Low-pressure Turbine Cascade Research Articles

Large Eddy Simulation of a Condensing Wet Steam Turbine Cascade

Abstract The influence of turbulence modeling approach by means of (U)RANS and large eddy simulation (LES) on the overall modeling of turbulent condensing wet steam flows is investigated using the example of a low-pressure steam turbine cascade. For an accurate numerical treatment of turbulence in the presence of shock waves, necessary for predictive scale-resolving computations, a hybrid flux treatment switches between a baseline nondissipative central flux in energy consistent split form and a shock-capturing upwind flux in shocked regions based on a shock sensor. Condensation is realized by a monodispersed Euler–Euler source term model, the equation of state by the highly efficient and accurate spline based table lookup method (SBTL). The numerical treatment is validated with a decay of homogeneous isotropic turbulence test case containing eddy shocklets. The measurement results of the condensing wet steam cascade are overall much better matched by LES compared to RANS and URANS. Analysis shows that the LES is much better able to account for the inherently unsteady nature of the spontaneous condensation process and its interaction with the trailing edge shock wave structure.

Numerical investigation on ultra-high-lift low-pressure turbine cascade aerodynamics at low Reynolds numbers using transition-based turbulence models

ABSTRACT The performance of ultra-high-lift (UHL) low-pressure turbine (LPT) is subject to complex flow phenomena (e.g. separation, transition and reattachment) which require advanced modelling for accurate numerical predictions. The feasibility and fidelity of three widely used transition-based turbulence models are evaluated in the Reynolds-Averaged Navier-Stokes (RANS) prediction of low-Reynolds number flows in linear UHL LPT cascade (T106C). All three transition models prove to capture the tendency that the size of separation bubble decreases with the increase of Reynolds number or inlet turbulence intensity. It turns out that intermittency factor-transition momentum thickness Reynolds number based shear stress transport turbulence model is the most accurate among the three models, expect for the clean inlet case at an isentropic outlet Reynolds number of . It is suggested that different viscosity ratios should be prescribed at the inlet for various models to mimic the effect of turbulence intensities precisely. In order to take into account the periodic wakes in computation, a moving cylindrical bar is added to the cascade inlet. The assessment of the capability of three models in predicting unsteady wake induced transition is carried out for selected Reynolds numbers. Some practical suggestions are given for the use of transition models based on RANS equations in simulation of the ultra-high-lift LPT cascade flows at low Reynolds numbers.

Steady and Unsteady RANS Modeling of Wake Effects and Grid Resolution Requirements in a Low-Pressure Turbine Cascade

Due to relative motion between rotors and stators in aircraft engines, periodic wakes are present in downstream blade rows, which exert significant influence at flow loss and engine efficiency. To quantify and reproduce this influence, a low-pressure turbine cascade is computed using steady and unsteady RANS methods in the flow solver TRACE by DLR and MTU Aero Engines. A thorough grid study is carried out and various aspects of grid resolution requirements are investigated for the setups and considered performance metrics respectively. A steady state transition model extension that has been developed and published by the authors is applied to the cascade flow at a number of operating points and validated with experimental data while being compared to the unsteady results as well as the steady state results without the wake effect extension. Further, a variation of wake-related parameters is carried out while discussing the effects modeled in the unsteady setup as well as the ability of the two steady state setups (with and without wake extension) to capture the trends identified by the unsteady results. A sufficiently accurate reproduction by the wake model extension enables steady simulations of the inherently unsteady effects in the aerodynamic design of the turbine, which results in an enormous saving of computational time and effort.

Studies on Secondary Flow Control in a Low Pressure Turbine Cascade for Aero-Engines by a Composite-Type Device

Studies on Secondary Flow Control in a Low Pressure Turbine Cascade for Aero-Engines by a Composite-Type Device

Compressible large eddy simulation of the unsteady evolution process in a LPT Cascade with incoming wakes

An in-house large eddy simulation (LES) code based on three-dimensional compressible N-S equations is used to research the impact of incoming wakes on unsteady evolution characteristic in a low-pressure turbine (LPT) cascade. The Mach number is 0.4 and Reynolds number is 0.6 × 105 (based on the axial chord and outlet velocity). The reduced frequency of incoming wakes is Fred = 0 (without wakes), 0.37 and 0.74. A detailed analysis of Reynolds stresses and turbulent kinetic energy inside the boundary layer has been carried out. Particular consideration is devoted to the transport process of incoming wakes and the intermittent property of the unsteady boundary layer. With the increase of reduced frequency, the inhibiting effect of wakes on boundary layer separation gradually enhances. The separation at the rear part of the suction side is weakened and the separation point moves downstream. However, incoming wakes lead to an increase in dissipation and aerodynamic losses in the main flow area. Excessive reduced frequency ( Fred = 0.74) causes the main flow area to become one of the main source areas of loss. An optimal reduced frequency exists to minimize the aerodynamic loss of the linear cascade.

Large Eddy Simulation of Secondary Flows in an Ultra-High Lift Low Pressure Turbine Cascade at Various Inlet Incidences

Abstract Increasing the blade loading of a low pressure turbine blade decreases the number of blades, thus improving the aero-engine performance in terms of the weight and manufacture cost. Many studies focused on the blade-to-blade flow field of ultra-high lift low pressure turbines. The secondary flows of ultra-high lift low pressure turbines received much less attention. This paper investigates the secondary flows in an ultra-high lift low pressure turbine cascade T106C by large eddy simulation at a Reynolds number of 100,000. Both time-averaged and instantaneous flow fields of this ultra-high lift low pressure turbine are presented. To understand the effects of the inlet angle, five incidences of ‒10°, ‒5°, 0, +5° and +10° are investigated. The case at the design incidence is analyzed first. Detailed data is used to illustrate the how the fluids in boundary layers develops into secondary flows. Then, the cases with different inlet incidences are discussed. The aerodynamic performances are compared. The effect of blade loading on the vortex structures is investigated. The horseshoe vortex, passage vortex and the suction side corner vortex are very sensitive to the loading of the front part of the blade.

A Comparison of Turbulence Levels From Particle Image Velocimetry and Constant Temperature Anemometry Downstream of a Low-Pressure Turbine Cascade at High-Speed Flow Conditions

AbstractThe development and verification of new turbulence models for Reynolds-averaged Navier–Stokes (RANS) equation-based numerical methods require reliable experimental data with a deep understanding of the underlying turbulence mechanisms. High accurate turbulence measurements are normally limited to simplified test cases under optimal experimental conditions. This work presents comprehensive three-dimensional data of turbulent flow quantities, comparing advanced constant temperature anemometry (CTA) and stereoscopic particle image velocimetry (PIV) methods under realistic test conditions. The experiments are conducted downstream of a linear, low-pressure turbine cascade at engine relevant high-speed operating conditions. The special combination of high subsonic Mach and low Reynolds number results in a low density test environment, challenging for all applied measurement techniques. Detailed discussions about influences affecting the measured result for each specific measuring technique are given. The presented time mean fields as well as total turbulence data demonstrate with an average deviation of ΔTu<0.4% and ΔC/Cref<0.9% an extraordinary good agreement between the results from the triple sensor hot-wire probe and the 2D3C-PIV setup. Most differences between PIV and CTA can be explained by the finite probe size and individual geometry.

Recognition of Structures Leading to Transition in a Low-Pressure Turbine Cascade: Effect of Reduced Frequency

Abstract The boundary layer developing over the suction side of a low-pressure turbine cascade operating under unsteady inflow conditions has been experimentally investigated. Time-resolved particle image velocimetry (PIV) measurements have been performed in two orthogonal planes, the blade-to-blade and a wall-parallel plane embedded within the boundary layer, for two different wake-reduced frequencies. Proper orthogonal decomposition (POD) has been used to analyze the data and to provide an interpretation of the most significant flow structures for each phase of the wake passing cycle. Detailed information on the most energetic turbulent structures at a particular phase is obtained with a newly developed procedure that overcomes the limit of classical phase average. The synchronization of the measurements in the two planes allows the computation of the characteristic dimension of boundary layer streaky structures that are responsible for transition. The largest and most energetic structures are observed when the wake centerline passes over the rear part of the suction side, and they appear practically the same for both reduced frequencies. The passing wake forces transition leading to the breakdown of the boundary layer streaks. Otherwise, the largest differences between the low and high reduced frequency are observed in the calmed region. The postprocessing of these two planes allowed computing the spacing of the streaky structures and making it nondimensional by the boundary layer displacement thickness observed for each phase. The nondimensional value of the streaks spacing is about constant, irrespective of the reduced frequency.

Numerical investigation of separation-induced transition in a low-pressure turbine cascade in a low-disturbance environment

In this paper, the separation-induced transition in an LPT (low-pressure turbine) cascade is investigated at low Reynolds number with DNS (direct numerical simulation). The transition process is accurately predicted giving good agreements between the DNS and experimental results. To illustrate the secondary instability of separation-induced transition in a low-disturbance environment, the results are comprehensively analyzed in both Fourier space and physical space. It is illustrated that the effect of hyperbolic instability dominates around the saddle point of hyperbolic streamlines. This instability mechanism is responsible for the emergence of the streamwise vortices in the braid region. Elongated and intensified because of the “stretching” effect of the background flow, these vortices become the most noticeable characteristic of the flow field. Fundamental modes of small spanwise wavelength are excited in the braid region, so as some low-frequency modes. The elliptical instability plays a minor role than hyperbolic instability. It is also observed that the fundamental mode with a larger spanwise wavelength is unstable in the vortex core which is associated with the deformation of the vortex core via elliptical instability. There is no convincing evidence for the existence of subharmonic instability.

Unsteady experimental and numerical investigation of aerodynamic performance in ultra-high-lift LPT

Detailed experimental and numerical investigations were performed for an ultra-high-lift front-loaded low-pressure turbine cascade (Zw = 1.58) with periodic wakes. The interaction mechanisms between the incoming wakes and endwall secondary flow were carefully examined. Wakes were produced by moving upstream rods, and flow field downstream of the cascade was measured using a seven-hole probe. Experimental results revealed that incoming wakes influenced not only the boundary layer development of the blade suction surface but also the complex endwall secondary vortex structures. On the suction surface: Incoming wakes clearly suppressed the suction side separation bubble at a low Reynolds number of 25000. Nevertheless, the effects of different wake passing frequencies were not significantly different at Re = 100000, and the profile losses under wake passing were even greater than in the absence of wakes. At the endwalls: Incoming wakes more strongly suppressed secondary flow at Re = 100000 than at Re = 25000, because the low-momentum fluid inside the incoming wakes clearly increased the endwall cross-passage pressure gradient at Re = 25000. The experimental results indicated that periodic wakes decreased the passage vortex and counter vortex core strength by 25% and 30%, respectively, at Re = 100000. Instantaneous results also demonstrated that endwall secondary vortices decreased significantly near the position of wakes passing.

Vortex dynamics mechanisms of separated boundary layer in a highly loaded low pressure turbine cascade

This paper investigates the vortex dynamics in the suction-side boundary layer on an aero-engine low pressure turbine blade at two different Reynolds numbers at which short and long laminar separation bubbles occur. Different vortical patterns are observed and investigated through large eddy simulation (LES). The results show that at the higher Reynolds number, streamwise streaks exist upstream of separation line. These streaks initiate spanwise undulation in the form of vortex tubes, which roll-up and shed from the shear layer due to the Kelvin–Helmholtz instability. The vortex tubes alternately pair together and eventually distort and break down to small-scale turbulence structures near the mean reattachment location and convect into a fully turbulent boundary layer. At the lower Reynolds number, streamwise streaks are strong and the separated flow is unable to reattach to the blade surface immediately after transition to turbulence. Therefore, bursting of short bubbles into long bubbles can occur, and vortex tubes have larger diameters and cover a part of the blade span. In this case vortex pairing does not occur and vortex shedding process is promoted mainly by flapping phenomenon. Moreover, the results of dynamic mode decomposition (DMD) analysis show a breathing motion as a source of unsteadiness in the separation location, which is accompanied by the flapping phenomenon.

On the mesh resolution of industrial LES based on the DNS of flow over the T106C turbine

A high-order Navier-Stokes solver based on the flux reconstruction (FR) or the correction procedure via reconstruction (CPR) formulation is employed to perform a direct numerical simulation (DNS) and large eddy simulations (LES) of a well-known benchmark problem – transitional flow over the low-pressure T106C turbine cascade. Hp-refinement studies are carried out to assess the resolution requirement. A 4th order (p3) simulation on the fine mesh is performed with a DNS resolution to establish a "converged" solution, including the mean pressure and skin-friction distributions, and the power spectral density in the wake. Then LES on the coarse and fine meshes with lower order schemes are conducted to assess the mesh and order dependence of the solution. In particular, we study the error in the transition location, the mean skin-friction distribution, and the mean lift and drag coefficients. These h- and p-refinement studies provide a much-needed guideline in h- and p- resolutions to achieve a certain level of accuracy for industrial LES applications.

Unsteady wakes-secondary flow interactions in a high-lift low-pressure turbine cascade

Detailed experimental measurements were conducted to study the interactions between incoming wakes and endwall secondary flow in a high-lift Low-Pressure Turbine (LPT) cascade. All of the measurements were conducted in both the presence and absence of incoming wakes, and numerical analysis was performed to elucidate the flow mechanism. With increasing Reynolds number, the influence of the incoming wakes on suppressing the secondary flow gradually increased owing to the greater influence of incoming wakes on reducing the negative incidence angle at higher Reynolds numbers, leading to a lower blade loading near the leading edge and suppression of the Pressure Side (PS) leg of the horseshoe vortex. However, the effect of unsteady wakes on suppressing the profile losses gradually became weaker owing to the reduced size of the Suction Side (SS) separation bubble and increased mixing loss in the free-flow region at high Reynolds numbers. Incoming wakes clearly improved the aerodynamic performance of the low-pressure turbine cascade at low Reynolds numbers of 25,000 and 50,000. In contrast, at the high Reynolds number of 100,000, the profile loss at the midspan and mass-averaged total losses downstream of the cascade were higher in the presence of wakes than in the absence of wakes, and the unsteady wakes exerted a negative influence on the aerodynamic performance of the LPT cascade.

Unsteady influences of blade loading distribution on secondary flow of ultra-high-lift LPT

Experimental measurements were performed to study the influence of blade loading distribution on the aerodynamic performance of an ultra-high-lift low-pressure turbine cascade (Zw = 1.58) in the presence and absence of incoming wakes. The flow structures in front-loaded and aft-loaded blade passages were comprehensively compared at various Reynolds numbers. Furthermore, the flow mechanisms responsible for the effect of the blade loading distribution on the secondary flow were discussed in detail through numerical calculations. At the low Reynolds number of 25,000, weaker secondary flow was observed for the aft-loaded blade under steady conditions owing to the lower cross-passage pressure gradient compared to the front-loaded blade. Unsteady wakes clearly improved the throughflow characteristics of the cascade passage, increased the blade loading, and strengthened the secondary flow in the aft-loaded blade passage. At the high Reynolds number of 100,000, unsteady wakes delayed generation of the passage vortex in both the front-loaded and aft-loaded blade passages and clearly weakened the secondary flow. The experimental results revealed that the upstream wakes reduced the strength of the passage vortex core in the aft-loaded and front-loaded blade profiles by 21.6% and 17.5%, respectively.

Large-Eddy Simulation and RANS Analysis of the End-Wall Flow in a Linear Low-Pressure Turbine Cascade, Part I: Flow and Secondary Vorticity Fields Under Varying Inlet Condition

Abstract In low-pressure turbines (LPTs), around 60–70% of losses are generated away from end-walls, while the remaining 30–40% is controlled by the interaction of the blade profile with the end-wall boundary layer. Experimental and numerical studies have shown how the strength and penetration of the secondary flow depends on the characteristics of the incoming end-wall boundary layer. Experimental techniques did shed light on the mechanism that controls the growth of the secondary vortices, and scale-resolving computational fluid dynamics (CFD) allowed to dive deep into the details of the vorticity generation. Along these lines, this paper discusses the end-wall flow characteristics of the T106 LPT profile at Re = 120 K and M = 0.59 by benchmarking with experiments and investigating the impact of the incoming boundary layer state. The simulations are carried out with proven Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) solvers to determine if Reynolds-averaged models can capture the relevant flow details with enough accuracy to drive the design of this flow region. Part I of the paper focuses on the critical grid needs to ensure accurate LES and on the analysis of the overall time-averaged flow field and comparison between RANS, LES, and measurements when available. In particular, the growth of secondary flow features, the trace and strength of the secondary vortex system, and its impact on the blade load variation along the span and end-wall flow visualizations are analyzed. The ability of LES and RANS to accurately predict the secondary flows is discussed together with the implications this has on design.

Impact of Number Representation for High-Order Implicit Large-Eddy Simulations

High-order numerical methods for unstructured grids combine the superior accuracy of high-order spectral or finite difference methods with the geometric flexibility of low-order finite volume or finite element schemes. Over the past few years, they have shown promise in enabling implicit large-eddy simulations within the vicinity of complex geometrical configurations. However, the cost of such simulations remains prohibitive. In this paper, the impact of number representation on the efficiency and efficacy of these methods is considered. Theoretical performance models are derived and assessed. Specifically, four test cases are considered: 1) the viscous Taylor–Green vortex, 2) flow over a circular cylinder, 3) flow through a T106c low-pressure turbine cascade, and 4) flow around a NACA 0021 in deep stall. It is found that the use of single (in lieu of double) precision arithmetic does not have a significant impact on accuracy. The performance of the resulting simulations was found to improve between 1.4 and 2.9 times.

Unsteady effects of periodic wake passing frequency on aerodynamic performance of ultra-high-lift low pressure turbine cascades

Flow losses and flow field data downstream of an ultra-high-lift aft-loaded low pressure turbine cascade were comprehensively measured for different incoming wake passing frequencies, and the mechanisms through which incoming wakes influence secondary flow were examined via numerical calculations. Reynolds numbers ranging from 25 000–100 000 (based on the axial chord and inlet velocity) were considered in both the presence and absence of the wakes. At low Reynolds numbers of 25 000 and 50 000, increasing wake passing frequency gradually suppressed the suction surface separation bubble and increased the cross-passage pressure gradient, and the unsteady wakes clearly improved the throughflow characteristics of the cascade passage. Furthermore, the larger separation bubble on the suction surface hindered the migration of the secondary vortices from the endwall to span. Consequently, incoming wakes were not beneficial for suppressing secondary flow at low Reynolds numbers. At the high Reynolds numbers of 80 000 and 100 000, increasing wake passing frequency afforded stronger inhibition of the secondary flow owing to the reduction in blade loading originating from the “negative jet” influence of the wakes. Transport of incoming wakes in the cascade passage caused the downstream migration of the position of the saddle point, which is also advantageous for decreasing secondary flow losses.

Characterization of Periodic Incoming Wakes in a Low-Pressure Turbine Cascade Test Section by Means of a Fast-Response Single Sensor Virtual Three-Hole Probe

This paper aims at evaluating the characteristics of the wakes periodically shed by the rotating bars of a spoked-wheel type wake generator installed upstream of a high-speed low Reynolds linear low-pressure turbine blade cascade. Due to the very high bar passing frequency obtained with the rotating wake generator (fbar = 2.4−5.6 kHz), a fast-response pressure probe equipped with a single 350 mbar absolute Kulite sensor has been used. In order to measure the inlet flow angle fluctuations, an angular aerodynamic calibration of the probe allowed the use of the virtual three-hole mode; additionally, yielding yaw corrected periodic total pressure, static pressure and Mach number fluctuations. The results are presented for four bar passing frequencies (fbar = 2.4/3.2/4.6/5.6 kHz), each tested at three isentropic inlet Mach numbers M1,is = 0.26/0.34/0.41 and for Reynolds numbers varying between Re1,is = 40,000 and 58,000, thus covering a wide range of engine representative flow coefficients (ϕ = 0.44−1.60). The measured wake characteristics show fairly good agreement with the theory of fixed cylinders in a cross-flow and the evaluated total pressure losses and flow angle variations generated by the rotating bars show fairly good agreement with theoretical results obtained from a control volume analysis.

The effect of endwall boundary layer and incoming wakes on secondary flow in a high-lift low-pressure turbine cascade at low Reynolds number

The endwall flow features are heavily dependent on the incoming boundary layer. It was particularly important to increase understanding the effect of inlet boundary layer thickness on endwall secondary flow under unsteady conditions. In present study, the influences of incoming wakes and various boundary layer thickness on endwall secondary flow were studied in a typical high-lift low-pressure turbine cascade, numerical calculation and experiment measurement of seven-hole probe were adopted at Re = 25,000 (based on the inlet velocity and the axial chord). Upstream wakes were simulated through moving rods upstream of the cascade. Detailed analysis was focused on the mechanisms of periodic wake influencing on the endwall vortex structures under thick endwall boundary layer condition. Influences of two different endwall boundary layer thickness on endwall secondary vortices structures were also comparatively analyzed. Under steady condition without wake, although thick incoming boundary layer reduces the cross-passage pressure gradient near endwall, more low momentum fluid inside thick endwall boundary layer is drawn into secondary vortices, finally resulting in stronger the pressure side leg of the leading edge horseshoe vortex and passage vortex, compared to the results of thin boundary layer condition. Under unsteady condition with thick inlet boundary layer, the “negative jet” effect of incoming wakes delays intersection of pressure side leg and suction side leg of leading edge horseshoe vortex on blade suction surface. The time-averaged strength of passage vortex and counter vortex core decreases by about 32%, and the underturning and overturning of endwall secondary flow is suppressed. The instantaneous results also indicate the endwall secondary vortices are reduced periodically at the position of wakes passing.

Numerical investigation on flow control effects of dynamic hump for turbine cascade at different Reynolds number and hump oscillating frequency

From takeoff to cruise, the operating Reynolds number (Re) of the low-pressure turbine (LPT) in an aero-engine decreases significantly, making the boundary layer near the aft portion of the blade surface susceptible to separation. This paper discusses the feasibility of using local dynamic surfaces as an active flow control method to suppress the laminar flow separation on the suction surface in PAKB LPT cascade at low Re. A local dynamic surface, shaped as a half-sinusoidal hump with an oscillation amplitude of 1 mm, was positioned just upstream of the peak velocity point. Unsteady Reynolds-Averaged Navier–Stokes (RANS) simulations were performed to investigate the aerodynamic performance of the cascades with and without dynamic hump. At first, the effect of Re was studied with the hump oscillating frequency at 200 Hz. Compared with uncontrolled cascade at various Re from 25,000 to 150,000, the profile losses of the cascade controlled by dynamic hump reduced significantly at low Re condition where the separation bubble was very large, and increased slightly at high Re condition where the separation bubble was small. Then the effect of the hump oscillation frequency was investigated under the Re of 25,000. It was found that the controlled cascade loss reached a minimum in a certain hump oscillating frequency range. At low Re of 25,000, the loss of controlled cascade was mainly composed of low-loss continuous laminar vortices that were attached to the suction surface. Continuous vortices with a certain spacing confined the low-energy fluid near the suction surface by energy exchange between the free flow and near-wall flow, which suppressed the large-scale flow separation.