Low-pressure Turbine Cascade Research Articles

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.

Effect of periodic wakes and a contoured endwall on secondary flow in a high-lift low-pressure turbine cascade at low Reynolds numbers

The use of a contoured endwall has the potential to suppress endwall secondary flow. Unsteady wakes affect not only the boundary layer characteristics of blade suction surface at blade midspan but also the endwall flow structures. The lack of understanding of the flow mechanism of the combined effects of periodic wakes and contoured endwall on secondary flow limits their roles. This paper presents a experimental and numerical investigation of the endwall secondary flow in a typical high-lift low-pressure turbine cascade. Wakes were produced by moving rods upstream of cascade, and the flow fields at the exit of cascade were measured using a seven-hole pressure probe. The study focused on the combined effect of the upstream wakes and the contoured endwall on the secondary flow as well as the underlying physical mechanisms. The influences of the Reynolds numbers and the contoured endwall on the performance of high-lift low-pressure turbine endwall regions were also discussed. At steady conditions without wakes, the total losses in the turbine cascade increased with decreasing Reynolds number; the most intense passage vortex, counter vortex and corner vortex were observed at a low Reynolds number of 25,000 (based on the axial chord and the inlet velocity). The contoured endwall decreased the cross-passage pressure gradient, and suppressed the passage vortex. Under unsteady conditions, the interaction between upstream wakes and the passage vortex results in reduction of the passage vortex. The combined effect of the contoured endwall and periodic wakes redistributed the endwall pressure and further decreased the cross-passage pressure gradient. Consequently, the intensities of the passage vortex and counter vortex decreased by 17% and 11% respectively, compared with the flat endwall cascade with wakes. Contoured endwall with wakes reduced secondary kinetic energy of cascade exit by 53.8% than the result of the flat endwall no wake. Which is beneficial to improve the aerodynamic performance of the high-lift low-pressure turbine.

Numerical Investigation of Active Flow Control of Low-Pressure Turbine Endwall Flow

The endwall flow in a highly loaded linear low-pressure turbine cascade has been numerically investigated for a chord-based Reynolds number of 100,000. Mean flow visualizations reveal a horseshoe vortex at the endwall-leading edge junction. The pressure side leg of the horseshoe vortex develops into a highly coherent passage vortex. The passage vortex is supported by a strong secondary flow that impinges on the endwall and spreads in the pitchwise direction. A dynamic mode decomposition reveals temporally growing three-dimensional modes that are susceptible to unsteady flow control. Based on the modal analysis and physical understanding of the mean flow topology, several different steady and unsteady flow control strategies were devised for weakening the passage vortex, countering the secondary flow, and reducing the suction side corner flow separation. Steady blowing that directly opposes the secondary flow effectively lowers the total pressure losses but requires large amplitudes, which makes it inefficient. Unsteady forcing is demonstrated to be an efficient way for decreasing the coherence of the passage vortex and for obtaining a moderate reduction of the total pressure losses.

On the Periodically Unsteady Interaction of Wakes, Secondary Flow Development, and Boundary Layer Flow in An Annular Low-Pressure Turbine Cascade: An Experimental Investigation

The experimental results reported in this contribution address the time-dependent impact of periodically unsteady wakes on the development of profile and end wall boundary layers and consequently on the secondary flow system. Experimental investigations are conducted on an annular 1.5 stage axial turbine rig at Ruhr-Universität Bochum’s Chair of Thermal Turbomachines and Aeroengines. The object under investigation is a modified T106 profile low-pressure turbine (LPT) stator row at a representative exit flow Reynolds number of 200,000. By making use of an annular geometry instead of a linear cascade, the influence of curvilinear end walls, nonuniform, increasing pitch across the span and radial flow migration can be represented. Incoming wakes are generated by a variable-speed driven rotor equipped with cylindrical bars. Special emphasis is put on the wake-induced recurrent formation, suppression, weakening, and displacement of individual vortices and separated flow regimes. For this, based on a comprehensive set of time-resolved measurement data, the interaction of impinging bar wakes and boundary layer flow and thus separation and its periodic manipulation along the passage end walls and on the blade suction surface are studied within the frequency domain.

Evaluation of an algebraic model for laminar-to-turbulent transition on secondary flow loss in a low-pressure turbine cascade with an endwall

The predictive qualities of a recently developed algebraic intermittency model for laminar-to-turbulent transition are analysed for the flow through a linear cascade of low-pressure turbine blades with an endwall. Both steady RANS (Reynolds-averaged Navier–Stokes) and time-accurate RANS (URANS) simulations are performed. The results are compared with reference LES (Large Eddy Simulation) by Cui et al. (2017, Numerical investigation of secondary flows in a high-lift low pressure turbine, Int. J. of Heat and Fluid Flow, vol. 63) and results by the local correlation-based intermittency transport model (LCTM) by Menter et al. (2015, A one-equation local correlation-based transition model. Flow Turbul. Combust., vol. 95) for laminar and turbulent endwall boundary layers at the cascade entrance. Good agreement is obtained with the reference LES and with results by the LCTM for the evolution through the cascade of the mass-averaged total pressure loss coefficient and for profiles of pitchwise-averaged total pressure loss coefficient at the cascade exit.

Interaction of wake disturbance with compressible transitional boundary layers in a low-pressure turbine cascade under rotor-stator interaction

Transition-process-resolving numerical simulations of compressible transitional flows subjected to rotor-stator interaction in a low-pressure turbine cascade were performed. Cases of small and large Strouhal numbers (St) of blade passage, and with and without isotropic free-stream turbulence (IFST) are compared. Cascade geometry is T106, and the computations are conducted at its design point condition. A higher-order finite difference method based on sixth-order compact difference and tenth-order filtering is employed. Variation in St and the intensity of IFST changes the frequency of turbulence inflow, the convection and propagation of turbulence, and the interaction of disturbance with boundary layers in the blade passage. The changes result in different pressure fluctuation in the passage. Because pressure fluctuation can convey information away from the passage, it is expected to be used for diagnosing boundary layer states around the blade indirectly away from the passage.

Experimental and Numerical Investigation of Secondary Flow Structures in an Annular Low Pressure Turbine Cascade Under Periodic Wake Impact—Part 2: Numerical Results

In this work, we present the results of the numerical investigations of periodic wake–secondary flow interaction carried out on a low pressure turbine (LPT) equipped with modified T106-profile blades. The numerical predictions obtained by means of unsteady Reynolds-averaged Navier–Stokes (URANS) simulations using a k-ω-model have been compared with measurements conducted in the same configuration and showed a good agreement. Based on the verified numerical data, the Q-criterion has been employed to characterize the secondary flow structures and accurately identify their origin. An analysis of the fundamental wake kinematics and the unsteady vortex migration revealed dominant interaction mechanisms such as the circumferential fluctuation of the pressure side horseshoe vortex (HSV) and its direct interaction with the passage vortex (PV) and the concentrated shed vortex (CSV). Finally, a correlation with the total pressure loss coefficient is provided and a link to the incoming wake structures is given.

Experimental and Numerical Investigation of Secondary Flow Structures in an Annular Low Pressure Turbine Cascade Under Periodic Wake Impact—Part 1: Experimental Results

Experimental studies have been conducted on a modified T106 low pressure turbine (LPT) profile in an annular 1.5 stage axial turbine rig at the Chair of Thermal Turbomachines and Aeroengines, Ruhr-Universität Bochum. The rig setup allows the highly resolved measurement of unsteady wake–stator flow interaction in both space and time. Incoming wakes are generated by a variable-speed driven rotor equipped with cylindrical bars. In the present paper, an experimental approach to the investigation of unsteady phenomena is proposed. Time-averaged and instantaneous measurement data from 2D flow field traverses at the stator exit are provided for the analysis of the periodically unsteady vortex formation, displacement, and suppression. Additional time-accurate blade pressure data are used to study the relationship between the flow structures downstream of the stator row and the immediate intermittent wake impact on the blades. Bar wake kinematics is also discussed in relation to the observations.

Synchronization of multi-plane measurement data by means of POD: application to unsteady boundary layer transition

Time-resolved particle image velocimetry measurements have been performed on the suction side of a low-pressure turbine cascade perturbed by periodic unsteady incoming wakes. The flow field has been investigated in two orthogonal planes. A wall-normal and a wall-parallel plane have been considered to identify and characterize the dimensions of the coherent structures developing within the boundary layer at the different wake passing phases. A new proper orthogonal decomposition (POD)-based procedure has been developed to sort the data acquired in the two planes. Particularly, the temporal coefficients of the POD modes related to wake migration have been used to provide phase-averaged data and synchronize the information obtained in the two measurement planes. It is shown that this technique does not require an external trigger, typically requested by previous available multi-plane phase-locking strategies. Moreover, POD has been applied to sub-ensembles of data at the same relative phase within the wake passing cycle. Thanks to the proposed procedure, the characteristic dimension of the eddies caused by the wake–boundary layer interaction has been investigated: at each phase of the wake passing cycle, the POD provides the representation of the most relevant flow features. It has been possible to observe the presence of boundary layer streaks and when they first occur within the wake passing period. One-dimensional auto-correlations of the velocity fields in the wall-parallel plane have been performed following the periodic perturbation induced by the passing wake in space and time. The auto-correlation provides the spacing between streaks, that, once scaled with the local boundary layer displacement thickness measured at the same phase in the wall-normal plane, is consistent with the available literature data for steady flows.

Prediction of endwall losses in a low pressure turbine cascade with an algebraic intermittency model

The predictive qualities of a newly developed algebraic intermittency model are analysed by simulation of the flow through a linear cascade of low pressure turbine (LPT) blades with endwalls. Both steady RANS (Reynolds-averaged Navier-Stokes) and time-accurate RANS (URANS) simulations are performed. The results are compared with reference LES (Large Eddy Simulation) by Cui et al. (2017, Numerical investigation of secondary flows in a high-lift low pressure turbine, Int. J. of Heat and Fluid Flow, vol. 63) for a turbulent endwall boundary layer (TBL) at the entrance to the cascade. Good agreement is obtained between simulations with the algebraic model and the reference LES for the evolution of the mass-averaged total pressure loss coefficient and for profiles of pitchwise-averaged turbulent kinetic energy.

Effects of free-stream turbulence on the global pressure fluctuation of compressible transitional flows in a low-pressure turbine cascade

PurposeThis paper aims to investigate global pressure fluctuations in compressible transitional flows in a low-pressure turbine cascade because of variations in the free-stream turbulence and its interaction with the boundary layers.Design/methodology/approachTransition process resolving numerical simulations are performed with different types of inflow turbulence. The unsteady three-dimensional fully compressible Navier–Stokes equations are solved using a sixth-order compact difference and a tenth-order filtering method. First, simulations of both K-regime and bypass transitions are conducted for a flat plate boundary layer to validate the use of the filter in computing different transition routes. Second, computations of the cascade flows are conducted. Cases of no free-stream turbulence, isotropic free-stream turbulence of 5 per cent and wakes from an upstream cylinder are compared. For wakes, variations in wake trajectory depending on the cylinder blade relative position are also taken into account.FindingsThe different transition routes are successfully reproduced by the present method even with strong filtering. When feedback phenomena occur near the trailing edge, high-frequency oscillations dominate in the flow field. Low-frequency oscillations become dominant when the blade boundary layer becomes turbulent. Thus, the effects of the free-stream turbulence and its interaction with the boundary layer appear as changes in the global pressure fluctuation.Originality/valueThe free-stream turbulence qualitatively affects global pressure fluctuations, which become a medium to convey boundary-layer information away from the cascade.

The Influence of Different Wake Profiles on Losses in a Low Pressure Turbine Cascade

Large eddy simulations were carried out in order to investigate the influence of unsteady incoming wakes with different profiles on the loss mechanisms of the high lift T106Alinear low-pressure turbine (LPT) cascade. Bars placed upstream of the LPT blade were set into rotation around their axis, thus generating circulation, as well as asymmetrical wake profiles. Three different rotation rates were simulated, yielding different wake parameters that were then compared to an actual turbine blade wake profile. Whereas the commonly-used non-rotating bars generated wakes with turbulent kinetic energy levels several times higher than that of an actual blade wake, the case with counter-clockwise rotation led to more rapid wake mixing. All three wakes were able to trigger boundary layer transition and thus intermittently prevent separation on the suction surface. However, the weaker the wakes, the larger and longer lasting the separation bubbles became, and an increase in profile losses could be observed. Interestingly, the configuration with the weakest wake and the largest separation bubble resulted in a reduction of the overall LPT loss.

Local Spatial Linear Stability Analysis of Low-Pressure Turbine Endwall Boundary Layer

An implicit large-eddy simulation of a low-pressure turbine cascade for an axial chord–based Reynolds number of 100,000 revealed steady longitudinal structures near the endwall that are reminiscent of crossflow vortices. A local spatial linear stability theory analysis with Newtonian eddy model of a profile extracted from the strongly curved section did not indicate any spatially growing steady modes. An unstable steady mode could be identified when the quasi-laminar assumption was made. The mode shape roughly agreed with the observed steady flow structures. Further analysis revealed that this mode was most strongly amplified when the frequency was nonzero. Based on a comparison of the Reynolds stress profiles upstream and inside the cascade, it is argued that the unstable mode may affect the mean flow through a modification of the Reynolds shear stresses.

Numerical Investigation of Secondary Flow and Loss Development in a Low-Pressure Turbine Cascade with Divergent Endwalls

Secondary flow and loss development in the T106Div-EIZ low-pressure turbine cascade are investigated utilizing (U)RANS simulations in cases with and without periodically incoming wakes at M a 2 t h = 0 . 59 and R e 2 t h = 2 × 10 5 . The predictions are compared to experimental data presented by Kirik and Niehuis (2015). The axial mid-span and overall loss development in the T106Div-EIZ and the T106A-EIZ in the steady case are analyzed regarding the effects caused by the different loading distributions and by the divergent endwall geometry. Furthermore, the entropy generation is analyzed in the T106Div-EIZ with periodically incoming wakes in several axial positions of interest and compared to the undisturbed steady case. It is found that in the front-loaded T106Div-EIZ, the incoming wakes cause a premature endwall loss production in the front part of the passage, resulting in a lower intensity of the secondary flow downstream of the passage and a redistribution of the loss generation components.

Experimental Investigation of Total Pressure Loss Development in a Highly Loaded Low-Pressure Turbine Cascade

Improvements in turbine design methods have resulted in the development of blade profiles with both high lift and good Reynolds lapse characteristics. An increase in aerodynamic loading of blades in the low-pressure turbine (LPT) section of aircraft gas turbine engines has the potential to reduce engine weight or increase power extraction. Increased blade loading means larger pressure gradients and increased secondary losses near the endwall. Prior work has emphasized the importance of reducing these losses if highly loaded blades are to be utilized. The present study analyzes the secondary flow field of the front-loaded low-pressure turbine blade designated L2F with and without blade profile contouring at the junction of the blade and endwall. The current work explores the loss production mechanisms inside the LPT cascade. Stereoscopic particle image velocimetry (SPIV) data and total pressure loss data are used to describe the secondary flow field. The flow is analyzed in terms of total pressure loss, vorticity, Q-Criterion, turbulent kinetic energy, and turbulence production. The flow description is then expanded upon using an implicit large eddy simulation (ILES) of the flow field. The Reynolds-averaged Navier–Stokes (RANS) momentum equations contain terms with pressure derivatives. With some manipulation, these equations can be rearranged to form an equation for the change in total pressure along a streamline as a function of velocity only. After simplifying for the flow field in question, the equation can be interpreted as the total pressure transport along a streamline. A comparison of the total pressure transport calculated from the velocity components and the total pressure loss is presented and discussed. Peak values of total pressure transport overlap peak values of total pressure loss through and downstream of the passage suggesting that the total pressure transport is a useful tool for localizing and predicting loss origins and loss development using velocity data which can be obtained nonintrusively.

A POD-Based Procedure for the Split of Unsteady Losses of an LPT Cascade

A time-resolved particle image velocimetry (TR-PIV) system has been employed to investigate the unsteady propagation of upstream wakes in a low-pressure turbine cascade. Data are obtained in the steady state condition and for two passing wake reduced frequencies. The study is focused on the identification and split of the different dynamics responsible for deterministic and random oscillations, thus loss generation by means of a new procedure based on proper orthogonal decomposition (POD). The paper takes advantage of the properties of POD that reduce the data set to a low number of modes that represent the most energetic dynamics of the system. It is clearly shown that the phase averaged flow field can be represented by a few number of POD modes related to the wake passing event for the unsteady cases. Proper orthogonal decomposition is also able to capture flow features affecting the instantaneous flow field not directly related to the wake passage (i.e., the vortex shedding phenomenon induced by the intermittent separation developing between adjacent wakes), which are smeared out in the phase averaged results. A procedure exploiting the biorthogonality condition of the POD modes, and the related temporal coefficients, has been developed for the quantification of the contribution due to the different POD modes to the overall turbulence kinetic energy production, or, equivalently, the mean flow energy dissipation rate. Results into the paper clearly show that losses due to wake migration, boundary layer and vortex shedding related phenomena can be distinguished and separately quantified for the different tested conditions.

Accurate Estimation of Profile Losses and Analysis of Loss Generation Mechanisms in a Turbine Cascade

The paper analyzes losses and the loss generation mechanisms in a low-pressure turbine (LPT) cascade by proper orthogonal decomposition (POD) applied to measurements. Total pressure probes and time-resolved particle image velocimetry (TR-PIV) are used to determine the flow field and performance of the blade with steady and unsteady inflow conditions varying the flow incidence. The total pressure loss coefficient is computed by traversing two Kiel probes upstream and downstream of the cascade simultaneously. This procedure allows a very accurate estimation of the total pressure loss coefficient also in the potential flow region affected by incoming wake migration. The TR-PIV investigation concentrates on the aft portion of the suction side boundary layer downstream of peak suction. In this adverse pressure gradient region, the interaction between the wake and the boundary layer is the strongest, and it leads to the largest deviation from a steady loss mechanism. POD applied to this portion of the domain provides a statistical representation of the flow oscillations by splitting the effects induced by the different dynamics. The paper also describes how POD can dissect the loss generation mechanisms by separating the contributions to the Reynolds stress tensor from the different modes. The steady condition loss generation, driven by boundary layer streaks and separation, is augmented in the presence of incoming wakes by the wake–boundary layer interaction and by the wake dilation mechanism. Wake migration losses have been found to be almost insensitive to incidence variation between nominal and negative (up to −9 deg) while at positive incidence, the losses have a steep increase due to the alteration of the wake path induced by the different loading distribution.

Effects of periodic wakes on the endwall secondary flow in high-lift low-pressure turbine cascades at low Reynolds numbers

Periodic wakes affect not only the surface boundary layer characteristics of low-pressure turbine blades and profile losses but also the vortex structures of the secondary flow and the corresponding losses. Thus, understanding the physical mechanisms of unsteady interactions and the potential to eliminate secondary losses is becoming increasingly important for improving the performance of high-lift low-pressure turbines. However, few studies have focused on the unsteady interaction mechanism between periodic wakes and endwall secondary flow in low-pressure turbines. This paper verified the accuracy of computational fluid dynamics by comparing experimental results and those of the numerical predictions by taking a high-lift low-pressure turbine cascade as the research object. Discussion was focused on the interaction mechanisms between the upstream wakes and secondary flow within the high-lift low-pressure turbine. The results indicated that upstream wakes have both positive and negative effects on the endwall flow, where the periodic wakes can decrease significantly the size of the separation bubble, prevent the formation of secondary vorticity structures at relatively high Reynolds numbers (100,000 and 150,000), and reduce the cross-passage pressure gradient of cascade. In addition, periodic wakes can improve the cascade incidence characteristic in terms of reducing the overturning and underturning of the secondary flow at downstream of the cascade all of which are beneficial for decreasing the endwall secondary losses, whereas more endwall boundary layer is involved in the main flow passage due to the wake transport, resulting in increased strength of the secondary flow at low Reynolds number of 25,000 and 50,000. Compared with the results without wakes, the total pressure loss for unsteady condition at the cascade exit decreases by 2.7% and 6.1% at high Reynolds number of 100,000 and 150,000, respectively. However, the secondary loss at unsteady flow conditions increases at low Reynolds number of 25,000 and 50,000.

Experimental and Numerical Characterization of Flow Through Highly Loaded Low-Pressure Turbine Cascade

An increase in loading of low-pressure turbine blades beyond the present state of the art will directly translate into lighter, smaller, and lower-cost turbine engines. Advances in aerodynamic modeling have permitted the design of highly loaded low-pressure turbine blades with good midspan loss performance. Earlier research has shown that forward loading can mitigate the low-Reynolds-number performance lapse at midspan but also results in increased secondary losses. Experiments and simulations were carried out to characterize the flow through a highly loaded linear cascade with front-loaded profiles at a Reynolds number based on axial chord and inflow velocity of 100,000. Endwall and suction surface oil-flow visualizations were obtained and particle image velocimetry measurements were taken in several axial planes inside and downstream of the cascade. Good agreement of the experimental and numerical data was achieved with respect to the trajectory of the passage vortex and the suction-side corner flow as well as the total pressure loss coefficient. A detailed analysis of the simulation data revealed a pronounced suction-side corner separation that can be characterized by streamwise vortices that have the same sense of rotation as the passage vortex.

Surface pressure characteristics of a highly loaded turbine blade at design and off-design conditions; a CFD methodology

The flow field passing through a highly loaded low pressure (LP) turbine cascade is numerically investigated at design and off-design conditions. The Field Operation And Manipulation (OpenFOAM) platform is used as the computational Fluid Dynamics (CFD) tool. In this regard, the influences of grid resolution on the results of k-e, k-ω, and large-eddy simulation (LES) turbulence models are investigated and compared with those of experimental measurements. A numerical pressure undershoot is appeared near the end of blade pressure surface which is sensitive to grid resolution and flow turbulence modeling. The LES model is able to resolve separation on both coarse and fine grid resolutions. In addition, the off-design flow condition is modeled by negative and positive inflow incidence angles. The numerical experiments show that a separation bubble generated on blade pressure side is predicted by LES. The total pressure drop has also been calculated at incidence angles between -20° and +8°. The minimum total pressure drop is obtained by k-ω and LES at design point.