Modern Low-pressure Turbine Research Articles

Improvements in the Prediction of Steady and Unsteady Transition and Mixing in Low-Pressure Turbines by Means of Machine-Learnt Closures

Abstract In this article, novel machine-learnt transition and turbulence models are applied to the prediction of boundary-layer transition and wake mixing in a low-pressure turbine (LPT) cascade, including unsteady inflow cases with incoming wakes. A laminar kinetic energy (LKE) transport approach is employed as baseline transition framework. The application of the machine-learnt explicit algebraic Reynolds stress models (EARSMs) takes advantage of a zonal strategy based on a newly developed sensing function that allows automated wake demarcation. This work compares the performance of several approaches that are based on the application of improved transition models without machine learnt EARSMs, baseline transition model with EARSMs trained for improved wake mixing, and new transition and turbulence closures that are simultaneously developed in a fully coupled way and aimed at improving both transition and wake mixing predictions in LPTs. The investigation is carried out on a cascade of industrial footprint, representative of modern LPT bladings. First, the various modeling frameworks are evaluated, without bar wakes (steady conditions), over a wide range of Reynolds number values. Unsteady Reynolds-averaged Navier–Stokes (URANS) analyses have also been carried out to investigate the predictive capabilities of machine-learnt closures in the presence of incoming bar wakes. Reynolds-averaged Navier–Stokes (RANS)/URANS results are scrutinized against large eddy simulation (LES) calculations and experimental data. It will be demonstrated that machine-learnt EARSMs are capable of producing realistic wake mixing when simply applied on top of the baseline transition models. However, the most accurate predictions will be shown to be provided by fully integrated machine-learnt transition/turbulence closures.

Prediction of flutter effects on transient flow structure and aeroelasticity of low-pressure turbine cascade using direct numerical simulations

The aerodynamic characteristics of advanced low-pressure turbines (LPTs) could be affected by the interaction between the transitional and turbulent flow and the dynamic behaviour of the blades. Consequently, analysing the details of the interactions between the transient flow, blade vibrations and the flutter occurrence over the blades of LPTs are essential in order to enhance the aerodynamic efficiency of the modern LPTs. The distinctive feature of the present analysis is performing high-fidelity simulations based on a DNS approach employing a 3D blade model to investigate the flutter instabilities in a T106A turbine at various inter blade phase angles (IBPAs) at different Reynolds numbers. The impacts of the flutter on the transient flow structure are examined by using a direct numerical simulation method. The results show that at IBPA=0∘, persistent patterns of vortex generation are detected with fluid flow mixing in the downward areas. For IBPA=180∘, however, the recirculation generated by the upper blades proceeds toward the lower ones and interferes with the shedding from the trailing edge which impact the wake structure in the downstream regions significantly. A three-dimensional frequency domain model based on the harmonic balance method is also proposed in this study to investigate the capabilities and limitations of frequency domain methods in predicting aeroelasticity and details of flow structures in LPTs.

Low-Pressure Turbine Cooling Systems

Modern low-pressure turbine engines are equipped with casings impingement cooling systems. Those systems (called Active Clearance Control) are composed of an array of air nozzles, which are directed to strike turbine casing to absorb generated heat. As a result, the casing starts to shrink, reducing the radial gap between the sealing and rotating tip of the blade. Cooling air is delivered to the nozzles through distribution channels and collector boxes, which are connected to the main air supply duct. The application of low-pressure turbine cooling systems increases its efficiency and reduces engine fuel consumption.

Direct Numerical Simulation of Interaction between Transient Flow and Blade Structure in a Modern Low-Pressure Turbine

A feature of a modern aeronautical Low-Pressure Turbine (LPT) is the high blade loadings with complex, transient and separated flow regimes. Most existing research have focused only on analysing the transient flow and flow separation in such turbines. The aerodynamics of a modern LPT, however, can be significantly influenced by the interaction between the unsteady flow field and the blade structure motion in a complex non-linear fashion which could lead to aeroelastic instabilities such as flutter. Therefore, the understanding of the mechanism of the interaction between the flow field unsteadiness and the blade structure in a modern LPT is essential to examine the vibration stress levels to ensure the blade mechanical integrity. The novelty of this paper, first and foremost, is using a high-fidelity direct numerical simulation method to explore the mechanism of flutter and forced response in a modern LPT, T106A turbine, and to study the effects of various sources of unsteadiness on the aeroelastic instabilities of the blade. Secondly, this paper investigates and identifies the adequate working ranges of the harmonic balance method, which has been widely used for the aeromechanical analysis of turbomachines, on predicting the behaviour of the highly unsteady flow due to fluid-structure interaction in an LPT. Another emphasis of this paper is the determination of the capability of the Unsteady Reynolds Averaged Navier–Stokes (URANS) model for the aeroelasticity analysis of an LPT involving the highly unsteady flow. This paper will bridge a key gap in the knowledge of aeroelasticity modelling and analysis of modern LPTs.

Delineating Loss Sources Within a Linear Cascade With Upstream Cavity and Purge Flow

Purge air is injected in cavities at the hub of axial turbines to prevent hot mainstream gas ingestion into interstage gaps. This process induces additional losses for the turbine due to an interaction between the purge and mainstream flow. This paper investigates the flow in a low-speed linear cascade rig with upstream hub cavity at a Reynolds number commonly observed in modern low-pressure turbine stages by the use of numerical simulation. Numerical predictions are validated by comparing against experimental data available. Three different purge mass flow rates are tested using three different rim seal geometries. Numerical simulations are performed using a large-eddy simulation (LES) solver on structured grids. An investigation of the different mechanisms associated with the turbine flow including cavity and purge air is intended through this simplified configuration. The underlying mechanisms of loss are tracked using an entropy formulation. Once described for a baseline case, the influence of purge flow and rim seal geometry on flow mechanisms and loss generation is described with the emphasis to obtain design parameters for losses reduction. The study quantifies loss generation due to the boundary layer on wetted surfaces and secondary vortices developing in the passage. The analysis shows different paths by which the purge flow and rim seal geometry can change loss generation including a modification of the shear layer between purge and mainstream, interaction with secondary vortices, and a modification of the flow behavior close to hub compared with a smooth configuration. The study shows the influence of purge flow rate and swirl on the strengthening of secondary vortices in the passage and the ability of axial overlapping rim seal to delay the development of secondary vortices compared with simple axial gaps.

On the Influence of an Acoustically Optimized Turbine Exit Casing Onto the Unsteady Flow Field Downstream of a Low Pressure Turbine Rotor

Modern low pressure turbines (LPT) are designed in order to fulfil a various number of requirements such as high endurance, low noise, high efficiency, low weight, and low fuel consumption. Regarding the reduction of the emitted noise, different designs of LPT exit guide vanes (aerodynamically and/or acoustically optimized) of the turbine exit casing (TEC) were tested, and their noise reduction capabilities and aerodynamic performance were evaluated. In particular, measurements of TEC-losses were performed, and differences in the losses were reported. Measurements were carried out in a one and a half stage subsonic turbine test facility at the engine relevant operating point approach. This work focuses on the study of the unsteady flow field downstream of an unshrouded LPT rotor. The influence on the upstream flow field of a TEC design including acoustically optimized vanes (inverse cut-off TEC) is investigated and compared with a second TEC configuration without vanes (Vaneless TEC), by means of fast response aerodynamic pressure probe (FRAPP) measurements. The second configuration served as a reference concerning the influence of turbine exit guide vanes (TEGVs) onto the upstream located LPT rotor. The interactions between the stator and rotor wakes, secondary flows, and the TEGVs potential effect are identified via modal decomposition according to the theory of Tyler and Sofrin. The main structures constituting the unsteady flow field are detected, and the role of the major interaction effects in the loss generation mechanism and in the acoustic emission is analyzed. This study based on the modal analysis of the unsteady flow field offers new insight into the main interaction mechanisms and their importance in the assessment of the aerodynamic and aeroelastic performance of modern LPT exit casings.

Scaling Three-Dimensional Low-Pressure Turbine Blades for Low-Speed Testing

The present activity was carried out in the framework of the Clean Sky European Research Project ITURB (optimal high-lift turbine blade aeromechanical design), aimed at designing and validating a turbine blade for a geared open-rotor engine. A cold-flow, large-scale, low-speed (LS) rig was built in order to investigate and validate new design criteria, providing reliable and detailed results while containing costs. This paper presents the design of an LS stage and describes a general procedure that allows to scale three-dimensional (3D) blades for LS testing. The design of the stator row was aimed at matching the test-rig inlet conditions and at providing the proper inlet flow field to the blade row. The rotor row was redesigned in order to match the performance of the high-speed (HS) configuration, compensating for both the compressibility effects and different turbine flow paths. The proposed scaling procedure is based on the matching of the 3D blade loading distribution between the real engine environment and the LS facility one, which leads to a comparable behavior of the boundary layer and hence to comparable profile losses. To this end, the datum blade is parameterized, and a neural-network-based methodology is exploited to guide an optimization process based on 3D Reynolds-averaged Navier–Stokes (RANS) computations. The LS stage performance was investigated over a range of Reynolds numbers characteristic of modern low-pressure turbines (LPTs) by using a multi-equation, transition-sensitive, turbulence model. Some comparisons with experimental data available within the project finally proved the effectiveness of the proposed scaling procedure.

Compressible Direct Numerical Simulation of Low-Pressure Turbines—Part I: Methodology

Modern low pressure turbines (LPT) feature high pressure ratios and moderate Mach and Reynolds numbers, increasing the possibility of laminar boundary-layer separation on the blades. Upstream disturbances including background turbulence and incoming wakes have a profound effect on the behavior of separation bubbles and the type/location of laminar-turbulent transition and therefore need to be considered in LPT design. Unsteady Reynolds-averaged Navier–Stokes (URANS) are often found inadequate to resolve the complex wake dynamics and impact of these environmental parameters on the boundary layers and may not drive the design to the best aerodynamic efficiency. LES can partly improve the accuracy, but has difficulties in predicting boundary layer transition and capturing the delay of laminar separation with varying inlet turbulence levels. Direct numerical simulation (DNS) is able to overcome these limitations but has to date been considered too computationally expensive. Here, a novel compressible DNS code is presented and validated, promising to make DNS practical for LPT studies. Also, the sensitivity of wake loss coefficient with respect to freestream turbulence levels below 1% is discussed.

Aerodynamic Design of High End Wall Angle Turbine Stages—Part I: Methodology Development

A design methodology is presented for turbines in an annulus with high end wall angles. Such stages occur where large radial offsets between the stage inlet and stage outlet are required, for example in the first stage of modern low pressure turbines, and are becoming more prevalent as bypass ratios increase. The turbine vanes operate within s-shaped ducts which result in meridional curvature being of a similar magnitude to the blade-to-blade curvature. Through a systematic series of idealized computational cases, the importance of two aspects of vane design are shown. First, the region of peak end wall meridional curvature is best located within the vane row. Second, the vane should be leant so as to minimize spanwise variations in surface pressure—this condition is termed “ideal lean.” This design philosophy is applied to the first stage of a low pressure turbine with high end wall angles.

Transition Mechanisms in Laminar Separated Flow Under Simulated Low Pressure Turbine Aerofoil Conditions

Based on detailed experimental work conducted at a low speed test facility, this paper describes the transition process in the presence of a separation bubble with low Reynolds number, low free-stream turbulence, and steady main flow conditions. A pressure distribution has been created on a long flat plate by means of a contoured wall opposite of the plate, matching the suction side of a modern low-pressure turbine aerofoil. The main flow conditions for four Reynolds numbers, based on suction surface length and nominal exit velocity, were varied from 80,000 to 300,000, which covers the typical range of flight conditions. Velocity profiles and the overall flow field were acquired in the boundary layer at several streamwise locations using hot-wire anemometry. The data given is in the form of contours for velocity, turbulence intensity, and turbulent intermittency. The results highlight the effects of Reynolds number, the mechanisms of separation, transition, and reattachment, which feature laminar separation-long bubble and laminar separation-short bubble modes. For each Reynolds number, the onset of transition, the transition length, and the general characteristics of separated flow are determined. These findings are compared to the measurement results found in the literature. Furthermore, the experimental data is compared with two categories of correlation functions also given in the literature: (1) correlations predicting the onset of transition and (2) correlations predicting the mode of separated flow transition. Moreover, it is shown that the type of instability involved corresponds to the inviscid Kelvin-Helmholtz instability mode at a dominant frequency that is in agreement with the typical ranges occurring in published studies of separated and free-shear layers.

Computational investigation of blade slotting on a high-load low-pressure turbine profile at various reynolds numbers: Part I—Slotting scheme’s verification

Boundary layer separation and reattachment is often an unavoidable feature of low pressure (LP) turbine, one of the main causes of this phenomenon is the high altitude low Reynolds number experienced by the modern LP turbine stage in aero-engine. Although an excellent turbine airfoil design can avoid flow separation on certain extent, but within flight envelope, LP turbine’s characteristic Reynolds number may varied greatly, so it will be still under the risk of the presence of separation bubble. In this two parts paper a new concept of slotted-blade was raised to testify the gain of the blade slotting. A high aerodynamic loading LP turbine blade IET-LPTA was under investigated with different Reynolds number. Computational results reveal that the blade slotting could be a way of choice to suppress separation bubble and reduce profile loss under the condition of low Reynolds number, although its position and geometry need to be further investigated.

Numerical Simulation of Bypass Transition by the Approach of Intermittency Transport Equation

This paper deals with the development of a numerical approach to predict freestream turbulence-induced (FST-induced) boundary layer bypass transition using an intermittency transport equation. An intermittency based transition model, which is critical for invoking transition onset according to Abu-Ghannam and Shaw correlation, is implemented into the proven Reynolds-Averaged N-S (RANS) solver. The intermittent behavior of the transitional flow is incorporated into the computation by modifying the eddy viscosity t µ , obtained from a turbulence model. Wilcox low Reynolds k ω − turbulence model is employed to calculate the eddy viscosity and others turbulent quantities. For validation, the present transition model is applied to the benchmark experiments of flat plate test cases of ERCOFTAC series and to predictions of a modern Low Pressure (LP) turbine flow. It follows from the detail comparisons of the calculated results with the relevant experimental data and other researchers’ simulations that the present model is capable to make a reasonable prediction of FST-induced bypass transition.

Effect of the Leakage Flows and the Upstream Platform Geometry on the Endwall Flows of a Turbine Cascade

This work describes the effect that the injection of leakage flow from a cavity into the mainstream has on the endwall flows and their interaction with a large pressure surface separation bubble in a low-pressure turbine. The effect of a step in hub diameter ahead of the blade row is also simulated. The blade profile under consideration is a typical design of modern low-pressure turbines. The tests are conducted in a low speed linear cascade. These are complemented by numerical simulations. Two different step geometries are investigated, i.e., a backward-facing step and a forward-facing step. The leakage tangential velocity and the leakage mass flow rate are also modified. It was found that the injection of leakage mass flow gives rise to a strengthening of the endwall flows independently of the leakage mass flow rate and the leakage tangential velocity. The experimental results have shown that below a critical value of the leakage tangential velocity, the net mixed-out endwall losses are not significantly altered by a change in the leakage tangential velocity. For these cases, the effect of the leakage mass flow is confined to the wall, as the inlet endwall boundary layer is pushed further away from the wall by the leakage flow. However, for values of the leakage tangential velocity around 100% of the wheel speed, there is a large increase in losses due to a stronger interaction between the endwall flows and the leakage mass flow. This gives rise to a change in the endwall flows’ structure. In all cases, the presence of a forward-facing step produces a strengthening of the endwall flows and an increase of the net mixed-out endwall losses when compared with a backward-facing step. This is because of a strong interaction with the pressure surface separation bubble.

Large-Eddy Simulation of Unsteady Surface Pressure Over a Low-Pressure Turbine Blade due to Interactions of Passing Wakes and Inflexional Boundary Layer

The unsteady pressure over the suction surface of a modern low-pressure (LP) turbine blade subjected to periodically passing wakes from a moving bar wake generator is described. The results presented are a part of detailed large-eddy simulation (LES) following earlier experiments over the T106 profile for a Reynolds number of 1.6×105 (based on the chord and exit velocity) and the cascade pitch to chord ratio of 0.8. The present LES uses coupled simulations of cylinder for wake, providing four-dimensional inflow conditions for successor simulations of wake interactions with the blade. The three-dimensional, time-dependent, incompressible Navier-Stokes equations in fully covariant form are solved with 2.4×106 grid points for the cascade and 3.05×106 grid points for the cylinder using a symmetry-preserving finite difference scheme of second-order spatial and temporal accuracy. A separation bubble on the suction surface of the blade was found to form under the steady state condition. Pressure fluctuations of large amplitude appear on the suction surface as the wake passes over the separation region. Enhanced receptivity of perturbations associated with the inflexional velocity profile is the cause of instability and coherent vortices appear over the rear half of the suction surface by the rollup of shear layer via Kelvin-Helmholtz (KH) mechanism. Once these vortices are formed, the steady-flow separation changes remarkably. These coherent structures embedded in the boundary layer amplify before breakdown while traveling downstream with a convective speed of about 37% of the local free-stream speed. The vortices play an important role in the generation of turbulence and thus to decide the transitional length, which becomes time dependent. The source of the pressure fluctuations on the rear part of the suction surface is also identified as the formation of these coherent structures. When compared with experiments, it reveals that LES is worth pursuing as an understanding of the eddy motions and interactions is of vital importance for the problem.

Aeroelastic Stability of Welded-in-Pair Low Pressure Turbine Rotor Blades: A Comparative Study Using Linear Methods

The aerodynamic damping of a modern low pressure turbine bladed-disk with interlock rotor blades is compared for the first time to that obtained when the rotor blades are welded in pairs through the lateral face of the shroud. The damping is computed solving the linearized Reynolds averaged Navier-Stokes equations on a moving grid. First the basics of the stabilizing mechanism of welding the rotor blades in pairs is investigated using two-dimensional analyses and the Panovsky and Kielb method. It is concluded that the stabilizing effect is due to the suppression of unsteady perturbations in one out of the two passages providing for the first time a physical explanation to engine data. Three-dimensional effects are then studied using the actual mode shapes of two bladed disks differing solely in the shroud boundary conditions. It is concluded that the increase in the aerodynamic damping, due to the modification of the mode shapes caused by welding the rotor blades in pairs, is smaller than that due to the overall raise of the reduced frequencies of a bladed disk with an interlock design. The modification of the flutter boundaries due to mistuning effects is assessed using the reduced order model known as the Fundamental Mistuning Model. A novel extension of the critical reduced frequency stability maps accounting for mistuning effects is derived and applied for both, the freestanding and welded-in-pair airfoils. The stabilizing effect of mistuning is clearly seen in these maps. Finally, the effect of mistuning on low-pressure-turbine bladed disks is studied. It is shown that the modification on the stability limit of the interlock bladed disk is negligible, while for the welded-in-pair configuration a 0.15% increase of the damping relative to the critical damping is found. This qualitative difference between both configurations had not been reported before.

Predictions of Transitional Flows in Low-Pressure Turbines Using Intermittency Transport Equation

A transport equation for the intermittency factor is employed to predict the transitional e ows in low-pressure turbine applications. The intermittent behavior of the transitional e ows is taken into account and incorporated into computations by modifying the eddy viscosity πt with the intermittency factor °. Turbulence quantities are predicted by using Menter’ s shear stress transport two-equation turbulence model, and the intermittency factor is obtained from the solution of a recently developed transport equation model. The transport equation model not only can reproduce the experimentally observed streamwise variation of the intermittency in the transition zone, but it also provides a realistic cross-stream variation of the intermittency proe le. The current model is applied to predictions of a modern low-pressure turbine experiment, and detailed comparisons of the computational results with the experimental data are presented. The model has been shown to be capable of predicting the low-pressure turbine e ow transition under a variety of Reynolds number and freestream turbulence conditions.

Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

Detailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil. Reynolds numbers based on wetted plate length and nominal exit velocity were varied from 50,000 to 300,000, covering cruise to takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2 and 7 percent) were set using passive grids. The location of boundary-layer separation does not depend strongly on the free-stream turbulence level or Reynolds number, as long as the boundary layer remains nonturbulent prior to separation. Strong acceleration prevents transition on the upstream part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition in the adverse pressure gradient region. Under low free-stream turbulence conditions, transition is induced by instability waves in the shear layer of the separation bubble. Reattachment generally occurs at the transition start. At Re=50,000 the separation bubble does not close before the trailing edge of the modeled airfoil. At higher Re, transition moves upstream, and the boundary layer reattaches. With high free-stream turbulence levels, transition appears to occur in a bypass mode, similar to that in attached boundary layers. Transition moves upstream, resulting in shorter separation regions. At Re above 200,000, transition begins before separation. Mean velocity, turbulence, and intermittency profiles are presented.