Passage Vortex Research Articles

Study on leakage flow characteristics of radial inflow turbines at rotor tip clearance

Tip clearance leakage flow in a radial inflow turbine rotor for microturbines under the stage environment is investigated using a three-dimensional viscous flow simulation. The results indicate that the scraping flow caused by relative motion between casing and rotor tip, and the pressure difference between pressure side and suction side at rotor tip, play important roles in tip clearance leakage flow. The more the rotor tip speed increases and tip clearance height decreases, the more the scraping effect acts. Though the leakage velocity of tip clearance at midsection and exducer regions changes less when the rotor rotational speed is changing, the distance between passage vortex and rotor suction side varies in evidence. Main leakage flow rate of tip clearance takes place at region of exducer tip and some seal configurations will be quite effective for cutting leakage flow if these configurations are arranged over midsection and exducer of the radial inflow rotor.

Numerical simulation of tip clearance flow passive control in axial turbine

This paper focuses on the effects of five different passive turbine tip clearance flow control methods on the tip clearance flow physics, which consists of a partial suction side squealer tip, a double squealer tip, a pressure side tip shelf with inclined squealer tip on a double squealer tip, a tip platform extension edge in pressure side and in suction side respectively. A pressure-correction based, 3D Reynolds-averaged Navier-Stokes equations CFD code with Reynolds Stress Model was adopted. The variable specific heat was considered. The detailed tip clearance flow field with different squealer rims was described with the streamline and the velocity vector. Accordingly, the mechanisms of five passive controls were elucidated; the effects of the passive controls on turbine efficiency and tip clearance flow field were illuminated. The results showed that the secondary flow loss near the outer casing including the tip leakage losses and the passage vortex losses could be reduced in all the five passive control methods. The turbine efficiency could be increased via the rational passive turbine tip clearance flow control. The Improved PS Squealer had the best effect on turbine efficiency, and the efficiency increased by 0.215%.

Film Cooling Effectiveness Distribution on a Gas Turbine Blade Platform With Inclined Slot Leakage and Discrete Film Hole Flows

A five-blade, linear cascade is used to experimentally investigate turbine blade platform cooling. A 30deg inclined slot upstream of the blades is used to model the seal between the stator and rotor, and 12 discrete film holes are located on the downstream half of the platform for additional cooling. The film cooling effectiveness is measured on the platform using pressure sensitive paint (PSP). Using PSP, it is clear that the film cooling effectiveness on the blade platform is strongly influenced by the platform secondary flow through the passage. Increasing the slot injection rate weakens the secondary flow and provides more uniform film coverage. Increasing the freestream turbulence level was shown to increase film cooling effectiveness on the endwall, as the increased turbulence also weakens the passage vortex. However, downstream, near the discrete film cooling holes, the increased turbulence decreases the film cooling effectiveness. Finally, combining upstream slot flow with downstream discrete film holes should be cautiously done to ensure coolant is not wasted by overcooling regions on the platform.

Optimized Shroud Design for Axial Turbine Aerodynamic Performance

This paper presents a comprehensive study of the effect of shroud design in axial turbine aerodynamics. Experimental measurements and numerical simulations have been conducted on three different test cases with identical blade geometry and tip clearances but different shroud designs. The first and second test cases are representative of a full shroud and a nonaxisymmetric partial shroud geometry while the third test case uses an optimized partial shroud. Partial shrouds are sometimes used in industrial application in order to benefit from the advantage of shrouded configuration, as well as reduce mechanical stress on the blades. However, the optimal compromise between mechanical considerations and aerodynamic performances is still an open issue due to the resulting highly three-dimensional unsteady flow field. Aerodynamic performance is measured in a low-speed axial turbine facility and shows that there are clear differences between the test cases. In addition, steady and time resolved measurements are performed together with computational analysis in order to improve the understanding of the effect of the shroud geometry on the flow field and to quantify the sources of the resultant additional losses. The flow field analysis shows that the effect of the shroud geometry is significant from 60% blade height span to the tip. Tip leakage vortex in the first rotor is originated in the partial shroud test cases while the full shroud case presents only a weak indigenous tip passage vortex. This results in a significant difference in the secondary flow development in the following second stator with associated losses that varies by about 1% in this row. The analysis shows that the modified partial shroud design has improved considerably the aerodynamic efficiency by about 0.6% by keeping almost unchanged the overall weight of this component, and thus blade root stresses. The work, therefore, presents a comprehensive flow field analysis and shows the impact of the shroud geometry in the aerodynamic performance.

Control of Rotor Tip Leakage Through Cooling Injection From the Casing in a High-Work Turbine

This paper presents an experimental investigation of a novel approach for controlling the rotor tip leakage and secondary flow by injecting cooling air from the stationary casing onto the rotor tip. It contains a detailed analysis of the unsteady flow interaction between the injected air and the flow in the rotor tip region and its impact on the rotor secondary flow structures. The experimental investigation has been conducted on a one-and-1/2-stage, unshrouded turbine, which has been especially designed and built for the current investigation. The turbine test case models a highly loaded, high pressure gas turbine stage. Measurements conducted with a two-sensor fast-response aerodynamic probe have provided data describing the time-resolved behavior of flow angles and pressures, as well as turbulence intensity in the exit plane of the rotor. Cooling air has been injected in the circumferential direction at a 30 deg angle from the casing tangent, opposing the rotor turning direction through a circumferential array of ten equidistant holes per rotor pitch. Different cooling air injection configurations have been tested. Injection parameters such as mass flow, axial position, and size of the holes have been varied to see the effect on the rotor tip secondary flows. The results of the current investigation show that with the injection, the size and the turbulence intensity of the rotor tip leakage vortex and the rotor tip passage vortex reduce. Both vortices move toward the tip suction side corner of the rotor passage. With an appropriate combination of injection mass flow rate and axial injection position, the isentropic efficiency of the stage was improved by 0.55 percentage points.

Effects of squealer rim height on aerodynamic losses downstream of a high-turning turbine rotor blade

The effects of squealer rim height on three-dimensional flows and aerodynamic losses downstream of a high-turning turbine rotor blade have been investigated for a typical tip gap-to-chord ratio of h/ c = 2.0%. The squealer rim height-to-chord ratio is changed to be h st/ c = 0.00 (plane tip), 1.37, 2.75, 5.51, and 8.26%. Results show that as h st/ c increases, the tip leakage vortex tends to be weakened and the interaction between the tip leakage vortex and the passage vortex becomes less severe. The squealer rim height plays an important role in the reduction of aerodynamic loss when h st/ c ⩽ 2.75%. In the case of h st/ c ⩾ 5.51%, higher squealer rim cannot provide an effective reduction in aerodynamic loss. The aerodynamic loss reduction by increasing h st/ c is limited only to the near-tip region within a quarter of the span from the casing wall.

Effects of Nozzle Secondary Vortices on Unsteady Hub-Endwall Flow of a Turbine Rotor

The unsteady flow field of an axial-flow turbine rotor was investigated experimentally using an LDV system. Detailed measurements of the time-averaged and time-dependent distributions of the velocity, the flow angle, and the turbulence intensity were carried out. The obtained data was analyzed from the viewpoints of both absolute (stationary) frame of reference and relative (rotating) frame of reference. This paper focuses on the effect of the turbine nozzle secondary vortices (a passage vortex and a trailing edge vortex) near the hub endwall on the flow field inside the rotor passage. The nozzle passage vortex are accumulated on the rotor suction surface because of both the negative jet effect of the nozzle passage vortex itself and the formation of the rotor passage vortex. The nozzle passage vortex induces large fluctuations of velocity and turbulence intensity inside the rotor.

Study on Secondary Flow within Low Aspect Ratio Steam Turbine Cascade (1st Report, Mechanism of Loss Generation)

In order to address recent environmental problems such as global warming due to CO2, emissions, continuous efforts are required to improve the performance of the steam turbine, which supports the base of energy conversion. Although the aerodynamic design technologies of steam turbine have been developed remarkably by applying the computational fluid dynamics (CFD), it is not easy to reduce the losses within a low aspect ratio cascade in the control stage and high-pressure stages even if the latest three-dimensional design are used. Then, the authors investigated the loss generation mechanism in a low aspect ratio cascade using the smoke visualization and detailed internal flow measurement with a micro pitot probe to develop the new technologies for reducing the secondary loss. This paper describes the evolution of horse-shoe vortex and passage vortex through a turbine blade in experimental studies, theoretical approach to explain the motion of the vortex and the effects of inlet boundary layer on passage vortex interaction analyzed by three-dimensional viscous flow analysis.

PIV Maps of Tip Leakage and Secondary Flow Fields on a Low-Speed Turbine Blade Cascade With Moving End Wall

New, detailed flow field measurements are presented for a very large low-speed cascade representative of a high-pressure turbine rotor blade with turning of 110deg and blade chord of 1.0m. Data were obtained for tip leakage and passage secondary flow at a Reynolds number of 4.0×105, based on exit velocity and blade axial chord. Tip clearance levels ranged from 0% to 1.68% of blade span (0% to 3% of blade chord). Particle image velocimetry was used to obtain flow field maps of several planes parallel to the tip surface within the tip gap, and adjacent passage flow. Vector maps were also obtained for planes normal to the tip surface in the direction of the tip leakage flow. Secondary flow was measured at planes normal to the blade exit angle at locations upstream and downstream of the trailing edge. The interaction between the tip leakage vortex and passage vortex is clearly defined, revealing the dominant effect of the tip leakage flow on the tip end-wall secondary flow. The relative motion between the casing and the blade tip was simulated using a motor-driven moving belt system. A reduction in the magnitude of the undertip flow near the end wall due to the moving wall is observed and the effect on the tip leakage vortex examined.

Computation of tip-leakage flow in a linear compressor cascade with a second-moment turbulence closure

Strong interaction among tip-leakage vortex, mainstream, passage vortex and turbulence in and behind the blade tip and casing in compressors is one of the major causes of energy losses, affecting the overall gas turbine performance. A number of experiments and recent large-eddy simulations of simplified generic tip-clearance configurations have substantially advanced the understanding of the complex vortical and turbulence structures and energy conversion in and behind a very narrow clearance space. However, the prediction of phenomena in real turbomachinery still relies on the one-point (Reynolds-averaged Navier–Stokes, RANS) Computational Fluid Dynamics which is still burdened by uncertainties related primarily to inadequate turbulence modelling, and also to insufficient accuracy of the numerical algorithms. In this paper we address both these issues by testing an advanced turbulence model and a numerical method in the computations of a tip-clearance flow in a three-dimensional linear compressor cascade. For closing the mean momentum equations we adopted a Second Moment Turbulence Closure (SMC) incorporating low- Re-number and wall proximity effects, which has earlier proved to reproduce accurately a series of generic two- and three-dimensional anisotropic and non-equilibrium flows relevant to turbomachinery. In order to diminish numerical uncertainties, the computations are performed using Petrov-Galerkin stabilized finite element code featuring equal order Q1–Q1 pairs. The solution scheme is based on an in-house parallel Overlapping Domain Decomposition approach featuring a FGMRes linear solver. The results, compared with experiments and the conventional linear low- Re-number eddy viscosity model demonstrate that the SMC here adopted is capable to reproduce all the phenomenological features related to tip-leakage flows, especially the evolution of the strong three-dimensionality and turbulence anisotropy, and can be considered a prospective model for predicting real turbomachinery flows.

Variation of Vortex Structure in a Compressor Cascade at Different Incidences

Based on experimental results, a numerical study was performed to analyze the flow pattern and the vortex structure in a compressor cascade at different incidences using the topological principles as well as the critical principles of the three-dimensional separation. The results show that the topology of the critical points on the suction surface varies with the incidence and that the separation type changes to closed separation from open separation. In addition, the passage vortex is a classical open separation. The horseshoe vortex and the comer vortex belong to closed separation. Finally, the authors propose a new model of the vortex structure in a compressor cascade.

Flow visualisation and heat/mass transfer in a linear turbine cascade with tip clearance

Flow visualisation and heat/mass transfer measurements in a linear turbine cascade with tip/casing clearances ranging from 0.60% to 6.90% of chord show the influence of blade tip geometry for a standard flat tip, a squealer tip and a geometry with a winglet on the pressure side and a squealer on the suction side of the blade. Oil-lampblack and oil-dot techniques show the surface flow on the blade endwall and tip while a laser light-sheet and a smoke wire indicate the three-dimensional flow in the tip clearance and blade passages. Local heat/mass transfer from the blade tips is measured using the naphthalene sublimation technique. Experiments with the standard flat tip blade clearly show that a separation bubble exists at the pressure side edge of the tip surface at all tip clearance levels. The effect of the tip leakage vortex on the secondary flows in the blade passage is also evident: with an increase of tip clearance, the horseshoe vortices disappear while the passage vortex is weakened and pushed away from the suction surface by a strengthened leakage vortex. The flow in the gap changes significantly for the squealer tip and the winglet-squealer tip cases. The surface flow visualisation identifies reattachment and recirculation regions on the blade tips. Photographic images, compared to detailed convective heat/mass transfer measurements, provide an interpretation of the heat/mass transfer results.

Stator-clocking effects on the unsteady interaction of secondary flows in a 1.5-stage unshrouded turbine

Effects of stator clocking on the unsteady interaction of stator and rotor secondary flows and performance are investigated in a 1.5-stage, high-pressure turbine. Due to the low aspect ratio design of the blade rows, vortical structures dominate the inlet flow field of the second stator row. Therefore the interaction of first stator and rotor secondary flows in relation to the stator-clocking position must be considered in order to achieve an optimized multi-stage performance. Four different stator-clocking positions are studied in this experimental investigation. The data comprise unsteady and steady probe measurements, which are acquired downstream of the rotor and the second stator blade row of a 1.5-stage, unshrouded turbine. A two-sensor fast response aerodynamic probe technique is used in the measurement campaign. It is found that multi-row interaction effects of vortical secondary flow features dominate the loss creation in turbine stages having low aspect ratio geometries. The periodic interaction of shed passage vortices of the first stator with secondary flow structures of the rotor creates regions characterized by reduced total pressure and increased turbulence, and entropy at fixed locations relative to the subsequent stator. Circumferential position of these interaction regions defines the spanwise distribution of total pressure loss in the downstream stator. Stator-clocking controls the spanwise distribution of loss in high-pressure turbines. However, the potential of reducing the loss is limited due to the three-dimensional nature of the flow, and hence, the non-uniform effect of a clocking position on the spanwise performance of a downstream stator.

Particle Image Velocimetry Measurements of the Three-Dimensional Flow in an Exhaust Hood Model of a Low-Pressure Steam Turbine

The three-dimensional flow structure inside an exhaust hood model of a low-pressure steam turbine was investigated using a particle image velocimetry (PIV) velocity field measurement technique. The PIV measurements were carried out in several selected planes under design operation conditions with simulated total pressure distribution and axial velocity profile. The mean flow fields revealed a complicated vortical flow structure and the major sources of energy loss. Vortices with different scales were observed inside the exhaust hood: a strong separation vortex (SV) behind the tip of the guide vane, a longitudinal vortex (LV) at the exhaust hood top, a large-scale passage vortex (PV) evolving throughout the flow path, and an end-wall vortex (EWV) in the region adjacent to the front end-wall. Both the SV and the large-scale PV seemed to consume large amounts of kinetic energy and reduce the pressure recovery ability. The results indicate that the steam guide vane and the bearing cone should be carefully designed so as to control the vortical flow structure inside the exhaust hood.

Investigation of the Flow Field in a High-Pressure Turbine Stage for Two Stator-Rotor Axial Gaps—Part II: Unsteady Flow Field

An extensive experimental analysis was carried out at Politecnico di Milano on the subject of unsteady flow in high pressure (HP) turbine stages. In this paper, the unsteady flow measured downstream of a modern HP turbine stage is discussed. Traverses in two planes downstream of the rotor are considered, and, in one of them, the effects of two very different axial gaps are investigated: the maximum axial gap, equal to one stator axial chord, is chosen to “switch off” the rotor inlet unsteadiness, while the nominal gap, equal to 1/3 of the stator axial chord, is representative of actual engines. The experiments were performed by means of a fast-response pressure probe, allowing for two-dimensional phase-resolved flow measurements in a bandwidth of 80kHz. The main properties of the probe and the data processing are described. The core of the paper is the analysis of the unsteady rotor aerodynamics; for this purpose, instantaneous snapshots of the rotor flow in the relative frame are used. The rotor mean flow and its interaction with the stator wakes and vortices are also described. In the outer part of the channel, only the rotor cascade effects can be observed, with a dominant role played by the tip leakage flow and by the rotor tip passage vortex. In the hub region, where the secondary flows downstream of the stator are stronger, the persistence of stator vortices is slightly visible in the maximum stator-rotor axial gap configuration, whereas in the minimum stator-rotor axial gap configuration their interaction with the rotor vortices dominates the flow field. A good agreement with the wakes and vortices transport models has been achieved. A discussion of the interaction process is reported giving particular emphasis to the effects of the different cascade axial gaps. Some final considerations on the effects of the different axial gap over the stage performances are reported.

Unsteady Flow Field of an Axial-Flow Turbine Rotor at a Low Reynolds Number

The unsteady flow field of an annular turbine rotor was investigated experimentally using a laser Doppler velocimetry (LDV) system. Detailed measurements of the time-averaged and time-resolved distributions of the velocity, flow angle, turbulence intensity, etc., were carried out at a very low Reynolds number condition, Reout=3.5×104. The data obtained were analyzed from the viewpoints of both an absolute (stationary) frame of reference and a relative (rotating) frame of reference. The effect of the turbine nozzle wake and secondary vortices on the flow field inside the rotor passage was clearly captured. It was found that the nozzle wake and secondary vortices are suddenly distorted at the rotor inlet, because of the rotating potential field of the rotor. The nozzle flow (wake and passage vortices) and the rotor flow (boundary layer, wake, tip leakage vortex, and passage vortices) interact intensively inside the rotor passage.

Experimental Investigation of Secondary Flow Structure in a Blade Passage With and Without Leading Edge Fillets

Velocity and pressure measurements are presented for a blade passage with and without leading edge contouring in a low speed linear cascade. The contouring is achieved through fillets placed at the junction of the leading edge and the endwall. Two fillet shapes, one with a linear streamwise cross-section (fillet 1) and the other with a parabolic cross-section (fillet 2), are examined. Measurements are taken at a constant Reynolds number of 233,000 based on the blade chord and the inlet velocity. Data presented at different axial planes include the pressure loss coefficient, axial vorticity, velocity vectors, and yaw and pitch angles. In the early stages of the development of the secondary flows, the fillets are seen to reduce the size and strength of the suction-side leg of the horseshoe vortex with associated reductions in the pressure loss coefficients and pitch angles. Further downstream, the total pressure loss coefficients and vorticity show that the fillets lift the passage vortex higher above the endwall and move it closer to the suction side in the passage. Near the trailing edge of the passage, the size and strength of the passage vortex is smaller with the fillets, and the corresponding reductions in pressure loss coefficients extend beyond the mid-span of the blade. While both fillets reduce pressure loss coefficients and vorticity, fillet 1 (linear fillet profile) appears to exhibit greater reductions in pressure loss coefficients and pitch angles.

Assessment of linear and non-linear two-equation turbulence models for aerothermal turbomachinery flows

Three linear two-equation turbulence models k − e, k − ω and k − 1 and a non-linear k − 1 model are used for aerodynamic and thermal turbine flow prediction. The pressure profile in the wake and the heat transfer coefficient on the blade are compared with experimental data. Good agreement is obtained with the linear k − 1 model. No significant modifications are observed with the non-linear model. The balance of transport equation terms in the blade wake is also presented. Linear and non-linear k − 1 models are evaluated to predict the three-dimensional vortices characterising the turbine flows. The simulations show that the passage vortex is the main origin of the losses.

Investigation for Loss Generation Mechanisms of Flow in Turbomachinery by Using Curved Square Duct

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 degrees by 10 degrees with fixing the length of arc for the mean radius of curvature of the bend or with fixing the mean radius of curvature.

Effect of Axial Spacing Between Stator and Rotor on Turbine Performance

Unsteady effect of rotor-stator interaction on turbine stage performance was investigated for two cases of axial spacings between stator and rotor, i.e. large and small axial spacings. Unsteady RANS simulations were performed to clarify several interesting features of the unsteady three-dimensional flow field in the turbine stage. Simulation results showed that the stator wake was convected from pressure side to suction side in the rotor. As a result, secondary flow was periodically generated by the wake passing through the rotor passage, and then the passage vortices in the rotor near the endwall were suppressed and fluctuated in the radial direction downstream of the rotor. It was found that turbine stage efficiency with the small gap was higher than that with the large gap because the wake entered the rotor before mixing.