful Controlled Flow design but incorporates the following new features: - Ultra High Lift for the mean section (at significantly reduced axial width) - Ultra High Back Surface Deflection for the mean section - forward leading edge sweep. The new guide delivers the same radial distribution of absolute fluid exit angle to the runner as the previous design. Confirmatory model turbine tests demonstrated that the new guide delivered a stage efficiency improvement of 0.35%, above an already very high datum level. The endwall sections of the guide are kept the same which maintains the mechanical strength of the diaphragm (same stress and deflection). Therefore, the new design can easily ‘slot-in’ and replace the previous design. The following will be described in detail: - History of the designs/background and design philosophy - Flow physics - Stage optimization and performance prediction - Probabilistic analysis and robustness of the design - Confirmatory model turbine testing and validation (comparison with design predictions).