Aerodynamic Robustness of End Wall Contouring Against Rim Seal Purge Flow

In the present study, the results of an experimental investigation are presented, which have been undertaken in the axial turbine facility LISA at ETH Zurich. The two test configurations consist of a one-and-a-half stage, unshrouded, highly loaded axial turbine with 3-dimensionally shaped blading representative of modern high pressure gas turbines. The two test configurations differ in the hub end walls: while one design has cylindrical end walls, the other design features non-axisymmetric end wall contouring (EWC). Both turbine designs have not been especially designed for the unsteady and complex interaction mechanisms of the hub rim seal purge flow with the main annulus flow. However, these turbine designs have been subject to measurements without (nominal) and with purge flow (0.8% of the main mass flow) with the purpose of studying the aerodynamic robustness of the performance of the stages with respect to the rim seal purge flow. In order to further analyze the robustness of both turbine designs, also measurements at off-design conditions have been taken. The steady and unsteady aerodynamic effects are measured, respectively, with pneumatic probes as well as with the in-house developed and manufactured Fast Response Aerodynamic Probe (FRAP) technology. With the aim of evaluating the aerodynamic performance and robustness of the end wall design, the one result of the experimental investigation is the quantification of the sensitivity of the stage efficiency with respect to the case with and without purge flow for both turbine designs. By means of the analysis of the time-resolved flow field and characterization of the secondary flow features, their reaction to the presence of purge flow is highlighted and used as an explanation for the efficiency deficits caused by the purge flow. The measurements show a benefit in stage efficiency of +0.2% by using the end wall contouring in the nominal case, confirming the design intention and effectiveness of the contoured end walls. However, the beneficial impact of the end wall contouring is taken back by a higher sensitivity of the stage efficiency with respect to the purge flow, which causes the efficiency benefit to vanish with the investigated purge flow injection rate of 0.8%. The off-design measurements show that also the sensitivity of the stage with end wall contouring with respect to the reduction of stage loading factor is by 1/3 higher than the one of the cylindrical end walls. The measurements imply that the cost of higher stage efficiency at nominal conditions by the use of end wall contouring is paid with a higher sensitivity of the stage to changes in the rotor incoming flow field and thus with a lower aerodynamic robustness of the turbine design.