Transport, convective

We now repeat the derivation of the steady-state heat transport limited moisture uptake model for the system described by VanCampen et al. [17], The experimental geometry is shown in Figure 9, and the coordinate system of choice is spherical. It will be assumed that only conduction and radiation contribute significantly to heat transport (convective heat transport is negligible), and since radiative flux is assumed to be independent of position, the steady-state solution for the temperature profile is derived as if it were a pure conductive heat transport problem. We have already solved this problem in Section m.B, and the derivation is summarized below. At steady state we have already shown (in spherical coordinates) that... 

In the limit of a vanishingly small time interval, this term represents the rate at which the extensive property N is transported convectively with the fluid motion across the control surfaces out of the control volume. Given that the fluid flow can be described by a vector field V, the convective transport flux across the area A of the control surface can be written as... 

Comparable or larger errors are introduced by unwanted convective mass transport. Convection is caused by physical motion of the solution, sometimes purposefully introduced for techniques such as rotating electrode voltammetry. When a quiet solution is desired, however, convective errors may arise at longer experiment times (slow scan rates) from mechanical vibrations of the solution. Convection is a particular problem for cells inside inert-atmosphere boxes, on which fans and vacuum pumps may be operative. Convection raises the current... 

Solar activity cycle—The periodic, roughly 11-year rise and fall in the number of active features, such as sunspots, prominences, and flares, in the Sun s atmosphere it is thought to be caused by the periodic tangling of the Sun s magnetic field by its rotation and the motion of heat-transporting convective bubbles of gas beneath its surface. 

Chemicals can also be transported within and between compartments. Transport may be via convection, diffusion, or bulk transport. Convection occurs when environmental contaminants dissolved or dispersed in air or water are carried along by air or water currents. Diffusion through air or water occurs relatively slowly, and is of importance only over small distances. However, diffusion (as well as... 

Dialysis is a diffusion-based separation process that uses a semipermeable membrane to separate species by vittue of their different mobilities in the membrane. A feed solution, containing the solutes to he separated, flows ou one side of the membrane while a solvent stream, die dialysate, flows on die other side (Fig. 21. -1). Solute transport across the membrane occurs by diffusion driven by the difference in solme chemical potential between the two membrane-solution interfaces. In practical dialysis devices, no obligatory transmembrane hydraulic pressure may add an additional component of convective transport. Convective transport also may occur if one stream, usually the feed, is highly concentrated, thus giving rise to a transmembrane osmotic gradient down which solvent will flow. In such circumstances, the description of solute transport becomes more complex since it must incorporate some function of die trans-membrane fluid velocity. 

Many codes have been used to calculate the transport, convective mixing, and combustion of hydrogen in containments, some of which are shortly described in the following. Some more CFD computer codes are being briefly described in the sections 8.4.2. on atmospheric dispersion and 8.5.1. and 8.5.2. on combustion processes. 

Hydrogen-air-steam combustion experiments have been conducted to simulate local detonations in a nuclear containment and to validate the respective numerical detonation codes. Experimental facilities used were the Battelle model containment and the containment of the decommissioned HDR in Germany as well as one-compartment vessels at Hanford and Nevada Test Site in the USA or the RUT facility in Russia. Many codes have been developed to simulate the transport, convective mixing, and combustion of hydrogen in a containment building. State-of-the-art are CFD models or lumped parameter models with successful reproduction of, e.g., thermal stratification phenomena. 

To facilitate the comparison between systems of different geometries and scales, we next rewrite the previous equation using dimensionless quantities. The Peclet number is a dimensionless number defined as the ratio of nondif-fusive transport (convection and electrical migration) to transport by diffusion in a particular system ... 

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