Centrifugal Stress Research Articles

Study of cracks in the last-stage rotor blade of a steam turbine and the corrosion fatigue properties of its materials

In this study, to clarify the failure mechanism of the last-stage rotor blade in the low-pressure cylinder of a steam turbine, the peculiarities of crack initiation and propagation on the inlet side of the last-stage rotor blade at a distance of 125–165 mm were analyzed, along with the corrosion fatigue properties of its materials. The results showed that crack initiation occurred at the tip of the pit due to a combination of factors: stress concentration at the tip of the pit, corrosion of the Cr-poor area near the prior austenite grain boundary, centrifugal tensile stress, and steam bending stress. The crack propagation could be divided into the initial intergranular and late transgranular propagation stages. The main reason for the initial intergranular propagation was stress corrosion, and the main reason for the later transgranular propagation was corrosion fatigue. High-frequency induction quenching technology can improve the microhardness of the blade's surface material and enhance the blade's resistance to water erosion, but it may also reduce the corrosion fatigue resistance of the blade material. The rotary bending corrosion fatigue test can effectively simulate the crack propagation process of the blade. These results are of great significance for the safe operation of the last-stage rotor blade in the low-pressure cylinder of a steam turbine.

The unique influence of 1000 °C high-speed rotation simulated turbine blade working conditions on microstructural degradation of single-crystal superalloy NiAlReRu

Turbine blades are mainly subjected to thermal stress and gradient centrifugal stress in operational conditions. The effects of centrifugal stress on the microstructural degradation of single crystal superalloys under a multiaxial stress state remain unclear. Here, the NiAlReRu component was exposed to high-speed rotation at 20,000 rpm and a high temperature of 1000 °C for 100 h. The analysis indicates that the centrifugal stress causes the distortion of dendrite stems and the aggregation of micropores. The centrifugal stress promotes the formation of interfacial dislocation networks, accompanied by more severe rafting of γ′ phases. These findings provide a comprehensive experimental understanding of the microstructural evolution under simulated turbine blade working conditions, which provides a reference for the subsequent tests under near-service conditions.

Optoelectronic Strain-Measurement System Demonstrated on Scaled-Down Flywheels.

Monitoring the strain in the rotating flywheel in a kinetic energy storage system is important for safe operation and for the investigation of long-term effects in composite materials like carbon-fiber-reinforced plastics. An optoelectronic strain-measurement system for contactless deformation and position monitoring of a flywheel was investigated. The system consists of multiple optical sensors measuring the local relative in-plane displacement of the flywheel rotor. A special reflective pattern, which is necessary to interact with the sensors, was applied to the surface of the rotor. Combining the measurements from multiple sensors makes it possible to distinguish between the deformation and in-plane displacement of the flywheel. The sensor system was evaluated using a low-speed steel rotor for single-sensor performance investigation as well as a scaled-down high-speed rotor made from PVC plastic. The PVC rotor exhibits more deformation due to centrifugal stresses than a steel or aluminum rotor of the same dimensions, which allows experimental measurements at a smaller flywheel scale as well as a lower rotation speed. Deformation measurements were compared to expected deformation from calculations. The influence of sensor distance was investigated. Deformation and position measurements as well as derived imbalance measurements were demonstrated.

Effect of impeller structure on aerodynamic performance and noise reduction of a small multi-blade centrifugal fan

In this study, the numerical simulation and orthogonal test are combined to study the effects of blade outlet angle, blade inlet angle, and blade number on the centrifugal fan flow characteristics, stress distribution, and noise control. The experimental tests were carried out to confirm the accuracy of the computational model. The computational fluid dynamics (CFD) technology was used to establish 16 groups of impeller models and derive the optimal parameter combination form. The results show that the blade inlet angle and outlet angle have more significant effects on efficiency and flow rate of fans. Among different optimize structures, the A8 fan with an inlet angle of 60°, an outlet angle of 180°, and a blade number of 45, has the best aerodynamic performance, with a 17.4% increase in flow rate, a 3.3% increase in efficiency, a 7.6% reduction in noise and a 23.0% reduction in impeller stress, compared to the original fan. Reasonable adjustment of the inlet and outlet angles greatly improves fluid distribution and suppresses flow separation. Furthermore, the noise can be reduced by lessening the disturbance between the volute tongue and wake flow, so the flow state gets greatly improved at the volute tongue region.

Characterisation of the creep cavitation process on grain boundaries in a polycrystalline nickel-base alloy 247

ABSTRACT Hot gas components such as gas turbine blades are loaded with constant centrifugal stress at high temperatures and thus the formation of creep cavitation on grain boundaries is expedited. Conventionally cast polycrystalline Nickel-base superalloys used as blade materials are prone to creep cavitation as unfavourably orientated grain boundaries exist. This research presents a diffusion-based probabilistic creep model which describes the creep cavitation process on grain boundaries. It includes the three mechanisms: pore nucleation, growth, and coalescence. The calibration of the model has been carried out by analysing Alloy 247 as-received specimens and specimens with pre-strain. For the evaluation of the pore numbers and sizes, a deep learning model for pore detection was trained on light microscopic and scanning electron microscopy images. Electron backscatter diffraction images are analysed for further investigations regarding grain orientations and grain boundary angles to the loading direction. The calibrated model allows predictions of pore size distributions over time.

Degradation and dislocation evolution of the nickel-based single crystal superalloy DD6 under the coupling high-speed rotating and high temperature environments

To model the centrifugal loads and complex geometries experienced by turbine blades during service, the nickel-based single crystal superalloy DD6 samples with a groove structure are designed and tested under the coupling conditions of high-speed rotating of 20,000 r/min and high temperature of 980 ℃. Scanning electron microscopy and transmission electron microscopy are used to analyze the microstructural evolution of γ-γ' phases and dislocation morphologies of 400 h and 1000 h samples. The results indicate that microstructural degradation of the γ-γ' phase attributes to the larger centrifugal tensile stress S11 in the applied physical stress compositions until deformation occurs. Our observations suggest that a/2 < 101 > -type dislocations initiate plastic deformation by slipping within the γ phase. However, their movement is constrained at the interface between the γ and γ' phases, resulting in their tendency to aggregate at this boundary. After the formation of stable interfacial dislocation networks, these dislocations then react to generate either a< 010 > -type dislocations or a/2 < 112 > -type dislocations, which cut into the γ' phase. However, under high stress conditions, the a/2 < 101 > -type dislocations are primarily cut directly in the form of dislocation pairs.

A 3D-XIGA rotating cracked model for vibration analysis of blades

Cracking of blades is one of the common failures in rotating machinery. Traditionally, the finite element method (FEM) is a typical tool to simulate the crack effects, which needs refined mesh processing and then limits computational efficiency. A three-dimensional (3D) rotating cracked model relying on the extended isogeometric analysis (XIGA) is developed for the vibration analysis of blades with inclined single and multiple cracks in this study. In the XIGA method, the representation of cracks can be well expressed by using enriched elements which allow modeling stress singularity ahead of the crack tip and discontinuity of the crack face precisely. The level set method (LSM) is used to identify the element type, so the number of elements is independent of the crack parameters. Considering penetrating cracks in the thickness direction, the 3D level set problem can be reduced to a 2D one. The stress and vibration displacement fields are exactly described by 3D elasticity theory combined with a two-stage solving progress. Numerical data of the established rotating cracked model with single or multiple cracks are obtained and compared with the existing results. The frequency steering and crossover phenomena due to the dimensional and locational parameters of the cracks are discussed in detail, which is crucial to accurately identify cracks. The influence of rotating speed and parameters of single and multiple cracks on the vibrational characteristics are also considered, which shows that cracks make the centrifugal stress field and frequency parameters more complex.

Jet Engine Gas Turbine Blades: Beyond the Super Alloys

The High Pressure Turbine (HPT) blades of a turbo-engine, whether used in land-based power- generating equipment or in aircraft propulsion systems, are among the most severely loaded engineering components; they operate at very high temperatures, they are subjected to very high centrifugal stresses, they are susceptible to creep and fatigue damage and they are also exposed to adverse conditions such as corrosion, oxidation, sulphidation, foreign object damage, etc. Nickel-based super alloys were used to make turbine blades in the 1940s and the Turbine Inlet Temperature (TIT) has been steadily increased over the years due to material innovations such as improved alloy chemistry and protective coatings, processing innovations such as directional solidification and single crystal blade fabrication and design innovations such as complex internal cooling channels that enable the blades to operate at temperatures beyond their melting point. With the improvements attainable with the super alloys reaching the limit, efforts are underway to reduce the weight of the blades by making them out of lower density materials such as intermetallic compounds of titanium and aluminum. With the reduction in the centrifugal forces, the weight of the supporting structures such as the bearings can also be reduced, resulting in gains in fuel consumption and reduction in the generation of exhaust gases. Intermetallic compounds such as titanium aluminides and nickel aluminides, composites using such intermetallics as matrices, and ceramic matrix composites with silicon carbide fibers in a silicon carbide matrix, are being introduced in a phased manner; some of these materials have already shown their potential in the Low Pressure Turbine (LPT) stages and are advancing through the higher temperature parts of the turbo-engine with the ultimate goal of replacing the heavier super alloys. These new materials are difficult to fabricate and innovative methods based on centrifugal casting, forging and additive manufacturing from powders are being developed. This paper describes some of these important innovative developments.

The High Pressure Gas Turbine Blades of Jet Engines

The modern turbo-engine is one of the finest examples of engineering ingenuity, complexity and usefulness. The turbo-engine, whether used in land-based power generating equipment or in aircraft propulsion, has had a profound impact on our lives. Increasing air-travel, rising fuel prices, and environmental concerns have all combined to increase the need for more fuel-efficient engines. The blades of the high-pressure turbine are the most severely loaded members in the turbo-engine. The various aspects of these blades are described. First, the major components of a turbo-engine as well as the different types of turbo-engines are briefly described. Then the development of the turbo-engine is placed in its historical perspective. Next, the severity of the blade loading conditions, such as high temperatures, high centrifugal stresses, corrosion, oxidation, sulphidation, foreign object damage, etc., and the resulting failure modes are summarized. Super alloys are the materials of choice for these blades. The incremental improvements in the alloy chemistry are briefly traced. Important developments in the blade manufacture, the different types of blade cooling channels and their machining, thermal barrier coatings to decrease the blade temperature while increasing the Turbine Inlet Temperature (TIT), and future trends are discussed. The future belongs inevitably to advanced ceramics and their composites, with coatings for environmental protection.

Simulation research on high speed centrifugal residual stress homogenization of turbine disk

At present, problems such as excessive residual stress, local stress concentration and machining deformation are common encountered in the production and processing of turbine disk. In order to further improve the machining quality and performance of turbine disk components, a high speed centrifugal stress homogenization and strengthening method with in-depth research and application value is proposed in this paper. The simulation research on the centrifugal stress homogenization effect of turbine disk forging blank is carried out, and the distribution of residual stress at different speeds is obtained, the influence of rotating speed on stress homogenization effect is summarized.

Study on the dynamic stability of circular saw blade during medium density fiberboard sawing process with thermo-mechanical coupling

The dynamic stability of the circular saw blade determines whether the sawing process can smoothly proceed. The sawing heat significantly affects the dynamic stability of the circular saw blade. This study proposes a sawing heat simulation method which allows to analyze the temperature, stress, and natural frequency of a blade sawing medium density fiberboard (MDF). The edge temperature of the saw blade in high-speed cutting is higher than that in low-speed cutting, which leads to the formation of a large temperature gradient and reduces the natural frequency of the saw blade. However, the centrifugal stress more significantly increases the natural frequency during high-speed cutting, the saw blade still has better dynamic stability than during low-speed cutting. The simulated saw blade temperature field and static natural frequency are consistent with the experimental results, which validates the proposed method. The proposed model allows to predict the saw blade temperature distributions and to effectively set the sawing parameters.

Performance Evaluation and Comparison of Three-Phase and Six-Phase Winding in Ultrahigh-Speed Machine for High-Power Application

This article investigates the influence of multiphase winding topologies in high-power ultra-high-speed machines (HP-UHSM) of 500 kr&#x002F;min. At this speed level, increasing the rotor&#x0027;s magnetic loading excites its critical bending resonances and leads to structural breakdown. On the other hand, increasing the stator&#x0027;s electric loading using the three-phase winding increases unwanted vibrations in a slotted stator and reduces the electromagnetic inter-action of the stator and rotor in a slotless stator. Consequently, the maximum output power level of UHSM (500 kr&#x002F;min or more) is limited to a few hundred watts only in the state-of-the-art. To over-come such a critical limitation, this article proposes a new design methodology for HP-UHSM, where the rotor&#x0027;s bending resonances and centrifugal stresses are restricted by limiting the maximum aspect ratio (<i>L&#x002F;D</i>), and an optimal multiphase winding is adopted in the slotless stator to increase the power level by effective electric loading. Also, a Multiphysics optimization is utilized to obtain the optimum magnetic loading and electric loading, where the bending resonance and other system limits are defined using multi-disciplinary design constraints. It is observed that the multiphase winding provides an added degree of freedom to increase the power level of UHSM without exciting the rotor&#x0027;s bending resonances and structural breakdown. Using the proposed method, a multiphase 2 kW 500 kr&#x002F;min HP-UHSM has been designed for the safety-critical AMEBA system and compared its Multiphysics performance with the three-phase machine having the same volume. Finally, exten-sive experiments are performed on both prototypes to validate the effectiveness of the proposed method. It is shown that the multi-phase HP-UHSM has no critical bending resonance below the 500 kr&#x002F;min, and it has 16.3&#x0025; higher output power with 1.18&#x0025; higher efficiency and 28.6&#x0025; lower back-EMF than the three-phase design.

Effects of multiple wetting incidents, shear and sliding friction on lubricant stability in SLIPS

Surface icing almost invariably derives from the precursory step of liquid water encountering the surface. Thus, slippery liquid infused porous surfaces, SLIPS, must possess steady wetting durability, and lubricant stability to function as a reliable hydro−/icephobic surface design especially in outdoor applications. Additionally, they should maintain their phobic performance under shear forces, and possess low sliding friction to act as a slippery, multirepellent surfaces. These characteristics are needed in variable applications ranging from moving and rotating blades to steady surfaces, operating in altering climate conditions. More profound durability testing is needed to examine the loss of surface functionality when the lubricant is depleted from the structure via various routes. In addition, the durability tests should be designed to serve the application-related purposes and thus, to reveal performance differences between slippery surfaces for further analysis and targeted end-use development. Here, we tested the wetting durability and stability of SLIPS with multicycle Wilhelmy plate by dipping the surfaces multiple times in water bath. Additionally, we examined the effects of centrifugal and friction-based shear stress to investigate the lubricant depletion from the structure. Tests that measure the durability and the stability of SLIPS designs are in great need in further developing functional slippery surfaces for real outdoor application coatings which encounter environmental stresses, e.g., wetting and icing. Acknowledging the material differences under specific stresses will guide designing the slippery surfaces towards more specific and functionable end-use applications.

Numerical simulation of airflow field in rotor spinning channel based on dynamics optimization

To respond to high-speed characteristics and complicated airflow of the air-extraction rotor, the paper takes advantage of ANSYS parametric design language to develop the parametric finite element modeling program by taking the geometric feature of the slip plane as the design variable, so as to propose a mathematical model applicable to dynamics optimization of the rotor. The optimal design parameters of the slip plane are obtained through dynamics optimization ( α ≈ 13.92°, L ≈ 11.17 mm), and the centrifugal stress and static deformation of the rotor body are further reduced while increasing its fundamental frequency by about 7.15%, so that it can adapt to higher safety critical speed. The RNG k- ε turbulence physics model is applied to the numerical simulation of the airflow field in the optimal spinning channel, and a computational domain of single-phase steady fluid with nonstructural mesh is constructed by using ICEM CFD and FLUENT software. On this basis, the simulation result with excellent convergence of the flow field is obtained through the SIMPLE algorithm and pressure-velocity coupled solution. In addition, by virtue of the two-dimensional and three-dimensional flow field characteristic analysis for key fields (like the rotor wall, fiber pipeline, false twister, collecting groove, inlet and outlet), the airflow field state in the spinning channel (like the static pressure, dynamic pressure, velocity vector field, turbulence intensity and streamline trajectory) is confirmed, which will be conducive to gain a deep insight into the open-end spinning mechanism of the air-extraction rotor.

Effect of Centrifugal Force on Power Output of a Spin-Coated Poly(Vinylidene Fluoride-Trifluoroethylene)-Based Piezoelectric Nanogenerator

While poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) film is an excellent piezoelectric material for mechanical energy harvesting, the piezoelectric output varies considerably with the spin coating conditions. Herein, we reported a systematic evaluation of the structural, electrical, mechanical, and microstructural properties of spin-coated P(VDF-TrFE) films obtained at various distances from the center, as well as under different rotational speeds. With increasing distance, the remnant polarization, dielectric constant, and crystallinity of the films increased, which resulted in enhanced piezoelectric power at the largest distance. With increasing rotational speed, the remnant polarization, dielectric constant, and crystallinity of the films initially increased and then decreased, while the Young’s modulus continuously increased. This resulted in an enhanced piezoelectric power at a given rotational speed. The piezoelectric power is proportional to the remnant polarization and inversely proportional to the Young’s modulus. The highest (2.1 mW) and lowest (0.5 mW) instantaneous powers were obtained at the largest (1.09 μC/cm2·GPa−1) and smallest (0.60 μC/cm2·GPa−1) value of remnant polarization over Young’s modulus, respectively. We explain these behaviors in terms of the centrifugal force-induced shear stress and grain alignment, as well as the thickness-dependent β-phase crystallization and confinement. This work implies that the spin coating conditions of distance and rotational speed should be optimized for the enhanced power output of spin-coated P(VDF-TrFE)-based piezoelectric nanogenerators.

Simulation studies on combined effect of variable geometry, rotation and temperature gradient on critical speed of gas turbine disc

PurposeThe present findings report a significant influence of disc profile and thickness on the order of excitation leading to critical speed condition. Certain transverse modes of vibration of the disc have been obtained to be more susceptible to get excited while recording the lowest critical speeds.Design/methodology/approachNumerical simulation using finite-element method has been adopted due to the complicated geometry, complex loadings and intricate analytical formulation. A comprehensive analysis of exclusive as well as combination of thermal and centrifugal loads has been taken up to determine the intensity and characteristics of the individual/combined effects.FindingsThe typical gas turbine disc profile has been analyzed to predict the critical speed under the factual working condition of an aero-engine. FEM analysis of uniform and variable thickness discs have been carried out under stationary, rotating and rotating-thermal considerations while emphasizing the effect of disc profile and thickness. Centrifugal stresses developed due to rotational effect result in unceasing stiffening of the discs with higher stiffening for a greater number of nodal diameters. On the other hand, a role reversal of thermal effect from stiffening to softening is figured out with increasing numbers of nodal diameters. However, the discs are subjected to an overall stiffening effect on account of the combined centrifugal and thermal loading, with the effect decreasing with an increase in disc thickness. Under the combined loading, the order of excitation leading to critical speed condition is dependent on disc profile and thickness. Moreover, the vibrational modes (0,1) and (0,2) are identified as more prominent adverse modes corresponding to lowest critical speeds.Practical implicationsThe present findings are expected to serve as guidelines during the design phase of gas turbine discs of aeroengine applications.Originality/valueThe present work deliberates on the simulation and analysis of gas turbine disc specific to aeroengine application. The real-life disc geometry has been analyzed with due consideration of major factual operating conditions to identify the critical speed. The identification of various critical speed using numerical analysis can help to reduce the number of experimental tests required for certification.

Structural design of the fluted shaped charge liner using multi-section optimization method

Spin effect of the small diameter shaped charge results in the centrifugal stress during the jet stretching process. Consequently, the jet scatters, which deceases the jet penetration capability. In the present study, a multi-section method was proposed to design the spin-compensation liner. The spin-compensation rate (SCR) of the liner was defined as the specific angular velocity that a fluted liner can offset. Based on the plain stress theory, SPH numerical method was applied to study the converging process of the 2D fluted structure. The spin-compensation mechanism of the fluted structure was illustrated. Then, nine cross sections were chosen along the liner axis equidistantly. On each of the section, a 2D fluted structure was designed to offset a given initial angular velocity. After, the optimized fluted structures were integrated into a 3D fluted liner. Jet appearances of the normal liner and the fluted liners under different initial angular velocities were compared, which verifies the practicality of the multi-sectional method. The multi-section optimization method provides a new efficient method of designing the shaped charge liner for a specific usage.

Measurements of debris flow entrainment and dynamics

The mechanisms of debris flows and their interaction with mitigation structures are still not well understood. Among the research challenges, only few entrainment measurements are available in literature, as entrainment is often masked by deposition on top. In this paper, we present a simple, cheap, and effective method to measure the entrainment depths. Flume experiments have therefore been performed to assess the influence of the initial debris flow volume and of an upstream flexible barrier on entrainment. To better understand the debris flow dynamics, the flow basal stresses have been measured. A high degree of liquefaction at the base of the debris flow is observed. A mitigation measure to reduce entrainment has also been studied. A compact flexible barrier was installed in the upstream part of the channel and is observed to deflect the flow along a curvilinear path. High normal stresses are measured at the base of the overflow, which are caused by the additional centrifugal stresses from the overflow. The results from the flume tests suggest that the flow interaction with an upstream flexible barrier may significantly influence the debris flow dynamics both upstream and downstream of the barrier.

Enzymatic inhibition of α-amylase and encapsulation of bioactive compounds by nanoemulsion from pulp extract Terminalia catappa Linn fruit

Bioactive compounds are antioxidants of great importance for the cosmetics and food industries. These antioxidants can inhibit enzymes or reduce oxidative reactions of free radicals, which may reflect in disease prevention. The extracts of Terminalia catappa fruit showed high concentrations of phenolic compounds (2.98 ± 0.38 mg/mL), which are considered a good source of antioxidants. The aqueous extract of T. catappa fruit proved to be efficient to deactivate α-amylase and complete inhibition of the enzyme was observed using an extract dosage of 0.8 mg/mL. The encapsulation of the extract of T. catappa fruit was performed by phase inversion using Crodamol OQ-LQ/distilled water/Tween 80 formulations. All formulations formed nanoemulsions, which showed good stability under centrifugal force stress and nanoemulsion droplets still had an average size of less than 69 nm. Although the average size of droplets was less than 200 nm, the characteristics of the nanoemulsions degenerated after 26 days, especially under temperatures of 25 and 35 °C. Therefore, the present study could expand the possibilities of the application of bioactive from T. catappa fruit for pharmaceutical purposes or cosmetic ones.

Dynamic modeling of rotary blade crack with regard to three-dimensional tip clearance

To investigate the dynamic characteristics of the three-dimensional blade tip clearance (3D-BTC) of rotary blade cracks, a novel dynamic model of the rotary blade crack with regard to the 3D-BTC is derived based on the Timoshenko beam theory. In this model, the modal shape functions of the blade are derived based on the transfer matrix method, and the axial and circumferential aerodynamic loads are introduced to better obtain the dynamic responses of the blade vibration. Then, a breathing crack model of the blade trailing edge crack is established by considering the coupling effects of the centrifugal stress, axial bending stress, and circumferential bending stress. The crack opening width is introduced into the breathing crack model to comprehensively depict the crack together with the crack depth and crack location. Moreover, a numerical calculation method for the 3D-BTC is proposed for the rotary blade with cracks. The dynamic model is validated by comparing it with the finite element model and the experiment. Finally, the effects of the blade trailing edge crack on the 3D-BTC are analyzed. The results show that the cracked blade's first-order natural frequency decreases with the crack depth increase, and it increases first and then decreases slightly when the crack location moves away from the blade root. The effects of crack opening width on the first-order natural frequency are relatively small due to its small size. In the meantime, the nonlinear damage indicators (NDIs) of the 3D-BTC are calculated. The results show that the NDIs can reflect the severity of the blade crack, and the proposed model can comprehensively analyze the effects of rotary blade crack on the 3D-BTC, which is beneficial for the diagnosis method for blade cracks based on the 3D-BTC.