Linear Turbine Cascade Research Articles

Turbine Blade Tip Leakage Flow Control by Unsteady Periodic Wakes of Upstream Blade Row

An experimental and numerical study of tip leakage flow control in axial turbine was performed by means of unsteady periodic wakes of upstream blade row. The flow field was investigated respectively in linear turbine cascade and turbine rotor. Focus was placed on the analysis of unsteady interaction mechanism between upstream periodic wakes and tip leakage vortices. The influence of the upstream periodic wakes on turbine performance was also discussed. The results indicate that the upstream periodic wakes can effectively reduce the strength of the tip leakage vortices. The loss measurements and computations at the exit of the turbine blade showed a decrease in stagnation pressure loss or entropy rise and an increase in efficiency. It means the engine specific fuel consumption and the exhaust gas temperature margin can be improved as well as increased turbine total efficiency.

20605 超高負荷直線タービン翼列内の流れに関する研究 : 入射角および翼端間隙の影響

An increase of turbine blade loading resulting in the downsizing and the decrease of weight of gas turbines can be achieved by the increase of blade turning angle. However, the aerodynamic performance of turbine cascade may be significantly deteriorated by the strengthened secondary flow. In this study, the oil-flow visualization and the blade surface static pressure measurements were performed for ultra-highly loaded linear turbine cascade with the turning angle of 160 degree in order to clarify the effects of the incidence angle and the tip-clearance on the secondary flow behavior. The present experimental results showed that the increase of incidence angle strengthens the horseshoe vortex and passage vortex, but not so influences the behavior of leakage vortex.

Numerical and experimental investigation of the turbulent flow through a low‐pressure turbine cascade in the endwall region

AbstractIn the present work, direct numerical simulations (DNS) of the flow through a low‐pressure linear turbine cascade T106 with parallel endwalls were conducted to investigate the effects of unsteady passing wakes of the upstream blade row on the secondary flow in the endwall region of the passage. The impact of the wakes on the secondary flow is discussed by means of the time‐averaged values. Furthermore, the results of DNS are compared with experimental data. (© 2013 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Global Approach to Turbomachinery Flow Control: Passage Vortex Control

A flow control scheme was implemented in a low-pressure turbine cascade that simultaneously mitigated profile and endwall losses using midspan vortex generator jets (VGJs) and endwall suction. The combined system had an approximate zero-net mass flux. During the design, a theoretical model was used that effectively predicted the trajectory of the passage vortex using inviscid results obtained from two-dimensional computational fluid dynamics (CFD). The model was used in the design of two flow control approaches: the removal and redirection approaches. The emphasis of the removal approach was the direct application of flow control along the passage vortex (PV) trajectory. The redirection approach attempted to alter the trajectory of the PV with the judicious placement of suction holes. A potential flow model was created to aid in the design of the redirection approach. The model results were validated using flow visualization and particle image velocimetry (PIV) in a linear turbine cascade. Detailed total pressure loss wake surveys were measured while matching the suction and VGJ mass flow rates for the removal and redirection approaches at ReCx = 25,000 and blowing ratio, B, of 2. When compared with the no control results, the addition of VGJs and endwall suction reduced the wake losses by 69% (removal) and 68% (redirection). The majority of the total pressure loss reduction resulted from the spanwise VGJs, while the suction schemes provided modest additional reductions (<2%). At ReCx = 50,000, the endwall control effectiveness was assessed for a range of suction rates without midspan VGJs. Area-averaged total pressure loss reductions of up to 28% were measured in the wake at ReCx = 50,000, B = 0, with applied endwall suction (compared to no suction at ReCx = 50,000), at which point the loss core of the PV was almost completely eliminated.

Cooling performance analysis of steam cooled gas turbine nozzle guide vane

As a new kind of advanced cooling technique, steam cooling has been applied in modern high temperature gas turbine blade cooling for improving the turbine efficiency. The superheated steam is selected as coolant to replace traditional compressor air as closed loop steam cooling for the internal convective cooling. This paper experimentally and computationally investigates the cooling performance of internal steam convective cooling in a nozzle guide vane with five smooth radial cooling ducts. Experiments are conducted on a linear turbine cascade at exit Mach numbers of 0.9, and exit Reynolds number of 1.2×106. Temperature and static pressure on the cooled vane surface are measured at the mid span for a range of coolant-to-mainstream temperatures ratio and coolant-to-mainstream mass flow ratio. The numerical investigations using the conjugate calculation technique are also performed to predict the complex three dimensional flow and heat transfer. The k–ω based Shear–Stress-Transport (SST) model is selected as the turbulence model. It can be found that the numerical results of vane temperature are underestimated compared with experimental data, especially at the trailing edge. The coolant steam has much higher cooling effectiveness than air, about 12%. The cooling effectiveness at the vane middle chord region is much higher than that at the leading and trailing region, by approximately 50% and 20%, respectively, which will lead to great temperature gradient and thermal stresses at the leading and trailing region. Therefore, more complicated cooling configuration besides convective cooling may be necessitated for this vane.

Computational Study of High-Lift Low-Pressure Turbine Cascade Aerodynamics at Low Reynolds Number

This study uses a Reynolds-averaged Navier–Stokes finite volume flow solver to simulate the flowfields around a two-dimensional linear turbine cascade model at a Reynolds number of 25,000. Three blade profiles have been simulated, including the aft-loaded Pack B, which has a nominal Zweifel loading coefficient equal to 1.15, the midloaded L1M (), and the front-loaded L2F (). All three blade profiles are known to be susceptible to varying degrees of laminar flow separation along the suction surface. Turbulence models used, which to the authors’ knowledge have been applied for the first time here, are the Abe–Kondoh–Nagano linear low- as well as the Kato–Launder modification. Time-accurate simulations, including fully laminar computations, are compared with experimental data and higher-order computations to judge the accuracy of the results, where it is shown that Reynolds-averaged Navier–Stokes simulations with appropriate turbulence modeling can produce both quantitatively and qualitatively similar separation behavior as seen in experiments. However, large-scale vortical motions predicted by the turbulence models and laminar solver advect downstream and instigate flow reattachment, a result the authors believe would be attenuated to some degree by three-dimensional vortex breakdown. Results collectively show that characterization of transition to turbulence as judged by analysis of the Reynolds shear stress is not sufficient in two dimensions, and further analysis including spectral methods may be necessary to better predict transition at low Reynolds number.

21003 トランスピレーション冷却を有する直線タービン動翼列内の流れの数値解析(GS-20【流体機械】)

21003 トランスピレーション冷却を有する直線タービン動翼列内の流れの数値解析(GS-20【流体機械】)

146 超高負荷直線タービン翼列内の流れに関する実験的研究 : 翼形状と入射角の影響(流体工学III,一般講演)

146 超高負荷直線タービン翼列内の流れに関する実験的研究 : 翼形状と入射角の影響(流体工学III,一般講演)

Application of Endwall Contouring to Transonic Turbine Cascades: Experimental Measurements at Design Conditions

An experimental investigation of the endwall flows in two high-turning turbine cascades was presented by Taremi et al. (2010, “Measurements of Endwall Flows in Transonic Linear Turbine Cascades: Part II—High Flow Turning,” ASME Conf. Proc., GT2010-22760, pp. 1343–1356). Endwall contouring was subsequently implemented in these cascades to control the secondary flows and reduce the total pressure losses. The current paper presents experimental results from these cascades to assess the effectiveness of endwall contouring in the transonic flow regime. The results include blade loadings, total pressure losses, streamwise vorticity and secondary kinetic energy distributions. In addition, surface flow visualization results are presented in order to interpret the endwall limiting streamlines within the blade passages. The flat-endwall and contoured-endwall cascades produce very similar midspan loading distributions and profile losses, but exhibit different secondary flows. The endwall surface flow visualization results indicate weaker interaction between the secondary flows and the blade suction surface boundary layers in the contoured cascades. Overall, the implementation of endwall contouring results in smaller and less intense vortical structures, and the reduction of the associated secondary kinetic energy (SKE) and exit flow angle variations. However, the mass-averaged losses at the main measurement plane, located 40% axial chord lengths downstream of the cascade (1.4CX), do not corroborate the numerically predicted improvements for the contoured cascades. This is in part attributed to slower mixing rates of the secondary flows in the compressible flow regime. The mass-averaged results at 2.0CX, on the other hand, show smaller losses for the contoured configurations associated with smaller SKE dissipation downstream of the cascades. Accordingly, the mixed-out row losses also show improvements for the contoured cascades.

Measurements of Losses and Reynolds Stresses in the Secondary Flow Downstream of a Low-Speed Linear Turbine Cascade

Experimental measurements of the mean and turbulent flow field were preformed downstream of a low-speed linear turbine cascade. The influence of turbulence on the production of secondary losses is examined. Steady pressure measurements were collected using a seven-hole pressure probe and the turbulent flow quantities were measured using a rotatable x-type hotwire probe. Each probe was traversed downstream of the cascade along planes positioned at three axial locations: 100%, 120%, and 140% of the axial chord (Cx) downstream of the leading edge. The seven-hole pressure probe was used to determine the local total and static pressure as well as the three mean velocity components. The rotatable x-type hotwire probe, in addition to the mean velocity components, provided the local Reynolds stresses and the turbulent kinetic energy. The axial development of the secondary losses is examined in relation to the rate at which mean kinetic energy is transferred to turbulent kinetic energy. In general, losses are generated as a result of the mean flow dissipating kinetic energy through the action of viscosity. The production of turbulence can be considered a preliminary step in this process. The measured total pressure contours from the three axial locations (1.00, 1.20, and 1.40Cx) demonstrate the development of the secondary losses. The peak loss core in each plane consists mainly of low momentum fluid that originates from the inlet endwall boundary layer. There are, however, additional losses generated as the flow mixes with downstream distance. These losses have been found to relate to the turbulent Reynolds stresses. An examination of the turbulent deformation work term demonstrates a mechanism of loss generation in the secondary flow region. The importance of the Reynolds shear stresses to this process is explored in detail.

Effects of freestream turbulence on bypass transition of separated boundary layer on low-pressure turbine airfoils

This paper presents experimental studies on bypass transition of separated boundary layer on low-pressure turbine airfoils, focusing on the effects of freestream turbulence on the transition process. Hot-wire probe measurements are performed on the suction side of an airfoil in the low-pressure linear turbine cascade at several Reynolds number conditions. Freestream turbulence is enhanced by use of turbulence grid located upstream of the cascade. The results of this experimental study show that the location of boundary layer separation does not strongly depend on the freestream turbulence level. However, as the freestream turbulence level increases, the size of separation bubble becomes small and the location of turbulent transition moves upstream. The size of separation bubble becomes small as the Reynolds number increases. At low freestream turbulence intensity, the velocity fluctuation due to Kelvin-Helmholtz instability is observed clearly in the shear layer of the separation bubble. At high freestream turbulence intensity, the streak structures appear upstream of the separation location, indicating bypass transition of attached boundary layer occurs at high Reynolds number.

Effects of an Upstream Cavity on the Secondary Flow in a Transonic Turbine Cascade

This paper examines experimentally the effects of an upstream cavity on the flow structures and secondary losses in a transonic linear turbine cascade. The cavity approximates the endwall geometry resulting from the platform overlap at the interface between stationary and rotating turbine blade rows. Previous investigations of the effects of upstream cavity geometries have been conducted mainly at low-speed conditions. The present work aims to extend such research into the transonic regime with a more engine representative upstream platform geometry. The investigations were carried out in a blow-down type wind tunnel. The cavity is located at 30 % of axial chord from the leading edge, extends 17 % of axial-chord in depth, and is followed by a smooth ramp to return the endwall to its nominal height. Two cascades are examined for the same blade geometry: the baseline cascade with a flat endwall and the cascade with the cavity endwall. Measurements were made at the design incidence and the outlet design Mach number of 0.80. At this condition, the Reynolds number based on outlet velocity is about 600,000. Off-design outlet Mach numbers of 0.69, and 0.89 were also investigated. Flowfield measurements were carried out at 40 % axial-chord downstream of the trailing edge, using a seven-hole pressure probe, to quantify losses and identify the flow structures. Additionally, surface flow visualization using an ultra-violet reactive dye was employed at the design Mach number, on the endwall and blade surfaces, to help in the interpretation of the flow physics. The experimental results also include blade-loading distributions, and the probe measurements were processed to obtain total-pressure loss coefficients, and streamwise vorticity distributions. It was found that the presence of the upstream cavity noticeably altered the structure and the strength of the secondary flow. Some effect on the secondary losses was also evident, with the cavity having a larger effect at the higher Mach number.

Unsteady inflow effects on the wake shed from a high-lift LPT blade subjected to boundary layer laminar separation

An experimental investigation on the near and far wake of a cascade of high-lift low-pressure turbine blades subjected to boundary layer separation over the suction side surface has been carried out, under steady and unsteady inflows. Two Reynolds number conditions, representative of take-off/landing and cruise operating conditions of the real engine, have been tested. The effect of upstream wake-boundary layer interaction on the wake shed from the profile has been investigated in a three-blade large-scale linear turbine cascade. The comparison between the wakes shed under steady and unsteady inflows has been performed through the analysis of mean velocity and Reynolds stress components measured at midspan of the central blade by means of a two-component crossed miniature hot-wire probe. The wake development has been analyzed in the region between 2% and 100% of the blade chord from the central blade trailing edge, aligned with the blade exit direction. Wake integral parameters, half-width and maximum velocity defects have been evaluated from the mean velocity distributions to quantify the modifications induced on the vane wake by the upstream wake. Moreover the thicknesses of the two wake shear layers have been considered separately in order to identify the effects of Reynolds number and incoming flow on the wake shape. The self-preserving state of the wake has been looked at, taking into account the different thicknesses of the two shear layers. The evaluation of the power density spectra of the velocity fluctuations allowed the study of the wake unsteady behavior, and the detection of the effects induced by the different operating conditions on the trailing edge vortex shedding.

123 トランスピレーション冷却を有する直線タービン静翼列内の流れの数値解析(流体工学II,一般講演)

123 トランスピレーション冷却を有する直線タービン静翼列内の流れの数値解析(流体工学II,一般講演)

Effects of turbine blade tip shape on total pressure loss and secondary flow of a linear turbine cascade

Leakage flow through the turbine blade tip clearance causes a strong leakage vortex near the suction side of the blade. The leakage vortex induces aerodynamics losses and a high heat transfer coefficient on the suction side of the near blade tip region. In this study, the effects of blade tip shape on the secondary flow and total pressure loss in a turbine cascade have been investigated experimentally. A total of 11 tip shapes including plane, single or double squealer, dimpled, chamfered, and grooved tips were tested. Tip clearances of 1.5% and 2.3% of the blade span were tested. The Reynolds number, based on the blade chord and cascade exit velocity, was 2.48×105. The secondary flow and total pressure were measured by a five-hole probe at one-chord downstream from the blade trailing edge. The total pressure loss coefficient increased as the tip clearance increased, and the effect of tip shape on secondary flow and total pressure loss was significant. Among the tested tip shapes, the double squealer tip and the grooved along pressure side tip performed best and both tips showed smaller total pressure loss coefficients than that of the plane tip.

Potential of specification of swirl in axisymmetric CFD methods for turbine blade aerodesign

A time-marching model for complete axial flow turbomachinery is adopted, which replaces the blades with hub-to-shroud streamsurfaces. The main effects of the real blades on the flow are modelled by blade forces. The inverse method seeks the hub-to-shroud geometry that produces a specified distribution of swirl in the blade regions. Swirl drives the spanwise distribution of specific rotor work. Euler equations are solved in the meridional plane using an upwind finite-volume scheme. The time-marching algorithm includes an evolutionary equation for each hub-to-shroud surface, which is solved by upwind finite differences. Since swirl is a monotonic function of the outflow angle, its specification allows the inverse method to converge more naturally and effectively than previous approaches to high-deflection solutions of the axisymmetric problem. A satisfactory agreement is found in comparing the numerical solution to (i) a theoretical solution for a linear turbine cascade and (ii) a conventional throughflow solution and a three-dimensional computational fluid dynamics (CFD) solution for a two-stage turbine.

Influence of Chord Length and Inlet Boundary Layer on the Secondary Losses of Turbine Blades

In this paper, results concerning the influence of chord length and inlet boundary layer thickness on the endwall loss of a linear turbine cascade are discussed. The investigations were performed in a low speed cascade tunnel using the turbine profile T40. The turning of 90 deg and 70 deg, the velocity ratio in the cascade from 1.0 to 3.5 as well as the chord length of 100 mm, 200 mm, and 300 mm were specified. In a measurement distance of one chord behind the cascade in main flow direction, an approximate proportionality of endwall loss and chord was observed in a wide range of velocity ratios. At small measurement distances (e.g., s2/l=0.4), this proportionality does not exist. If a part of the flow path within the cascade is approximately incorporated, a proportionality to the chord at small measurement distances can be obtained, too. Then, the magnitude of the endwall loss mainly depends on the distance in main flow direction. At velocity ratios near 1.0, the influence of the chord decreases rapidly, while at a velocity ratio of 1.0, the endwall loss is independent of the chord. By varying the inlet boundary layer thickness, no correlation of displacement thickness and endwall loss was achieved. A calculation method according to the modified integral equation by van Driest delivers the wall shear stress. Its influence on the endwall loss was analyzed.

Upstream wake–secondary flow interactions in the endwall region of high-loaded turbines

A detailed experimental and numerical investigation of the unsteady interaction of secondary flow vortices in turbine endwall region was performed with the effect of upstream periodic wakes. The flow field was investigated respectively in a linear turbine cascade and a turbine rotor. The study revealed the physical mechanisms of unsteady interaction between upstream wake and secondary vortices. The influence of the upstream wake on the performance of turbine endwall region was also discussed. The flow field at the exit of the turbine blade row showed a decrease in passage vortex strength and loss due to the upstream wake transport. Two interaction mechanisms are proposed whereby passage vortex loss decreases. They are the upstream wake–pressure side leg of the horseshoe vortex interaction and the upstream wake–passage vortex interaction. The transport of upstream wake can suppress the development of pressure side leg of the horseshoe vortex and passage vortex because of the “negative jet” influence of the wake.

Tip gap height effects on the aerodynamic performance of a cavity squealer tip in a turbine cascade in comparison with plane tip results: part 1—tip gap flow structure

Tip gap height effects on the flow structure over a cavity squealer tip have been investigated in a linear turbine cascade for power generation, in comparison with the corresponding plane tip results. Oil film flow visualizations are conducted on the tip surface and casing wall for tip gap height-to-chord ratios of h/c = 1.0, 2.0, and 3.0%. The squealer tip has a recessed cavity enclosed by a full length squealer with its rim height-to-chord ratio of 5.51%. The results show that most of in-coming fluid entering the tip gap inlet for the cavity squealer tip is entrapped by the suction-side squealer rim, and the cavity fluid is discharged into the blade flow passage over the suction-side squealer rim in the region from the mid-chord to the trailing edge. Regardless of h/c, the cavity squealer tip makes the leakage flow zone narrower than the plane tip, and is superior to the plane tip in reducing the tip leakage mass flow rate. A qualitative flow model describing full flow features over the cavity squealer tip is suggested. In this flow model, the tip gap exit area is classified into four different regions, and the tip gap height effects on the discharge characteristics in each region are discussed in detail.

Tip gap height effects on the aerodynamic performance of a cavity squealer tip in a turbine cascade in comparison with plane tip results: part 2—aerodynamic losses

Tip gap height effects on aerodynamic losses downstream of a cavity squealer tip have been investigated in a linear turbine cascade for power generation, in comparison with plane tip results. Three-dimensional flow fields are measured with a five-hole probe for tip gap height-to-chord ratios of h/c = 0.5, 1.0, 1.5 and 2.0%. The cavity squealer tip has a full length squealer with its rim height-to-chord ratio of 5.51%. For a fixed value of h/c, the tip leakage vortex for the cavity squealer tip is always weaker than that for the plane tip, and the flow field in the passage vortex region for the cavity squealer tip is less influenced by the tip leakage flow than that for the plane tip. For the cavity squealer tip, there is no appreciable change in local aerodynamic loss with h/c in the passage vortex region, but local aerodynamic loss in the tip leakage vortex region increases with h/c. The roles of the cavity squealer tip in reducing aerodynamic loss in comparison with the plane tip case are twofold: (1) the cavity squealer tip decreases the leakage flow discharge in the region from the leading edge to the mid-chord, which leads to an aerodynamic loss reduction in the passage vortex region and (2) it also decreases the leakage flow discharge downstream of the mid-chord, which results in an aerodynamic loss reduction in the tip leakage vortex region.