2,968 research outputs found
Using Magnetic Bearing Orbit Information to Maximize Centrifugal Compressor Efficiency at Off-Design Conditions
Active magnetic bearings used on oil-free centrifugal refrigeration compressors have lower stiffness than conventional oil-lubricated journal or rolling element bearings. The lower stiffness of these bearings makes them sensitive to internal flow instabilities that are precursors of rotating stall or compressor surge. At operating conditions far away from surge the internal flow is very stable and the magnetic bearings keep the shaft centered, resulting in a minimal bearing orbit. The internal flow instabilities that arise when the compressor approaches the surge limit result in some radially fluctuating forces on the shaft. The active magnetic bearings correct for these fluctuating radial forces on the shaft. The bearing orbit increases with the size of these radial forces. Optimum compressor efficiency occurs close to surge at incipient stall conditions when maximum internal flow pressure recovery occurs. The positional feedback system of the active magnetic bearing control loop system indicates the bearing orbit which relates to compressor efficiency. At high flow conditions capacity is controlled by compressor speed for the imposed pressure ratio. At low flow conditions a combination of variable speed and a means of range extension (e.g. inlet guide vanes, variable geometry diffuser, or flow recirculation) are required to control capacity at the imposed pressure ratio and guarantee stable compressor operation. The bearing orbit signal can be used to determine which speed/geometry combination gives the highest compressor efficiency. If the bearing orbit is below a minimum value the compressor runs too close to choke and a reduction in speed combined with an opening of the diffuser throat area would increase compressor efficiency. If, on the other hand, the bearing orbit is above a maximum value the compressor runs too close to surge and an increase in speed combined with a closing of the diffuser throat area is required to increase compressor efficiency and guarantee surge-free compressor. Experience with this control scheme will be illustrated for a newly developed 350 ton two-stage centrifugal compressor where the variable geometry hardware is replaced with a controlled internal flow recirculation
Reduced hypothalamic-pituitary-adrenal axis activity in chronic multi-site musculoskeletal pain : partly masked by depressive and anxiety disorders
Peer reviewedPublisher PD
Origin of line tension for a Lennard-Jones nanodroplet
The existence and origin of line tension has remained controversial in
literature. To address this issue we compute the shape of Lennard-Jones
nanodrops using molecular dynamics and compare them to density functional
theory in the approximation of the sharp kink interface. We show that the
deviation from Young's law is very small and would correspond to a typical line
tension length scale (defined as line tension divided by surface tension)
similar to the molecular size and decreasing with Young's angle. We propose an
alternative interpretation based on the geometry of the interface at the
molecular scale
Kinetic Characterization and X-ray Structure of a Mutant of Haloalkane Dehalogenase with Higher Catalytic Activity and Modified Substrate Range
Conversion of halogenated aliphatics by haloalkane dehalogenase proceeds via the formation of a covalent alkyl-enzyme intermediate which is subsequently hydrolyzed by water. In the wild type enzyme, the slowest step for both 1,2-dichloroethane and 1,2-dibromoethane conversion is a unimolecular enzyme isomerization preceding rapid halide dissociation. Phenylalanine 172 is located in a helix-loop-helix structure that covers the active site cavity of the enzyme, interacts with the Clβ of 1,2-dichloroethane during catalysis, and could be involved in stabilization of this helix-loop-helix region of the cap domain of the enzyme. To obtain more information about the role of this residue in dehalogenase function, we performed a mutational analysis of position 172 and studied the kinetics and X-ray structure of the Phe172Trp enzyme. The Phe172Trp mutant had a 10-fold higher kcat/Km for 1-chlorohexane and a 2-fold higher kcat for 1,2-dibromoethane than the wild-type enzyme. The X-ray structure of the Phe172Trp enzyme showed a local conformational change in the helix-loop-helix region that covers the active site. This could explain the elevated activity for 1-chlorohexane of the Phe172Trp enzyme, since it allows this large substrate to bind more easily in the active site cavity. Pre-steady-state kinetic analysis showed that the increase in kcat found for 1,2-dibromoethane conversion could be attributed to an increase in the rate of an enzyme isomerization step that preceeds halide release. The observed conformational difference between the helix-loop-helix structures of the wild-type enzyme and the faster mutant suggests that the isomerization required for halide release could be a conformational change that takes place in this region of the cap domain of the dehalogenase. It is proposed that Phe172 is involved in stabilization of the helix-loop-helix structure that covers the active site of the enzyme and creates a rigid hydrophobic cavity for small apolar halogenated alkanes.
A Chiller Control Algorithm for Multiple Variable-speed Centrifugal Compressors
Capacity control of a chiller with multiple variable-speed centrifugal compressors running in parallel between a common evaporator and condenser under reduced load conditions can be achieved by speed variation and/or adjusting the number of compressors in operation. These different capacity control schemes result in modifications of chiller efficiency under part-load conditions. The decision in selecting the number of operational compressors matching the compressor rotational speed to reach a desired part-load capacity depends not only on the required partial load but also on the required pressure ratio at that part-load condition. To illustrate the control dilemma, the paper will start with an example that shows the need for a different control strategy for two different chiller pressure ratios at equal part-load capacity. This difference in partial load control can be planned by observing the topological variation of the efficiency islands on the variable-speed centrifugal compressor map. For a given refrigerant the efficiency of a compressor is a function of only two process variables: head (isentropic enthalpy rise) and volumetric flow rate on the compressor map. With that information a multiple compressor control algorithm has been developed that can estimate optimum chiller efficiency for any head/flow combination the chiller with multiple compressors might encounter. The improvement in chiller part-load efficiency using this algorithm can be substantial as will be illustrated in the paper for a two compressor chiller configuration
Height Distribution and Orientation of Colloidal Dumbbells Near a Wall
Geometric confinement strongly influences the behavior of microparticles in
liquid environments. However, to date, nonspherical particle behaviors close to
confining boundaries, even as simple as planar walls, remain largely
unexplored. Here, we measure the height distribution and orientation of
colloidal dumbbells above walls by means of digital in-line holographic
microscopy. We find that while larger dumbbells are oriented almost parallel to
the wall, smaller dumbbells of the same material are surprisingly oriented at
preferred angles. We determine the total height-dependent force acting on the
dumbbells by considering gravitational effects and electrostatic particle-wall
interactions. Our modeling reveals that at specific heights both net forces and
torques on the dumbbells are simultaneously below the thermal force and energy,
respectively, which makes the observed orientations possible. Our results
highlight the rich near-wall dynamics of nonspherical particles, and can
further contribute to the development of quantitative frameworks for
arbitrarily-shaped microparticle dynamics in confinement.Comment: Ruben W. Verweij and Stefania Ketzetzi contributed equally to this
work. 17 pages and 7 figure
Stability of additive-free water-in-oil emulsions
We calculate ion distributions near a planar oil-water interface within
non-linear Poisson-Boltzmann theory, taking into account the Born self-energy
of the ions in the two media. For unequal self-energies of cations and anions,
a spontaneous charge separation is found such that the water and oil phase
become oppositely charged, in slabs with a typical thickness of the Debye
screening length in the two media. From the analytical solutions, the
corresponding interfacial charge density and the contribution to the
interfacial tension is derived, together with an estimate for the
Yukawa-potential between two spherical water droplets in oil. The parameter
regime is explored where the plasma coupling parameter exceeds the
crystallization threshold, i.e. where the droplets are expected to form
crystalline structures due to a strong Yukawa repulsion, as recently observed
experimentally. Extensions of the theory that we discuss briefly include
numerical calculations on spherical water droplets in oil, and analytical
calculations of the linear PB-equation for a finite oil-water interfacial
width.Comment: 9 pages, 4 figures, accepted by JPCM for proceedings of LMC
Regulation of Immunity to Respiratory Syncytial Virus by Dendritic Cells, Toll-Like Receptors, and Notch
The activation and maintenance of pulmonary viral disease is regulated at multiple levels and determined by the early innate response to the pathogenic stimuli. Subsequent activation events that rely directly and indirectly on the virus itself can alter the development and severity of the ensuing immunopathologic responses. In the present review we outline several interconnected mechanisms that rely on the early recognition of viral nucleic acid for the most appropriate anti-viral immune responses, including TLRs and Notch activation in DCs and T cells. Deviation or persistence of the immune response to respiratory viruses may impact significantly on the severity of the responses. While these mechanisms are likely similar in most respiratory viral infections, this review will focus on findings with respiratory syncytial virus (RSV) infections.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63262/1/vim.2007.0110.pd
Towards photophoretically levitating macroscopic sensors in the stratosphere
Photophoretic forces could levitate thin 10 centimeter-scale structures in
Earths stratosphere indefinitely. We develop models of the thermal
transpiration lofting force on a bilayer sandwich structure under stratospheric
conditions driven by radiative fluxes in the thermal-infrared and solar-band.
Similar structures have been levitated in the laboratory. Lofting is maximized
when the layers are separated by an air gap equal to the mean free path (MFP),
when about half of the layers surface area consists of holes with radii <
MFP, and when the top layer is solar-transmissive and infrared-emissive while
the bottom layer is solar-absorptive and infrared-transmissive. We describe a
preliminary design of a 10 cm diameter device that combines a levitating
structure made of two membranes 2 m apart with the support structure
required for stiffness and orientation control. We limit the design to
components that could be fabricated with available methods. Structural analysis
suggests that the device would have sufficient strength to withstand forces
that might be encountered in transport, deployment, and flight. Our models
predict a payload capacity of about 300 mg at 25 km altitude and our analysis
suggests it could support bidirectional radio communication at over 10 Mb/s and
could have limited navigational abilities. Such devices could be useful for
atmospheric science or telecommunications, and similar devices might be useful
on Mars. Structures a few times larger might have payloads of a few grams.Comment: Main: 14 pages, 4 figures. Supporting information: 7 pages, 13
figure
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