Liquid flow, internal mechanisms

A study of how a sohd dries may be based on the internal mechanism of liquid flow or on the effec t of the external conditions of temperature, humidity, air flow, state of subdivision, etc., on the diying rate of the sohds. The former procedure generally requires a fundamental study of the internal condition. The latter procedure, although less fundamental, is more generally used because the results have greater immediate application in equipment design and evaluation. 

The structure of the solid determines the mechanism for which internal liquid flow may occur. These mechanisms can include (1) diffusion in continuous, homogeneous solids, (2) capillary flow in granular and porous sohds, (3) flow caused by shrinkage and pressure gradients, (4) flow caused by gravity, and (5) flow caused by a vaporization-condensation sequence. 

All these factors are the external variables. The internal mechanism of liquid flow does not affect the constant rate. 

For nitration a nitrator (7) equipped with cooling coils and a mechanical stirrer comprising a cylindrical rotor equipped internally with turbine paddles is used. At a sufficiently high rate of rotation, the liquid flows rapidly through the rotor in one direction thus ensuring correct circulation between the coils. 

As pointed out, common modes of interface in the case of liquid-liquid two-phase flow are slug flow and parallel flow (Figure 15.9). In the case of slug flow, two mechanisms are known to be responsible for the mass transfer between the two liquids (a) internal circulation [66, 75, 84] takes place within each slug and (b) the concentration gradients between adjacent slugs lead to the diffusion between the phases. In the case of the parallel pattern, the flow is laminar and the transfer of molecules between the two phases is supposed to occur only by diffusion. 

Flooding is the most common of the hydraulic constraints likely to be encountered. There are two mechanisms that cause it. The first, downcomer flooding, arises if the maximum internal liquid rate is exceeded. Liquid flows through the downcomer under gravity. The level of liquid built up in the downcomer is as result of a balance between the pressure drop across it and the head of liquid held above it on the tray. As the flow increases, the pressure drop increases (with the square of the flow) and so the head must increase. Ultimately the level reaches the tray above and the tray ceases to provide any separation. 

The devolatilization of a component in an internal mixer can be described by a model based on the penetration theory [27,28]. The main characteristic of this model is the separation of the bulk of material into two parts A layer periodically wiped onto the wall of the mixing chamber, and a pool of material rotating in front of the rotor flights, as shown in Figure 29.15. This flow pattern results in a constant exposure time of the interface between the material and the vapor phase in the void space of the internal mixer. Devolatilization occurs according to two different mechanisms Molecular diffusion between the fluid elements in the surface layer of the wall film and the pool, and mass transport between the rubber phase and the vapor phase due to evaporation of the volatile component. As the diffusion rate of a liquid or a gas in a polymeric matrix is rather low, the main contribution to devolatilization is based on the mass transport between the surface layer of the polymeric material and the vapor phase. 

Current breakup models need to be extended to encompass the effects of liquid distortion, ligament and membrane formation, and stretching on the atomization process. The effects of nozzle internal flows and shear stresses due to gas viscosity on liquid breakup processes need to be ascertained. Experimental measurements and theoretical analyses are required to explore the mechanisms of breakup of liquid jets and sheets in dense (thick) spray regime. 

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