Passage Vortex Research Articles

Combustor Turbine Interface Studies—Part 2: Flow and Thermal Field Measurements

Most turbine inlet flows resulting from the combustor exit are nonuniform in the near-platform region as a result of cooling methods used for the combustor liner. These cooling methods include injection through film-cooling holes and injection through a slot that connects the combustor and turbine. This paper presents thermal and flow field measurements in the turbine vane passage for a combustor exit flow representative of what occurs in a gas turbine engine. The experiments were performed in a large-scale wind tunnel facility that incorporates combustor and turbine vane models. The measured results for the thermal and flow fields indicate a secondary flow pattern in the vane passage that can be explained by the total pressure profile exiting the combustor. This secondary flow field is quite different than that presented for past studies with an approaching flat plate turbulent boundary layer along the upstream platform. A counter-rotating vortex that is positioned above the passage vortex was identified from the measurements. Highly turbulent and highly unsteady flow velocities occur at flow impingement locations along the stagnation line.

An Experimental and Computational Study of the Formation of a Streamwise Shed Vortex in a Turbine Stage

Shear layers shed by aircraft wings roll up into vortices. A similar, though far less common, phenomenon can occur in the wake of a turbomachine blade. This paper presents experimental data from a new single-stage turbine that has been commissioned at the Whittle Laboratory. Two low-aspect ratio stators have been tested with the same rotor row. Surface flow visualization illustrates the extremely strong secondary flows present in both NGV designs. These secondary flows lead to conventional passage vortices, but also to an intense vortex sheet which is shed from the trailing edge of the blades. Pneumatic probe traverses show how this sheet rolls up into a concentrated vortex in the second stator design, but not in the first. A simple numerical experiment is used to model the shear layer instability and the effects of trailing edge shape and exit yaw angle distribution are investigated. It is found that the latter has a strong influence on shear layer rollup: inhibiting the formation of a vortex downstream of NGV 1 but encouraging it behind NGV 2.

Investigation for Loss Generation Mechanisms of Flow in Turbomachinery by Using Curved Square Duct (1st Report, Influence of Inlet Velocity Distribution)

The secondary flow within a passage of a turbomachinery exhibits a complex flow pattern by the effects of the centrifugal and the Coriolis forces. In this secondary flow, the passage vortex generates a major part of the losses. However, the mechanism of the loss generation by the secondary flow has not been fully clarified yet. In this point of view, the passage vortex is closely examined by the numerical computations using the curved square ducts as fundamental models for the generation of the passage vortex. The inlet boundary layer thickness and the inlet velocity profiles are chosen as the major parameters affecting the generation of passage vortex in the present study. The computed results exhibited 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.

Numerical Simulation on the Flow Field in a Turbine Stage with Upstream Flow Injection from the Outer Casing: Effects of the Injection Angle.

Flow injection is being used in steam turbines for some combined or geothermal plants in order to increase its total thermal efficiency. It can be easily imagined, however, that such a flow injection is accompanied by additional aerodynamic loss due to the flow mixing or the change in local flow angle. Preliminary three-dimensional steady numerical simulations are conducted for the flow field inside the chamber from which the secondary flow is injected. The results showed a small variation of the flow velocity at the injection slot and provided useful information concerning the magnitude of the pitchwise variation of the injection angle. Further numerical simulations in a turbine stage were then performed by prescribing different injection angle distribution at the injection slot. The turbine stage computations revealed the existence of a high loss region generated from the interaction of the injected flow and the passage vortex near the tip of the nozzle vanes. It was found that injection angles orientated to the suction side of the stator provided lower loss values, while cases where flow was injected to the pressure side minimized the flow angle deviation.

Influence of the state of the inlet endwall boundary layer on the interaction between pressure surface separation and endwall flows

This paper describes the effect of the state of the inlet boundary layer (laminar or turbulent) on the structure of the endwall flow on two different profiles of low-pressure (LP) turbine blades (solid thin and hollow thick). At present the state of the endwall boundary layer at the inlet of a real LP turbine is not known. The intention of this paper is to show that, for different designs of LP turbine, the state of the inlet boundary layer affects the performance of the blade in very different ways. The testing was completed at low speed in a linear cascade using area traversing, flow visualization and static pressure measurements. The paper shows that, for a laminar inlet boundary layer the two profiles have a similar loss distribution and structure of endwall flow. However, for a turbulent inlet boundary layer the two profiles are shown to differ significantly in both the total loss and endwall flow structure. The pressure side separation bubble on the solid thin profile is shown to interact with the passage vortex, causing a higher endwall loss than that measured on the hollow thick profile.

Predictions of External Heat Transfer for Turbine Vanes and Blades With Secondary Flowfields

Detailed heat transfer distributions on the endwall and along the vane/blade surface are essential for component mechanical integrity and life predictions. Due to secondary flows, high gradients in heat transfer are present at the endwall and at the vane or blade surface itself where the passage vortex influences the mainstream flow. This paper documents the benchmarking of three turbulence models: 1) k-ε realizable with wall functions, 2) k-ε realizable with two layer model, and 3) the V2F model for endwall and surface heat transfer and flowfield predictions. Benchmark experimental data from a scaled-up low speed rig for both a stator and rotor geometry are used for comparisons of heat transfer and flowfield. While the k-ε realizable turbulence models give a good prediction of the secondary flow pattern, the heat transfer at the endwall and at the surface is not well predicted due to the inadequate modeling of near wall turbulence. The V2F model gives better agreement with the experiments on the endwall and vane midspan heat transfer is also well predicted, although transition occurs too far upstream on the suction surface. The results from this study represent the feasibility of CFD utilization as a predictive tool for local heat transfer distributions on a vane/blade endwall.

블레이드 앞전 3차원 형상 변형에 의한 터빈 캐스케이드 내의 이차유동 제어

The blade leading edge is modi fied to control the secondary flow generated in the turbine cascade with fence by intensifying the suction side branch of the horseshoe vortex. The incompressible Navier-Stokes equations are numerically solved with a high Reynolds number k- € turbulence closure model for investigating the vortical flows in the turbine cascade. The computational results of total pressure loss coefficients in the wake region are first compared with experiments for validation. The structure and strength of the passage vortex near the suction surface are examined by testing various geometrical parameters of the turbine blade leading edge.<br/>

Topology and vortex structures of a curving turbine cascade with tip clearance (II)—Topological flow pattern and vortex structure in the transverse section of a blade cascade

By means of ink trace visualization of the flows in conventional straight, positively curved and negatively curved cascades with tip clearance, and measurement of the aerodynamic parameters in transverse section, and by appling topology theory, the topological structures and vortex structure in the transverse section of a blade cascade were analyzed. Compared with conventional straight cascade, blade positive curving eliminates the separation line of the upper passage vortex, and leads the secondary vortex to change from close separation to open separation, while blade negative curving effects merely the positions of singular points and the intensities and scales of vortex.

Topology and vortex structures of a curving turbine cascade with tip clearance (I)—Experimental model and topological flow patterns on both endwalls and blade surfaces

By means of ink trace visualization of the flows in conventional straight, positively curved and negatively curved cascades with tip clearance, and measurement of the aerodynamic parameters in the transverse section, and by appling topology theory, the structures on both endwalls and blade surfaces were analyzed. Compared with conventional straight cascade, blade positive curving eliminates the separation line of the upper passage vortex and leads the secondary vortex to change from close separation to open separation, while blade negative curving effects merely the positions of singular points and the intensities and scales of vortex.

터빈 로터의 익단 간극이 성능에 미치는 영향에 대한 수치해석적 연구

The effect of tip clearance is important part for turbomachinery performance. Tip leakage flow due to tip clearance is mixed with passage vortex. Large amount of loss is generated at the mixing region and overall performance of turbomachinery is reduced. Numerical calculation of the lst stage rotor of GE7FA gas turbine is carried out to investigate tip clearance effect on performance, pitchwise variations of velocity profiles, pressure distributions and loss coefficients. A commercial code, CFX-TascFlow is validated in this study.

Transonic Rotor/Stator Interaction Using a 3-D Linearised Unsteady Viscous Flow Model

The paper first summarizes the forced response problem in turbomachinery and reviews various numerical methods for the simulation of unsteady flows. A particular technique, based on the linearisation of the unsteady Favre-averaged Navier-Stokes equations on three-dimensional mixed-element grids of tetrahedra, hexahedra and wedges, is described in some detail. The methodology was applied to a NGV/rotor interaction benchmark case for which detailed steady and unsteady flow measurements are available. The steady-state flow, calculated using a non-linear viscous representation, was described in detail with emphasis on features such as separation, horseshoe and passage vortices, tip leakage and shock structure since these are likely to influence the unsteady flow. The sources of unsteadiness on the rotor passage were evaluated from the steady-state solution at the NGV outlet. The disturbances were split into vortical, entropic and potential waves, the Fourier components of which were considered separately. The summation of the vortical and entropic waves was used as a rotor inlet boundary condition in order to assess the wake/rotor unsteady interaction. Similarly, potential waves were used to study the potential/rotor interaction. The results obtained from these two types of unsteady interactions were superimposed and compared with experimental data. Good qualitative and, in most cases, quantitative agreement was obtained, a finding which suggests that the unsteady flowfield generated by the relative blade motion can be considered to be a quasi-linear phenomenon for the particular HP turbine studied. Finally, the mechanisms of wake/rotor and potential/rotor interactions were studied in some detail and it was concluded that the former was strong in the crown of the blade while the latter was dominant in the leading edge region.

Secondary flow and forced convection heat transfer near endwall boundary layer fences in a 90° turning duct

The present study deals with the secondary flow and heat transfer aspects of endwall boundary layer fences in 90° turning ducts. Boundary layer fences have recently been re-introduced as additional components to achieve favorable aerothermal effects in turbine/compressor passages and internal cooling systems. The turbine passage or internal coolant channel is simulated by a 90° turning duct ( Re D=360,000) to study the aerothermal interaction of the boundary layer fence and the passage flow that is dominated by the passage vortices. Specifically, a single boundary layer fence of varying dimensions is attached to a heated endwall of the duct. The current study adds to the currently small number of investigations on the use of endwall boundary layer fences and their aerothermal interaction with the passage flow system.

Two-equation turbulence models for prediction of heat transfer on a transonic turbine blade

Two versions of the two-equation k– ω model and a shear stress transport (SST) model are used in a three-dimensional, multi-block, Navier–Stokes code to compare the detailed heat transfer measurements on a transonic turbine blade. It is found that the SST model resolves the passage vortex better on the suction side of the blade, thus yielding a better comparison with the experimental data than either of the k– ω models. However, the comparison is still deficient on the suction side of the blade. Use of the SST model does require the computation of distance from a wall, which for a multi-block grid, such as in the present case, can be complicated. However, a relatively easy fix for this problem was devised. Also addressed are issues such as (1) computation of the production term in the turbulence equations for aerodynamic applications, and (2) the relation between the computational and experimental values for the turbulence length scale, and its influence on the passage vortex on the suction side of the turbine blade.

Blade-Row Interaction in a High-Pressure Turbine

This paper presents a study of the three-dimensional e owe eld within the blade rows of a single-stage highpressure axial turbine (low-speed, large-scale ). Measurements have been performed in the stationary and rotating frames of reference. Time-mean data have been obtained using e ve-hole pneumatic probes. The transport mechanisms of the stator wake and passage vortices through the rotor blade row have been studied using smoke e ow visualization. Furthermore, unsteady measurements have been carried out using a three-axis hot wire. Steady and unsteady numerical simulations have been performed using a structured three-dimensional Navier ‐Stokes solver to further understand the blade-row interactions. The transport of the stator viscous e ow through the rotor blade-row is described. The rotor passage vortices are affected by the transport of the stator secondary e ow. It is observed that the stator secondary e ow vortices are convected through the downstream rotor blade-row in a similar but not identical way to the wake. At the hub the kinematic interaction between the stator and the rotor passage vorticeshas two effects. First,thesuction sideleg of the statorpassagevortex isdisplaced radially upwards over the developing rotor passage vortex at the hub. Additionally, the pressure side leg of the stator passage vortex is entrained into the rotor passage vortex. The predicted e owe eld was interrogated from the perspective of loss production. The contribution of the unsteady e ow to the stage loss has been evaluated using unsteady numerical simulations. The effect of stator viscous e ow transport on the rotor e ow angles is also discussed in brief. Finally, a kinematic model is proposed for the transport of the secondary-e ow vortices in the downstream blade-row based on the understanding obtained from the measurements and numerical simulations.

Measurements of Endwall Heat(Mass) Transfer Coefficient in a Linear Turbine Cascade Using Naphthalene Sublimation Technique

Heat (mass) transfer characteristics have been investigated on the endwall of a large-scale linear turbine cascade. Its profile is based on the mid-span of the first-stage rotor blade in a industrial gas turbine. By using the naphthalene sublimation technique, local heat (mass) transfer coefficients are measured for two different free-stream turbulence intensities of 1.3% and 4.7%. The results show that local heat (mass) transfer Stanton number is widely varied on the endwall, and its distribution depends strongly on the three-dimensional vortical flows such as horseshoe vortices, passage vortex, and corner vortices. From this experiment, severe heat loads are found on the endwall near the blade suction side as well as near the leading and trailing edges of the blade. In addition, the effect of the free-stream turbulence on the heat (mass) transfer is also discussed in detail.

Investigation of internal flow in ultra - highly loaded turbine cascade by PIV method

The highly loaded turbine blades are able to reduce both the number of blades and the stages of turbojet-engines. In this study, PIV(Particle Image Velocimetry) method is used for the measurements of the secondary flow in ultra-highly loaded turbine blade cascades. The results obtained by the PIV method clearly show the complicated behavior of the secondary flow in the cascade. The horseshoe vortex and the passage vortex are observed inside the cascade. Moreover, the wake generated by the accumulation of the low energy fluid by the passage vortex near the suction side and that discharged toward downstream of the trailing edge has been recognized.

Reduction of Secondary Flow Losses in Turbine Cascades by Leading Edge Modifications at the Endwall

Experimental results are presented which show the influence on the secondary flow and its losses by a profile modification of the leading edge very close to the endwall. The investigation was carried out with a well-known turbine profile that originally was developed for highly loaded low pressure turbines. The tests were done in a low speed cascade wind tunnel. The geometrical modification was achieved by a local thickness increase; a leading edge endwall bulb. It was expected that this would intensify the suction side branch of the horse-shoe (hs-) vortex with a desirable weakening effect on the passage vortex. The investigated configuration shows a reduction of secondary losses by 2.1 percent points that represents approximately 50 percent of these losses compared to the reference profile. Detailed measurements of the total pressure field behind the cascade are presented for both the reference and the modified profile. The influence of the modified hs-vortex on the overall passage vortex can be clearly seen. The results of a numerical analysis are compared with the experimental findings. A numerical analysis shows that the important details of the experimental findings can be reproduced. Quantitative values are locally different. The theoretical approach taken cannot yet be used for an exact prediction of the loss reduction. However, the analysis of the interaction and the resulting tendencies are considered to be valid. Hence, theoretical investigations as a guideline for the design of a leading edge bulb at the endwall are a valuable tool.

Tip Clearance Effects in a Turbine Rotor: Part I—Pressure Field and Loss

This paper presents an experimental investigation of the effects of the tip clearance flow in an axial turbine rotor. The effects investigated include the distribution and the development of the pressure, the loss, the velocity, and the turbulence fields. These flow fields were measured using the techniques of static pressure taps, rapid response pressure probes, rotating five-hole probes, and Laser Doppler Velocimeter. Part I of this paper covers the loss development through the passage, and the pressure distribution within the passage, on the blade surfaces, on the blade tip, and on the casing wall. Regions with both the lowest pressure and the highest loss indicate the inception and the trace of the tip leakage vortex. The suction effect of the vortex slightly increases the blade loading near the tip clearance region. The relative motion between the turbine blades and the casing wall results in a complicated pressure field in the tip region. The fluid near the casing wall experiences a considerable pressure difference across the tip. The highest total pressure drop and the highest total pressure loss were both observed in the region of the tip leakage vortex, where the loss is nearly twice as high as that near the passage vortex region. However, the passage vortex produces more losses than the tip leakage vortex in total. The development of the loss in turbine rotor is similar to that observed in cascades. Part II of this paper covers the velocity and the turbulence fields.

Secondary Flow Measurements in a Turbine Passage With Endwall Flow Modification

A flow modification technique is introduced in an attempt to allow increased turbine inlet temperatures. A large-scale two half-blade cascade simulator is used to model the secondary flow between two adjacent turbine blades. Various flow visualization techniques and measurements are used to verify that the test section replicates the flow of an actual turbine engine. Two techniques are employed to modify the endwall secondary flow, specifically the path of the passage vortex. Six endwall jets are installed at a location downstream of the saddle point near the leading edge of the pressure side blade. These wall jets are found to be ineffective in diverting the path of the passage vortex. The second technique utilizes a row of 12 endwall jets whose positions along the centerline of the passage are based on results from an optimized boundary layer fence. The row of jets successfully diverts the path of the passage vortex and decreases its effect on the suction side blade. This can be expected to increase the effectiveness of film cooling in that area. The row of jets increases the aerodynamic losses in the passage, however. Secondary flow measurements are presented showing the development of the endwall flow, both with and without modification. [S0889-504X(00)01004-7]

Three-Dimensional Flow Analysis of Turbine Blade Cascades with Leading-Edge Ejection

The results of two different cooling cone gurations in a turbine cascade, one with slot ejection and one with shower-head ejection, are presented. Experimental and numerical results of the pressure distribution and the three-dimensional e owe eld are compared. Thenumerical results correspond well with the experimental data. The investigations are performed for e lm-cooled cases with a blowing ratio of M = 1.1. It can be shown that complete discretization of the cooling channels and the plenum is necessary to obtain good aerodynamic results of the main e ow because the complex e ow structure in the ejection channels and the plenum has a signie cant ine uence on the calculated aerodynamic behavior of the blades. In the leading-edge area, the interaction of the cooling e uid jets and the main e ow leads to mixture vortices and e ow separations. Furthermore, the ine uence of secondary e ow phenomena on the distribution of the cooling e uid along the blade surface is presented. On the suction side, the passage vortex leads to a displacement of cooling e uid in the direction of the midspan.