Self-consistent Mean Field theory in weakly ionized media

We present a self-consistent mean field theory of the dynamo in 3D and turbulent diffusion in 2D in weakly ionized gas. We find that in 3D, the backreaction does not alter the beta effect while it suppresses the alpha effect when the strength of a mean magnetic field exceeds a critical value. These results suggest that a mean field dynamo operates much more efficiently in weakly ionized gas compared to the fully ionized gas. Furthermore, we show that in 2D, the turbulent diffusion is suppressed by back reaction when a mean magnetic field reaches the same critical strength, with the upper bound on turbulent diffusion given by its kinematic value. Astrophysical implications are discussed

THREE-DIMENSIONAL SIMULATIONS OF TRANSIENT RESPONSE OF PEM FUEL CELLS

Transients have utmost importance in the lifetime and performance degradation of PEM fuel cells. Recent studies show that cyclic transients can induce hygro-thermal fatigue. In particular, the amount of water in the membrane varies significantly during transients, and determines the ionic conductivity and the structural properties of the membrane. In this work, we present three-dimensional time-dependent simulations and analysis of the transport in PEM fuel cells. U-sections of anode and cathode serpentine flow channels, anode and cathode gas diffusion layers, and the membrane sandwiched between them are modeled using incompressible Navier-Stokes equations in the gas flow channels, Maxwell-Stefan equations in the channels and gas diffusion layers, advection-diffusion-type equation for water transport in the membrane and Ohm’s law for ionic currents in the membrane and electric currents in gas diffusion electrodes. Transient responses to step changes in load, pressure and the relative humidity of the cathode are obtained from simulations, which are conducted by means of a third party finite-element package, COMSOL

A Comparison of Fick and Maxwell-Stefan Diffusion Formulations in PEMFC Cathode Gas Diffusion Layers

This paper explores the mathematical formulations of Fick and Maxwell-Stefan diffusion in the context of polymer electrolyte membrane fuel cell cathode gas diffusion layers. Formulations of diffusion combined with mass-averaged Darcy flow are considered for three component gases. Fick formulations can be considered as approximations of Maxwell-Stefan in a certain sense. For this application, the formulations can be compared computationally in a simple, one dimensional setting. We observe that the predictions of the formulations are very similar, despite their seemingly different structure. Analytic insight is given to the result. In addition, it is seen that for both formulations, diffusion laws are small perturbations from bulk flow. The work is also intended as a reference to multi-component gas diffusion formulations in the fuel cell setting.Comment: 12 pages, submitted to the Journal of Power Source

On the Ionisation Fraction in Protoplanetary Disks II: The Effect of Turbulent Mixing on Gas--phase Chemistry

We calculate the ionisation fraction in protostellar disk models using two different gas-phase chemical networks, and examine the effect of turbulent mixing by modelling the diffusion of chemical species vertically through the disk. The aim is to determine in which regions of the disk gas can couple to a magnetic field and sustain MHD turbulence. We find that the effect of diffusion depends crucially on the elemental abundance of heavy metals (magnesium) included in the chemical model. In the absence of heavy metals, diffusion has essentially no effect on the ionisation structure of the disks, as the recombination time scale is much shorter than the turbulent diffusion time scale. When metals are included with an elemental abundance above a threshold value, the diffusion can dramatically reduce the size of the magnetically decoupled region, or even remove it altogther. For a complex chemistry the elemental abundance of magnesium required to remove the dead zone is 10(-10) - 10(-8). We also find that diffusion can modify the reaction pathways, giving rise to dominant species when diffusion is switched on that are minor species when diffusion is absent. This suggests that there may be chemical signatures of diffusive mixing that could be used to indirectly detect turbulent activity in protoplanetary disks. We find examples of models in which the dead zone in the outer disk region is rendered deeper when diffusion is switched on. Overall these results suggest that global MHD turbulence in protoplanetary disks may be self-sustaining under favourable circumstances, as turbulent mixing can help maintain the ionisation fraction above that necessary to ensure good coupling between the gas and magnetic field.Comment: 11 pages, 7 figures; accepted for publication in A &

Linearized Kompaneetz equation as a relativistic diffusion

We show that Kompaneetz equation describing photon diffusion in an environment of an electron gas, when linearized around its equilibrium distribution, coincides with the relativistic diffusion discussed in recent publications. The model of the relativistic diffusion is related to soluble models of imaginary time quantum mechanics. We suggest some non-linear generalizations of the relativistic diffusion equation and their astrophysical applications (in particular to the Sunyaev-Zeldovich effect).Comment: 12 page

Diffusion NMR Methods Applied to Xenon Gas for Materials Study

We report initial NMR studies of i) xenon gas diffusion in model heterogeneous porous media, and ii) continuous flow laser-polarized xenon gas. Both areas utilize the Pulsed Gradient Spin Echo techniques in the gas-phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients - a brief overview of this area is provided in the introduction. The heterogeneous or multiple-length scale model porous media consisted of random packs of mixed glass beads of two different sizes. We focus on observing the approach of the time-dependent gas diffusion coefficient, D(t), (an indicator of mean squared displacement) to the long-time asymptote, with the aim of understanding the long-length scale structural information that may be derived from a heterogeneous porous system. The Pade approximation is used to interpolate D(t) data between the short and long time limits. Initial studies of continuous flow laser-polarized xenon gas demonstrate velocity-sensitive imaging of much higher flows than can generally be obtained with liquids (20 - 200 mm/s). Gas velocity imaging is, however, found to be limited to a resolution of about 1 mm/s due to the high diffusivity of gases compared to liquids. We also present the first gas-phase NMR scattering, or diffusive-diffraction, data: namely, flow-enhanced structural features in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack.Comment: single pdf file including all figure