61 research outputs found
An Investigation of Nonlinear Flow Oscillations in a High-Pressure Centrifugal Pump
High-pressure multistage pumps and their coupled piping systems, typically used in the process and power generation industry, can experience dangerous system-level instabilities. This can occur at flow coefficients well away from the surge limit and in the absence of cavitation. Such a pumping system and a related new kind of instability are the focus of this paper. A system-wide instability was observed at 0.05 times rotor frequency for flow coefficients near maximum head rise but at negative slope, thus on the stable side of the head rise characteristic. A previous study based on system-level experiments concluded that this instability differs from classical surge, cavitation surge, rotating stall, and rotating cavitation, but the underlying mechanism and necessary flow conditions remain unknown. This paper investigates the root cause of the system-wide pump instability, employing a systematic analysis of the impact of geometry changes on pump stability and performance. It is found that the upstream influence of the unsteady flow separation in the return channel leads to a time-varying incidence angle change on the volute tongue which causes periodic ingestion of low-stagnation pressure fluid into the diffuser passages. This sets up a limit cycle, promoting the system-wide instability. With the instability mechanism determined, the pump is redesigned to remove the flow separation while maintaining performance at design conditions. Unsteady numerical simulations demonstrate improved efficiency and pressure recovery at low flow coefficients. A time accurate calculation also indicates stable operation at all relevant flow conditions. The paper resolves a long-standing pump stability problem and provides design guidelines for reliable and improved performance, important to the chemical processing and power generation industry
In situ epitaxial MgB2 thin films for superconducting electronics
A thin film technology compatible with multilayer device fabrication is
critical for exploring the potential of the 39-K superconductor magnesium
diboride for superconducting electronics. Using a Hybrid Physical-Chemical
Vapor Deposition (HPCVD) process, it is shown that the high Mg vapor pressure
necessary to keep the MgB phase thermodynamically stable can be achieved
for the {\it in situ} growth of MgB thin films. The films grow epitaxially
on (0001) sapphire and (0001) 4H-SiC substrates and show a bulk-like of
39 K, a (4.2K) of A/cm in zero field, and a
of 29.2 T in parallel magnetic field. The surface is smooth with a
root-mean-square roughness of 2.5 nm for MgB films on SiC. This deposition
method opens tremendous opportunities for superconducting electronics using
MgB
A crossover randomised controlled trial of oral mandibular advancement devices for obstructive sleep apnoea-hypopnoea (TOMADO)
Rationale Mandibular advancement devices (MADs)
are used to treat obstructive sleep apnoea-hypopnoea
syndrome (OSAHS) but evidence is lacking regarding
their clinical and cost-effectiveness in less severe disease.
Objectives To compare clinical- and cost-effectiveness
of a range of MADs against no treatment in mild to
moderate OSAHS.
Measurements and methods This open-label,
randomised, controlled, crossover trial was undertaken at
a UK sleep centre. Adults with Apnoea-Hypopnoea Index
(AHI) 5–<30/h and Epworth Sleepiness Scale (ESS) score
≥9 underwent 6 weeks of treatment with three nonadjustable
MADs: self-moulded (SleepPro 1; SP1);
semi-bespoke (SleepPro 2; SP2); fully-bespoke MAD
(bMAD); and 4 weeks no treatment. Primary outcome
was AHI scored by a polysomnographer blinded to
treatment. Secondary outcomes included ESS, quality of
life, resource use and cost.
Main results 90 patients were randomised and 83
were analysed. All devices reduced AHI compared with
no treatment by 26% (95% CI 11% to 38%, p=0.001)
for SP1, 33% (95% CI 24% to 41%) for SP2 and 36%
(95% CI 24% to 45%, p<0.001) for bMAD. ESS was
1.51 (95% CI 0.73 to 2.29, p<0.001, SP1) to 2.37
(95% CI 1.53 to 3.22, p<0.001, bMAD) lower than no
treatment (p<0.001 for all). Compliance was lower for
SP1, which was the least preferred treatment at trial exit.
All devices were cost-effective compared with no
treatment at a £20 000/quality-adjusted life year (QALY)
threshold. SP2 was the most cost-effective up to
£39 800/QALY.
Conclusions Non-adjustable MADs achieve clinically
important improvements in mild to moderate OSAHS and
are cost-effective
Thermodynamics and thin film deposition of MgB2 superconductors
The recently discovered superconductor MgB2 with Tc at 39 K has great potential in superconducting microelectronics. Thermodynamics studies with the calculation of phase diagrams (CALPHAD) modelling technique show that due to the high volatility of Mg, MgB2 is only thermodynamically stable under fairly high Mg overpressures for likely in situ growth temperatures. This provides a helpful insight into the appropriate processing conditions for MgB2 thin films, including the identification of the pressure–temperature region for adsorption-controlled growth. The initial MgB2 thin films were made by pulsed laser deposition followed by in situ annealing. The cross-sectional transmission electron microscopy reveals a nanocrystalline mixture of textured MgO and MgB2 with very small grain sizes. A zero-resistance transition temperature of 34 K and a zero-field critical current density of 1.3 × 106 A cm−2 were obtained. The qualities of these films are limited by the thermodynamic stability conditions, which favour deposition techniques that can maintain a high flux of Mg.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48989/2/u20333.pd
Deposition and Properties of Superconducting MgB 2 Thin Films
The recently discovered superconductor MgB 2 with T c at 39 K has great potential in superconducting electronics. In this paper, we review the deposition techniques used for MgB 2 thin films in the light of a thermodynamic study of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique. This thermodynamic study identifies a growth window in the pressure–temperature phase diagram, in which the magnesium pressure is very high for likely in situ growth temperatures. A Hybrid Physical–Chemical Vapor Deposition (HPCVD) technique that successfully achieves such a high Mg pressure is shown to produce in situ epitaxial MgB 2 thin films with bulk superconducting properties.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45120/1/10948_2004_Article_474022.pd
Probing domain microstructure in ferroelectric Bi4Ti3O12Bi4Ti3O12 thin films by optical second harmonic generation
The domain microstructure in an epitaxial thin film of Bi4Ti3O12Bi4Ti3O12 on a SrTiO3(001)SrTiO3(001) substrate is studied by second harmonic generation measurements. The input polarization dependence of the second harmonic signal exhibits spatial symmetries that reflect the presence of eight different domain variants present in the film. A theoretical model is presented that explains the observed symmetries and extracts quantitative information on the nonlinear optical coefficients of the material and statistics of domain variants present in the film area being probed. The following ratios of nonlinear coefficients and birefringence was determined: d12/d11=−3.498±0.171,d12/d11=−3.498±0.171, ∣d26/d12∣=0.365±0.010,∣d26/d12∣=0.365±0.010, ∣d26/d11∣=1.273±0.036,∣d26/d11∣=1.273±0.036, and ∣nb−na∣=0.101±0.018∣nb−na∣=0.101±0.018 (at 532 nm). © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70014/2/JAPIAU-89-2-1387-1.pd
Erratum: “Probing domain microstructure in ferroelectric Bi4Ti3O12Bi4Ti3O12 thin films by optical second harmonic generation” [J. Appl. Phys. 89, 1387 (2001)]
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69409/2/JAPIAU-89-9-5230-1.pd
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