Numerical Investigation of Stator Shroud Leakage Effects in a 1.5-Stage Low-Speed Axial Compressor

Authors

Michael Hopfinger - Research Assistant Institute of Turbomachinery and Flight Propulsion
Volker Gümmer - Head of the Institute of Turbomachinery and Flight Propulsion

Abstract

Gaps and leakage mass flows are always occurring between steady and rotating parts in turbomachinery. Basically, the stator gap can be established in two different ways. When there is a hub gap between the stator and rotating hub this design style is referred to as cantilevered design. Alternatively, the stator can be designed in shrouded configuration with a shroud cavity and a sealing arrangement. Due to the static pressure rise across the stator vane, the flow enters the shroud cavity and recirculates into the main flow path. The leakage flow interacts with the main flow and strongly affects the endwall flow development. Losses occurring due to the low momentum of the leakage fluid and mixing processes are responsible for a significant amount of compressor performance degradation. This paper describes the numerical investigations of shroud leakage effects in a 1.5-stage low-speed compressor. The tandem stator vanes comprising a front and a rear vane represents a novelty in this configuration. This research is aimed at understanding the influence of different leakage mass flows on the loss generation in tandem stators. Focus is put on yielding a correlation between leakage mass flow, efficiency and operating range. The magnitude of leakage mass flow depends primarily on the pressure rise across the stator and the sealing arrangement. Consequently, the sealing mass flow is controlled by different throttle conditions and gap widths between sealing fins and shroud. Initially, the flow structures within the cavity and the sealing arrangement and the resulting losses are investigated. Subsequently, the mixing process of the leakage flow and the main gaspath flow in front of stator vane is examined. Special emphasis is put on the influence on endwall loss generation. The influence of the leakage flow on the inlet conditions of the front vane at endwall regions is investigated. The contribution of the leakage flow on endwall loss growth within the flow channel will be discussed for a number of selected operating points. The generation of secondary flow phenomena such as channel and corner vortices are considered. Focus is put on locations of high loss regions at front and rear vane and their downstream propagation. The boundary layer development especially at the tandem gap area which is characterized by high blade-to-blade interaction is analyzed. Finally, it is examined how flow structures downstream of the rear vane are affected by the mass flow entering the sealing.