Turbulent mixing model

Norris, A. T. and S. B. Pope (1991). Turbulent mixing model based on ordered pairing. 

An improved turbulent mixing model. Combustion Science and Technology 28, 131-135. 

The aim of a turbulent mixing model Is to assess the Importance of mixing within the photic zone, and the way In which turbulence can mix water containing species that have either been photochemlcally produced or depleted, from the photic zone down to depths below the mixed layer. As Indicated In the Introduction, the aim of this Chapter Is not to discuss the consequences of horizontal transport. For the oceans, such considerations are usually not necessary bearing In mind the homogeneity that Is typically at a scale of several hundred kilometers, the velocity of horizontal transport, and the lifetimes of many photochemical species of a few hours to a few days (1 ). 

The turbulent mixing model was set up In this case with an Initial temperature profile of neutral stratification between the surface and 80 m, and then allowed to vary according to surface wind stress and heat flux. The Initial deviation of the currents from geostrophy was assumed to be zero. The surface heat flux was allowed to vary dlelly In a sinusoidal manner with a small constant heat flux superimposed, simulating net heating In summer and cooling in winter. 

The phenomenon of concentration polarization, which is observed frequently in membrane separation processes, can be described in mathematical terms, as shown in Figure 30 (71). The usual model, which is weU founded in fluid hydrodynamics, assumes the bulk solution to be turbulent, but adjacent to the membrane surface there exists a stagnant laminar boundary layer of thickness (5) typically 50—200 p.m, in which there is no turbulent mixing. The concentration of the macromolecules in the bulk solution concentration is c,. and the concentration of macromolecules at the membrane surface is c. 

The model used here is a slightly modified version of the standard Fluent model [2]. Two possible reaetion rates are ealeulated, the kinetie reaetion rate Rj, and a seeond reaetion rate R that is eontrolled by the turbulent mixing. The kinetie reaetion rate for speeies i is ealeulated as ... 

Skaret presents a general air and contaminant mass flow model for a space where the air volume, ventilation, filtration, and contaminant emission have been divided for both the zones and the turbulent mixing (diffusion) between the zones is included. A time-dependent behavior of the concentration in the zones with constant pollutant flow rate is presented. 

Baldyga, J. and Bourne, J.R., 1984c. A fluid mechanical approach to turbulent mixing and chemical reaction. Part III Computational and experimental results for the new micromixing model. Chemical Engineering Communications, 28, 259-281. 

The axial dispersion model is readily extended to nonisothermal reactors. The turbulent mixing that leads to flat concentration profiles will also give flat temperature profiles. An expression for the axial dispersion of heat can be written in direct analogy to Equation (9.14) ... 

J. Kim, S. H. Chung, K. Y. Ahn, and J. S. Kim, Simulation of a diffusion flame in turbulent mixing layer by the flame hole dynamics model with level-set method. Combust. Theory Model. 10(2) 219-240, 2006. 

The uncertainty in the predicted CHF of rod bundles depends on the combined performance of the subchannel code and the CHF correlation. Their sensitivities to various physical parameters or models, such as void fraction, turbulent mixing, etc., are complementary to each other. Therefore, in a comparison of the accuracy of the predictions from various rod bundle CHF correlations, they should be calculated by using their respective, accompanied computer codes.The word accompanied here means the particular code used in developing the particular CHF correlation of the rod bundle. To determine the individual uncertainties of the code or the correlation, both the subchannel code and the CHF correlation should be validated separately by experiments. For example, the subchannel code THINC II was validated in rod bundles (Weismanet al., 1968), while the W-3 CHF correlation was validated in round tubes (Tong, 1967a). 

The three models were calculated with the same chemical and physical inputs with the only exception of convection, for which we adopted the Full Spectrum of Turbulence convective model (FST, Canuto Mazzitelli 1991), and the MLT model (Vitense 1953) with two values of the free parameter connected to the mixing length a = 1.7 (the standard value, used to reproduce the evolution of the Sun) and a = 2.1. 

Jurado E (2006) Modelling the ocean atmosphere exchanges of persistent organic pollutants (POPs). PhD thesis, Universitat Politecnica de Catalunya, Barcelona, Spain Jurado E, Zaldvar JM, Marinov D, Dachs J (2007) Fate of persistent organic pollutants in the water column Does turbulent mixing matter Mar Poll Bull 57 441 151 Kissa E (2001) Fluorinated Surfactants and Repellents. Marcel Dekker Inc. 

Baldyga, I., and Henczka, M., Turbulent mixing and parallel chemical reactions in a pipe application of a closure model, Recents Progres en Genie des Procedes 11, 341-348 (1997). Bermingham, S. K., Kramer, H. I. M., and Van Rosmalen, G. M. Comp. Chem. Eng. 22, 355-362... 

The CFD model for the reaction given in Eq. (60) in the limit where the first reaction is very fast must account for fluctuations in and Y2 due to turbulent mixing. In general, this is done by solving for their joint PDF (Fox, 2003), denoted here by / ((, y). There are several ways this can be accomplished ... 

Dispersion models describe the airborne transport of toxic materials away from the accident site and into the plant and community. After a release the airborne toxic material is carried away by the wind in a characteristic plume, as shown in Figure 5-1, or a puff, as shown in Figure 5-2. The maximum concentration of toxic material occurs at the release point (which may not be at ground level). Concentrations downwind are less, because of turbulent mixing and dispersion of the toxic substance with air. 

The dominant transport process from water is volatilization. Based on mathematical models developed by the EPA, the half-life for M-hexane in bodies of water with any degree of turbulent mixing (e.g., rivers) would be less than 3 hours. For standing bodies of water (e.g., small ponds), a half-life no longer than one week (6.8 days) is estimated (ASTER 1995 EPA 1987a). Based on the log octanol/water partition coefficient (i.e., log[Kow]) and the estimated log sorption coefficient (i.e., log[Koc]) (see Table 3-2), ii-hexane is not expected to become concentrated in biota (Swann et al. 1983). A calculated bioconcentration factor (BCF) of 453 for a fathead minnow (ASTER 1995) further suggests a low potential for -hcxanc to bioconcentrate or bioaccumulate in trophic food chains. 

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