Hydrodynamic instability mechanism

Hydrodynamic instability mechanism. Here, instabilities occur in the vapor-liquid interfaces leading to the breakdown of the vapor release mechanisms and to vapor accumulation at the surface leading to critical heat flux. 

Hydrodynamic Instability Mechanism. This mechanism was suggested by Zuber (Zuber [159], Zuber et al. [160]) the original Zuber hypothesis was for an infinite flat plate, and the situation is illustrated conceptually in Fig. 15.63. 

Landau instabilities are the hydrodynamic instabilities of flame sheets that are associated neither with acoustics nor with buoyancy but instead involve only the density decrease produced by combustion in incompressible flow. The mechanism of Landau instability is purely hydrodynamic. In principle, Landau instabilities should always be present in premixed flames, but in practice they are seldom observed (26,27). 

For the hydrodynamic instability model, Lienhard and Dhir (1973b) extended the Zuber model to the CHF on finite bodies of several kinds (see Sec. 2.3.1, Fig. 2.18). Lienhard and Hasan (1979) proposed a mechanical energy stability criterion The vapor-escape wake system in a boiling process remains stable as long as the net mechanical energy transfer to the system is negative. They concluded that there is no contradiction between this criterion and the hydrodynamic instability model. 

Neal, L. G., S. M. Zivi, and R. W. Wright, 1967, The Mechanisms of Hydrodynamic Instabilities in Boiling Channel, Euratom Rep., Proc. Symp. on Two-Phase Flow Dynamics, Eindhoven, The Netherlands. (6)... 

The breakup or bursting of liquid droplets suspended in liquids undergoing shear flow has been studied and observed by many researchers beginning with the classic work of G. I. Taylor in the 1930s. For low viscosity drops, two mechanisms of breakup were identified at critical capillary number values. In the first one, the pointed droplet ends release a stream of smaller droplets termed tip streaming whereas, in the second mechanism the drop breaks into two main fragments and one or more satellite droplets. Strictly inviscid droplets such as gas bubbles were found to be stable at all conditions. It must be recalled, however, that gas bubbles are compressible and soluble, and this may play a role in the relief of hydrodynamic instabilities. The relative stability of gas bubbles in shear flow was confirmed experimentally by Canedo et al. (36). They could stretch a bubble all around the cylinder in a Couette flow apparatus without any signs of breakup. Of course, in a real devolatilizer, the flow is not a steady simple shear flow and bubble breakup is more likely to take place. 

FIGURE 9.8. Schematic illustration of the mechanism of hydrodynamic instability. 

Since the concern here is with the destruction of a contiguous laminar flame in a turbulent field, consideration must also be given to certain inherent instabilities in laminar flames themselves. There is a fundamental hydrodynamic instability as well as an instability arising from the fact that mass and heat can diffuse at different rates i.e., the Lewis number (Le) is nonunity. In the latter mechanism, a flame instability can occur when the Le number (D/a) is less than 1. 

Mechanism of rupture. Black films. The mechanism of hydrodynamic instability of thin foam films was analyzed in [278, 279, 411], The stability of ultrathin films is governed by a competition between capillary forces and the molecular component of the disjoining pressure. An instability can arise when dU/dh > 0 and the capillary pressure is not too large. This is possible if... 

Hydrodynamic instabilities 3.11.1 Homogeneous instability in shear flow The anisotropic properties of nematics give rise to certain novel instability mechanisms that are not encountered in the classical problem of hydro-dynamic instability in ordinary liquids. Theoretical work on electro-hydrodynamic instability stimulated systematic studies on two other types of convective processes, viz, thermal and hydrodynamic instabilities, and it was soon established that the basic mechanisms involved in all three cases are closely similar. " In this section we shall examine the problem of hydrodynamic instabilities in nematics. 

As shown in Figure 26.1, the wide gap opens up between the particle and continuum paradigms. This gap cannot be spanned using statistical mechanical methods only. The existing theoretical models to be applied in the mesoscale are based on heuristics obtained via downscaling of macroscopic models and upscaling particle approach. Simphfied theoretical models of complex fluid flows, e.g., flows in porous media, non-Newtonian fluid dynamics, thin film behavior, flows in presence of chemical reactions, and hydrodynamic instabilities formation, involve not only vah-dation but should be supported by more accurate computational models as well. However, until now, there has not been any precisely defined computational model, which operates in the mesoscale, in the range from 10 A to tens of microns. 

Dyson-type equations have been used extensively in quantum electrodynamics, quantum field theory, statistical mechanics, hydrodynamic instability and turbulent diffusion studies, and in investigations of electromagnetic wave propagation in a medium having a random refractive index (Tatarski, 1961). Also, this technique has recently been employed to study laser light scattering from a macromolecular solution in an electric field. 

Instabilities caused by the flow of matter have been known for a long time and their study constitutes a central task of hydrodynamics and its applications [1], The driving force of these instabilities are the spatial gradients of the flow velocity field when spatially separated elements are in relative motion, they exert destabilizing mechanical, electrical or electromagnetic forces on each other. The hydrodynamic system may be just a single species which is often simply referred to as matter or fluid , regardless of its chemical nature. Perhaps the simplest example of a hydrodynamic instability is the Kelvin-Helmholtz instability of in viscid shear flow [1]. 

Studies of propylene random copolymers have recently gained importance over isotactic polypropylene in applications requiring high clarity, flexibility and low-temperature performance (Maier and Calafut, 1998). Traditionally, the enhancement in mechanical performance of melt-spun fibres relies primarily on the control of molecular chain orientation and crystalline structure development through take-up speed, drawing ratio and quenching conditions. Heterogeneous particulate reinforcement of polymers often leads to phase separation, increases the melt viscosity and creates hydrodynamic instabilities. 

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