1,720,994 research outputs found
Fretting wear of alloy steels at the blade tip of steam turbines
Full text linkIn order to reduce blade resonant vibration amplitude in turbomachinery, blades are assembled with a mutual interlocking at the tip. The aim of this study is to investigate the wear mechanism at the contact interface of the blade shroud in steam turbines. Experimental data are available concerning the wear mechanism at interfaces of aircraft engines blades, while the literature regarding the same effect on steam turbines is less rich. Moreover, the transposition of the results from the aero-engine to the steam turbine is difficult, because materials and working conditions are different. To overcome this lack of knowledge an experimental campaign was set up to investigate this wear mechanism under the specific conditions and with the distinctive materials used in steam turbines.
Two base materials (alloy steels) were tested under different conditions: surface treatment (with and without laser quenching), temperature and normal load. Dissipated energies were determined from the hysteresis loops measured during the tests and were correlated to the test conditions. Profiles of worn surfaces were measured, and volume losses were accurately computed with a procedure that takes into account the roughness of the surfaces.
Experiments were conducted both at room and low temperature (150 °C). At room temperature the surface temperature increased to 60-70 °C, due to the heat generated in the wear process. Comparison of volume losses at room and low temperature showed that at 150°C the volume losses decreased dramatically. This behavior was explained with a brittle-ductile transition. In other words, the same wear mechanism, adhesion and abrasion respectively in stick and gross slip condition, give very different results for a small softening effect of the material. Moreover, experimental results showed much more sensitive wear rates to the heat treatment than to the steel type
Innovative adaptive penalty in surrogate-assisted robust optimization of blade attachments
No full textThis paper proposes the combination of the adaptive penalty method based on design space sampling and evolutionary optimization towards the solution of a multi-objective blade attachment robust design problem. An adaptive penalty function based on Latin hypercube sampling was applied to tackle the non-feasible spaces inside the searching domain. Implementing this method provided a reduction in time to convergence. A genetic algorithm (GA) was used as an optimizer to minimize the stress state in critical areas of the attachment. The state of stress was computed using a finite-element model denoted as high-fidelity model. To reduce the call back to the high-fidelity model, a meta-model (also denoted as surrogate model) was developed and embedded in the GA to reduce the computational time. Using the surrogate model instead of the high-fidelity model also provided a reduction in the time needed to find the optimum. Besides, in order to obtain the most robust solution among the optimums given by the Pareto front, the same Kriging surrogate model was employed to perform a global sensitivity analysis
Fretting wear damage mechanism of CoMoCrSi coatings
No full textSuperalloy coatings of the CoMoCrSi family (e.g. Tribaloy® T800) are applied to mitigate wear effects at high temperature. These coatings are extensively used on the contact surfaces of the shroud of turbine blades. If severe wear occurs on these contact surfaces the blade interlocking decreases, reducing the stiffness of the assembly, altering its dynamic behaviour, and increasing the risk of fatigue failure. Fretting is the expected damage mechanics on these mating surfaces. The study presented in this paper investigates the fretting damage mechanism of interfaces coated with CoMoCrSi alloys. The experimental plan includes fifteen combinations of the test parameters: two contact geometries, three deposition processes, four temperatures, three normal loads and three strokes. Wear at different number of cycles was also explored. Moreover, two types of contact geometries were investigated, namely point contact (sphere-on-flat) and flat-on-flat. The friction coefficient was computed using the hysteresis loops measured during the fretting tests. The topography of the contact surfaces was measured at different fretting wear cycles to estimate the volume loss. Wear grooves were observed by scanning electron microscopy. Results of point contact experiments at room temperature exhibited a steady friction coefficient independent of the normal load. Wear volumes showed a sharp increasing in wear rate at high dissipated energy while the trend was linear at lower dissipated energy. Oxidation was found more dependent on substrate than on temperature, stroke and wear cycles. Wear volumes and wear rate on flat-on-flat specimens coated with welded T800 were higher at 400 C than at room temperature and at high temperature (800 C). At room temperature, wear volumes of welded T800 applied by single layer were much higher than in dual layer. At room temperature and at low dissipated energy the wear rate of the point contact geometry was lower than flat-on-flat. At high energy, the wear rate of point contact tends to the flat-on-flat wear rate
The effect of friction damping on the dynamic response of vibrating structures: an insight into model validation
No full textDry friction is widely used in turbomachinery in the form of under platform dampers to limit resonant vibration and avoid high-cycle fatigue failures of the blades. Most test rigs that are used to investigate the behavior of dampers aim to evaluate their performance by reduction in blade vibration amplitude. This approach is insufficient to understand local nonlinearities of the contact and influence of blade dynamics on UPDs behavior. A newly developed test rig provides the user with an unprecedented set of information: it measures contact forces and relative displacements between dampers and blade together with the overall blade response. This controlled environment, together with a state-of-the-art numerical model of the test rig, is used to provide an insight into the subject of model validation. The presented experimental and numerical study of the damper is used to highlight the relevance of an accurate representation of the constraints induced by friction contacts and to discuss the adequacy of state-of-the-art contact models
Effect of variation in contact friction on the performance of the under-platform dampers
Full text linkUnder platform-dampers (UPDs) are commonly used devices in turbomachinery to mitigate the turbine blade vibrations caused by the periodically fluctuating stresses. These dampers are placed in the underside of two adjacent blades and vibration energy is partly dissipated by the friction at the blade/damper interfaces. As a result, the vibration amplitude is reduced with beneficial effects on the blade fatigue life. At LAQ AERMEC a novel test rig has been developed to accurately measure the response of a single turbine blade and the kinematics/dynamics of two adjacent UPDs. In this newly developed test rig, each damper is in contact with the under-platform of the blade on one side and with ground/fixed platform on its other side. The dampers are pressed against the blade platform by static forces applied by dead weights. A static force is also radially applied to root of the blade to clamp it to the rig, simulating the effect of the actual centrifugal force in operating conditions. Finally, a transverse periodic excitation is applied in order to excite the blade's first resonances. In this paper, the performance of different UPDs in terms of reduction of the blade vibration amplitude and shift in resonance frequency is studied at two different contact friction conditions (normal and low friction). Low friction conditions are obtained by introducing a thin layer of oil between the damper-blade contact interfaces. Experiments are performed on a real turbine blade to investigate the semi-cylindrical dampers. This profound study of UPDs provides a strong basis to understand the effect of damper with different contact conditions, to limit the blade vibratio
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Unbalanced Langmuir kinetics affects TASEP dynamical transitions: mean-field theory
Full text linkIn a previous study we developed a mean-field theory of dynamical transitions for the totally-asymmetric simple-exclusion process with open boundaries and Langmuir kinetics, in the so-called balanced regime, characterized by equal binding and unbinding rates. Here we show that simply including the possibility of unbalanced rates gives rise to an unexpectedly richer dynamical phase diagram. In particular, the current work predicts an unusual type of dynamical transition, which exhibits certain similarities with first-order phase transitions of equilibrium systems. We also point out that different types of dynamical transition are accompanied by different structural changes in the (mean-field) relaxation spectrum
High-frequency nonlinear vibration analysis through low-frequency stereo-camera systems
Full text linkThis paper describes a new methodology that expands the capabilities of low-frame, high-resolution stereo-camera systems in studying the dynamic behavior of components in the presence of nonlinear phenomena. A new downsampling technique called the Smoothed Harmonics Analysis (SHA) is proposed. This technique addresses the limitations due to the low-frame rate cameras for the study of high-frequency periodic steady-state nonlinear oscillations. SHA enables accurate reconstruction of downsampled signals, thus opening up numerous potential applications. The feasibility of this technique is demonstrated by analyzing the motion of a beam with nonlinear behavior. The nonlinearity is caused by intermittent contact while the beam is subjected to harmonic excitation
Unbalanced Langmuir kinetics affects TASEP dynamical transitions: Mean-field theory
Full text linkIn a previous study we developed a mean-field theory of dynamical transitions for the totally-asymmetric simple-exclusion process with open boundaries and Langmuir kinetics, in the so-called balanced regime, characterized by equal binding and unbinding rates. Here we show that simply including the possibility of unbalanced rates gives rise to an unexpectedly richer dynamical phase diagram. In particular, the current work predicts an unusual type of dynamical transition, which exhibits certain similarities with first-order phase transitions of equilibrium systems. We also point out that different types of dynamical transition are accompanied by different structural changes in the (mean-field) relaxation spectrum
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