139 research outputs found
Oil film measurement in polytetrafluoroethylene-faced thrust pad bearings for hydrogenerator applications
There is a growing trend in the replacement of the babbit facing in thrust pad bearings with a composite polytetrafluoroethylene (PTFE) surface layer. The PTFE-faced bearings have been shown to allow a greater specific pressure, reduce thermal crowning, and, in some cases, negate the need for an oil-lift (jacking) system. These designs of bearing require new methods for the measurement of oil film thickness both to assist in their development and for plant condition monitoring. In this work, an ultrasonic method of oil film measurement is evaluated for this purpose. An ultrasonic transducer is mounted on the back face of the thrust pad. Pulses are generated and transmitted through the pad material, bonding interlayer, and PTFE surface layer. The proportion of the wave that reflects back from the oil film layer is determined. This is then related to the oil film thickness using a series of calibration experiments and a spring stiffness model. In practice, the reflected signal is difficult to distinguish, in the time domain, from other internal reflections from the pad. Signals are compared with reflections when no oil film is present and processing is carried out in the frequency domain. Experiments have been performed on a full size PTFE-faced thrust pad destined for a hydroelectric power station turbine. The instrumented pad was installed in a test facility and subjected to a range of loading conditions both with and without oil lift. Although there were some problems with the robustness of the experimental procedure, oil films were successfully measured and used to study the effect of the oil-lift system on film formation. © IMechE 2006
Antihydrogen formation dynamics in a multipolar neutral anti-atom trap
Antihydrogen production in a neutral atom trap formed by an octupole-based
magnetic field minimum is demonstrated using field-ionization of weakly bound
anti-atoms. Using our unique annihilation imaging detector, we correlate
antihydrogen detection by imaging and by field-ionization for the first time.
We further establish how field-ionization causes radial redistribution of the
antiprotons during antihydrogen formation and use this effect for the first
simultaneous measurements of strongly and weakly bound antihydrogen atoms.
Distinguishing between these provides critical information needed in the
process of optimizing for trappable antihydrogen. These observations are of
crucial importance to the ultimate goal of performing CPT tests involving
antihydrogen, which likely depends upon trapping the anti-atom
Search For Trapped Antihydrogen
We present the results of an experiment to search for trapped antihydrogen
atoms with the ALPHA antihydrogen trap at the CERN Antiproton Decelerator.
Sensitive diagnostics of the temperatures, sizes, and densities of the trapped
antiproton and positron plasmas have been developed, which in turn permitted
development of techniques to precisely and reproducibly control the initial
experimental parameters. The use of a position-sensitive annihilation vertex
detector, together with the capability of controllably quenching the
superconducting magnetic minimum trap, enabled us to carry out a
high-sensitivity and low-background search for trapped synthesised antihydrogen
atoms. We aim to identify the annihilations of antihydrogen atoms held for at
least 130 ms in the trap before being released over ~30 ms. After a three-week
experimental run in 2009 involving mixing of 10^7 antiprotons with 1.3 10^9
positrons to produce 6 10^5 antihydrogen atoms, we have identified six
antiproton annihilation events that are consistent with the release of trapped
antihydrogen. The cosmic ray background, estimated to contribute 0.14 counts,
is incompatible with this observation at a significance of 5.6 sigma. Extensive
simulations predict that an alternative source of annihilations, the escape of
mirror-trapped antiprotons, is highly unlikely, though this possibility has not
yet been ruled out experimentally.Comment: 12 pages, 7 figure
Recommended from our members
TEMPERATURE AND THERMAL STRESS DISTRIBUTIONS IN CIRCULAR CYLINDERS. Technical Report ME-11
Recommended from our members
Granular flow in the marginal ice zone
The region of sea ice near the edge of the sea ice pack is known as the marginal ice zone (MIZ), and its dynamics are complicated by ocean wave interaction with the ice cover, strong gradients in the atmosphere and ocean and variations in sea ice rheology. This paper focuses on the role of sea ice rheology in determining the dynamics of the MIZ. Here, sea ice is treated as a granular material with a composite rheology describing collisional ice floe interaction and plastic interaction. The collisional component of sea ice rheology depends upon the granular temperature, a measure of the kinetic energy of flow fluctuations. A simplified model of the MIZ is introduced consisting of the along and across momentum balance of the sea ice and the balance equation of fluctuation kinetic energy. The steady solution of these equations is found to leading order using elementary methods. This reveals a concentrated region of rapid ice flow parallel to the ice edge, which is in accordance with field observations, and previously called the ice jet. Previous explanations of the ice jet relied upon the existence of ocean currents beneath the ice cover. We show that an ice jet results as a natural consequence of the granular nature of sea ice
- …