Convective mixing physical processes

The slowest chemical time-scale which is decoupled must be faster than that of the physical processes. This is the only restriction which applies, however, there are no restrictions on the nature of the perturbation which may be caused by diffusion, convection, mixing processes etc. The technique has been successfully applied to laminar reacting flows and diffusion flames and could potentially be applied to autoignition systems, although it is unlikely that the degree of reduction will be as great as that found for diffusion flames [144]. 

The physical processes responsible for the mixing of chemical species — besides convection predominantly Eddington-Sweet circulations and shear instabilities — do also... 

As their name suggests, these models are based on the physical principles of diffusion and convection, which govern the mixing process. According to the flow pattern, the reactor is divided into different zones with different flow characteristics. 

Diffusion, which occurrs in essentially all matter, is one of the most ubiquitous phenomena in nature. It is the process of transport of materials driven by an external force field and the gradients of pressure, temperature, and concentration. It is the net transport of material that occurs within a single phase in the absence of mixing either by mechanical means or by convection. The rates of different technical as well as many physical, chemical, and biological processes are directly influenced by diffusive mass transfer, and also the efficiency and quality of processes are governed by diffusion [1]. 

Diffusion is not restricted to molecular transfer through stagnant layers of solid or fluid. It also takes place in fluid phases by physical mixing and by the eddies of turbulent flow, just as heat flow may occur by convection. This is called eddy diffusion. Sometimes the diffusion process is accompanied by bulk flow of the mixture in a direction parallel to the direction of diffusion, and it is often associated with heat flow. 

A chemical reaction is initiated by bringing the required reactants into intimate contact. Efficient execution of this process often defines the efficiency of the reaction and, therefore, requires considered examination when designing a reaction system. The physical transport of a component along a concentration gradient by molecular diffusion and turbulent convection is known as mass transfer. The transport of mass through an interface between two media or phases of the same medium is extremely important, since chemical reactions are normally coupled to the mass transfer efficiency. The mass transfer coefficient is used to quantify the efficiency of mixing in macrofluidic systems and is given by... 

The analogy between the problem of convective diffusion in a channel with semipermeable walls and the problem of mixed heterogeneous reaction makes it possible to consider these problems together. Conclusions that are true for one problem would be true for the other problem as well, even though these processes are distinct from both physical and chemical viewpoints. Besides, the analogy with chemical reactions allows us to introduce a dimensionless parameter of the problem (see (6.16)) - the Damkoler number ... 

Moreover the effect of chemical reaction rate constant and also of some physical parameters of liquid flows (density, viscosity) on conditions of characteristic macroscopic fronts formation in turbulent flows limited by impenetrable wall allows supposing the various nature of reaction and mixing fronts formation. In the first case kinetic and diffusion process parameters are determinant, and in the second - preliminary convective and turbulent transfer. The influence of density and viscosity of liquid flows, i.e. parameters determining hydrodynamic regime of liquid flows in tubular canals on conditions of reaction and mixing plan front formation shows the important role of hydrodynamic constituent also in general case under corresponding macrostructures formation. 

Moreover, the influence of the chemical reaction rate constant and some other physical parameters of the liquid flow (density, viscosity) on the conditions of macroscopic front formation in turbulent flows, allow us to make an assumption about the differences in the nature of the reaction front and mixing front formation. In the first case, the key parameters of the process are the kinetic and diffusion parameters in the second case, however, the key parameters of the process are the convective and turbulent transfer. The influence of density and viscosity, i.e., the parameters which define the hydrodynamic motion mode of the liquid flow in the tubular channels, on the... 

Mixing can be broadly defined as a process to reduce the non-uniformity of a composition. The basic mechanism of mixing is to induce physical relative motion of the ingredients. The types of motion that can occur are molecular diffusion, turbulent motion, and convective motion. The first two types of motion are essentially limited to gases and low-viscosity liquids. Convective motion is the predominant motion in high-viscosity liquids, such as polymer melts. As discussed in Section 5.3.3, polymer melts are not capable of turbulent motion as a result of their high viscosity motion in polymer melts is always by laminar flow. 

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