Passage Vortex Research Articles

Experimental Investigation of Flow Structure and Nusselt Number in a Low-Speed Linear Blade Passage With and Without Leading-Edge Fillets

The potential of contouring the leading edge of a blade to control the development of the secondary flows in the blade passage and to reduce the thermal loading to the end wall is investigated experimentally. Fillets placed at the junctions of the leading edge and the end wall are used for contouring. Four different types of fillet profiles are tested in a low-speed linear cascade a Reynolds numbers of 233,000 based on the inlet velocity. Images of instantaneous smoke flow patterns show a smaller horseshoe vortex along the leading edge with the fillets. In the passage, the fillets cause the passage vortex to be located closer to the suction surface. Upstream of the throat, the normalized axial vorticity values for the passage vortex and the turbulence intensity levels are smaller with the fillets compared to the baseline. For the leading-edge fillet with a concave profile, the end-wall Nusselt number distributions show significant reductions compared to the baseline.

Vortex-Wake-Blade Interaction in a Shrouded Axial Turbine

This paper presents time-resolved flow field measurements at the exit of the first rotor blade row of a two stage shrouded axial turbine. The observed unsteady interaction mechanism between the secondary flow vortices, the rotor wake and the adjacent blading at the exit plane of the first turbine stage is of prime interest and analyzed in detail. The results indicate that the unsteady secondary flows are primarily dominated by the rotor hub passage vortex and the shed secondary flow field from the upstream stator blade row. The analysis of the results revealed a roll-up mechanism of the rotor wake layer into the rotor indigenous passage vortex close to the hub endwall. This interesting mechanism is described in a flow schematic within this paper. In a second measurement campaign the first stator blade row is clocked by half a blade pitch relative to the second stator in order to shift the relative position of both stator indigenous secondary flow fields. The comparison of the time-resolved data for both clocking cases showed a surprising result. The steady flow profiles for both cases are nearly identical. The analysis of the probe pressure signal indicates a high level of unsteadiness that is due to the periodic occurrence of the shed first stator secondary flow field.

Influence of Blade Leading Edge Geometry on Turbine Endwall Heat (Mass) Transfer

The secondary flows, including passage and other vortices in a turbine cascade, cause significant aerodynamic losses and thermal gradients. Leading edge modification of the blade has drawn considerable attention as it has been shown to reduce the secondary flows. However, the heat transfer performance of a leading edge modified blade has not been investigated thoroughly. Since a fillet at the leading edge blade is reported to reduce the aerodynamic loss significantly, the naphthalene sublimation technique with a fillet geometry is used to study local heat (mass) transfer performance in a simulated turbine cascade. The present paper compares Sherwood number distributions on an endwall with a simple blade and a similar blade having a modified leading edge by adding a fillet. With the modified blades, a horseshoe vortex is not observed and the passage vortex is delayed or not observed for different turbulence intensities. However, near the blade trailing edge the passage vortex has gained as much strength as with the simple blade for low turbulence intensity. Near the leading edge on the pressure and the suction surface, higher mass transfer regions are observed with the fillets. Apparently the corner vortices are intensified with the leading edge modified blade.

Fan-Shaped Hole Effects on the Aero-Thermal Performance of a Film-Cooled Endwall

The present paper investigates the effects of a fan-shaped hole endwall cooling geometry on the aero-thermal performance of a nozzle vane cascade. Two endwall cooling geometries with four rows of holes were tested, for different mass flow rate ratios: the first configuration is made of cylindrical holes, whereas the second one features conical expanded exits and a reduced number of holes. The experimental analysis is mainly focused on the variations of secondary flow phenomena related to different injection rates, as they have a strong relationship with the film cooling effectiveness. Secondary flow assessment was performed through downstream 3D aerodynamic measurements, by means of a miniaturized 5-hole probe. The results show that at high injection rates, the passage vortex and the 3D effects tend to become weaker, leading to a strong reduction of the endwall cross flow and to a more uniform flow in spanwise direction. This is of course obtained at the expense of a significant increase of losses. The thermal behavior was then investigated through the analysis of adiabatic effectiveness distributions on the two endwall configurations. The wide-banded thermochromic liquid crystals (TLC) technique was used to determine the adiabatic wall temperature. Using the measured distributions of film-cooling adiabatic effectiveness, the interaction between the secondary flow vortices and the cooling jets can be followed in good detail all over the endwall surface. Fan-shaped holes have been shown to perform better than cylindrical ones: at low injection rates, the cooling performance is increased only in the front part of the vane passage. A larger improvement of cooling coverage all over the endwall is attained with a larger mass flow rate, about 1.5% of core flow, without a substantial increase of the aerodynamic losses.

An Empirical Prediction Method for Secondary Losses in Turbines—Part I: A New Loss Breakdown Scheme and Penetration Depth Correlation

Despite its wide use in meanline analyses, the conventional loss breakdown scheme is based on a number of assumptions that are known to be physically unsatisfactory. One of these assumptions states that the loss generated in the airfoil surface boundary layers is uniform across the span. The loss results at high positive incidence presented in a previous paper (Benner, M. W., Sjolander, S. A., and Moustapha, S. H., 2004, ASME Paper No. GT2004-53786.) indicate that this assumption causes the conventional scheme to produce erroneous values of the secondary loss component. A new empirical prediction method for secondary losses in turbines has been developed, and it is based on a new loss breakdown scheme. In the first part of this two-part paper, the new loss breakdown scheme is presented. Using data from the current authors’ off-design cascade loss measurements, it is shown that the secondary losses obtained with the new scheme produce a trend with incidence that is physically more reasonable. Unlike the conventional loss breakdown scheme, the new scheme requires a correlation for the spanwise penetration depth of the passage vortex separation line at the trailing edge. One such correlation exists (Sharma, O. P., and Butler, T. L., 1987, ASME J. Turbomach., 109, pp. 229–236.); however, it was based on a small database. An improved correlation for penetration distance has been developed from a considerably larger database, and it is detailed in this paper.

Investigation for Loss Generation Mechanisms of Flow in Turbomachinery by Usin Curved Square Duct (3rd Report, Influence of Curved Angle)

In a series of present studies, the flows within the stationary curved ducts are analyzed numerically with the aerodynamic or the geometrical parameters affecting the loss generation caused by the passage vortex within a passage of a turbomachinery. In the former reports, the inlet boundary layer thickness and the inlet velocity distortion were taken as the aerodynamic parameter and the aspect ratio of the cross section was chosen as the geometrical parameter. In this report, the effects of the curved angle of the bend on the loss generation caused by the passage vortex are examined by relating the bend of curved duct to the blade to blade surface of an axial flow turbine cascade. The curved angle is changed from 90 to 160 degree by 10 degree with fixing the length of arc for the mean radius of curvature of the bend or fixing the mean radius of curvature.

Effect of vane-rotor interaction on the unsteady flowfield downstream of a transonic high pressure turbine

This paper analyses the effect of the stator-rotor interference on the stage exit flow field of a transonic turbine operated under engine representative conditions in the von Karman Institute (VKI) compression tube facility. The test programme comprises three pressure ratios and two Reynolds numbers. The time-averaged radial distributions of total pressure and temperature downstream of the rotor are affected by the hub passage vortices and the interaction between the tip passage vortex and the rotor tip leakage vortex. The azimuthal distributions exhibit significant non-uniformities with a periodicity of one vane pitch. The amplitude of this non-uniformity is very sensitive to the pressure ratio. A simple model shows that, contrary to the common belief, the transport of the vane wake and secondary flows across the rotor is not enough to explain the magnitude of the variations. The pitch-wise vane exit static pressure distribution, which is significantly distorted owing to the transonic regime of the vane, should be taken into account. The amplitude of the time-resolved fluctuations of pressure and temperature, which are due to rotor blade passing events, also varies significantly depending on the probe location in a vane pitch. The measured time-accurate rotor relative inlet total pressure suggests that the rotor relative exit undergoes periodic excursions in the transonic regime.

Investigation of a Novel Secondary Flow Feature in a Turbine Cascade With End Wall Profiling

A novel secondary flow feature, previously unreported for turbine blading as far as the authors are aware, has been discovered. It has been found that it is possible to separate part of the inlet boundary layer on the blade row end wall as it is being over-turned and rolled up into the passage vortex. This flow feature has been discovered during a continuing investigation into the aerodynamic effects of non-axisymmetric end wall profiling. Previous work, using the low speed linear cascade at Durham University, has shown the potential of end wall profiling for reducing secondary losses. The latest study, the results of which are described here, was undertaken to determine the limits of what end wall profiling can achieve. The flow has been investigated in detail with pressure probe traversing and surface flow visualization. This has found that the inlet boundary locally separates, on the early suction side of the passage, generating significant extra loss which feeds directly into the core of the passage vortex. The presence of this new feature gives rise to the unexpected result that the secondary flow, as determined by the exit flow angle deviations and levels of secondary kinetic energy, can be reduced while at the same time the loss is increased. CFD was found to calculate the secondary flows moderately well compared with measurements. However, CFD did not predict this new feature, nor the increase in loss it caused. It is concluded that the application of non-axisymmetric end wall profiling, although it has been shown to be highly beneficial, can give rise to adverse features that current CFD tools are unable to predict. Improvements to CFD capability are required in order to be able to avoid such features, and obtain the full potential of end wall profiling.

349 曲がりダクトによるターボ機械内部の損失生成機構の解明(G05-6 流体機械(1),G05 流体工学)

In a passage of turbomachinery, the complex secondary flow is produced by the effects of the centrifugal forces induced by the passage curvature as well as its rotation and the Coriolis force. The passage vortex (PV) in the secondary flow generates a major part of the losses. Curved ducts are considered to be fundamental models to clarify the behavior of PV. In this study, the analyses of the flow within curved duct are performed with the numerical technique in order to examine the effects of the rotational speed on the formation of the passage vortex by relating the curvature of a curved duct to that on the meridional plane of a centrifugal impeller. The computed results revealed the relationships among the centrifugal forces induced by the passage curvature, the Coriolis force caused by the rotation and the formation of PV.

끝틈새가 선회각이 큰 터빈 동익 익렬 후류영역에서의 3차원유동 및 압력손실에 미치는 영향

The effect of tip clearance height on the three-dimensional flow and aerodynamic loss in the wake region of a high-turning turbine rotor cascade has been investigated with a miniature cone-type five-hole probe. Distributions of velocity magnitude, secondary velocity vectors, and total-pressure loss coefficient are presented for three tip gap-to-span ratios of h/s = 0.0, 0.5 and 1.0 percent. The result shows that with the increment of h/s, tip leakage vortex tends to be intensified and aerodynamic loss due to the leakage vortex is increased as well. In the case of h/s = 1.0 percent, aerodynamic loss in the tip-leakage flow region is found dominant in comparison with that in the passage vortex region. With increasing h/s, mass-averaged secondary loss coefficient has a greater portion in the mass-averaged total-pressure loss coefficient.

Vortex Transport and Blade Interactions in High Pressure Turbines

This paper presents a study of the three-dimensional flow field within the blade rows of a high-pressure axial flow steam turbine stage. Half-delta wings were fixed to a rotating hub to simulate an upstream rotor passage vortex. The flow field is investigated in a low-speed research turbine using pneumatic and hot-wire probes downstream of the blade row. The paper examines the impact of the delta wing vortex transport on the performance of the downstream blade row. Steady and unsteady numerical simulations were performed using structured three-dimensional Navier-Stokes solver to further understand the flow field. The loss measurements at the exit of the stator blade showed an increase in stagnation pressure loss due to the delta wing vortex transport. The increase in loss was 21% of the datum stator loss, demonstrating the importance of this vortex interaction. The transport of the stator viscous flow through the rotor blade row is also described. The rotor exit flow was affected by the interaction between the enhanced stator passage vortex and the rotor blade row. Flow underturning near the hub and overturning towards the midspan was observed, contrary to the classical model of overturning near the hub and underturning towards the midspan. The unsteady numerical simulation results were further analyzed to identify the entropy producing regions in the unsteady flow field.

The Influence of Leading-Edge Geometry on Secondary Losses in a Turbine Cascade at the Design Incidence

The paper presents detailed experimental results of the secondary flows from two large-scale, low-speed linear turbine cascades. The aerofoils for the two cascades were designed for the same inlet and outlet conditions and differ mainly in their leading-edge geometries. Detailed flow field measurements were made upstream and downstream of the cascades using three and seven-hole pressure probes and static pressure distributions were measured on the aerofoil surfaces. All measurements were made exclusively at the design incidence. The results from this experiment suggest that the strength of the passage vortex plays an important role in the downstream flow field and loss behavior. It was concluded that the aerofoil loading distribution has a significant influence on the strength of this vortex. In contrast, the leading-edge geometry appears to have only a minor influence on the secondary flow field, at least for the design incidence.

Secondary Flows and Loss Caused by Blade Row Interaction in a Turbine Stage

A study of the three-dimensional stator-rotor interaction in a turbine stage is presented. Experimental data reveal vortices downstream of the rotor which are stationary in the absolute frame—indicating that they are caused by the stator exit flowfield. Evidence of the rotor hub passage vortices is seen, but additional vortical structures away from the endwalls, which would not be present if the rotor were tested in isolation, are also identified. An unsteady computation of the rotor row is performed using the measured stator exit flowfield as the inlet boundary condition. The strength and location of the vortices at rotor exit are predicted. A formation mechanism is proposed whereby stator wake fluid with steep spanwise gradients of absolute total pressure is responsible for all but one of the rotor exit vortices. This mechanism is then verified computationally using a passive-scalar tracking technique. The predicted loss generation through the rotor row is then presented and a comparison made with a steady calculation where the inlet flow has been mixed out to pitchwise uniformity. The loss produced in the steady simulation, even allowing for the mixing loss at inlet, is 10% less than that produced in the unsteady simulation. This difference highlights the importance of the time-accurate calculation as a tool of the turbomachine designer.

Experimental and Numerical Investigation of the Unsteady Surface Pressure in a Three-Stage Model of an Axial High Pressure Turbine

This paper presents the results of a numerical and experimental investigation of the unsteady pressure field in a three-stage model of a high pressure steam turbine. Unsteady surface pressure measurements were taken on a first and second stage stator blade, respectively. The measurements in the blade passage were supplemented by time resolved measurements between the blade rows. The explanation of the origin of the unsteady pressure fluctuations was supported by unsteady three-dimensional computational fluid dynamic calculations of which the most extensive calculation was performed over two stages. The mechanisms affecting the unsteady pressure field were: the potential field frozen to the upstream blade row, the pressure waves originating from changes in the potential pressure field, the convected unsteady velocity field, and the passage vortex of the upstream blade row. One-dimensional pressure waves and the unsteady variation of the pitchwise pressure gradient due to the changing velocity field were the dominant mechanisms influencing the magnitude of the surface pressure fluctuations. The magnitude of these effects had not been previously anticipated to be more important than other recognized effects.

Investigation for Loss Generation Mechanisms of Flow in Turbomachinery by Using Curved Square Duct (2nd Report, Influence of Cross-Sectional Aspect Ratio)

In the secondary flow within a passage of a turbomachinery, the passage vortex generates a major part of the losses. In a series of studies, the passage vortex is closely analyzed by the numerical computations using curved ducts as fundamental models for the generation of the passage vortex. The flows in the curved ducts are examined by relating the bend of curved duct to the blade-to-blade surface of an axial flow turbine cascade and to the meridional plane of a centrifugal impeller. The cross-sections of passage of a turbomachinery have various aspect ratios. In this study, the cross-sectional aspect ratio is chosen as the major parameter affecting the loss generation cuased by the passage vortex. The computed results clarified that the decrease of the aspect ratio strengthened the passage vortex and consequently, increased the loss caused by the passage vortex.

Influences of Blade Bowing on Flowfields of Turbine Stator Cascades

To understand the influences of blade positive and negative bowing on flowfields of turbine stator cascades with low aspect ratio, six sets of cascades with different bowed angles of the blades were designed and tested. A five-hole pitot microprobe, which was located upstream and downstream of the cascades, was used to survey the flowfields in detail. The results were different from other published results with a different kind of turbine stator blade profile. When a blade has positive or negative bowing, the passage vortices do not change, whereas the characteristics of the trailing vortices, such as the intensity, locations, size, and structure, do change distinctly. In the case of blade positive bowing, the local spanwise distribution of the yaw flow angle at the cascade exit is significantly affected, with very little changes in the cascade overall loss.

Numerical investigation of loss generation mechanisms of flow in turbomachinery by using curved square duct

The secondary flow within a passage of turbomachinery exhibits a complex flow pattern by the effect of the centrifugal and the Coriolis forces. The passage vortex in this secondary flow generates a major part of the losses. However, the mechanism of the loss generation has not been fully clarified yet. In this point of view, the passage vortex is closely examined by the computational method using the two-dimensional curved square ducts as fundamental models. The inlet boundary layer thickness and the inlet velocity distortion are considered to be the major parameters affecting the generation of passage vortex in the present study. The computed results revealed that the passage vortex gave the predominant effects for the generation of loss not only in the breakdown process but also in the development process.

Secondary flow effects in relatively narrow channels

Secondary flow effects were discussed in numerous papers at the past ISAIF Symposia, mainly in connection with turbine or compressor cascades[1]. This paper will complement these papers by looking at the problem from the channel (or blade passages) geometry point of view. If we describe as secondary flows any flows in planes perpendicular to the main flow direction, then there are at least three kinds of secondary flows in a typical turbine rotor cascade: Quite often all the secondary flow vortices merge downstream into a passage vortex with a non-negligible contribution to the channel (cascade) losses, and it is worth investigating the individual contributions to these losses to take them into account in the design procedure.

Local Mass/Heat Transfer on Turbine Blade Near-Tip Surfaces

Local mass transfer measurements on a simulated high-pressure turbine blade are conducted in a linear cascade with tip clearance, using a naphthalene sublimation technique. The effects of tip clearance (0.86–6.90% of chord) are investigated at an exit Reynolds number of 5.8×105 and a low turbulence intensity of 0.2%. The effects of the exit Reynolds number 4−7×105 and the turbulence intensity (0.2 and 12.0%) are also measured for the smallest tip clearance. The effect of tip clearance on the mass transfer on the pressure surface is limited to 10% of the blade height from the tip at smaller tip clearances. At the largest tip clearance high mass transfer rates are induced at 15% of curvilinear distance Sp/C by the strong acceleration of the fluid on the pressure side into the clearance. The effect of tip clearance on the mass transfer is not very evident on the suction surface for curvilinear distance of Ss/C<0.21. However, much higher mass transfer rates are caused downstream of Ss/C≈0.50 by the tip leakage vortex at the smallest tip clearance, while at the largest tip clearance, the average mass transfer is lower than that with zero tip clearance, probably because the strong leakage vortex pushes the passage vortex away from the suction surface. High mainstream turbulence level (12.0%) increases the local mass transfer rates on the pressure surface, while a higher mainstream Reynolds number generates higher local mass transfer rates on both near-tip surfaces.

Numerical study on flow characteristics at blade passage and tip clearance in a linear cascade of high performance turbine blade

A numerical analysis has been conducted in order to simulate the characteristics of complex flow through linear cascades of high performance turbine blade with/without tip clearance by using a pressure-correction based, generalized 3D incompressible Navier-Stokes CFD code. The development and generation of horseshoe vortex, passage vortex, leakage vortex, tip vortex within tip clearance, etc. are clearly identified through the present simulation which uses the RNG k-e turbulent model with wall function method and a second-order linear upwind scheme for convective terms. The present simulation results are consistent with the generally known tendency that occurs in the blade passage and tip clearance. A 3D model for secondary and leakage flows through turbine cascades with/without tip clearance is also suggested from the present simulation results, including the effects of tip clearance height.