Instantaneous fluid exchange

Unfortunately, the thin lamella resists instantaneous fluid exchange. According to the simple Reynolds parallel-film model QA,15) adopted below, this hydrodynamic resistance is inversely 3... 

A recirculation pump is to provide a constant mass flow to the reactor, regardless of the instantaneous fluid density. The pump is to be equipped with a variable-speed drive motor to maintain the desired mass flow rate throughout the start-up cycle. The pump is to be located in proximity to the heat exchanger within the shield building. 

Movement of a soluble chemical throughout a water body such as a lake or river is governed by thermal, gravitational, or wind-induced convection currents that set up laminar, or nearly frictionless, flows, and also by turbulent effects caused by inhomogeneities at the boundaries of the aqueous phase. In a river, for example, convective flows transport solutes in a nearly uniform, constant-velocity manner near the center of the stream due to the mass motion of the current, but the friction between the water and the bottom also sets up eddies that move parcels of water about in more randomized and less precisely describable patterns where the instantaneous velocity of the fluid fluctuates rapidly over a relatively short spatial distance. The dissolved constituents of the water parcel move with them in a process called eddy diffusion, or eddy dispersion. Horizontal eddy diffusion is often many times faster than vertical diffusion, so that chemicals spread sideways from a point of discharge much faster than perpendicular to it (Thomas, 1990). In a temperature- and density-stratified water body such as a lake or the ocean, movement of water parcels and their associated solutes will be restricted by currents confined to the stratified layers, and rates of exchange of materials between the layers will be slow. 

Interaction of alkali with rock minerals in reservoir sand is complicated. Somerton and Radke (11) classified these interactions into surface exchange and hydrolysis, congruent and in congruent dissolution reactiaiSr and insoluble salt formation by reaction with hardness ions in the pore fluids and exchanged from sand surfaces. These interactions may also be classified into reversible adsorption or non-reversible chemical consurrp-tion, and kinetic controlled or instantaneous reactions. 

The calculation is carried out for an open-loop circuit, that is, a circuit in which the entrance conditions to the reactor core are constant in time. This implies that the external loop removes all the heat generated in the core. The core proper is a right circular cylinder of height h and radius po. All fluid particles and heat are transported around the circuit at a uniform velocity V, and it is assumed that thermal equilibrium and density changes occur instantaneously with variations in temperature. Moreover, it is assumed that no lateral exchange of heat occurs between adjacent fluid streamlines i.e., the reactor is taken to be a line from the standpoint of heat generation in the fuel. Thus all axial paths through the core are identical. 

Although the momentum exchange between fluid and solid occurs instantaneously at the half time step, in calculating U,(t -h h) we have made the assumption that the hydrodynamic force is distributed over the time step. We actually attempted to derive an update for the velocity assuming that the hydrodynamic force acts over a very small fraction of the time step, but this has not led to a sensible result as yet. It is not entirely clear if the assumption that the hydrodynamic force acts over the whole time step is valid, and does not, for example, produce an artificial dissipation. To resolve this question will require a detailed analysis of the fully coupled system, along the lines given in Sect. 4.5 for the simpler case of frictional coupling. A similar analysis for solid-fluid boundary conditions is an open area for further research. 

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