Computational fluid dynamics realizability

It is common within the industry to characterize chemical processes in terms of one or a few global reaction steps, assigning an Arrhenius rate expression to describe the rate of each reaction. If knowledge of the detailed chemistry is inadequate or the chemical scheme is to be combined with computational fluid dynamics for a complex flow description, a simplified chemistry may be necessary. It is important, however, to realize that such a chemical description can only be used for the narrow range of conditions (temperature, composition, etc.) for which it is developed. Any extrapolation outside these conditions may be erroneous or even disastrous. 

Since it will take several years to realize such an integral software toolbox, individual approaches with separate steps have to be applied to meet gradually the requirements of microreactor design. Standard software for computational fluid dynamics is directly applicable in this context, and there are also powerful software tools for the simulation of special steps in microfabrication processes. However, there has been rather little experience with materials for microreactors, optimization of microreactor design, and, in particular, the treatment of interdependent effects. Consequently, a profound knowledge of the basic properties and phenomena of microreaction technology just described is absolutely essential for the successful design of microreaction devices. 

Multiscale descriptions of particle-droplet interactions in spray processing of composite particles are realized based on Multiphase Computational Fluid Dynamics (M-CFD) models, in which processes such as liquid atomization and particle-droplet mixing spray (macro-scale), particle-droplet collision (mesoscale), and particle penetration into droplet (micro-scale) are taken into account as shown in Fig. 18.52. Thereby, the incorporation efficiency and sticking efficiency of solid particles in matrix particles are correlated with the operatiOTi conditions and material properties. 

The approach proposed in the previous section makes it possible to develop the most rigorous model of reacting gas mixtures, since it takes into account the detailed state-to-state vibrational and chemical kinetics in a flow. However, practical implementation of this method leads to serious difficulties. The first important problem encountered in the realization of the state-to-state model is its computational cost. Indeed, the solution of the fluid dynamics equations coupled to the equations of the state-to-state vibrational and chemical kinetics requires numerical simulation of a great number of equations for the vibrational level populations of all molecular species. The second fundamental problem is that experimental and theoretical data on the state-specific rate coefficients and espiecially on the cross sections of inelastic processes are rather scanty. Due to the above problems, simpler models based on quasi-stationary vibrational distributions are rather attractive for practical applications. In quasi-stationary approaches, the vibrational level populations are expressed in terms of a... 

This finding has significant consequences for molecular dynamics. If the dynamic behavior of sufficiently dense Lennard-Jones fluids is dominated by Vii(r), as was shown for static properties, then one can realize an enormous saving of computer time by using the purely repulsive part of the spherical or ellipsoidal Lennard-Jones potential. Since the computer time used in molecular dynamics depends partially on the number of interactions experienced per molecule, the savings factor is of the order (2 /2.5) = 0.09. 

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