Chemical source term rate constants

ALOHA has a comprehensive chemical source term library (>700 pure chemicals). The code can address many types of pipe and tank releases, including two-phased flows from pressurized/ cryogenic chemicals. The user may enter a constant or variable vapor source rate and duration of... 

Finally, to conclude our discussion on coupling with chemistry, we should note that in principle fairly complex reaction schemes can be used to define the reaction source terms. However, as in single-phase flows, adding many fast chemical reactions can lead to slow convergence in CFD simulations, and the user is advised to attempt to eliminate instantaneous reaction steps whenever possible. The question of determining the rate constants (and their dependence on temperature) is also an important consideration. Ideally, this should be done under laboratory conditions for which the mass/heat-transfer rates are all faster than those likely to occur in the production-scale reactor. Note that it is not necessary to completely eliminate mass/heat-transfer limitations to determine usable rate parameters. Indeed, as long as the rate parameters found in the lab are reliable under well-mixed (vs. perfect-mixed) conditions, the actual mass/ heat-transfer rates in the reactor will be lower, leading to accurate predictions of chemical species under mass/heat-transfer-limited conditions. 

Figure 56 uses the example of associate formation between the ionic defect O and the electronic defect h to emphasize that the strict treatment requires the solution of coupled diffusion-reaction relationships, describing the general (electro-)chemical reaction scheme with individual diffusion or rate constants as parameters (cf. Section VI. 2). Source terms (q) must be taken into account in the relevant continuity equations, e.g., for defect B that can be created by... 

Two other source terms need Co be considered. First, there Is the possibility of dry or wet deposition of a chemical species to the water surface from the atmosphere. The rate of dry deposition depends on Che concentration of Che species In Che gas phase, the concentration In surface water, Che Henry s Law constant, and the reactivity of Che species with seawater constituents, as well as the wind speed and the aerosol In the air adjacent to the water surface. e rate of wet deposition depends on Che concentration of the species In the precipitation and Che frequency and duration of precipitation events. Little Information is currently available about these processes, although there Is some evidence that a rainstorm can Inject significant quantities of 2 2 upper... 

Eq (1) states that there is a balance of the heat evolved in the chemical reaction, the heat conducted from the site of the reaction and the increase in the temp of the system. It is the term q which both.determines the expl props of a reactant and is the source of mathematical obstacles to finding the soln of eq (1). This is so because the peculiar nature of expl reaction requires a mathematical expression for q which will allow a very rapid change of reaction rate within a narrow temp range. The conventional two-constant Arrhenius term satisfies the requirement, providing the exothermicity of the... 

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