Parametrical Investigation of Turbine Stages With Open Tip Clearance for the Purpose of Stage Efficiency Increase

Abstract

The aerodynamic loss due to tip leakage vortex of rotor blades represents a significant part of viscous losses in axial flow turbines. The mixing of leakage flow with the main rotor passage flow causes losses and reduces turbine stage efficiency. Many measures have been proposed to reduce the loss in the tip clearance area. In this paper the reduction of the tip clearance loss due to changes made to the blade tip section profile is presented. The blade tip profile was modified to decrease the pressure gradient between pressure surface and suction surface. This approach allows the reduction of tip leakage and tip vortex strength and consequently the reduction of tip clearance losses. A 3D Navier-Stokes solver with q-ω turbulence model is used to analyze the flow in the turbine with various tip section profiles. Test data of three single-stage experimental turbines have been used to validate analytical predictions: • Highly loaded turbine stage with a pressure ratio π0T = 3.2 and reaction degree ρmean = 0.5. • Two turbines with a pressure ratio π0T = 3.9. (One with high degree of reaction ρmean = 0.55; the other with low degree of reaction ρmean = 0.26). The numerical investigation of the influence of various tip section profiles on stage efficiency was carried out in the range of relative tip clearance 0.5%–2.4% with the objective of a decreasing the influence of the tip clearance on the stage efficiency.

Gas Turbine Engine Performance Estimation and Prediction

“…Several research works have investigated the relation between fault modes and consequent degradation symptoms of the turbines [168][169][170][171], whereas fewer works have growth of symptoms 1 st 15 th 10 th studied the growth of the degradation symptoms over time [1]. Beside the effect of time, it is expected to experience faster degradation under higher levels of the GTE power, which lead to harsher operating condition for the components.…”

“…The dominant fault modes in the nozzle and the turbine section are known as: a) increase of the surface roughness in the nozzle vanes and the turbine blades[165], b) increase of the tip clearance in the turbine, and c) material removal and deformation of the profile[166,167]. The fault modes manifest their symptoms as deviations in the turbine performance map, i.e., under a constant operating condition, all the fault modes lead to loss of isentropic efficiency in the turbine, whereas the tip clearance and material removal from the profile lead to increase of the mass flow[168][169][170][171]. When the GTE is running, the only directly measurable fault is the tip clearance using advanced instrumentation, while the other two fault modes remain inaccessible for direct measurement.…”