Axial Flow Turbine Research Facility Research Articles

A simultaneous use of a leading-edge fillet and a non-axisymmetrically contoured endwall in a turbine stage

Secondary flow minimization is a crucial problem in a turbine passage. In the present paper, three different strategies are investigated to reduce the secondary-flow-related total pressure loss. These are leading-edge fillet, non-axisymmetric endwall contouring, and combining these two approaches. An experimental investigation in a single-stage turbine facility and RANS-based computations are performed to evaluate the designs. The experiments are carried out in the large-scale Axial Flow Turbine Research Facility AFTRF. A reference Flat Insert is installed on the nozzle guide vane passage's hub surface to allow future endwall design implementation during the experimental phase. The stereolithography manufactured reference insert has a constant thickness with a cylindrical shape. This investigation also uses four different leading-edge fillets, and they are attached to cylindrical Flat Insert initially. The same fillets are also attached to a contoured endwall. Simultaneous use of a leading-edge fillet and a non-axisymmetric contoured endwall for secondary flow control is the main objective of this research. Total pressure measurements are taken at the rotor inlet plane with a Kiel probe. The probe traversing is completed along one vane pitch and from 8% to 38% span. The leading-edge fillets effectively managed to weaken the horseshoe vortex occurring in front of the leading-edge. In one of the fillet cases sitting on the Flat Insert, the computational results at the NGV exit showed that the mass-averaged loss value was reduced by 1.31%. Three fillet designs decreased the area-averaged loss with a maximum reduction of 15.06%.

Performance of Partial and Cavity Type Squealer Tip of a HP Turbine Blade in a Linear Cascade

Three-dimensional highly complex flow structure in tip gap between blade tip and casing leads to inefficient turbine performance due to aerothermal loss. Interaction between leakage vortex and secondary flow structures is the substantial source of that loss. Different types of squealer tip geometries were tried in the past, in order to improve turbine efficiency. The current research deals with comparison of partial and cavity type squealer tip concepts for higher aerothermal performance. Effects of squealer tip have been examined comprehensively for an unshrouded HP turbine blade tip geometry in a linear cascade. In the present paper, flow structure through the tip gap was comprehensively investigated by computational fluid dynamic (CFD) methods. Numerical calculations were obtained by solving three-dimensional, incompressible, steady, and turbulent form of the Reynolds-averaged Navier-Stokes (RANS) equations using a general purpose and three-dimensional viscous flow solver. The two-equation turbulence model, shear stress transport (SST), has been used. The tip profile belonging to the Pennsylvania State University Axial Flow Turbine Research Facility (AFTRF) was used to create an extruded solid model of the axial turbine blade. For identifying optimal dimensions of squealer rim in terms of squealer height and squealer width, our previous studies about aerothermal investigation of cavity type squealer tip were utilized. In order to obtain the mesh, an effective parametric generation has been utilized using a multizone structured mesh. Numerical calculations indicate that partial and cavity squealer designs can be effective to reduce the aerodynamic loss and heat transfer to the blade tip. Future efforts will include novel squealer shapes for higher aerothermal performance.

Casing grooves to improve aerodynamic performance of a HP turbine blade

Highly three-dimensional and complex flow structure in the tip gap between a blade tip and the casing leads to significant inefficiency in the aerodynamic performance of a turbine. The interaction between the tip leakage vortex and the main passage flow is a substantial source of aerodynamic loss. The present research deals with the effect of groove type casing treatment on the aerodynamic performance of a linear turbine cascade. Grooved casings are widely used in compressors in order to improve the stall margin whereas limited studies are available on turbines. In this study, various circumferential grooves are investigated using the computational approach for a single stage axial turbine blade. The specific HP turbine airfoil under numerical investigation is identical to the rotor tip profile of the Axial Flow Turbine Research Facility (AFTRF) of the Pennsylvania State University. The carefully measured aerodynamic flow quantities in the AFTRF are used for initial computational quality assessment purposes. Numerical calculations are obtained by solving the three-dimensional, incompressible, steady and turbulent form of the Reynolds-Averaged Navier–Stokes (RANS) equations. A two-equation turbulence model, Shear Stress Transport (SST) k-ω is used in the present set of calculations. Current results indicate that groove type casing treatment can be used effectively in axial turbines in order to improve the aerodynamic performance. Detailed flow visualizations within the passage and numerical calculations reveal that a measurable improvement in the aerodynamic performance is possible using the specific circumferential grooves presented in this paper.

An aerothermal study of the influence of squealer width and height near a HP turbine blade

Highly three-dimensional and complex flow structure within the tip gap of an axial flow turbine is a substantial source of aerodynamic loss and heat transfer due to the interaction between the tip leakage vortex, secondary flows and the main passage flow. Most contemporary shroudless high pressure (HP) turbine designs employ squealer tips for durability, structural, aerodynamic design and heat transfer reasons. The present research deals with the influence of squealer width and height on the aerothermal performance of a HP turbine blade. In this study, four different squealer heights and seven squealer width values are investigated using a computational approach for an axial turbine blade depicting an E3 “Energy Efficient Engine” design. The specific HP turbine airfoil under investigation is identical to the rotor tip profile of the Axial Flow Turbine Research Facility (AFTRF) of the Pennsylvania State University. Numerical calculations are performed by solving the three-dimensional, steady and turbulent form of the Reynolds-Averaged Navier-Stokes (RANS) equations. A two-equation turbulence model, Shear Stress Transport (SST) k-ω is used in the present set of calculations. The current numerical predictions show a very good agreement with the extensive aerodynamic measurements obtained in the nozzle guide vane passages of AFTRF. The results indicate that determining proper squealer width and height is crucial to obtain better aerothermal performance in the form of reduced aerodynamic loss and heat transfer to the tip platform. Extensive numerical analysis within the tip gap reveals that increasing squealer height and reducing squealer width increases cavity volume leading to enlarged vortical structures near the pressure side and suction side of the cavity. Because of this enhanced vortical activity in the tip cavity, a blockage to the incoming pass-over flow is introduced and as a result tip leakage mass flow rate is reduced. While the tip leakage flow rate tends to decrease with increased height and reduced width, there is a strong effect from the squealer width and height combination due to the presence of complex interactions in the tip gap region. From a heat transfer point of view, decreasing squealer width and increasing squealer height noticeably reduces the overall Nu‾ on the blade tip platform. Nu‾ on the cavity floor, blade tip and squealer side walls are reduced depending on the increasing height and decreasing width values.

Factors Influencing Computational Predictability of Aerodynamic Losses in a Turbine Nozzle Guide Vane Flow

This paper deals with the computational predictability of aerodynamic losses in a turbine nozzle guide vane (NGV) flow. The paper shows that three-dimensional (3D) computations of Reynolds-Averaged Navier Stokes (RANS) equations have the ability to adequately represent viscous losses in the presence of laminar flows, transitional regions, and fully turbulent flow areas in the NGV of an high pressure (HP) turbine stage. The Axial Flow Turbine Research Facility (AFTRF) used for the present experimental results has an annular NGV assembly and a 29-bladed HP turbine rotor spinning at 1330 rpm. The NGV inlet and exit Reynolds numbers based on midspan axial chord are around 300,000 and 900,000, respectively. A general purpose finite-volume 3D flow solver with a shear stress transport (SST) k–ω turbulence model is employed. The current computational study benefits from these carefully executed aerodynamic experiments in the NGV of the AFTRF. The grid independence study is performed with static pressure coefficient distribution at the midspan of the vane and the total pressure coefficient at the NGV exit. The effect of grid structure on aerodynamic loss generation is emphasized. The flow transition effect and the influence of corner fillets at the vane–endwall junction are also studied. The velocity distributions and the total pressure coefficient at the NGV exit plane are in very good agreement with the experimental data. This validation study shows that the effect of future geometrical modifications on the turbine endwall surfaces will be predicted reasonably accurately. The current study also indicates that an accurately defined turbine stage geometry, a properly prepared block-structured/body-fitted grid, a state-of-the-art transitional flow implementation, inclusion of fillets, and realistic boundary conditions coming from high-resolution turbine experiments are all essential ingredients of a successful turbine NGV aerodynamic loss quantification via computations. This validation study forms the basis for the successful future generation of nonaxisymmetric endwall surface modifications in AFTRF research efforts.

Aerodynamic character of partial squealer tip arrangements in an axial flow turbine. Part I: Detailed aerodynamic field modifications via three dimensional viscous flow simulations around baseline tip

This paper deals with the viscous flow simulations of the complex tip leakage flow in Axial Flow Turbine Research Facility (AFTRF). Special attention is paid to the 3D structure of the tip leakage flow mechanisms in a baseline tip configuration with no desensitisation. Although past experimental studies provide much insight into the physical understanding of the tip region aerodynamics, there are still many areas of the flow-field in which experiments are extremely difficult to perform. Fine details of the entrance flow near the pressure side where the tip leakage jet starts to form, the leakage jet formation between the tip surface and outer casing, the re-circulatory flow zone very near the pressure side corner, the interaction area of the tip vortex with the conventional passage flow, the influence of the relative motion of the outer casing and leakage flow reversal can be visualised with excellent resolution. After the presentation of the measured inlet boundary conditions and a grid independency study, the baseline tip flow simulations are discussed in detail.

Aerodynamics of Tip Leakage Flows Near Partial Squealer Rims in an Axial Flow Turbine Stage

This paper deals with an experimental investigation of aerodynamic characteristics of full and partial-length squealer rims in a turbine stage. Full and partial-length squealer rims are investigated separately on the pressure side and on the suction side in the “Axial Flow Turbine Research Facility” (AFTRF) of the Pennsylvania State University. The streamwise length of these “partial squealer tips” and their chordwise position are varied to find an optimal aerodynamic tip configuration. The optimal configuration in this cold turbine study is defined as the one that is minimizing the stage exit total pressure defect in the tip vortex dominated zone. A new “channel arrangement” diverting some of the leakage flow into the trailing edge zone is also studied. Current results indicate that the use of “partial squealer rims” in axial flow turbines can positively affect the local aerodynamic field by weakening the tip leakage vortex. Results also show that the suction side partial squealers are aerodynamically superior to the pressure side squealers and the channel arrangement. The suction side partial squealers are capable of reducing the stage exit total pressure defect associated with the tip leakage flow to a significant degree.

Mainstream Aerodynamic Effects Due to Wheelspace Coolant Injection in a High-Pressure Turbine Stage: Part I—Aerodynamic Measurements in the Stationary Frame

The relative aerodynamic and performance effects associated with rotor–NGV gap coolant injections were investigated in the Axial Flow Turbine Research Facility (AFTRF) of the Pennsylvania State University. This study quantifies the effects of the coolant injection on the aerodynamic performance of the turbine for radial cooling, impingement cooling in the wheelspace cavity and root injection. Overall, it was found that even a small quantity (1 percent) of cooling air can have significant effects on the performance character and exit conditions of the high pressure stage. Parameters such as the total-to-total efficiency, total pressure loss coefficient, and three-dimensional velocity field show local changes in excess of 5, 2, and 15 percent, respectively. It is clear that the cooling air disturbs the inlet end-wall boundary layer to the rotor and modifies secondary flow development, thereby resulting in large changes in turbine exit conditions.

Mainstream Aerodynamic Effects Due to Wheelspace Coolant Injection in a High-Pressure Turbine Stage: Part II—Aerodynamic Measurements in the Rotational Frame

The current paper deals with the aerodynamic measurements in the rotational frame of reference of the Axial Flow Turbine Research Facility (AFTRF) at the Pennsylvania State University. Stationary frame measurements of “Mainstream Aerodynamic Effects Due to Wheelspace Coolant Injection in a High Pressure Turbine Stage” were presented in Part I of this paper. The relative aerodynamic effects associated with rotor–nozzle guide vane (NGV) gap coolant injections were investigated in the rotating frame. Three-dimensional velocity vectors including exit flow angles were measured at the rotor exit. This study quantifies the secondary effects of the coolant injection on the aerodynamic and performance character of the stage main stream flow for root injection, radial cooling, and impingement cooling. Current measurements show that even a small quantity (1 percent) of cooling air can have significant effects on the performance and exit conditions of the high-pressure turbine stage. Parameters such as the total pressure coefficient, wake width, and three-dimensional velocity field show significant local changes. It is clear that the cooling air disturbs the inlet end-wall boundary layer to the rotor and modifies secondary flow development thereby resulting in large changes in turbine exit conditions. Effects are the strongest from the hub to midspan. Negligible effect of the cooling flow can be seen in the tip region.

Three-Dimensional Flowfield Downstream of an Axial-Flow Turbine Rotor

A systematic and comprehensive investigation was performed to provide detailed data on the threedimensional viscous flow phenomena downstream of a modern turbine rotor and to understand the flow physics such as the origin, nature, development of wakes, secondary flow, and leakage flow. The experiment was carried out in the Axial Flow Turbine Research Facility (AFTRF) at The Pennsylvania State University, with velocity measurements taken with a 3-D LDV System. Two radial traverses at 1% and 10% of chord downstream of the rotor were performed. Sufficient spatial resolution was maintained to resolve blade wake, secondary flow, and tip leakage flow. The wake deficit is found to be substantial, especially at 1% of chord downstream of the rotor. At this location, negative axial velocity occurs near the tip, suggesting flow separation in the tip clearance region. Cross-correlations are mainly associated with the velocity gradient of the wake deficit. The radial velocities, both in the wake and in the endwall region, are found to be substantial. Two counter-rotating secondary flows are identified in the blade passage, with one occupying the half span close to the casing and the other occupying the half span close to the hub. The tip leakage flow is restricted to 10% immersion from the blade tip. There are strong vorticity distributions associated with these secondary flows and tip leakage flow. The passage averaged data are in good agreement with design values.