Macro-segregation

Powder Techniques. Highly alloyed materials made by the processes described are particularly susceptible to segregation of alloying elements during solidification both on a macro- and a microscale. Much plastic working was necessary to minimise this susceptibiUty before service appHcations. 

The concept of a well-stirred segregated reactor which also has an exponential residence time distribution function was introduced by Dankwerts (16, 17) and was elaborated upon by Zweitering (18). In a totally segregated, stirred tank reactor, the feed stream is envisioned to enter the reactor in the form of macro-molecular capsules which do not exchange their contents with other capsules in the feed stream or in the reactor volume. The capsules act as batch reactors with reaction times equal to their residence time in the reactor. The reactor product is thus found by calculating the weighted sum of a series of batch reactor products with reaction times from zero to infinity. The weighting factor is determined by the residence time distribution function of the constant flow stirred tank reactor. 

The "Filtration Model" as proposed in this study represents the overall rate of the solute transfer (macro-segregation) in zone refining very well for a wide range of experimental conditions. 

Using the proposed model along with the two generalized correlations for the experimental transmission coefficients in the partially solidified zone, a fairly close prediction of the solute redistribution (macro-segregation) of eutectic-forming mixtures after a single zone pass can be made. 

The world-line of each macro-organism begins after branching and segregation. 

Schmidt L. Fredriksson H., Formation of macro segregation and centre-line cracks in continuously cast steel. Ironmaking and Steelmaking (Quarterly), 2 (1975) 61-67... 

High between-iocation variance is most likely attributable to poor macro-mixing which can result from incomplete blending or segregation. Additionally, sampling errors... 

These models are not discussed here and the cited papers may be referred to for details of model equations. When macroscale and microscale segregation exist together (bottom left case of Fig. 5.5), none of the cited models are adequate. For such systems, it is necessary to include detailed interaction of fluid mechanics, mixing and reactions in the mathematical model. Various modeling approaches to simulate reactive flow processes with macro- and microscale segregation are discussed briefly below. 

The influence of the ultrasonic treatment of a melt on the zone (macro) segregation is much stronger. As is known, the conditions necessary for zone segregation... 

In ideal pipe reactor, the liquid elements do not exchange their contents with their surroundings (in axial direction). This is described by the term complete segregation (degree of segregation of 1) and the liquid is named macro-liquid. In a stirred tank, which is ideally back-mixed on a molecular level, micro-liquid exists with a zero degree of segregation. 

Consider two limiting cases to help explain the effect of segregation on a single reaction Feed streams containing reactants A and B are available, each first as a micro-fluid (free to mix) and then as a macro-fluid (segregation maintained) [15]. Micro-fluids A and B behave in the expected manner and reaction occurs. However, upon mixing of macro-fluids, no reaction takes place because molecules of A cannot contact molecules of B (except at the interface, which has a zero volume in this idealization). These two situations are illustrated in Figure 9.20. 

Macro scale (CCM level) adhesion phenomena, polymer segregation in catalyst layer, mechanical properties of electrode and membrane... 

Mixing of the starting materials (macro- and micromixing with the limiting cases of ideal micromixing and complete segregation). 

Commonly a minor component is to be dispersed within the bulk of the material and then mixture quality analysed at a very small scale of scrutiny. The cohesive mixture is usually of good quality when analysed at a large scale of scrutiny but at a small scale of scrutiny a high, and unacceptable, intensity of segregation can occur. To understand the nature of this problem the mixture and the structure of the mixture have to be examined on a micro as well as a macro scale. 

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