Journal articles on the topic 'Synchronous thermal instability'
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1
Balbahadur, A. C., and R. G. Kirk. "Part II—Case Studies for a Synchronous Thermal Instability Operating in Overhung Rotors." International Journal of Rotating Machinery 10, no. 6 (2004): 477–87. http://dx.doi.org/10.1155/s1023621x04000478.
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In Part I, a theoretical model was developed for a synchronous thermal instability that is caused by differential viscous shearing in bearings of overhung rotors. This second part used computer programs, which were based on the theoretical model, to examine various case studies that pertain to this thermal instability. Both plain and tilting pad journal bearing rotors were examined and good agreement was found between the theoretical predictions and the practical results.
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Balbahadur, A. C., and R. G. Kirk. "Part I—Theoretical Model for a Synchronous Thermal Instability Operating in Overhung Rotors." International Journal of Rotating Machinery 10, no. 6 (2004): 469–75. http://dx.doi.org/10.1155/s1023621x04000466.
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Atheoretical model has been developed for a synchronous thermal instability that is caused by differential viscous shearing in bearings of overhung rotors. This model employs an unbalance threshold criterion for instability instead of utilizing a traditional frequency-domain stability analysis. The current model will be used to investigate several case studies for both plain and tilting pad journal bearing rotors in the second part of this article.
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Singh, A. Narain, W. Doorsamy, and W. A. Cronje. "Thermal Instability Analysis of a Synchronous Generator Rotor using Direct Mapping." SAIEE Africa Research Journal 109, no. 1 (March 2018): 4–14. http://dx.doi.org/10.23919/saiee.2018.8531795.
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Bhadauria, Beer S., Atul K. Srivastava, Nirmal C. Sacheti, and Pallath Chandran. "Gravity Modulation of Thermal Instability in a Viscoelastic Fluid Saturated Anisotropic Porous Medium." Zeitschrift für Naturforschung A 67, no. 1-2 (February 1, 2012): 1–9. http://dx.doi.org/10.5560/zna.2011-0045.
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The present paper deals with a thermal instability problem in a viscoelastic fluid saturating an anisotropic porous medium under gravity modulation. To find the gravity modulation effect, the gravity field is considered in two parts: a constant part and an externally imposed time-dependent periodic part. The time-dependent part of the gravity field, which can be realized by shaking the fluid, has been represented by a sinusoidal function. Using Hill’s equation and the Floquet theory, the convective threshold has been obtained. It is found that gravity modulation can significantly affect the stability limits of the system. Further, we find that there is a competition between the synchronous and subharmonic modes of convection at the onset of instability. Effects of various parameters on the onset of instability have also been discussed.
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Balbahadur, A., and R. Kirk. "Part I?Theoretical Model for a Synchronous Thermal Instability Operating in Overhung Rotors." International Journal of Rotating Machinery 10, no. 6 (November 1, 2004): 469–75. http://dx.doi.org/10.1080/10236210490504021.
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Balbahadur, A., and R. Kirk. "Part II?Case Studies for a Synchronous Thermal Instability Operating in Overhung Rotors." International Journal of Rotating Machinery 10, no. 6 (November 1, 2004): 477–87. http://dx.doi.org/10.1080/10236210490504067.
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Olsson, Karl-Olof. "Some Unusual Cases of Rotor Instability." Journal of Vibration and Acoustics 125, no. 4 (October 1, 2003): 477–81. http://dx.doi.org/10.1115/1.1606692.
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In the paper presented here three peculiar cases of rotor instability are presented. The first is a tilting-pad bearing used as an intershaft bearing in a two-spool gas turbine. It was initially chosen for its good stability properties. However, in a deeper study it was found that those benefits are greatly reduced when both inner shaft and outer bearing are rotating. The second case comprises a turbine which showed a very strange performance with vibrations alternating between heavy and mild amplitudes. The periods were a couple of minutes long. After a more elaborate instrumentation was installed it was found that vibrations were synchronous and that the rotor experienced continuous changes of vibrations interrupted by sudden changes of amplitude. This revealed that nonlinearity and thermal effects coupled to the rotor vibrations must be incorporated to understand the behavior. Many hypotheses were tested and the one coming closest to explaining the case was one incorporating thermal bending caused by heat conduction through an annulus partly filled with oil. The third case is a simple one of more pure curiosity, where a rotor was provided with a substantial amount of inner damping, and just according to the school book showed dynamic instability.
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Yoshida, Yoshiki, Yoshifumi Sasao, Kouichi Okita, Satoshi Hasegawa, Mitsuru Shimagaki, and Toshiaki Ikohagi. "Influence of Thermodynamic Effect on Synchronous Rotating Cavitation." Journal of Fluids Engineering 129, no. 7 (January 10, 2007): 871–76. http://dx.doi.org/10.1115/1.2745838.
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Synchronous rotating cavitation is known as one type of cavitation instability, which causes synchronous shaft vibration or head loss. On the other hand, cavitation in cryogenic fluids has a thermodynamic effect on cavitating inducers because of thermal imbalance around the cavity. It improves cavitation performances due to delay of cavity growth. However, relationships between the thermodynamic effect and cavitation instabilities are still unknown. To investigate the influence of the thermodynamic effect on synchronous rotating cavitation, we conducted experiments in which liquid nitrogen was set at different temperatures (74K, 78K, and 83K). We clarified the thermodynamic effect on synchronous rotating cavitation in terms of cavity length, fluid force, and liquid temperature. Synchronous rotating cavitation occurs at the critical cavity length of Lc∕h≅0.8, and the onset cavitation number shifts to a lower level due to the lag of cavity growth by the thermodynamic effect, which appears significantly with rising liquid temperature. Furthermore, we confirmed that the fluid force acting on the inducer notably increases under conditions of synchronous rotating cavitation.
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Bhadauria, B. S. "Unsteady Heating of Rayleigh-Benard Convection." Zeitschrift für Naturforschung A 59, no. 4-5 (May 1, 2004): 266–74. http://dx.doi.org/10.1515/zna-2004-4-511.
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The linear thermal instability of a horizontal fluid layer with time-periodic temperature distribution is studied with the help of the Floquet theory. The time-dependent part of the temperature has been expressed in Fourier series. Disturbances are assumed to be infinitesimal. Only even solutions are considered. Numerical results for the critical Rayleigh number are obtained at various Prandtl numbers and for various values of the frequency. It is found that the disturbances are either synchronous with the primary temperature field or have half its frequency. - 2000 Mathematics Subject Classification: 76E06, 76R10.
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OR, A. C., and R. E. KELLY. "The effects of thermal modulation upon the onset of Marangoni–Bénard convection." Journal of Fluid Mechanics 456 (April 9, 2002): 161–82. http://dx.doi.org/10.1017/s0022112001007510.
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The effects of thermal modulation with time on the thermocapillary instability of a thin horizontal fluid layer with a deformable free surface are investigated on the basis of linear stability theory. First, a sinusoidal heating with a mean component is applied at the lower wall, corresponding to boundary conditions either in the form of prescribed temperature or heat flux. For finite-wavelength convection the thermal modulation exerts a strong effect, giving rise to a family of looped regions of instability corresponding to alternating synchronous or subharmonic responses. In the case of prescribed heat flux, the critical curve consists of significantly fewer loops than in the case of prescribed temperature. Thermal modulation with moderate modulation amplitude tends to stabilize the mean basic state, and optimal values of frequency and amplitude of modulation are determined to yield maximum stabilization. However, large-amplitude modulation can be destabilizing. A basic state with zero mean is then considered and the critical Marangoni number is obtained as a function of frequency. The effects of modulation are also investigated in the long-wavelength limit. For the case of prescribed temperature, the modulation does not affect the onset of the long-wavelength mode associated with the mean basic state and a purely oscillating basic state is always stable with respect to long-wavelength disturbances. For the case of prescribed heat flux both at the wall and free surface, by contrast, thermal modulation exerts a significant effect on the onset of convection from a mean basic state and long-wavelength convection can occur even for a purely oscillating basic state. The modulation can be stabilizing or destabilizing, depending on the frequency.
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Hu, Qilong, Min Zhu, and Jiangang Yang. "Study on Thermal Unstable Vibration of Rotor under Journal Whirl with Large Amplitude in Journal Bearing." International Journal of Rotating Machinery 2020 (January 25, 2020): 1–12. http://dx.doi.org/10.1155/2020/1980759.
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To investigate the thermal unstable vibration caused by journal whirls with large amplitude in journal bearing, an analysis model of lubricant film thickness is established. The journal surface temperature distribution is solved, and the reason for journal surface temperature difference appearance and its influence on rotor vibration are analyzed. Taking a turbogenerator as an example, the journal surface temperature difference and the induced rotor thermal bending under synchronous whirl in the bearing are calculated. Meanwhile, an engineering vibration fault with its treatment is presented. Results show that, the journal surface circumferential temperature difference is caused by viscous shearing within lubricant film under journal whirls with large amplitude in journal bearing. The direction of temperature difference is related to the direction of unbalanced force acting on journal. The temperature difference causes rotor thermal bending, which can be converted to a thermal unbalance on the rotor. The rotor vibration is caused by both thermal and initial unbalance. When the rotor is running below or at the critical speed, the vibration is on the increase until it leads to instability of the rotor eventually. When the rotor is running above the critical speed, the rotor vibration fluctuates periodically. Reducing the initial (mechanical) unbalances decreases the rotor vibration and the journal surface circumferential temperature difference.
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Varlamov, Ilya, Stanislav Parnikov, Igor Ievenko, Dmitry Baishev, and Kazuo Shiokawa. "Registration of synchronous geomagnetic pulsations and proton aurora during the substorm on March 1, 2017." EPJ Web of Conferences 254 (2021): 02012. http://dx.doi.org/10.1051/epjconf/202125402012.
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Data of synchronous geomagnetic pulsations and proton aurora registrations were analyzed during the substorm on March 1, 2017 at Zhigansk (L=4.5, induction magnetometer), Maimaga (L=4, all-sky imager and Yakutsk (L = 3.3, induction magnetometer) stations, simultaneously with satellite measurement of EMIC waves. Ground-based registration of proton aurora is very difficult due to the fact that their intensity is much lower than the aurora intensity caused by precipitations of electrons, but in the event of substorm activity at the zenith of Maimaga station, a narrow (1 degree in latitude) proton arc was observed. Irregular pulsations of the diminishing periods (IPDPs) in the range of Pc1 geomagnetic pulsations associated with the injection of energetic protons were recorded simultaneously at Zhigansk and Yakutsk stations. This is the first report when STEVE (Strong Thermal Emission Velocity Enhancement) was observed in the course of a substorm with the onset at 12:45 UT after the decay of Pc1-associated proton arc. It is shown that the proton arc and geomagnetic pulsations are a consequence of ion-cyclotron instability in the area of the outer plasmasphere overlapping by energetic protons.
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Shahani, Z. A., A. A. Hashmani, and M. M. Shaikh. "Steady State Stability Analysis and Improvement using Eigenvalues and PSS: A Case Study of a Thermal Power Plant in Jamshoro, Pakistan." Engineering, Technology & Applied Science Research 10, no. 1 (February 3, 2020): 5301–6. http://dx.doi.org/10.48084/etasr.3318.
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The efficient handling and distribution of electrical power consist one of the most complex and appealing research problems. Due to the interconnection of different power plants and intensity of load, which gradually changes due to continuous change in load on generating units, careful treatment of the small disturbances in a power system which may lead to severe disturbance is necessary. Stability is an essential part in electrical power system operation and control. The stability problem is related with the behavior of synchronous machine after the power system is subjected to trouble. This work presents Steady State Stability (SSS) analysis of a Jamshoro Thermal Power Plant (JTPP) by using eigenvalue analysis of the different cases by varying load at three different positions. A mathematical model has been used for the JTPP with real data in order to examine the behavior of the system and to find the eigenvalues. A Simulink model of the JTTP for waveform analysis in MATLAB/Simulink has been used without and with Power System Stabilizer (PSS). Numerical quantification of the eigenvalues under the examined cases categorizes the stability of the system. The waveforms of the system are analyzed, and in cases of instability, the proposed procedure utilizing PSS helps in maintaining the system’s usual working conditions. The eigenvalue analysis and simulation results show the behavior of synchronous machines when loading changes gradually. The existing system becomes stable after more swings, whereas by using PSS in the existing system, stable regimes are attained in less time. The obtained results demonstrate the effectiveness of the proposed solution for SSS examination and securing of the disturbances of the JTPP.
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Duc Tung, Doan, Le Van Dai, and Le Cao Quyen. "Subsynchronous Resonance and FACTS-Novel Control Strategy for Its Mitigation." Journal of Engineering 2019 (July 14, 2019): 1–14. http://dx.doi.org/10.1155/2019/2163908.
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The subsynchronous resonance (SSR) is an important problem in the power system, and especially the series compensated transmission lines may cause SSR in the turbine generators, such that it leads to the electrical instability at subsynchronous frequencies and potential turbine-generator shaft failures. Taking the Vietnamese Vungang thermal plants as an example, a shaft failure of Vungang I thermal power unit has occurred on November 24, 2015, due to SSR. The main cause for this failure is a resonance caused by the series capacitors on the 500 kV grid. This paper analyzes the SSR based on the location of shaft cracks and turbine generator mode shape for Vungang I and II thermal power plants. On the basis of that, it develops a novel control strategy for each Flexible AC Transmission system (FACTS) device as the thyristor controlled series compensator (TCSC), static VAR compensator (SVC), and static synchronous compensator (STATCOM). Then they are comparable to one another in order to choose a feasible solution for mitigating the SSR. The effectiveness of the proposed control strategy is verified via time domain simulation of the Vietnamese 500/220 kV transmission system using EMTP-RV and PSS/E programs. The obtained results show that the proposed strategy for SVC can be applied to immediately solve the difficulties encountering in the Vietnamese power system.
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Lin, Feng. "(Battery Division Early Career Award Sponsored by Neware Technology Limited) Design, Synthesis, and Characterization of Cathode Microstructures in Lithium Batteries." ECS Meeting Abstracts MA2022-02, no. 3 (October 9, 2022): 210. http://dx.doi.org/10.1149/ma2022-023210mtgabs.
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The propagation of redox reactions governs the electrochemical properties of battery materials and their critical performance metrics in battery cells. The recent research progress, especially aided by advanced analytical techniques, has revealed that incomplete and heterogeneous redox reactions prevail in many electrode materials. Advanced high-capacity cathode materials are mostly polycrystalline materials that exhibit complex charge distribution (the valence state distribution of the redox-active cations) due to the presence of numerous constituting grains and grain boundaries. The redox reactions in individual grains typically do not proceed concurrently due to their distinct geometric locations in polycrystalline particles. As a result, these unsynchronized local redox events collectively induce heterogeneous and anisotropic charge distribution, building up intergranular and intragranular stress. Therefore, these polycrystalline materials may exhibit weak mechanical stability, leading to undesired chemomechanical breakdown during battery operation. Grain engineering in polycrystalline materials provides a large playground to modulate the materials properties beyond controlling the chemical composition, and electronic and crystal structures. In particular, the anisotropic ion-conducting pathways in layered oxides make the grain crystallographic orientation a critical factor in determining the modality of the redox reactions in these materials. This presentation will discuss our recent progress in the design, synthesis, and characterization of cathode microstructures in lithium batteries. First, we will discuss how the charge distribution is guided by grain crystallographic orientations in polycrystalline battery materials. We elucidate the spatially resolved charge distribution in lithium layered oxides with different grain crystallographic arrangements and establish a model to quantify their charge distributions. While the holistic “surface-to-bulk” charge distribution prevails in polycrystalline particles, the crystallographic orientation-guided redox reaction governs the charge distribution in the local charged nanodomains. Compared to the randomly oriented grains, the radially aligned grains exhibit a lower cell polarization and higher capacity retention upon battery cycling. The radially aligned grains create less tortuous lithium-ion pathways, thus improving the charge homogeneity as statistically quantified from over 20 million nanodomains in polycrystalline particles. This study provides an improved understanding of the charge distribution and chemomechanical properties of polycrystalline battery materials. Second, we will discuss how the grain arrangement affects the thermal stability of polycrystalline cathode materials in rechargeable batteries. We performed a systematic in situ study on the Ni-rich polycrystalline cathode materials to investigate the fundamental degradation mechanism of charged cathodes at elevated temperatures, which is essential for tailoring material properties and improving performance. Using multiple microscopy, scattering, thermal, and electrochemical probes, we decoupled the major contributors to the thermal instability from intertwined factors. Based on our findings, the cathode grain microstructure has a forgotten yet important role in the thermal stability of polycrystalline rechargeable batteries. Oxygen release, as an important process during the thermal runaway, can be regulated through engineering grain arrangements. The grain arrangement can modulate the macroscopic crystallographic transformation pattern and oxygen diffusion length in layered cathodes to offer more possibilities for cathode material design and synthesis. Third, we will discuss our new understanding of particle behaviors in composite cathodes. We capture and quantify the particle motion during the solidification of battery electrodes and reveal the statistics of the dynamically evolving motion in the drying process, which has been challenging to resolve. We discover that the particle motion exhibits a strong dependence on its geometric location within a drying electrode. Our results also imply that the final electrode quality can be controlled by balancing the solvent evaporation rate and the particle mobility in the region close to the drying surface. We formulate a network evolution model to interpret the regulation and equilibration between electrochemical activity and mechanical damage of these particles. Through statistical analysis of thousands of particles using x-ray phase-contrast holotomography in a Ni-rich cathode, we found that the local network heterogeneity results in asynchronous activities in the early cycles, and subsequently the particle assemblies move toward a synchronous behavior. Our study pinpoints the chemomechanical behavior of individual particles and enables better designs of the conductive network to optimize the utility of all the particles during operation.
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ZHAO, Yanjun, Guohua NI, Wei LIU, Hongmei SUN, Siyuan SUI, Dongdong LI, Huan ZHENG, Zhongyang MA, and Chi ZHANG. "Dynamic characteristics of multi-arc thermal plasma in four types of electrode configurations." Plasma Science and Technology 24, no. 5 (April 13, 2022): 055407. http://dx.doi.org/10.1088/2058-6272/ac4ee7.
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Abstract The enhanced volume of thermal plasma is produced by a multi-arc thermal plasma generator with three pairs of discharge electrodes driven by three directed current power suppliers. Combined with a high-speed camera and an oscilloscope, which acquire optical and electric signals synchronously, the dynamic behavior of different kinds of multi-arc discharge adjusted by the electrode arrangement is investigated. Also, the spatial distributions and instability of the arc discharge are analyzed in four electrode configurations using the gray value statistical method. It is found that the cathodic arcs mainly show a contracting state, while the anodic arcs have a trend of transition from shrinkage to a diffusion-like state with the increase of the discharge current. As a result of the adjustment of the electrode configuration, a high temperature region formed in the center of the discharge region in configurations of adjacent electrodes with opposite flow distribution and opposite electrodes with swirl flow distribution due to severe fluctuation of arcs. The discharge voltage rises with increased discharge current in this novel multi-arc plasma generator. It is also found that anode ablation mainly occurs on the conical surface at the copper electrode tip, while cathode erosion mainly occurs on the surface of the inserted tungsten and the nearby copper.
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Guo, Zenglin, and Gordon Kirk. "Morton Effect Induced Synchronous Instability in Mid-Span Rotor–Bearing Systems—Part I: Mechanism Study." Journal of Vibration and Acoustics 133, no. 6 (October 4, 2011). http://dx.doi.org/10.1115/1.4004665.
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The Morton Effect in rotor-bearing systems may lead to an unstable operation. In Part I, the mechanism of the Morton Effect–induced thermal instability in the mid-span rotor systems is studied. First, the equivalent thermal induced imbalance is introduced and its magnitude and directions are assumed, to represent the viscous thermal effect on the rotor systems. Then, the simplified rotor and bearing models are adopted for the derivation of analytical expressions. The results show that there exists a threshold of instability due to the Morton Effect in the mid-span rotors. Based on the assumptions of linear isotopic bearing supports, this threshold speed takes a simple form, which is determined by the support stiffness and the introduced equivalent coefficient of thermal effect, for the rigid or elastic rotors, with the thermal imbalance acting in the same direction as the response displacement. The threshold of instability is also obtained for the system with the thermal imbalance acting perpendicular to the response displacement, where the supporting damping plays a role. For a perspective view of the system stability, a stability map for the damped rigid mid-span rotors with the thermal imbalance having arbitrary phase difference is generated. It shows that the stable operating regions of the system are bounded by two curves of threshold of instability, named the first and second threshold speeds of instability, respectively. The Morton Effect–induced instability thresholds are actually affected by both the magnitude and relative phase of the thermal imbalance. The mechanism of the Morton Effect–induced thermal instability of mid-span rotors supported by linear isotropic bearings can be explained through the fact that the Morton Effect introduces either negative stiffness or negative cross-coupled stiffness. In addition, the Morton Effect also has a comprehensive impact on both the amplitude and phase lag of the steady-state unbalance response. It may shift both curves in a manner dependent on the relative magnitude and direction of the thermal imbalance.
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Shin, Dongil, Alan B. Palazzolo, and Xiaomeng Tong. "Squeeze Film Damper Suppression of Thermal Bow-Morton Effect Instability." Journal of Engineering for Gas Turbines and Power 142, no. 12 (December 1, 2020). http://dx.doi.org/10.1115/1.4048602.
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Abstract The Morton effect (ME) is a synchronous vibration problem in turbomachinery caused by the nonuniform viscous heating around the journal circumference, and its resultant thermal bow (TB) and ensuing synchronous vibration. This paper treats the unconventional application of the SFD for the mitigation of ME-induced vibration. Installing a properly designed squeeze film damper (SFD) may change the rotor's critical speed location, damping, and deflection shape, and thereby suppress the vibration caused by the ME. The effectiveness of the SFD on suppressing the ME is tested via linear and nonlinear simulation studies employing a three-dimensional (3D) thermohydrodynamic (THD) tilting pad journal bearing (TJPB), and a flexible, Euler beam rotor model. The example rotor model is for a compressor that experimentally exhibited an unacceptable vibration level along with significant journal differential heating near 8000 rpm. The SFD model includes fluid inertia and is installed on the nondrive end bearing location where the asymmetric viscous heating of the journal is highest. The influence of SFD cage stiffness is evaluated.
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Shin, Dongil, and Alan B. Palazzolo. "Tilting Pad Journal Bearing Misalignment Effect on Thermally Induced Synchronous Instability (Morton Effect)." Journal of Tribology 143, no. 3 (September 21, 2020). http://dx.doi.org/10.1115/1.4048164.
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Abstract This paper investigates the influence of misaligned journal bearing effects on the thermally induced rotor instability (Morton effect “ME”) problem. The Morton effect is caused by uneven viscous heating of the journal in a fluid film bearing, which causes thermal bending, especially in rotors with an overhung disc or coupling weight. The thermally induced bending in the shaft may cause a vibration instability, which results in an excessive level of synchronous vibration. Previous research focused on parametric studies of the rotor and bearing design parameters, including overhung mass, bearing radial clearance, and lubricant viscosity. The present study investigates the influence of journal misalignment on the Morton effect. A coupled fluid-thermal-structural, three-dimensional finite element model (FEM) is developed to simulate fluid film pressures and temperatures, and shaft temperatures and vibrations. Simulations were conducted with different ratios of journal misalignment, and different pad-pivot types to determine their effect on the phenomenon. The simulation results indicate that the amplitude of the misalignment angle affects the instability speed range (ISR) caused by the Morton effect under certain conditions.
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Lee, Jung Gu, and Alan Palazzolo. "Morton Effect Cyclic Vibration Amplitude Determination for Tilt Pad Bearing Supported Machinery." Journal of Tribology 135, no. 1 (December 20, 2012). http://dx.doi.org/10.1115/1.4007884.
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This paper presents theoretical models and simulation results for the synchronous, thermal transient, instability phenomenon known as the Morton effect. A transient analysis of the rotor supported by tilting pad journal bearing is performed to obtain the transient asymmetric temperature distribution of the journal by solving the variable viscosity Reynolds equation, a 2D energy equation, the heat conduction equation, and the equations of motion for the rotor. The asymmetric temperature causes the rotor to bow at the journal, inducing a mass imbalance of overhung components such as impellers, which changes the synchronous vibrations and the journal's asymmetric temperature. Modeling and simulation of the cyclic amplitude, synchronous vibration due to the Morton effect for tilting pad bearing supported machinery is the subject of this paper. The tilting pad bearing model is general and nonlinear, and thermal modes and staggered integration approaches are utilized in order to reduce computation time. The simulation results indicate that the temperature of the journal varies sinusoidally along the circumferential direction and linearly across the diameter. The vibration amplitude is demonstrated to vary slowly with time due to the transient asymmetric heating of the shaft. The approach's novelty is the determination of the large motion, cyclic synchronous amplitude behavior shown by experimental results in the literature, unlike other approaches that treat the phenomenon as a linear instability. The approach is benchmarked against the experiment of de Jongh.
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Saravanan, S., and T. Sivakumar. "Thermovibrational Instability in a Fluid Saturated Anisotropic Porous Medium." Journal of Heat Transfer 133, no. 5 (February 1, 2011). http://dx.doi.org/10.1115/1.4003013.
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A comprehensive investigation is made to understand the effect of harmonic vibration on the onset of convection in a horizontal anisotropic porous layer heated either from below or from above. The layer is subject to vertical mechanical vibrations of arbitrary amplitude and frequency. The porous medium is assumed to be both mechanically and thermally anisotropic, and Brinkman’s law is invoked to model the momentum balance. Both continued fraction and Hill’s infinite determinant methods are used to determine the convective instability threshold with the aid of Floquet theory. The synchronous and subharmonic resonant regions of dynamic instability are determined and their critical boundaries are found. The results show that anisotropy in permeability favors convection whereas that in thermal conductivity suppresses it with a wider cellular pattern at the instability threshold. The influence of vibration parameters and heating condition on the anisotropy effects and the competition between the synchronous and subharmonic modes are discussed. This study also reveals the existence of a closed disconnected instability region in certain areas of the parameter space for the first time in literature.
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Tong, Xiaomeng, Alan Palazzolo, and Junho Suh. "A Review of the Rotordynamic Thermally Induced Synchronous Instability (Morton) Effect." Applied Mechanics Reviews 69, no. 6 (October 20, 2017). http://dx.doi.org/10.1115/1.4037216.
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The Morton effect (ME) is a thermally induced instability problem that most commonly appears in rotating shafts with large overhung masses and supported by fluid-film bearings. The time-varying thermal bow, due to the asymmetric journal temperature distribution, may cause intolerable synchronous vibrations that exhibit a hysteresis behavior with respect to rotor speed. First discovered by Morton in the 1970s and theoretically analyzed by Keogh and Morton in the 1990s, the ME is still not fully understood by industry and academia experts. Traditional rotordynamic analysis generally fails to predict the potential existence of ME-induced instability in the design stage or troubleshooting process, and the induced excessive rotor vibrations cannot be effectively suppressed through conventional balancing, due to the continuous fluctuation of vibration amplitude and phase angle. In recent years, a fast growing number of case studies of ME have sparked academic interest in analyzing the causes and solutions of ME, and engineers have moved from an initial trial and error approach to more research inspired modification of the rotor and bearing. To facilitate the understanding of ME, the current review is intended to give the most comprehensive summary of ME in terms of symptoms, causes, prediction theories, and solutions. Published case studies in the past are also analyzed for ME diagnosis based on both the conventional view of critical speed, separation margin (SM), and the more recent view of the rotor thermal bow and instability speed band shifting. Although no universal solutions of ME are reported academically and industrially, recommendations to help avoid the ME are proposed based on both theoretical predictions and case studies.
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Guo, Zenglin, and Gordon Kirk. "Morton Effect Induced Synchronous Instability in Mid-Span Rotor–Bearing Systems, Part 2: Models and Simulations." Journal of Vibration and Acoustics 133, no. 6 (October 12, 2011). http://dx.doi.org/10.1115/1.4004666.
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The mechanism of the Morton Effect induced synchronous instability has been discussed in Part 1, using an assumption of isotropic linear bearings. The second part of the current study will now focus on the more realistic systems, mid-span rotors supported by the hydrodynamic journal bearings. First, the models to calculate the thermal bending of the shaft and the temperature distribution across the journal surface are established. This can be used to calculate the equivalent thermal imbalance. The calculations of the temperature difference and its equivalent thermal imbalance using hydrodynamic plain journal bearing models are conducted and discussed with the comparison to the analytical results obtained in Part 1. It shows that the thermal imbalance induced by the Morton Effect may increase to the level of the mechanical imbalance and then its influence on the system stability should be included. The suggested thermal bending model also partially explains that the mid-span rotors are less liable to be influenced by the Morton Effect induced instability than are the overhung configurations, because of the restraining effect between two supports. Finally, a symmetric mid-span rotor - hydrodynamic journal bearing system is calculated to show its stability performance. The results show the inclusion of the Morton Effect may lead to an unstable operation of the system. Considering the existence of the oil film self-induced vibration due to the dynamic characteristics of fluid film bearings, the Morton Effect may make a further negative impact on the stability of the system. The simulation results of the unbalance response show that the Morton Effect changes the shapes of the whirling orbits and makes them no longer the standard elliptical orbits around the static equilibriums.
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Suh, Junho, and Alan Palazzolo. "Three-Dimensional Thermohydrodynamic Morton Effect Analysis—Part II: Parametric Studies." Journal of Tribology 136, no. 3 (April 25, 2014). http://dx.doi.org/10.1115/1.4027310.
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This paper presents simulation results corresponding with the theoretical Morton effect model explained in Part I, where the 3D finite element models of bearing, shaft, and fluid film are adopted. In addition, it explains how thermal bow induced imbalance force develops in the spinning journal with time and how the vibration level is affected by the thermal bow vector. Shaft asymmetric thermal expansion induced by nonuniform journal heating is simulated, which is one of the unique contributions of this research. The effect of changes in: (1) thermal boundary condition around the pad, (2) lubricant supply temperature, (3) initial mechanical imbalance, (4) pivot stiffness, (5) film clearance, and (6) pad material are studied. Cooling the pad and the lubricant, using a pad with a low thermal expansion coefficient, soft pivot, and reducing the initial imbalance are found to be the best remedies for the thermal induced synchronous rotor instability problem.
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Suh, Junho, and Alan Palazzolo. "Three-Dimensional Thermohydrodynamic Morton Effect Simulation—Part I: Theoretical Model." Journal of Tribology 136, no. 3 (April 25, 2014). http://dx.doi.org/10.1115/1.4027309.
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The present study is focused on accurate prediction of the Morton effect problem including journal asymmetric heating and the corresponding long period amplitude oscillations using a nonlinear time transient rotor-dynamic simulation. This paper presents a theoretical model of thermal induced synchronous instability problems in a nonlinear rotor–bearing system, and suggests a new computational algorithm for the nonlinear transient analysis of the Morton effect where the dynamic and thermal problems are combined. For the analysis of the Morton effect problem, a variable viscosity Reynolds equation and a 3D energy equation are coupled via temperature and viscosity, and solved simultaneously. Three-dimensional heat transfer equations of bearing and shaft are modeled by a finite element method, and thermally coupled with the fluid film via a heat flux boundary condition. Asymmetric heat flux into the synchronously whirling rotor is solved by the orbit time averaged heat flux from fluid film to the spinning shaft surface. The journal orbit is calculated by the nonlinear transient dynamic analysis of a rotor–bearing system with a variable time step numerical integration scheme. For the computation time reduction, modal coordinate transformation is adopted in dynamic and thermal transient analysis. Thermal bow effect makes a significant change to the dynamic behavior of a rotor–bearing system, and a thermal hysteresis bode plot, that is one of the characteristics of the Morton effect problem, is presented with time varying spin speed.
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Tong, Xiaomeng, and Alan Palazzolo. "Double Overhung Disk and Parameter Effect on Rotordynamic Synchronous Instability—Morton Effect—Part I: Theory and Modeling Approach." Journal of Tribology 139, no. 1 (August 16, 2016). http://dx.doi.org/10.1115/1.4033888.
Full textAbstract:
The Morton effect (ME) is a thermally induced instability problem that most commonly appears in rotating shafts with large overhung masses, outboard of the bearing span. The time-varying thermal bow due to the asymmetric journal temperature distribution may cause intolerable synchronous vibrations that exhibit a hysteresis behavior with respect to rotor speed. The fully nonlinear transient method designed for the ME prediction, in general, overhung rotors is proposed with the capability to perform the thermoelastohydrodynamic analysis for all the bearings and model the rotor thermal bow at both overhung ends with equivalent distributed unbalances. The more accurate nonlinear, coupled, double overhung approach is shown to provide significantly different response prediction relative to the more approximate linear method based using bearing coefficients and the single-overhung method, which assumes that the ME on both rotor ends can be decoupled. The flexibility of the bearing pad and pivot is investigated to demonstrate that the pivot flexibility can significantly affect the rotordynamics and ME, while the rigid pad model is generally a good approximation.
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Shin, Dongil, and Alan B. Palazzolo. "Tilting Pad Bearing Pivot Friction and Design Effects on Thermal Bow-Induced Rotor Vibration." Journal of Tribology 143, no. 12 (March 29, 2021). http://dx.doi.org/10.1115/1.4050427.
Full textAbstract:
Abstract The Morton effect (ME) is a thermally induced vibration problem observed in a rotor supported by hydrodynamic bearings. The journal’s synchronous orbiting induces nonuniform viscous heating on its circumference, and the ensuing thermal bow often causes unacceptable vibration levels in the rotor. This paper investigates the influence of the tilting pad journal bearing (TPJB)’s pivot design on the severity and instability speed range of ME vibration. Simulations are conducted with two different types of pivots: cylindrical (CYL) and spherical (SPH), which produce different pad degrees-of-freedom and nonlinear pivot stiffness due to their geometries. The friction between pad and pivot, which only exists with the spherical pivot, is modeled, and its impact on the ME is evaluated. The example rotor model, as obtained from the literature, is single overhung, with experimentally measured excessive vibration and large journal temperature differentials, near 8000 rpm. The bearing and journal are modeled with three-dimensional (3D) finite elements, and the shaft with flexible beam elements for ME simulation. Nonlinear transient simulations are carried out for a wide operating speed range with varying pivot design parameters. Simulation results indicate that the predicted ME instability is sensitive to the pivot shape, pivot flexibility, and pad-pivot friction.
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Kim, Tae Ho, and Luis San Andrés. "Effect of Side Feed Pressurization on the Dynamic Performance of Gas Foil Bearings: A Model Anchored to Test Data." Journal of Engineering for Gas Turbines and Power 131, no. 1 (October 2, 2008). http://dx.doi.org/10.1115/1.2966421.
Full textAbstract:
Comprehensive modeling of gas foil bearings (GFBs) anchored to reliable test data will enable the widespread usage of these bearings into novel high speed turbomachinery applications. GFBs often need a forced cooling gas flow, axially fed through one end of the bearing, for adequate thermal management. This paper presents rotordynamic response measurements on a rigid rotor supported on GFBs during rotor speed run-up and coastdown tests with the GFBs supplied with increasing feed gas pressures to 2.8bars. Rotor speed run-up tests to 35krpm show that the bearing end side feed gas pressurization delays the onset speed of rotor subsynchronous whirl motions. The test data validate closely the predictions of the threshold speed of instability and the whirl frequency ratio derived from a GFB model that implements the axial evolution of gas circumferential flow velocity as a function of the imposed side feed pressure. Rotor speed coastdown tests from 25krpm with a low feed pressure of 0.35bar evidence a nearly linear synchronous rotor response for small and moderately large imbalance mass distributions. A structural finite element rotordynamics model integrates linearized synchronous speed GFB force coefficients and predicts synchronous responses, amplitude, and phase angles, agreeing with the test data. The analysis and measurements demonstrate the profound effect of the end side feed gas pressurization on the rotordynamic performance of GFBs.
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Fan, Xiaohua, Yixiu Gan, Miaowen Tan, and Wenhe Wang. "Theoretical study on thermal decomposition mechanism of 1-nitroso-2-naphthol." Scientific Reports 12, no. 1 (November 21, 2022). http://dx.doi.org/10.1038/s41598-022-24638-z.
Full textAbstract:
Abstract1-nitroso-2-naphthol has thermal instability of thermal decomposition, spontaneous combustion and even explosion. Its thermal decomposition characteristics were tested by synchronous thermal analyzer (TGA/DSC); The activation energy of the thermal decomposition process was calculated by Kissinger method; The infrared absorption characteristic spectra of the gas products produced in the thermal decomposition process were measured by TGA/DSC-FTIR, and the thermal decomposition reaction process was speculated. The results show that the initial temperature (Tonset) of TGA exothermic decomposition of 1-nitroso-2 naphthol is between 129.01 and 155.69 °C, and the faster the heating rate(β), the higher the Tonset, but the faster the thermal decomposition rate, the greater the heat release and the worse the thermal stability. The activation energy (E) of the thermal decomposition process is 83.323 kJ/mol calculated by Kissinger method. The dynamic test results of TGA/DSC-FTIR show that the main reaction of 1-nitroso-2 naphthol during heating is intermolecular dehydration to form ether, and the secondary reaction is decomposition into aliphatic nitro compounds, carbonyl compounds and amines. Sodium hydroxide will reduce the thermal stability of 1-nitroso-2 naphthol. After adding sodium hydroxide, the thermal decomposition process of 1-nitroso-2 naphthol has changed. The main reaction is that 1-nitroso-2-naphthol reacts with sodium hydroxide to produce sodium nitrophenol, which is further decomposed into aliphatic nitro compounds. The research results have guiding significance for finding the reasonable conditions and temperature of 1-nitroso-2 naphthol during storage and transportation.
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Yu, Haifeng, Yueqiang Cao, Long Chen, Yanjie Hu, Xuezhi Duan, Sheng Dai, Chunzhong Li, and Hao Jiang. "Surface enrichment and diffusion enabling gradient-doping and coating of Ni-rich cathode toward Li-ion batteries." Nature Communications 12, no. 1 (July 27, 2021). http://dx.doi.org/10.1038/s41467-021-24893-0.
Full textAbstract:
AbstractCritical barriers to layered Ni-rich cathode commercialisation include their rapid capacity fading and thermal runaway from crystal disintegration and their interfacial instability. Structure combines surface modification is the ultimate choice to overcome these. Here, a synchronous gradient Al-doped and LiAlO2-coated LiNi0.9Co0.1O2 cathode is designed and prepared by using an oxalate-assisted deposition and subsequent thermally driven diffusion method. Theoretical calculations, in situ X-ray diffraction results and finite-element simulation verify that Al3+ moves to the tetrahedral interstices prior to Ni2+ that eliminates the Li/Ni disorder and internal structure stress. The Li+-conductive LiAlO2 skin prevents electrolyte penetration of the boundaries and reduces side reactions. These help the Ni-rich cathode maintain a 97.4% cycle performance after 100 cycles, and a rapid charging ability of 127.7 mAh g−1 at 20 C. A 3.5-Ah pouch cell with the cathode and graphite anode showed more than a 500-long cycle life with only a 5.6% capacity loss.
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Childs, Dara W., and Rohit Saha. "A New, Iterative, Synchronous-Response Algorithm for Analyzing the Morton Effect." Journal of Engineering for Gas Turbines and Power 134, no. 7 (May 23, 2012). http://dx.doi.org/10.1115/1.4005973.
Full textAbstract:
Morton Effect problems involve the steady increase in rotor synchronous-response amplitudes due to differential heating across a fluid-film bearing that is induced by synchronous response. The present work presents a new computational algorithm for analyzing the Morton Effect. Previous approaches were based on Eigen or Nyquist analyses for stability studies and predicted an onset speed of instability. The present algorithm starts with a steady state elliptical orbit produced by the initial imbalance distribution, which is decomposed into a forward-precessing circular orbit and a backwards-precessing circular orbit. A separate (and numerically intensive) calculation based on the Reynolds equation plus the energy equation gives predictions for the temperature distributions induced by these separate orbits for a range of orbit radius-to-clearance ratios. Temperature distributions for the forward and backward orbits are calculated and added to produce the net temperature distribution due to the initial elliptic orbit. The temperature distribution is assumed to vary linearly across the bearing and produces a bent-shaft angle across the bearing following an analytical result due to Dimoragonas. This bent-shaft angle produces a synchronous rotor excitation in the form of equal and opposite moments acting at the bearing’s ends. For a rotor with an overhung section, the bend also produces a thermally induced imbalance. The response is due to: (1) the initial mechanical imbalance, (2) the bent-shaft excitation, and (3) the thermally-induced imbalance are added to produce a new elliptic orbit, and the process is repeated until a converged orbit is produced. For the work reported, no formal stability analysis is carried out on the converged orbit. The algorithm predicts synchronous response across the rotor’s speed range plus the speed where the response amplitudes becomes divergent by approaching the clearance. Predictions are presented for one example from the published literature, and elevated vibration levels are predicted well before the motion diverges. Synchronous-response amplitudes due to Morton Effect can be orders of magnitude greater than the response due only to mechanical imbalance, particularly near rotor critical speeds. For the example considered, bent-shaft-moment excitation produces significantly higher response levels than the mechanical imbalance induced by thermal bow. The impact of changes in: (1) bearing length-to-diameter ratio, (2) reduced lubricant viscosity, (3) bearing radius-to-clearance ratio and (4) overhung mass magnitude are investigated. Reducing lubricant viscosity and/or reducing the overhung mass are predicted to be the best remedies for Morton Effect problems.
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Yang, Jongin, and Alan Palazzolo. "Three-Dimensional Thermo-Elasto-Hydrodynamic Computational Fluid Dynamics Model of a Tilting Pad Journal Bearing—Part II: Dynamic Response." Journal of Tribology 141, no. 6 (April 30, 2019). http://dx.doi.org/10.1115/1.4043350.
Full textAbstract:
Part II presents a novel approach for predicting dynamic coefficients for a tilting pad journal bearing (TPJB) using computational fluid dynamics (CFD) and finite element method (FEM), including fully coupled elastic deflection, heat transfer, and fluid dynamics. Part I presented a similarly novel, high fidelity approach for TPJB static response prediction which is a prerequisite for the dynamic characteristic determination. The static response establishes the equilibrium operating point values for eccentricity, attitude angle, deflections, temperatures, pressures, etc. The stiffness and damping coefficients are obtained by perturbing the pad and journal motions about this operating point to determine changes in forces and moments. The stiffness and damping coefficients are presented in “synchronously reduced form” as required by American Petroleum Institute (API) vibration standards. Similar to Part I, an advanced three-dimensional thermal—Reynolds equation code validates the CFD code for the special case when flow Between Pad (BP) regions is ignored, and the CFD and Reynolds pad boundary conditions are made identical. The results show excellent agreement for this validation case. Similar to the static response case, the dynamic characteristics from the Reynolds model show large discrepancies compared with the CFD results, depending on the Reynolds mixing coefficient (MC). The discrepancies are a concern given the key role that stiffness and damping coefficients serve instability and response predictions in rotordynamics software. The uncertainty of the MC and its significant influence on static and dynamic response predictions emphasizes a need to utilize the CFD approach for TPJB simulation in critical machines.