Optimized Shroud Design for Axial Turbine Aerodynamic Performance

Abstract

This paper presents a comprehensive study of the effect of shroud design in axial turbine aerodynamics. Experimental measurements and numerical simulations have been conducted on three different test cases with identical blade geometry and tip clearances but different shroud designs. The first and second test cases are representative of a full shroud and a nonaxisymmetric partial shroud geometry while the third test case uses an optimized partial shroud. Partial shrouds are sometimes used in industrial application in order to benefit from the advantage of shrouded configuration, as well as reduce mechanical stress on the blades. However, the optimal compromise between mechanical considerations and aerodynamic performances is still an open issue due to the resulting highly three-dimensional unsteady flow field. Aerodynamic performance is measured in a low-speed axial turbine facility and shows that there are clear differences between the test cases. In addition, steady and time resolved measurements are performed together with computational analysis in order to improve the understanding of the effect of the shroud geometry on the flow field and to quantify the sources of the resultant additional losses. The flow field analysis shows that the effect of the shroud geometry is significant from 60% blade height span to the tip. Tip leakage vortex in the first rotor is originated in the partial shroud test cases while the full shroud case presents only a weak indigenous tip passage vortex. This results in a significant difference in the secondary flow development in the following second stator with associated losses that varies by about 1% in this row. The analysis shows that the modified partial shroud design has improved considerably the aerodynamic efficiency by about 0.6% by keeping almost unchanged the overall weight of this component, and thus blade root stresses. The work, therefore, presents a comprehensive flow field analysis and shows the impact of the shroud geometry in the aerodynamic performance.

Comparative investigation of tip leakage flow and its effect on stage performance in shrouded and unshrouded turbines

“…Therefore, the discussion in this investigation is limited to the results obtained from steady-state simulations. Based on the consideration of steady-state calculations, 'Stage' interface (mixing plane approach) was used; 33,42,43 here, the circumferentially average flow information is transferred from the upstream side of the interface to the downstream side, that is, meridional variation of the flow is maintained across the interface. However, the stage averaging at the interface incurs a one-time mixing loss, and this loss is equivalent to assuming that the physical mixing supplied by the relative motion between stator and rotor domains is sufficiently large to cause any upstream velocity profile to mix out prior to entering the downstream blade row.…”

Effect of in-service burnout effect on the transonic leakage flows over cavity tip model

“…One way to seal the blade tip is to interconnect the blades using a shroud. 7,8 However, this method increases the blades weight and centrifugal loading. Hence there is a strong motivation to use unshrouded turbine blades with no interconnection between the blades but in the presence of the so-called leakage flows over the blades tip surface.…”

Numerical investigations on leakage performance and flow mechanism of spiral groove seal for shrouded blade in axial turbine

“…Harvey 9 designed a partial shroud structure with winglets for the high-pressure turbine, which can significantly reduce blade cooling requirements and tip leakage loss. Porreca et al, [10][11][12] Rebholz et al, 13 and Liu 14,15 studied shroud reduction on the turbine blade, by comparing the influences of full and partial shroud structure on leakage flow and turbine performance, improved and optimized partial shroud structures were brought up. Li 16 applied four configurations with or without labyrinth seal arranged on casing and rotor blade tip for computational simulations and found that the double-side labyrinth seal achieved the highest turbine stage efficiency.…”