L. Porreca

Turbulence Measurements and Analysis in a Multistage Axial Turbine

This paper presents turbulence measurements and detailed flow analysis in an axial turbine stage. Fast response aerodynamic probes were used to resolve aperiodic fluctuations along the three directions. Assuming incompressible flow, the effective turbulence level and Reynolds stress are retrieved by evaluating the stochastic velocity component out of the measured time-resolved pressure and flow angle fluctuations along the streamwise, radial, and circumferential direction. A comparison between turbulence intensity and measured total pressure shows that flow structures with higher turbulence level are identified in the region of loss cores at the exit of the second stator passage. Turbulence intensity is evaluated under isotropic and nonisotropic assumption in order to quantify the departure from isotropic conditions. The measurements show that locally the streamwise fluctuating component can be twice bigger than the radial and tangential component. The current analysis shows that multisensor fast response aerodynamic probes can be used to provide information about the mean turbulence levels in the flow and the Reynolds stress tensor, in addition to the measurements of unsteady total pressure loss.

Fluid Dynamics and Performance of Partially and Fully Shrouded Axial Turbines

A unique comparative experimental and numerical investigation carried out on two test cases with shroud configurations, differing only in the labyrinth seal path, is presented in this paper. The blade geometry and tip clearance are identical in the two test cases. The geometries under investigation are representative of an axial turbine with a full and partial shroud, respectively. Global performance and flow field data were acquired and analysed. Computational simulations were carried out to complement the investigation and to facilitate the analysis of the steady and unsteady flow measurements. A detailed comparison between the two test cases is presented in terms of flow field analysis and performance evaluation. The analysis focuses on the flow effects reflected on the overall performance in a multi-stage environment. Strong interaction between the cavity flow and the blade tip region of the rotor blades is observed up to the blade midspan. A marked effect of this interaction can be seen in the downstream second stator where different vortex structures are observed. Moreover, in the partial shroud test case, a strong tip leakage vortex is developed from the first rotor and transported through the downstream blade row. A measurable change in the second stage efficiency was observed between the two test cases. In low aspect ratio blades within a multi-stage environment, small changes in the cavity geometry can have a significant effect on the mainstream flow. The present analysis has shown that an integrated and matched blade-shroud aerodynamic design has to be adopted to reach optimal performances. The additional losses resulting from small variations of the sealing geometry could result in a gain of up to one point in the overall stage efficiency.

Multistage Aspects and Unsteady Effects of Stator and Rotor Clocking in an Axial Turbine With Low Aspect Ratio Blading

This paper presents the outcome of a recent study in clocking-related flow features and multistage effects occurring in high-pressure turbine blade geometries. The current investigation deals with an experimentally based systematic analysis of the effects of both stator-stator and rotor-rotor clocking. Due to the low aspect ratio of the turbine geometry, the flow field is strongly three-dimensional and is dominated by secondary flow structures. The investigation aims to identify the flow interactions involved and the associated effects on performance improvement or degradation. Consequently a three-dimensional numerical analysis has been undertaken to provide the numerical background to the test case considered. The experimental studies were performed in a two-stage axial research turbine facility. The turbine provides a realistic multi-stage environment, in which both stator blade rows and the two rotors can be clocked relative to each other. All blade rows have the same blade number count, which tends to amplify clocking effects. Unsteady and steady measurements were obtained in the second stage using fast response aerodynamic probes (FRAP) and miniature pneumatic 5-hole probes. The current comprehensive investigation has shown that multistage and unsteady flow effects of stator and rotor clocking in low aspect ratio turbines are combined in a nonlinear fashion caused by axial and radial redistribution of low energy fluid. The integral result of clocking on stage efficiency is compensated by competing loss generating mechanisms across the span.Copyright

Optimized Shroud Design for Axial Turbine Aerodynamic Performance

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.

Investigation of 3D Unsteady Flows in a Two-Stage Shrouded Axial Turbine Using Stereoscopic PIV and FRAP: Part II — Kinematics of Shroud Cavity Flow

This paper presents an experimental study of the behavior of leakage flow across shrouded turbine blades. Stereoscopic particle image velocimetry and fast response aerodynamic probe measurements have been conducted in a low-speed two-stage axial turbine with a partial shroud. The dominant flow feature within the exit cavity is the radially outward motion of the main flow into the shroud cavity. The radial migration of the main flow is induced by flow separation at the trailing edge of the shroud due to a sudden area expansion. The radially outward motion is the strongest at mid-pitch as a result of interactions between vortices formed within the cavity. The main flow entering the exit cavity divides into two streams. One stream moves upstream towards the adjacent seal knife and reenters the main flow stream. The other stream moves downstream due to the interaction with the thin seal leakage flow layer. Closer to the casing wall, the flow interacts with the underturned seal leakage flow and gains swirl. Eventually, axial vorticity is generated due to these complex flow interactions. This vorticity is generated by a vortex tilting mechanism and gives rise to additional secondary flow. Due to these fluid motions combined with a contoured casing wall, three layers (the seal leakage layer, cavity flow layer, and main flow) are formed downstream of the shroud cavity. This result is different from the two-layer structure which is found downstream of conventional shroud cavities. The seal leakage jet formed through the seal clearance still exists at 25.6 percent axial chord downstream of the second rotor. This delay of complete dissipation of the seal leakage jet and its mixing with the cavity flow layer is due to the contoured casing wall. Time-averaged flow downstream of the shroud cavity shows the upstream stator’s influence on the cavity flow. The time-averaged main flow can be viewed as a wake flow induced by the upstream stator whose separation at the shroud trailing edge induces pitchwise non-uniformity of the cavity flow.Copyright

Investigation of Three-Dimensional Unsteady Flows in a Two-Stage Shrouded Axial Turbine Using Stereoscopic PIV—Kinematics of Shroud Cavity Flow

This paper presents an experimental study of the behavior of leakage flow across shrouded turbine blades. Stereoscopic particle image velocimetry and fast response aerodynamic probe measurements have been conducted in a low-speed two-stage axial turbine with a partial shroud. The dominant flow feature within the exit cavity is the radially outward motion of the main flow into the shroud cavity. The radial migration of the main flow is induced by flow separation at the trailing edge of the shroud due to a sudden area expansion. The radially outward motion is the strongest at midpitch as a result of interactions between vortices formed within the cavity. The main flow entering the exit cavity divides into two streams. One stream moves upstream toward the adjacent seal knife and reenters the main flow stream. The other stream moves downstream due to the interaction with the thin seal leakage flow layer. Closer to the casing wall, the flow interacts with the underturned seal leakage flow and gains swirl. Eventually, axial vorticity is generated due to these complex flow interactions. This vorticity is generated by a vortex tilting mechanism and gives rise to additional secondary flow. Because of these fluid motions combined with a contoured casing wall, three layers (the seal leakage layer, cavity flow layer, and main flow) are formed downstream of the shroud cavity. This result is different from the two-layer structure, which is found downstream of conventional shroud cavities. The seal leakage jet formed through the seal clearance still exists at 25.6% axial chord downstream of the second rotor. This delay of complete dissipation of the seal leakage jet and its mixing with the cavity flow layer is due to the contoured casing wall. Time-averaged flow downstream of the shroud cavity shows the upstream stator’s influence on the cavity flow. The time-averaged main flow can be viewed as a wake flow induced by the upstream stator whose separation at the shroud trailing edge induces pitchwise non-uniformity of the cavity flow.

INVESTIGATION OF 3D UNSTEADY FLOWS IN A TWO STAGE SHROUDED AXIAL TURBINE USING STEREOSCOPIC PIV AND FRAP PART I: INTERSTAGE FLOW INTERACTIONS

A detailed flow analysis has been carried out in a two-stage shrouded axial turbine by means of intrusive and non-intrusive measurement techniques. Multi-sensor Fast Response Aerodynamic Probe (FRAP) and 3D-PIV system were applied at two locations downstream of the first and second rotors. Several radial planes were measured focusing on the blade tip region in order to obtain a unique set of steady and unsteady velocity data. The investigation deals with the aerodynamics and kinematics of flow structures downstream of the first and second rotors and their interaction with the main flow in a partially shrouded turbine typical of industrial application. The first part of this work is focused on the flow field downstream of the first rotor while the second part studies the leakage flow in the cavity of the second rotor and its interaction with the main stream. The interstage region is characterized by interactions between the tip passage vortex and a vortex caused by the recessed shroud platform design. Flow coming from the blade passage suddenly expands and migrates radially in the cavity region causing a localized total pressure drop. The time evolution of these vortical structures and the associated downstream unsteady loss generation are analyzed. The partial shroud design adopted in this geometry is beneficial in terms of blade stress and thermal load; however flow field downstream of the first rotor is highly three dimensional due to the intense interaction between cavity and main streams. A flow interpretation is provided and suggestions for improved design are finally addressed based on the steady and unsteady flow analysis.© 2006 ASME

Aerothermal Analysis of a Partially Shrouded Axial Turbine

This work presents the results of the aerodynamic and thermostructural analysis of three different shrouded axial turbine configurations. The blade geometry of the turbine stages and the tip clearances of the test cases under investigation are identical although the shroud design is varied. The first test case is representative of a full shroud geometry, and the second and third test cases adopt different partial shroud arrangements. The aerodynamic characteristics of these geometries have been investigated in previous studies, which are summarized in this paper. The influence of the shroudgeometry on the heat load andmechanical stresses has been evaluated by a conjugate heat transfer approach coupled with finite element analysis. The combination of aerodynamic, centrifugal, and thermal loads has been applied to the solid blade model. Results show that the full shroud test case has higher mechanical stresses on the blade root but lower stress concentrations on the blade/shroud components. The partial shrouded cases have the lowest blade root stress. The last geometry is an optimized partially shrouded design, showing an improved lifetime achieved by a better stress distribution over the blade shroud compared to the other two cases. The effect of the shroud configuration on the aerodynamic performance, heat load, and mechanical stresses has been summarized, quantified, anddiscussed in detail. The combination of aerodynamicmeasurements and computational analysis shows that an optimum between aerodynamic performance and blade life is achieved by applying a small modification to the partial shroud geometry.

Interstage Flow Interactions and Loss Generation in a Two-Stage Shrouded Axial Turbine

The aerodynamics and kinematics of flow structures, including the loss generation mechanisms, in the interstage region of a two-stage partially shrouded axial turbine are examined. The nonaxisymmetric partial shroud introduces highly three-dimensional unsteady interactions, the details of which must be understood in order to optimize the design of the blade/shroud. Detailed measurements of the steady and unsteady pressure and velocity fields are obtained using a two-sensor fast response aerodynamic probe and stereoscopic particle image velocimetry. These intrusive and nonintrusive measurement techniques yield a unique data set that describes the details of the flow in the interstage region. The measurements show that a highly three-dimensional interaction occurs between the passage vortex and a vortex caused by the recessed shroud platform design. Flow coming from the blade passage suddenly expands and migrates radially upward in the cavity region, causing a localized relative total pressure drop. Interactions of vortex and wake structures with the second stator row are analyzed by means of the combination of the measured relative total pressure and nondeterministic pressure unsteadiness. The analysis of the data gives insight on unsteady loss mechanisms. This study provides improved flow understanding and suggests that the design of the blade/shroud and second stator leading edge may be further improved to reduce unsteady loss contribution.