110 research outputs found
Model of the meniscus of an ionic liquid ion source.
A simple model of the transfer of charge and ion evaporation in the meniscus of an ionic-liquid ion source working in the purely ionic regime is proposed on the basis of order-of-magnitude estimates which show that, in this regime, _i_ the flow in the meniscus is dominated by the viscosity of the liquid and is affected very little by the mass flux accompanying ion evaporation, and _ii_ the effect of the space charge around the evaporating surface is negligible and the evaporation current is controlled by the finite electrical conductivity of the liquid. The model predicts that a stationary meniscus of a very polar liquid undergoing ion evaporation is nearly hydrostatic and can exist only below a certain value of the applied electric field, at which the meniscus attains its maximum elongation but stays smooth. The electric current vs applied electric field characteristic displays a frozen regime of negligible ion evaporation at low fields and a conduction-controlled regime at higher fields, with a sharp transition between the two regimes owing to the high sensitivity of the ion evaporation rate to the electric field. A simplified treatment of the flow in the capillary or liquid layer through which liquid is delivered to the meniscus shows that the size of the meniscus decreases and the maximum attainable current increases when the feeding pressure is decreased, and that appropriate combinations of feeding pressure and pressure drop may lead to high maximum currents
THE SPIRAL WAVE INSTABILITY INDUCED BY A GIANT PLANET. I. PARTICLE STIRRING IN THE INNER REGIONS OF PROTOPLANETARY DISKS
We have recently shown that spiral density waves propagating in accretion
disks can undergo a parametric instability by resonantly coupling with and
transferring energy into pairs of inertial waves (or inertial-gravity waves
when buoyancy is important). In this paper, we perform inviscid
three-dimensional global hydrodynamic simulations to examine the growth and
consequence of this instability operating on the spiral waves driven by a
Jupiter-mass planet in a protoplanetary disk. We find that the spiral waves are
destabilized via the spiral wave instability (SWI), generating hydrodynamic
turbulence and sustained radially-alternating vertical flows that appear to be
associated with long wavelength inertial modes. In the interval , where denotes the semi-major axis of the planetary orbit
(assumed to be 5~au), the estimated vertical diffusion rate associated with the
turbulence is characterized by . For the disk model considered here, the diffusion rate is such that
particles with sizes up to several centimeters are vertically mixed within the
first pressure scale height. This suggests that the instability of spiral waves
launched by a giant planet can significantly disperse solid particles and trace
chemical species from the midplane. In planet formation models where the
continuous local production of chondrules/pebbles occurs over Myr time scales
to provide a feedstock for pebble accretion onto these bodies, this stirring of
solid particles may add a time constraint: planetary embryos and large
asteroids have to form before a gas giant forms in the outer disk, otherwise
the SWI will significantly decrease the chondrule/pebble accretion efficiency.Comment: Accepted for publication in the The Astrophysical Journal, 19 pages,
12 figures, 1 tabl
Diffuse charge and Faradaic reactions in porous electrodes
Porous electrodes instead of flat electrodes are widely used in electrochemical systems to boost storage
capacities for ions and electrons, to improve the transport of mass and charge, and to enhance reaction rates.
Existing porous electrode theories make a number of simplifying assumptions: (i) The charge-transfer rate is
assumed to depend only on the local electrostatic potential difference between the electrode matrix and the pore
solution, without considering the structure of the double layer (DL) formed in between; (ii) the charge-transfer
rate is generally equated with the salt-transfer rate not only at the nanoscale of the matrix-pore interface, but also
at the macroscopic scale of transport through the electrode pores. In this paper, we extend porous electrode theory
by including the generalized Frumkin-Butler-Volmer model of Faradaic reaction kinetics, which postulates charge
transfer across the molecular Stern layer located in between the electron-conducting matrix phase and the plane
of closest approach for the ions in the diffuse part of the DL. This is an elegant and purely local description of the
charge-transfer rate, which self-consistently determines the surface charge and does not require consideration of
reference electrodes or comparison with a global equilibrium. For the description of the DLs, we consider the
two natural limits: (i) the classical Gouy-Chapman-Stern model for thin DLs compared to the macroscopic pore
dimensions, e.g., for high-porosity metallic foams (macropores >50 nm) and (ii) a modified Donnan model for
strongly overlapping DLs, e.g., for porous activated carbon particles (micropores <2 nm). Our theory is valid
for electrolytes where both ions are mobile, and it accounts for voltage and concentration differences not only on
the macroscopic scale of the full electrode, but also on the local scale of the DL. The model is simple enough to
allow us to derive analytical approximations for the steady-state and early transients. We also present numerical
solutions to validate the analysis and to illustrate the evolution of ion densities, pore potential, surface charge,
and reaction rates in response to an applied voltage
Debye-Hueckel solution for steady electro-osmotic flow of a micropolar fluid in a cylindrical microcapillary
Analytic expressions for the speed, flux, microrotation, stress, and couple
stress in a micropolar fluid exhibiting steady, symmetric and one-dimensional
electro-osmotic flow in a uniform cylindrical microcapillary were derived under
the constraint of the Debye-Hueckel approximation, which is applicable when the
cross-sectional radius of the microcapillary exceeds the Debye length, provided
that the zeta potential is sufficiently small in magnitude. As the aciculate
particles in a micropolar fluid can rotate without translation, micropolarity
influences fluid speed, fluid flux, and one of the two non-zero components of
the stress tensor. The axial speed in a micropolar fluid intensifies as the
radius increases. The stress tensor is confined to the region near the wall of
the microcapillary but the couple stress tensor is uniform across the
cross-section.Comment: 19 page
Technoeconomic Analysis of Biofuel Production and Biorefinery Operation Utilizing Geothermal Energy
The comet Halley dust and gas environment
Quantitative descriptions of environments near the nucleus of comet P /Halley have been developed to support spacecraft and mission design for the flyby encounters in March, 1986. To summarize these models as they exist just before the encounters, we review the relevant data from prior Halley apparitions and from recent cometary research. Orbital elements, visual magnitudes, and parameter values and analysis for the nucleus, gas and dust are combined to predict Halley's position, production rates, gas and dust distributions, and electromagnetic radiation field for the current perihelion passage. The predicted numerical results have been useful for estimating likely spacecraft effects, such as impact damage and attitude perturbation. Sample applications are cited, including design of a dust shield for spacecraft structure, and threshold and dynamic range selection for flight experiments. We expect that the comet's activity may be more irregular than these smoothly varying models predict, and that comparison with the flyby data will be instructive.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43774/1/11214_2004_Article_BF00175326.pd
Effect of electro-osmotic flow on energy conversion on superhydrophobic surfaces
It has been suggested that superhydrophobic surfaces, due to the presence of
a no-shear zone, can greatly enhance transport of surface charges, leading to a
considerable increase in the streaming potential. This could find potential use
in micro-energy harvesting devices. In this paper, we show using analytical and
numerical methods, that when a streaming potential is generated in such
superhydrophobic geometries, the reverse electro-osmotic flow and hence current
generated by this, is significant. A decrease in streaming potential compared
to what was earlier predicted is expected. We also show that, due to the
electro-osmotic streaming-current, a saturation in both the power extracted and
efficiency of energy conversion is achieved in such systems for large values of
the free surface charge densities. Nevertheless, under realistic conditions,
such microstructured devices with superhydrophobic surfaces have the potential
to even reach energy conversion efficiencies only achieved in nanostructured
devices so far
Start-Up Electroosmotic Flow of Multi-Layer Immiscible Maxwell Fluids in a Slit Microchannel
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