Blade Cascade Research Articles

On the stall flutter occurrence in a blade cascade set to turbine and compressor geometry

A blade cascade allowing free pitch movement of five blades was developed in Institute of Thermomechanics. This model is introduced to study the phenomenon of stall flutter existing in rotary bladed wheels of steam turbines and other devices. This article describes how to induce the stall flutter for prescribed boundary conditions (as inlet velocity, various angles of attack, etc.) and gives survey about flow field differences around the cascade set to both the turbine and the compressor geometry. This experimental research utilized time-resolved Particle Image Velocimetry (PIV) to measure and to evaluate statistical quantities of the wake behind the NACA0010 profiles of the cascade. Also dynamical analysis was performed in the form of Fast-Fourier transform and also phase-locked measurement was applied. Gained knowledge will be utilized to design advanced model of the cascade allowing stall flutter examination without the use of ball bearings.

CFD Simulation of Transonic Flow Through the Tip-Section Turbine Blade Cascade Intended for the Long Turbine Blade

The paper deals with numerical simulations of transonic flow through the turbine blade cascade consisting of flat profiles. The cascade is one of variants of the tip section of ultra-long blades, which were designed for the last stage of the steam turbine. CFD simulations were realized by means of the ANSYS CFX commercial software using the γ-Reθ bypass transition model completed by the two-equation SST turbulence model. Some simulations were made only by the SST turbulence model for comparison. Numerical results were compared with experimental data. Calculations performed for two nominal regimes and two computational domains. In addition to the standard computational domain, the calculation was performed for a domain with an extended output part for the suppression of reflected shock waves. The interaction of the inner branch of the exit shock wave with the boundary layer on the blade suction side leads in the both flow regimes to the flow separation followed by the transition to turbulence. The flow structure in the blade cascade obtained for the extended domain corresponds well to experimental results.

CFD Investigation of the test facility for forced blade flutter research

With the increasing share of renewable power resources turbomachines need to be operated under a wider range of operating conditions including highly off-design regimes. Under such regimes an undesirable phenomenon of blade flutter might occur and possibly destroy the machine. To prevent this, intensive research is conducted by research teams worldwide. Blade flutter research program at the Institute of Thermomechanics of the Czech academy of sciences (IT CAS) mainly aims to advance experimental techniques for investigation of sonic and transonic blade flutter. For this purpose, the new sophisticated test facility was designed and manufactured. As part of the design process, the CFD computations were conducted in order to investigate the flow field in the test facility. This paper presents results of these computations with detailed analysis of flow structures occurring during the air flow through the stationary blade cascade.

Aerodynamic and dynamic loading in a blade cascade designed for flutter research

Flow-induced vibration of turbine and compressor blades, so called blade flutter, represents a serious problem for designers and operators of large turbomachines. The research of mechanisms leading to this dangerous aeroelastic instability, which can occur especially in modern long and slender blades, is hindered by lack of experimental data. A new experimental setup for controlled flutter testing has been designed in cooperation of the Institute of Thermomechanics of the Czech Academy of Sciences and Faculty of Mechatronics of the Technical University of Liberec. The test section consists of five planar blades placed in a transonic wind tunnel, with high-frequency torsional oscillation of the middle blade driven by an electric motor. The contribution presents the results of first measurements, namely the static pressure distribution for various inlet Mach numbers, aerodynamic moments and deformation of the middle blade due to inertial loads during high-frequency oscillation.

Wake characteristics of a steam turbine rotor tip linear cascade blade in subsonic flow

Experimental investigations have been carried out on a steam turbine rotor tip linear cascade to determine the effect of Mach number on the flow and total pressure losses at zero degree of incidence. Static pressure distribution on the blade surfaces have been measured. Exit flow survey has been carried out using a precalibrated five hole probe at the corresponding Mach number. The exit Mach number has been varied from 0.27 to 0.52. The effect of Mach number on various parameters such as lift coefficient, total pressure loss coefficient, mass averaged exit flow parameters and wake parameters has been found to be small. Wake self-similarity has been observed for velocity profiles at all Mach numbers. The wake half widths and integral thicknesses increase slowly with Mach number. Shape factors are in the range of 1.1 to 1.4, indicating turbulent boundary layers on the suction and pressure surfaces of the blade.

Modified model of reduction condensing losses strategy into the wet steam flow considering efficient energy of steam turbine based on injection of nano-droplets

In this study according to the challenges of the power generation industry and condensing losses of wet steam flow, the strategy of Nano-droplets injection in nucleation zone is used. The strategy as such have been adopted to reduce the condensing losses of the flow and increase the efficient energy of the steam turbine. In the first step, after performing the sensitivity analysis on the computational domain of the non-equilibrium condensation model, the results are validated with the experimental data. Also, the SST k-ω turbulence model is utilized to simulate the latency created by the presence of Nano-droplets in the created domain. In the second step, the measurement criterion for injection effects in different locations of the nucleation zone is defined as efficient energy. This energy for the steam turbine are calculated as the energy of flow between the cascade blade, which it has the lowest wet losses of liquid phase and the highest temperature, velocity and internal energy. Finally, based on the Modified case, the efficient energy parameters have been presented, which result in a decrease of approximately 45% 27.5%, 21.4% 44% and 18%, for the condensation losses, droplets radius size, liquid mass fraction, the erosion rate ratio and wetness losses, respectively. Additionally an increasing trend of 18.25% and 0.2% for the efficient energy and Mach number has been observed.

Flow Characteristics of the Nozzle Blade Cascade in the Mode of the Joint Operation with the Radial Diffuser

The obtained research data are given for the nozzle cascade used by a small-size gas turbine of an average fanning in combination with the radial diffuser. Aerodynamic characteristics of the nozzle blade cascade were determined in a wide range of a change in the Reynolds number varying from 4∙105 to 106 and the reduced velocity varying in the range of 0.4 to 1.13. The flow rate coefficient of the nozzle cascade was derived for all modes using the integral methods and the drainages behind the cascade. The kinetic energy loss coefficient and the flow angles were calculated using the measurement data of flow parameters in three control modes that were obtained due to the use of orientable pneumometric probes. When the expansion degree of the convergent –divergent annular duct behind the cascade is equal to 1.43 the flow in the narrow section of this duct is “enlocked” in the mode when the reduced velocity behind the cascade is equal to 1.127. At such velocity the Reynolds number 106 is self-similar for the flow rate coefficient. At lower values of Reynolds number, the decrease of it is accompanied by an intensive decrease in the flow rate coefficient for all the values of the reduced velocity. For the Reynolds number lower than 7∙105 an increase in the velocity results in a decreased flow rate coefficient. When this number exceeds 8∙105 an increase in the velocity results in an increase of the flow coefficient up to the moment when the flow is “enlocked” in the nozzle cascade.

Influence of blade vibrations on aerodynamic performance of axial compressor in gas turbine: Direct numerical simulation

Flutter instability because of the blade vibrations is a major problem in axial compressor cascades. This paper investigates the effects of blade oscillations on aerodynamic performance and turbulent flow characteristics of modern gas-turbine compressors. Highly accurate direct numerical simulations (DNS) are performed based on the spectral-hp element method to predict the flow instabilities, pressure fluctuations and vortex generation on the surface of the compressor blades by considering all complex physical parameters at different vibration frequencies (0 < f < 8.28 Hz). The main novelty of this study is to consider the aeroelastic vibrations of the compressor blades in DNS analysis. The novel proposed method enables the detailed analysis of complex structure fluid interactions, which cannot be obtained using other methods such as RANS models. The simulations revealed that the blade oscillations have a huge impact on the vortex generation and laminar separation bubble on the suction side of the blade. Moreover, the amplitude of unsteady pressure distribution is greater at 90% span, with the phage angle deviating between 100° and 350°. Strong fluctuations are observed at the centre of the wake region of the vibrating blade cascade. These fluctuations are activated by the flow disturbances resulting from the blade oscillation. Moreover, intense pressure fluctuations happen in the wake region of the oscillating compressor blade compared with the stationary one. The separating point occurred 8.28% sooner on the suction surface of the vibrating blade with k = 0.4 (Xsep/C = 0.288) compared to the stationary blade with Xsep/C = 0.314.

Characterization of Supersonic Compressible Fluid Flow Using High-Speed Interferometry.

This paper presents a very effective interference technique for the sensing and researching of compressible fluid flow in a wind tunnel facility. The developed technique is very sensitive and accurate, yet easy to use under conditions typical for aerodynamic labs, and will be used for the nonintrusive investigation of flutter in blade cascades. The interferometer employs a high-speed camera, fiber optics, and available “of-the-shelf” optics and optomechanics. The construction of the interferometer together with the fiber optics ensures the high compactness and portability of the system. Moreover, single-shot quantitative data processing based on introducing a spatial carrier frequency and Fourier analysis allows for almost real-time quantitative processing. As a validation case, the interferometric system was successfully applied in the research of supersonic compressible fluid discharge from a narrow channel in a wind tunnel. Density distributions were quantitatively analyzed with the spatial resolution of about . The results of the measurement revealed important features of the flow pattern. Moreover, the measurement results were compared with Computational Fluid Dynamics (CFD) simulations with a good agreement.

The best angle of hot steam injection holes in the 3D steam turbine blade cascade

Steam turbines play an undeniable role in the power generation cycle. In this paper, in order to increase the efficiency of steam turbines, hot steam is injected between the steam turbine blades in 3D mode. Therefore, by defining five comparative criteria (liquid mass fraction, condensation loss, kinetic energy, hot steam injection ratio, and economic cost), injections with holes in various angles are investigated. For this purpose, the effects of the angle of the hot steam injection holes are discussed at the suction side on the two-phase flow parameters. The optimal angle for hot steam injection holes is selected (−60°), at which liquid mass fraction, condensation loss, and kinetic energy are reduced by 40%, 22%, and 1%, respectively. In addition, economic costs and hot steam injection holes are 0.56 ($/hour) and 0.07 for the suggested case.

Effects of a Single Blade Incidence Angle Offset on Adjacent Blades in a Linear Cascade

The paper presents a numerical and experimental investigation of the effect of incindence angle offset in a two-dimensional section of a flat blade cascade in a high-speed wind tunnel. The aim of the current work is tp determine the aerodynamic excitation forces and approximation of the unsteady blade-loading function using a quasi-stationary approach. The numerical simulations were performed with an in-house finite-volume code built on the top of the OpenFOAM framework. The experimental data were acquired for regimes corresponding to the numerical setup. The comparison of the computational and experimental results is shown for the static pressure distributions on three blades and upstream and downstream of the cascade. The plot of the aerodynamic moments acting on all five blades shows that the adjacent blades are significantly influenced by the angular offset of the middle blade.

Acoustic and hydrodynamic power of wave scattering by an infinite cascade of plates in mean flow

We study the balance of outgoing and incoming power for acoustic wave scattering by a cascade of flat blades in uniform subsonic mean flow. In nonzero mean flow the Kutta condition at the trailing edge of the cascade plates leads to the production of vorticity in the form of unsteady vortex sheets attached at the trailing edges, which results in a hydrodynamic power flux and the exchange of energy between the unsteady vorticity and the acoustic field. Although the scattering problem itself has been subject to much previous research, a comprehensive study of the composition of the outgoing energy flux for cascade scattering has received little previous attention. As an application of our detailed study we show how the time-averaged energy balance can be used to explain certain symmetries of the field in the angle of incidence with respect to the cascade face when there is no mean flow and the effect of zero acoustic reflection at certain angles of incidence with mean flow. We also show that, depending on the angle of incidence of the acoustic wave and the velocity of the mean flow, the vortex sheets can either absorb or generate acoustic energy. This can, for certain parameter values, result in a net increase in outgoing sound power over the incident sound power, as we demonstrate on a number of numerical examples that are computed using the semi-analytical solution described in recent work by Maierhofer and Peake (J. Sound Vib., vol. 481 (2020), 115418).

Experimental Investigation of Effects of Reynolds Number and Incidence Angle on Secondary Flow within a Linear Blade Cascade

Experimental Investigation of Effects of Reynolds Number and Incidence Angle on Secondary Flow within a Linear Blade Cascade

Applications of little blades in a high-load compressor cascade with leaned blade

The development of boundary layer affects the compressor cascade performance to a certain extent. Therefore, the compound lean and little blades are selected to redistribute the boundary layer, and the influences of these two flow control technologies on the axial compressor cascade performance are further studied. The calculated results showed that appropriate high pressure region on the blade suction surface near the end-wall is helpful to reduce the total pressure loss of compressor cascade, which can be achieved by positive lean technique. Meanwhile, the maximum stable operation boundary can be expanded by the application of positive leaned blade. On the other hand, the introduction of negative lean angle not only increases the total pressure loss of cascade, but reduces the stable operation range. As the little blades are introduced in the negative lean compressor cascade, the stable operation range is significantly improved by the introduction of little blades. Especially the cascade with −10° lean angle, the maximum stable operation boundary is increased from 1° to 6°. In the positive lean compressor cascade, although more low-energy fluid is accumulated on the blade suction surface near the mid-span, the little blades still show an active role in reducing the total pressure loss and expending the stable operation range, because the influence range of induced vortex reaches 30%span. The results provide a reference for improving the aerodynamic performance of compressor stator, especially when more low-energy fluid is blocked in the range near the mid-span.

Novel Focal Sweep Strategy for Optical Aerothermal Measurements of Film-Cooled Gas Turbine Blades With Highly Inclined Viewing Angle

Abstract The viewing angle for optical aerothermal measurements on turbine surfaces is often limited by the turbine structure, requiring the optical system to have a large depth of field (DoF). Although the DoF can be increased by decreasing the lens aperture, this approach is impractical as a large aperture is essential to maintain an acceptable signal-to-noise ratio (SNR). To solve these problems in the optical aerothermal measurements of film-cooled gas turbine blades, an approach combining the focal-sweep method and three-dimensional (3D) reconstruction is proposed. The focal-sweep method is used to obtain all-in-focus images at an inclined viewing angle, following which the two-dimensional image is restored through 3D reconstruction. Thus, 3D point clouds with both a large DoF and high SNR can be produced. The developed method was validated via flat-plate film cooling experiments using pressure-sensitive paint at three blowing ratios of 0.4, 0.8, and 1.2, as well as three viewing angles. The measured adiabatic effectiveness contours demonstrate that the proposed method can produce all-in-focus measurements at highly inclined viewing angles, albeit at the price of slightly higher noise. In flat-plate experiments, the maximum relative difference is measured to be 6% between results obtained by conventional method at normal view and the proposed method at highly inclined view. Furthermore, the proposed method was applied to the turbine blade cascade film cooling experiment at a highly inclined viewing angle, and successfully reconstructed the 3D point cloud of the cooling effectiveness at the curved turbine blade surface.

A meshless method for subsonic stall flutter analysis of turbomachinery 3D blade cascade

A meshless method for subsonic stall flutter analysis of turbomachinery 3D blade cascade

A Novel Dehumidification Strategy to Reduce Liquid Fraction and Condensation Loss in Steam Turbines.

Massive droplets can be generated to form two-phase flow in steam turbines, leading to erosion issues to the blades and reduces the reliability of the components. A condensing two-phase flow model was developed to assess the flow structure and loss considering the nonequilibrium condensation phenomenon due to the high expansion behaviour in the transonic flow in linear blade cascades. A novel dehumidification strategy was proposed by introducing turbulent disturbances on the suction side. The results show that the Wilson point of the nonequilibrium condensation process was delayed by increasing the inlet superheated level at the entrance of the blade cascade. With an increase in the inlet superheated level of 25 K, the liquid fraction and condensation loss significantly reduced by 79% and 73%, respectively. The newly designed turbine blades not only remarkably kept the liquid phase region away from the blade walls but also significantly reduced 28.1% averaged liquid fraction and 47.5% condensation loss compared to the original geometry. The results provide an insight to understand the formation and evaporation of the condensed droplets inside steam turbines.

Performance Study of Novel Compressor Blades in a Two-Dimensional Cascade—Transonic Regime

Performance Study of Novel Compressor Blades in a Two-Dimensional Cascade—Transonic Regime

Development of the Universal Modelling Method mathematical model and the practice of its application

We present the latest improvements to the Universal Modelling Method for optimal gas-dynamic design of centrifugal compressors. The calculation of the gas velocity at the inlet to the impeller blade cascade was corrected. The flow restructuring scheme at the inlet to the impeller blade cascade has been refined. In accordance with this, the mathematical model of the non-incidence gas inlet to an impeller has been improved. A new approach to modelling the velocity diagram on the blade surfaces is proposed, based on the flow parameters corresponding to the non-incidence flow inlet. A mathematical model with quasi-three-dimensional approach to calculating losses in 3D impellers has been developed. The method of gas-dynamic functions is applied to the calculation of flow parameters in control sections. The new mathematical model was identified based on the test results of model stages and compressors. The mathematical model showed a 0.525% error in modelling efficiency of stages with 2D impellers for the design flow rate. For stages with 3D impellers, the modelling error is 0.889%. To verify the new model, the characteristics of 11 low-flow-rate model stages were calculated. The efficiency modelling error for the design flow rate is 1.08%. A series of model stages with a range of design flow rates coefficient 0.15–0.015 and design loading factor of 0.5 was designed. Four of these model stages were tested on the Gas dynamics of turbomachines laboratory ECC-55 test rig. The measured gas-dynamic characteristics confirmed those declared in the design.

Experimental Investigation of Wash Fluid Preheating on the effectiveness of online Compressor Washing in a Suction Tunnel Compressor Cascade

Online compressor washing is a promising method of preventing/ recovering the effects of fouling on the compressor blades. However, proper strategies need to be implemented for online compressor washing to be effective since it is conducted when the engine is in operation. This study presents an experimental investigation of wash fluid preheating on the effectiveness of online compressor washing in a suction wind tunnel compressor cascade. Crude oil was uniformly applied on the compressor cascade blade surfaces using a roller brush, and carborundum particles were ingested into the tunnel to create accelerated fouled blades. Demineralized water was preheated to 500C using the heat coil provided in the tank. Washing of the fouled blades were conducted using single flat fan nozzle, where the preheated and non preheated demineralized water were used separately to wash the fouled blades. When fouled blades washed with preheated demineralized and the non-preheated were compared, it was observed that there was little or no difference in terms of total pressure loss coefficient and exit flow angle. However, when the fouled and washed cases were compared, there was a significant difference in total pressure loss coefficient and exit flow angle.