Rheokinetic liquid

The conclusion of a review is a review of the review , i.e. an abstraction of the third order, if a theoretical analysis of a specific problem is conridered as an abstraction of the first order and the review of such analysis, i.e. the present work on the whole, as an abstraction of the second order. Such an approach reminds one of a view of an artist working in the field of nonrepresentational art , who escapes from the sxirrounding practical world. It is hardly probable that this would be fruitful in the present case. Therefore, concluding the discussion of the results of analyzing the flow of polymerizing liquids ( rheokinetic liquids), we would like to make only a few general remarks. 

These equations must be supplemented by a kinetic equation for the time dependence of the degree of conversion P(t), and the dependence of the viscosity of a reactive mass on (3, temperature, and (perhaps) shear rate, if the reactive mass is a non-Newtonian liquid. The last two terms in the right-hand side of Eq. (2.89) are specific to a rheokinetic liquid. The first reflects the input of the enthalpy of polymerization into the energy balance, and the second represents heat dissipation due to shear deformation of a highly viscous liquid (reactive mass). 

As a first approximation, the several authors assumed a fixed constant parabolic velocity profile.193,194 However, this approach is generally inadequate for a rheokinetic liquid because, first, real velocity profiles have a very different shape (as will be demonstrated below), and second,... 

Figure 4.25. Profiles of axial velocities in the flow of a rheokinetic liquid through a tubular reactor. Row rate increases from (a) to (c). The dashed lines denote the boundary between the high-viscosity product (high degree of conversion) and the low-viscosity reactant (breaking through stream).

Hydrodynamics of "rheokinetic liquids is a theoretical basis for many production processes, namely, RIM-process, processing of items from thermosetting resins, molding of elastomers, plastisols, etc. It is important to note that different physicochemical processes are superimposed [1-3], first of all heat exchange and flow [4], because rheokinetic phenomena are always accompanied by heat output and occur under nonisothermal conditions. Then, it is necessary to allow for the possibility of crystallization of the product taking place [5, 6], etc. 

It should be noted that all investigations of flow stability of polymerizing liquids are few in number and have been carried out up till now only for unidimensional problems. The problem of stability of steady rheokinetic two-dimensional flows to local hydrodynamic perturbations has not been discussed in the literature yet. Obviously the problem can be solved (the solution is difficult from the technical point of view), for example, by numerical methods solving the problem on unsteady development of the flow of polymerizing mass directly after a forced local change of the profile of the flow velocity. 

A complete analytical examination of the role of distribution of the flow velocity over the radius of a tube is obviously impossible. A formulated problem for a complete description of the flow of rheokinetic liquid seems to be quite difficult and it is clear that the first steps in investigating a two-dimensional flow were based on very simple assumptions. In a number of works [43,44], the authors took a fixed parabolic profile which is incorrect in principle for the flow of polymerizing media and leads to important mistakes. This is demonstrated very well in Ref. [45] where the possibility for styrene polymerization in a tubular reactor has been estimated it hse been shown that, if a real distribution of flow velocities and residence times over the radius is taken into account, the answer must be negative, in Ref. [44] however, a positive answer is obtained for an a priori parabolic profile. 

R tly, theoretical investigatimis were carried out of the effect of hydrocfynamics of polymerizing liquids [Pg.132]

However, rheokinetic effects cause the develproducts accumulate on the walls and in the axial zone the flow is accelerated, i.e. the feed rate of the reactants increases. As a result, the Vf = U,. equilibrium is violated, the front line is distmted and its central part is displaced towards the ouq ut. Consequently, the temperatiure becomes lower, the rate of combustion dr<, and the feed—combustion equilibrium is violated still more. Also, the frcmt region is cooled down and is transferred out of the tube. Therefore, for a rheokinetic liquid (polymerizing medium with a sharp viscosity growth), a low-temperature condition for the process is the only steady-state solution. The polymerization front normal to the flow can exist only as an unsteady state and this solution is unstable. 

Consideration of a two-dimensional flow pattern was a fundamental step in the analysis of the flow of rheokinetic curing liquids, as well as in the case of a sharp but limited viscosity growth. The loss of the possibility to flow leads to a stronger distortion of all characteristics over the section of Ae channel as compared to a usual pattern and to new qualitative regularities. This was demonstrated by Vaganov [91], where the ana-... 

We think that it is an urgent matter to solve concrete problems taking into accoimt all possible peculiarities different empirical relationships (rheological, kinetic), different geometry, various types of representing boundary conditions, organization of heat and mass transfer, etc. The absence of investigations of the effect of non-Newtonian properties of a liquid on the flow mechanisms of rheokinetic media seems to be a gap in the field of theoretical analysis. 

The important problems include the control of polymerization reactors containing rheokinetic liquids. These problems have not been solved in many respects even for more simple situations. Both mathematical modelling and physical understanding of the process are the key problems [112]. 

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