Critical speed

About: Critical speed is a research topic. Over the lifetime, 2764 publications have been published within this topic receiving 31365 citations.

Experimental Investigations of Resonance Vibration Control for Noncollocated AMB Flexible Rotor Systems

Abstract: The resonance vibration control of active magnetic bearing (AMB) supported flexible rotors is a challenging topic in the industrial applications. This work addresses the two issues on the noncollocated AMB flexible rotor systems while passing through the critical speed. A modal separation scheme is established to enhance the observability of the bending mode, and a damping optimization procedure of the electromagnetic force is proposed to provide the maximum control efficiency. First, the rotor imbalance analysis is carried out, and several possible approaches to minimize the vibration of the flexible rotor are presented. Then, the detailed descriptions of the bending mode extraction scheme using notch filters and phase-lead compensators are discussed for a noncollocated flexible rotor system. The root locii of the closed-loop AMB-rotor system with and without additional phase-lead compensator are also performed. A solution to determining the phase angles between the measured rotor displacements to the bearing forces is established using the physical modeling and experimental identification. Finally, simulation and experimental results on a 10 kW magnetically suspended centrifugal compressor show the effectiveness of the proposed methods.

Stabilizing a 46 MW Multistage Utility Steam Turbine Using Integral Squeeze Film Bearing Support Dampers

Abstract: A 46 MW 5,500 rpm multistage single casing utility steam turbine experienced strong subsynchronous rotordynamic vibration of the first rotor mode; preventing full load operation of the unit. The root cause of the vibration stemmed from steam whirl forces generated at secondary sealing locations in combination with flexible rotor-bearing system. Several attempts were made to eliminate the subsynchronous vibration by modifying bearing geometry and clearances, which came short of enabling full load operation.The following paper presents experimental tests and analytical results focused on stabilizing a 46 MW 6,230kg utility steam turbine experiencing subsynchronous rotordynamic instability. The paper advances an integral squeeze film damper (ISFD) solution, which was implemented to resolve the subsynchronous vibration and allow full load and full speed operation of the machine. The present work addresses the bearing-damper analysis, rotordynamic analysis, and experimental validation through waterfall plots, and synchronous vibration data of the steam turbine rotor. Analytical and experimental results show that using ISFD improved the stability margin by a factor of 12 eliminating the subsynchronous instability and significantly reducing critical speed amplification factors. Additionally, by using ISFD the analysis showed significant reduction in interstage clearance closures during critical speed transitions in comparison to the hard mounted tilting pad bearing configuration.Copyright © 2014 by ASME

Bifurcation Analysis of a Turbocharger Rotor Supported by Floating Ring Bearings

Abstract: Today, rotors of high-speed turbochargers are commonly supported by floating ring bearings due to their low costs and reduced power losses. A well known effect of such rotor bearing-systems is the occurrence of self-excited vibrations. In order to study the different nonlinear vibration effects with the methods of numerical continuation, a perfectly balanced flexible turbocharger rotor is considered which is supported by two identical floating ring bearings. Here, the bearing forces are modeled by applying the short bearing theory for both fluid films. After deriving the equations of motion of the turbocharger rotor, bifurcation analyses are carried out with both rigid and flexible model. Thereby, the main focus of the investigation is on the limit-cycle oscillation of higher amplitudes, which may cause rotor damage.In the lower speed range of operation the equilibrium position of the turbocharger rotor becomes unstable by a Hopf bifurcation emerging limit-cycle oscillations. By increasing the rotor speed the limit-cycle may lose its stability by a torus bifurcations leading into an area of quasi-periodic vibrations of the system. Further torus bifurcations, which emanate stable limit-cycles again, and various jump phenomena are also observed. For higher speed ranges a saddle-node bifurcation may occur from which stable limit-cycle oscillations of high amplitudes arise. The rotor speed, where this saddle-node bifurcation takes place, may be defined as a nonlinear critical speed of the turbocharger system supported by floating ring bearings. In the range of the nonlinear critical speed the bifurcation behavior of the turbocharger in floating ring bearings is quite complicated, since a further stable solution coexists beside the critical limit-cycle oscillation.

Influence of probe dynamic characteristics on the scanning speed for white light interference based AFM

Abstract: The dynamic characteristics of an atomic force probe are important for rapid and accurate measurement in white light interference (WLI) based atomic force microscope (AFM). The tip of the probe will fly from the surface when its dynamic characteristics are poor, which will cause measurement error. Generally such tip flight can be avoided by reducing the scanning speed, which will decrease the error at the cost of measurement efficiency. In this paper, a dynamical model of atomic force probe is presented to analyze the causes of tip flight in the measurement process. A numerical simulation is performed on a typical sample surface to investigate the influence of profile, preloading and probe parameters on scanning speed. Experimental testing is conducted on a self-developed WLI based AFM, and the experimental results agree well with that of the theory. The maximum scanning speeds of the probe for a sample are tested under certain conditions. It is shown that for a certain probe, the tip flight occurs at the upwards points of the measured sample when the scanning speed exceeds the critical speed, which is constrained by the preloading.

Tests of Airfoils Designed to Delay the Compressibility Burble

Abstract: Development of airfoil sections suitable for high-speed applications has generally been difficult because little was known of the flow phenomenon that occurs at high speeds. A definite critical speed has been found at which serious detrimental flow changes occur that lead to serious losses in lift and large increases in drag. This flow phenomenon, called the compressibility burble, was originally a propeller problem, but with the development of higher speed aircraft serious consideration must be given to other parts of the airplane. Fundamental investigations of high-speed airflow phenomenon have provided new information. An important conclusion of this work has been the determination of the critical speed, that is, the speed at which the compressibility burble occurs. The critical speed was shown to be the translational velocity at which the sum of the translational velocity and the maximum local induced velocity at the surface of the airfoil or other body equals the local speed of sound. Obviously then higher critical speeds can be attained through the development of airfoils that have minimum induced velocity for any given value of the lift coefficient. Presumably, the highest critical speed will be attained by an airfoil that has uniform chordwise distribution of induced velocity or, in other words, a flat pressure distribution curve. The ideal airfoil for any given high-speed application is, then, that form which at its operating lift coefficient has uniform chordwise distribution of induced velocity. Accordingly, an analytical search for such airfoil forms has been conducted and these forms are now being investigated experimentally in the 23-inch high-speed wind tunnel. The first airfoils investigated showed marked improvement over those forms already available, not only as to critical speed buy also the drag at low speeds is decreased considerably. Because of the immediate marked improvement, it was considered desirable to extend the thickness and lift coefficient ranges for which the original forms had been designed before further extending the investigation.