Vane Cascade Research Articles

Endwall film-cooling in a cascade with filleted vane, upstream discrete slots, and holes

ABSTRACT In vane cascades, large filleted vanes influence aerodynamics, while upstream endwall film-cooling arrangement impacts aero-thermal performance. This study explores experimental investigations on endwall flows and film-cooling effectiveness in a filleted vane cascade using discrete slots and holes. Two fillet profiles are examined covering the leading-edge area to either the throat or 62% of the axial chord. Film-cooling employs two slots and two rows of holes in the leading-edge region with inlet blowing ratios ranging from 1.0 to 2.8. Results show variations in total pressure loss and Nusselt number between filleted and un-filleted vanes, highlighting optimal film-cooling effectiveness with the smaller fillet profiles.

Sensitivity of the endwall flow in a linear vane cascade to blade fillet geometry

Based on the blade chord and inlet velocity, the current computational study uses a linear vane cascade with a large filleted blade-endwall junction with a 2.01 x 105 Reynolds number. Three fillets with related profiles are explored. To evaluate the upshots of geometric differences in a fillet attached to the endwall flow-field, the height and endwall-width of the fillets are changed. The RANS k-ω turbulent model is used in the computations, and the results are compared to experimental results from a similar cascade without the fillet. The computed results of the secondary flow-field in the endwall region along the cascade are compared for baseline (no fillet) and filleted passages. As a result of diminished leading-edge and passage vortices, the fillets lower pitchwise pressure gradients, flow separation, axial vorticity, and overall pressure losses when compared to the baseline. The pros of fillets on endwall secondary flows are however unaffected by fillet's geometric changes.

Aerodynamic tunnel for tests of turbine annular cascades

Aerodynamic test rigs are necessary for experimental testing of turbomachines and investigation of possible ways to improve machine performance. Existing installations give higher losses and do not work efficiently at off-design operating modes. An operating part with an adjustable radial diffuser was designed in order to determine the characteristics of turbine annular cascades. Experimental studies and computational verifications showed satisfactory results at various operating modes. The regulated backwall of the radial diffuser ensured supersonic velocity values behind the cascade and overall stable operation in a wide range of Mach numbers up to 1.3 when using compressors of comparatively small capacity. The optimum positions of the regulated backwall were determined, which provided a deep vacuum behind the cascade, as well as 1.5 times Mach number increase compared to the turbine cascade without a diffuser. Changing the inlet channel geometry at supersonic modes leads to an increase in the diffuser efficiency. Additionally, it was determined that the use of the turbine vane cascade in the test rig flow path is not necessary during calculation studies, but instead an axisymmetric vaneless converging area can be applied and give satisfactory results as well as reduce the time spent on calculations. The computational model can be used to optimize the design of an aerodynamic tunnel outlet area.

Analysis of vortex characteristics in hump region of reversible pump-turbine based on omega vortex identification method

As the core equipment of pumped storage power stations, reversible pump-turbines will frequently switch between different working modes during operation, and it is easy to appear hump region under pump condition, which will greatly affect the performance of the pumped storage unit. Therefore, in order to explore the causes of the hump region, this paper takes the model reversible pump-turbine as the research object. First, the unit speed–unit flow characteristics are compared with the model test results under different working conditions. Then, based on the omega vortex identification method, the vortex distribution in the flow channel of the unit is analyzed. By analyzing the flow characteristics of the flow components of the unit under different flow rates in the hump region, the internal flow law of the hump region is revealed. It is found that when the flow rate decreases, the rising head cannot offset the head lost by the hydraulic loss, thus forming the hump region, and the unstable flow gradually appears in the guide vane and runner area. There are unstable phenomena, such as flow separation, in the guide vane area. There is a sudden change of radial pressure in the vaneless area between the guide vane and the runner; that is, the unstable vortex almost occupies the whole flow channel due to the change of pressure in this area, resulting in the deterioration of the instability of the unit operation. When the flow rate is large, the radial pressure mutation zone mainly exists between the guide vane cascades, making it easy to form an unstable vortex. The research results can provide a theoretical reference for improving the stability of reversible pump-turbines.

Computational and Experimental Investigation of the Intensity and Scale of Flow Turbulence Influence on Losses in a Vane Cascade

Computational and Experimental Investigation of the Intensity and Scale of Flow Turbulence Influence on Losses in a Vane Cascade

Performance Evaluation of Damaged Blades Using Numerical Simulation and Machine Learning

The formulation of maintenance strategies for gas turbines requires an accurate and rapid assessment of damaged blade performance. Variety and uncertainty in the three-dimensional geometric deformation of gas turbine vanes and blades increase the difficulty of degradation analysis. In this paper, the effects on the aerodynamic performance of partial damage to turbine nozzle guide vanes are investigated experimentally and numerically. A methodology for constructing a deformation–degradation model for high-pressure gas turbine vanes/blades is proposed based on machine learning. A database consisting of 670 numerical simulation results with various deformation patterns is constructed for the vane cascade. A mapping is constructed between the matrices defining geometric deformations and the total pressure loss coefficient of the cascades based on a convolutional neural network model. The accuracy of this deformation–degradation model is verified against experimental results. We find that deformation on the suction side of the blade has the most significant influence on aerodynamic performance. The deformation–degradation model constructed for a specific cascade provides a rapid evaluation of aerodynamic performance based on rough geometrical measurements without additional experiments or numerical simulations, providing a potential tool in the precise repair of gas turbines.

Numerical flow field investigation around guide vane of a high head Francis turbine

Guide vanes are the most eroded component of the Francis turbine, and flow instabilities around guide vanes influence the flow field at the runner inlet. Guide vane cascade can be considered an alternative method to investigate the flow field, which maintains flow similarity with the prototype turbine. The objective of this paper is to numerically analyze the flow field around the guide vane cascade. A single guide vane cascade numerical model is developed to perform the simulation using commercial software ANSYS 2022. Two turbulence models, shear stress transport k-ω and standard k-ε, perform steady-state simulation at the best efficiency point. The pressure and velocity distribution are obtained at mid-span, and around the pressure and suction sides of the guide vane. Numerical simulation on a scaled prototype full turbine has also been performed to measure the Francis turbine’s torque and efficiency. The modified Bhilangana- III guide vane profile is installed in the B-III hydropower plant in India and taken as a reference Francis turbine. The velocity and pressure distributions obtained around guide vanes of the scaled full turbine are compared with the velocity and pressure distributions of the single guide vane cascade. A difference of 7.25 % in maximum velocity and 9.40 % in maximum pressure at the mid-span of the guide vane is found between the guide vane cascade and scaled full turbine at the best efficiency point. The overall efficiency of the scaled turbine is also measured, and a difference of 0.83 % between the numerical result and scaled hill chart at the best efficiency point was found.

The Effects of Mainstream Reynolds Number and Blowing Ratio on Film Cooling of Gas Turbine Vanes

Film cooling performance was evaluated numerically for three mainstream Reynolds numbers and four blowing ratios (BR). A computational model based on finite volume discretization was used to solve an incompressible and transient flow over a NACA 4412 cascade vane. Several passive scalars were included in the model to evaluate the condition of adiabatic temperature and constant temperature for the surface vane. For the adiabatic temperature condition, the film effectiveness mainly depends on the jet trajectory and recirculation zones. For the constant temperature condition, the net heat flux reduction (NHFR) varies according to the boundary layer separation and reattachment. Consequently, misleading conclusions could be drawn if only one of the two approaches is adopted. For instance, the mainstream Reynolds number Re∞ = 3615 reached a maximum average effectiveness lower than 0.3 with an average NHFR of 0.25. However, for Re∞ = 10,845 the maximum average effectiveness was close to 0.45, but with negative average NHFR values. This finding demonstrates the need to explore new indicators like jet trajectory, convective coefficient and skin friction coefficient, as presented in this paper.

Upstream Endwall Film-Cooing in a Vane Cascade with Cylindrical Shape Holes

To overcome the disadvantages of cylindrical holes in film cooling, complex geometries of the fan-shaped diffused holes are employed in cascade investigations. The present experiment employs a new design of a diffused hole for film cooling that is formed by diffusing a cylindrical hole smoothly and only in the forward direction. The aerothermal performances in a linear vane cascade are compared between an array of simple cylindrical holes and an array of diffused-cylindrical holes by employing them in the cascade upstream endwall. The objectives are to increase the aerothermal performance of the cylindrical holes in the gas-turbine passage film cooling. The measurements of the temperature, velocity, flow angle, and total-pressure losses are obtained at the inlet Reynolds number of , as well as the coolant-to-mainstream density ratio of 1.0 and temperature ratios between 0.94 and 1.0. Four inlet blowing ratios of film-cooling flow are tested. The results show less coolant migration into the boundary layer and passage vortex for the diffused holes than for the cylindrical holes. The passage vortex becomes weaker, and the overall total-pressure losses at the passage exit are lower for the diffused holes. The local and average adiabatic film-cooling effectivenesses along the endwall are always higher for the diffused holes.

Experimental study on the outlet flow field and cooling performance of vane-shaped pre-swirl nozzles in gas turbine engines

The pre-swirl nozzles are a crucial part of gas turbine's cooling pre-swirl system, because they directly affect the cooling performance of the system and the life of the turbine blades. Vane-shaped pre-swirl nozzles, including cascade vane nozzles (VN) and vane-shaped hole nozzles (VSH) proposed in recent years, are reported to have higher system temperature drop potential. In this paper, the pre-swirl performance as well as the cooling effect of these two types of nozzles were studied and compared experimentally. A five-hole probe was used to measure the flow field at nozzle exits under the outlet conditions of Mach number 0.2 and 0.3, and the comparisons of global deviation angle and pre-swirl effectiveness were carried out. The variations of pre-swirl effectiveness (η) with aerodynamic parameters, such as pressure ratio and Reynolds number in the range of 3 × 104 to 8 × 105 were also obtained with the help from numerical simulations. The cooling performance of two nozzles were further tested by a system test rig with rotating parts downstream of the two nozzles. Total pressure and outlet flow angle at nozzle exit illustrate that VN have obvious boundary layer separation from the suction surface, and the secondary endwall flow loss is high due to their narrow flow passages. As a result, when Mach number is 0.2, the deviation angle for VN is about 3° and the pre-swirl effectiveness is only 0.77. Well-designed VSH have improved flow passages with appropriate height to pitch ratio and a smaller trailing edge, and they suppress boundary layer separation and secondary flow development. This contributes to a significantly lower deviation angle near 0°, and an increased pre-swirl effectiveness of 0.86. In addition, the pre-swirl effectiveness of these two vane shaped nozzles will first increase with Reynolds number and then tend to a constant at Reynolds number about 4 × 105 and beyond. The maximum effectiveness of VN is 0.85 and that of VSH is about 0.94 and is 10.6% higher. Meanwhile, the measured temperature drop effectiveness of the pre-swirl system with VSH reaches 0.49, achieving an 8.9% improvement over the system with VN at 0.45. These results are consistent with the theoretical linear relationship between system temperature drop and nozzle pre-swirl effectiveness. The investigations in this paper also conclude that the pre-swirl effectiveness of nozzles is a more accurate parameter in classifying the nozzle performance and system cooling effect than traditionally used discharge coefficient.

The Influence of Turbulence and Reynolds Number on Endwall Heat Transfer in a Vane Cascade

Abstract Endwall heat transfer measurements have been acquired in a vane cascade over a range of turbulence conditions and Reynolds numbers using an array of small commercial infrared (IR) cameras. The linear cascade was tested over five inlet turbulence conditions ranging from low turbulence (0.7%) to high turbulence (17.4%) and three exit chord Reynolds numbers ranging from 500,000 to 2,000,000. The small commercial IR cameras made by Therm-App have a resolution of 384 by 288 pixels and were connected to individual smartphones to record the images. The cascade was modified with small zinc selenide windows to provide IR access for the cameras. The five cameras were calibrated against a constant temperature test plate and the output images were adjusted for the fisheye effect and thermal droop at the edges. The large-scale low-speed cascade, used in the endwall heat transfer study, was configured in a four-vane three full passage arrangement. The vane design includes a large leading and aft loading. This same cascade has been used in the acquisition of vane surface heat transfer distributions, vane suction surface heat transfer visualizations, and vane surface film cooling distributions. This paper includes comparisons with two large eddy simulation calculations, which were conducted prior to the acquisition of the heat transfer data. The influence of the secondary flows on the endwall heat transfer distributions, including the leading edge horseshoe vortex system, is particularly visible at lower turbulence levels and lower Reynolds numbers. However, at higher turbulence levels, the influence of secondary flows is less visible but the influence of Reynolds number and turbulence on transition can be discerned.

Heat Transfer Coefficient and Adiabatic Wall Temperature Measurements on High-Pressure Turbine Nozzle Guide Vanes With Representative Inlet Swirl and Temperature Distortions

Abstract Combustor exit conditions in modern gas turbines are generally characterized by significant temperature distortions and swirl degree, which in turn is responsible for very high turbulence intensities. These distortions have become particularly important with the introduction of lean combustion, as a mean to control NOx pollutant emissions. For this reason, combustor–turbine interaction studies have recently gained a lot of importance. Past studies have focused on the description of the effects of turbulence, swirl degree, and temperature distortions on the behavior of the high-pressure stages of the turbine, both considering them as separated aspects and accounting for their combined impact. Aspects like pressure losses, hot streaks migration, and film-cooling behavior have been widely investigated. Even if some studies have focused on the characterization of the heat transfer coefficient (HTC) on the nozzle guide vane external surface, none of them have addressed this aspect from a purely experimental point of view. Indeed, when inlet conditions are characterized by both swirl and temperature distortions, they represent a severe challenge for the commonly adopted measurement techniques. The work presented in this paper was carried out on a non-reactive, annular, three-sector test rig made by a non-reactive combustor simulator and a nozzle guide vane cascade; it is able to create a representative combustor outflow, characterized by all the flow characteristics described before. A novel experimental approach, which was developed in a previous work, was exploited to experimentally retrieve the heat transfer coefficient and the adiabatic wall temperature distributions on a non-cooled nozzle guide vane. Temperature measurements on the cascade inlet and outlet planes were also used to provide boundary conditions and achieve a better understanding of the investigated phenomena. The results allowed to evidence the effect of the inlet swirl on the heat transfer coefficient distribution, as well as the evolution of the temperature distribution on the vane surface moving through the cascade, constituting the first attempt to evaluate these aspects from a purely experimental point of view.

Impact of Slots on the Aerodynamic Performance of the Variable Inlet Guide Vane Cascade of a Centrifugal Compressor

This study reconstructed the flow field of a symmetrical variable inlet guide vane in a centrifugal compressor through the passive control method of vane slots. Based on the high-fidelity numerical simulation model verified by experiments, the influence of different slot forms on the flow field was investigated, and the passive control mechanism was revealed. The results demonstrated that the vane slot method can effectively suppress the suction surface separation and broaden the range of low-loss incidence angles. Overall, the 50_30 slotted vane achieves the best flow field control, with a 65.6% reduction in the total pressure loss coefficient and a 2.3° reduction in the deviation angle, respectively, at a 25° incidence angle. The linear characteristics of the pre-swirl grade variation curve with variable inlet guide vane incidence angles are also improved. Furthermore, changing the slot outlet angle has the most significant influence on the aerodynamic performance as it changes the throat width of the location, thereby affecting the flow rate and momentum of the jet. Finally, the impact of the velocity varies in the first self-similarity region on the slotted vane. The results indicate that, in contrast to the baseline vane, the suppression effect of the slot jet on the flow separation improves with the inlet velocity, whereas the deviation angle of the slotted vane declines with the inlet velocity. Meanwhile, the higher the incoming flow velocity, the better the slotted jet can inhibition of flow separation.

Numerical Investigations on Effect of Inflow Parameters on Development of Secondary Flow Field for Linear Low-Pressure Turbine Cascade

Abstract Endwall flows contribute the most crucial role in loss generation for axial flow turbine and compressor blades. These losses lead to modifying the blade loading and overall performance in terms of stable operating range. The present study aimed to determine the endwall flow streams in a low-speed low-pressure linear turbine cascade vane using a numerical approach. The study includes two sections. The first section includes an attempt to understand different secondary flow streams available at the endwall. The location of the horseshoe vortex and subsequent vortex patterns are identified in the section. The selection of a suitable turbulence model among shear stress transport (SST) k–ω and SST γ–θ to identify endwall flow streams is studied prior to the section. The steady-state numerical study is performed using Reynolds Averaged Navier–Stokes equations closed by the SST γ–θ turbulence model. The computational results are validated with experimental results available in the literature and are found to be in good agreement. The study is extended for different inflow conditions in a later section. The second section includes the effect of flow incidence and turbulence intensity on the endwall secondary flow field. Four different inflow incidences are considered for the study. The inlet turbulence intensities are varied by 1% and 10% for each case. The results revealed different secondary flow patterns at an endwall and found the change in behavior with inflow conditions. SST γ–θ turbulence model with lower turbulence intensity is more suitable to identify such flow behavior.

Конструктивные особенности лопастных решеток рабочих колес, перекачивающих газожидкостные смеси, позволяющие снизить объем газовых каверн

We employed numerical simulation methods to investigate how design features of impeller vane cascades in a centrifugal pump processing gas and liquid mixtures will affect the magnitude of gas caverns. We derived a mathematical model for multiphase flow of incompressible fluid. We performed hydrodynamic computations for various impeller vane cascade designs. We identified regions of local gas separation in commercially available and improved cascades that may result in gas plug formation and pump failure. The paper investigates the effect of the following parameters on the magnitude of a gas cavern: the angle of attack, pressure feed, pressure gradient, and the presence of through holes and transverse cutouts in a single-tier vane cascade. We consider design features of stacked van cascades and investigate how the vane length and vane distribution uniformity in the blading section of a stacked vane system affect gas cavern magnitudes. We selected the optimum vane cascade design for multiphase impellers. The paper then indicates further lines of research.

Intermittent cavitating flow field in tandem cascades of Kaplan turbines

Within the framework of the classical turbine design setup, we present a numerical investigation of unsteady cavitating flow in tandem cascades of guide vanes and rotor blades. The practical relevance of the present investigation is ensured by choosing real blade profiles and domain geometry for the large Kaplan turbines (9.5 m rotor diameter) from the Iron Gates I hydropower plant on the Danube River. The 2D tandem cascade setup is obtained with a conformal mapping of the circular guide vane cascade into a straight cascade, while the rotor blade cascade is by default a straight one for axial turbines. Both single-phase and two-phase cavitating flows results are presented and analysed. The single-phase liquid flow displays rather modest pressure and tangential force fluctuations, with the characteristic blade passing frequency, due to the interaction of the rotor blades with the wakes of guide vanes. The two-phase cavitating flow displays a completely different dynamics in comparison to the liquid flow, with periodic cavitation growth and collapse at a specific frequency lower than the blade-passing one. Moreover, the tangential force on the rotor blade displays severe intermittency and the force fluctuations are almost one order of magnitude larger than in the single-phase flow.

Optimization of guide vanes for variable speed Francis turbines – Human controlled vs. automated approach

Guide vanes of hydraulic turbines can maximise turbine efficiency and directly influence on turbine operating characteristics. A hydraulic analysis is carried out on geometrically parameterised guide vane foils, to maximise efficiency and potentially observe which configuration gives operating range expansion. Variable-speed operated turbines in hydropower plants tend to run with improved performance at off-design operating points due to the speed variations. A combination of variable speed turbine operation and guide vane shape optimization gives a win-win situation in water to energy usage ratio. Human controlled developed designs and CFD calculations were previously done. The obtained results are compared with automated approach in ANSYS Workbench using Design of Experiments. The numerical tests were made on parametrically developed guide vane cascades, where the turbine efficiency is the evaluated parameter and it is observed in the range of several rotational speeds and calculated flow rates. The cascade design which produces less turbine efficiency deviations for the tested rotational speed range is later exported and hydraulically and geometrically compared with the developed design from the manual approach. The tested configurations of the guide vanes are developed on the basis of Francis 99 turbine model from the Waterpower Laboratory at the Norwegian University of Science and Technology in the existing geometric space for the guide vanes and blades axis positioning circumference.

DEVELOPMENT OF A LIFETIME PRESSURE SENSITIVE PAINT PROCEDURE FOR HIGH-PRESSURE VANE TESTING

Abstract Optical measurements based on fast response Pressure Sensitive Paint (PSP) provide enhanced spatial resolution of the pressure field. This paper presents lifetime PSP at 20 kHz, with precise calibrations, and results from a demonstration in an annular vane cascade. The laser lifetime PSP methodology is first evaluated in a linear wind tunnel with a converging-diverging nozzle followed by a wavy surface. This test section is fully optically accessible with maximum modularity. A data reduction procedure is proposed for the PSP calibration, and optimal pixel binning is selected to reduce the uncertainty. In the annular test section, laser lifetime PSP was used to measure the time-averaged static pressure field on a section of the suction surface of a high-pressure turbine vane. Tests were performed at engine representative conditions in the Purdue Big Rig for Annular Stationary Turbine Analysis module at the Purdue Experimental Turbine Aerothermal Lab. The 2-D pressure results showed a gradual increase of pressure across the spanwise and flow directions, corroborated with local static pressure taps and computational results. The variation in PSP thickness was measured as a contribution to the uncertainty. The discrete Fourier transform of the unsteady pressure signal showed increased frequency content in wind-on compared to wind-off conditions at the mid-span and 30% span. Compared to the mid-span, the hub end wall region had an increase in frequencies and pressure amplitude. This result was anticipated given the expected presence of secondary flow structures at the near hub region.

Numerical Predictions of Turbine Cascade Secondary Flows and Heat Transfer With Inflow Turbulence

Abstract Turbine passage secondary flows are studied for a large rounded leading edge airfoil geometry considered in the experimental investigation of Varty et al. (“Vane Suction Surface Heat Transfer in Regions of Secondary Flows: The Influence of Turbulence Level, Reynolds Number, and the Endwall Boundary Condition,” ASME J. Turbomach., 140(2), p. 021010) using high resolution large eddy simulation. The complex nature of secondary flow formation and evolution are affected by the approach boundary layer characteristics, components of pressure gradients tangent and normal to the passage flow, surface curvature, and inflow turbulence. This paper presents a detailed description of the secondary flows and heat transfer in a linear vane cascade at exit chord Reynolds number of 5 × 105 at low and high inflow turbulence. Initial flow turning at the leading edge of the inlet boundary layer leads to a pair of counter-rotating flow circulation in each half of the cross plane that drives the evolution of the pressure side and suction side of the near-wall vortices such as the horseshoe and leading edge corner vortex. The passage vortex for the current large leading edge vane is formed by the amplification of the initially formed circulation closer to the pressure side (PPC) which strengthens and merges with other vortex systems while moving towards the suction side. The predicted suction surface heat transfer shows good agreement with the measurements and properly captures the augmented heat transfer due to the formation and lateral spreading of the secondary flows towards the vane midspan downstream of the vane passage. Effects of various components of the secondary flows on the endwall and vane heat transfer are discussed in detail.

Parametric Design Tool for Development of a Radial Guide Vane Cascade for a Variable Speed Francis Turbine

Hydropower as a part of the family of renewable energy sources represents an engineering and scientific field which inspires researchers to work on development of the systems and sub-systems in a way of optimizing the whole energy transformation process to obtain more efficient, flexible and reliable hydropower operation with the best possible water to energy ratio. This research is part of a Horizon 2020 HydroFlex project by the Norwegian University of Science and Technology (NTNU), where the main goal is development of a flexible hydropower generation. The guide vane cascade is one of the most crucial stationary sub-systems of the hydraulic turbine and is a subject of this study. Its re-design for obtaining a quality “flow-feeding” of a variable speed high head Francis turbine is developed. Having this goal in mind, a MATLAB code was generated, based on several key parameters, such as initial energy conditions as net head and turbine discharge at best efficiency point (BEP). Turbine runner geometrical constraints are taken into account during this process, while using recommendations for some initial guide vane calculations such as their number, inlet and outlet diameter, guide vane axis diameter, delivery angles etc. Using an inverse Euler turbine equation, the operating range of the turbine was calculated for a variable speed and discharge conditions, keeping the shock-free flow for all states at the runner’s inlet, as it is the most favourable inflow condition. For those operating points, the flow streamlines angles were obtained at the guide vanes leading and trailing edges. With an interpolating mathematical functions between the angles of the leading and trailing edges, the camber lines of the hydrofoils were obtained for further guide vane cascade geometry development. This algorithm can be implemented on any given runner geometry. The guide vane design is then exported into ANSYS Workbench for further numerical tests, such as CFD simulations for verifying the hydrodynamic characteristics and FEM analysis for verifying the structural integrity of this sub-system for variable speed operating conditions.