Experimental and numerical investigations of aerodynamic aspects of hot gas ingestion in rotor-stator systems with superimposed cooling mass flow

Abstract

Modern "High Temperature Gas Turbines" need very complex cooling configurations. Not only the turbine vanes and blades, but also the turbine discs, are cooled by air flow extracted from the compressor. Therefore, rotor-stator systems with a superimposed cooling mass flow are found in many constructions of gas turbines. The hot gas ingress into the wheelspace between stator and rotor, taking place under special conditions, is one of the major problems in these systems. An experimental set-up has been built to get a better knowledge of the aerodynamics of such configurations. The ingress of hot gas into the wheelspace with different sealing configurations has been considered by the measurements of surface pressure and velocities (using the LDA-measurement technique) in the shrouded rotor-stator-system. A variable mainstream flow (Ma-number up to 0.7, Re-number up to 2.106) - with and without nozzle guide vanes - has been realized, to obtain flow situations approximating those in real gas turbines. Parallel to the experiments, the flow structure has been calculated numerically using a modern multiblock finite-volume-scheme with non-orthogonal body-fitted grids. The influence of the mainstream flow has also been taken into account. Two different versions of the k-ε-turbulence model have been used to describe the turbulent effects of the flow. Important features of the experimental results are as follows: hot gas ingress can not only occur on the stator but also on the rotor side of the wheelspace depending on the conditions between the cooling gas and the mainstream flow. In real systems with pressure and velocity gradients in the circumferential direction, hot gas ingestion can not be completely prevented. It was only in the 2-D-axisymmetric case, without guide nozzles in the mainstream, that hot gas ingress could be suppressed completely. The numerical data has been compared to the solution obtained by similitude theory as well as to the measurements. It was found that the data correspond reasonably well. The numerical results confirm the conclusions drawn by experiment about the physical mechanism of hot gas ingress into the wheelspace.