Experimental Investigation of the Interaction between Purge and Main Annulus Flow upstream of a Guide Vane in a Low Pressure Turbine

In modern gas turbines or jet engines, the ingestion of hot gas through the hub-side gap between rotor and stator blades has to be prevented successfully in order to shield the turbine discs from high temperatures. For this purpose, compressor air is fed into the rotor-stator wheelspace. There it purges and hence seals the unavoidable gap between the rotor and the stator row which is referred to as the rim seal. In general, this so-called purge flow has detrimental effects. On the one hand, it does not fully contribute to the thermodynamic cycle of the engine. On the other hand, it spoils the main flow aerodynamics when it re-enters into the turbine. To reduce these detrimental effects and minimise the necessary amount of purge flow, a detailed understanding of the interaction between purge and main annulus flow is necessary. For this purpose, this topic has been investigated by several projects. Whereas in most of these projects the interaction between purge and main annulus flow has been investigated upstream of a rotor row in a high pressure turbine, in this thesis the purge flow injection upstream of the stator row of a low pressure turbine is examined. This aims to broaden the general understanding of the interaction between purge and main annulus flow. Furthermore, a comparison of the effects upstream of a rotor row and upstream of a stator row is carried out. For this investigation numerous experiments have been conducted during the EU Project MAGPI using the Large Scale Turbine Rig (LSTR) at Technische Universitat Darmstadt. This test rig is equipped with different measurement techniques, including pressure taps, 5-hole probes, PIV and a tracer gas system. In their combination these measurement techniques offer the possibility to study the purge flow induced effects in great detail. The measurements reveal a significant influence of purge flow on the flow field within the main annulus and the rim seal. With increasing purge flow, a negative incidence develops at the stator row. This increases the crossflow within the passage and amplifies secondary flows. In addition to this amplification of secondary flows, which is generally similar to results observed for purge flow injection upstream of a rotor row, a significant influence of the purge flow delivery is observed. Especially in low pressure turbines, the purge flow is often supplied through discrete holes in the rotor drum. At sufficiently high pressure ratios, the purge flow may leave these holes in the form of discrete jets. These so-called drive arm hole jets are examined to influence the flow field in the rim seal significantly in the current configuration. That is, they form a purge flow filled vortex in the rim seal, which suppresses the typical flow structure. In addition, these jets promote hot gas ingestion. Overall, the method of the purge flow delivery is assessed to play a significant role on the effects caused by purge flow injection. Furthermore, the flow field measurements yield a detailed insight into the interaction between purge and main annulus flow. Based on the results examined in this thesis, two different design proposals havebeen elaborated, which aim to reduce losses and hence increased the efficiency of the turbine