Phenomena at Low Velocities

A commonly known example of stable dissipative structures is the tur bulence (the generation of internal vortices) in quickly flowing gas or fluid. The stream, which is laminar at low velocities, jumps to the turbu lent state when passing through the bifurcation point, which is determined by the Reynolds number dependent on a combination of kinetic and vis cous parameters of the fluid medium. Tornadoes and storms that can be seen from space are amphtudinous phenomena of dissipative structures that can arise in the strongly non equflibrium atmosphere. 

In convective vaporization, the same boiling regimes are encountered, but modified by the net motion of the two-phase fluid past the surface. At low velocities or high heat fluxes, the convection effect is small, and nucleate boiling dominates. At higher velocities, the heat-transfer rate is dominated by the two-phase mixture sweeping across the surface. It is still important to avoid transition and film boiling, but the onset of these phenomena is complicated by many factors. (See [1, 34].)... 

CHEN Dai-xtm, WANG Zhang-rui, Non-Darcy Phenomena of Gas Flow at Low Velocity in Tight Porous Media , Journal of Chongqing University, 23(2000), 25- 27. 

Today there is a consensus that the current system thermal-hydraulic computer codes are not sufficiently validated for some conditions and for some phenomena relevant to natural circulation (low pressure, low driving heads, effect of non-condensables, boron transport at low velocities, etc. ). It is important to have assurance that the natural circulation systems are effective during all sequences in which they are required to function, and to define the needs for computer codes capable of taking into account all the important phenomena. Extensive work is needed to develop such codes and high quality experimental data are necessary to... 

Figure 3.30. Schematic showing the impact phenomena of multiple droplets on a substrate surface spreading pattern at low impact velocities (fop) and splashing mechanism at high impact velocities (bottom).

It should be pointed out that for a low pressure gas the radial- and axial diffusion coefficients are about the same at low Reynolds numbers (Rediffusion effects may be important at velocities where the dispersion effects are controlled by molecular diffusion. For Re = 1 to 20, however, the axial diffusivity becomes about five times larger than the radial diffusivity [31]. Therefore, the radial diffusion flux could be neglected relative to the longitudinal flux. If these phenomena were also present in a high-pressure gas, it would be true that radial diffusion could be neglected. In dense- gas extraction, packed beds are operated at Re > 10, so it will be supposed that the Peclet number for axial dispersion only is important (Peax Per). 

Reductions in heat transfer coefficients (up to 25%) were found at low values of average vibrational velocity, v = 2FH. However, in every case as the vibrational velocity was increased heat transfer coefficients increased significantly. Since the goal of this study was to investigate possible improvements in heat transfer using vibration, it was considered appropriate to concentrate on the improvements. Results and analyses, presented previously, neglected data below the threshold values of vibrational intensity required for significant improvement in heat transfer rate. These reductions may be unique to the experimental apparatus utilized however, since others such as Jackson (5) have reported such decreases, this is not felt to be the case. Rather, it seems likely that some not yet identified fluid flow phenomena are involved. 

The particular characteristics of the water binding at low vapour pressures, gives rise to some very interesting phenomena with regard to the diffusion of water in cellulose and to the velocity of conditioning, if cellulose is exposed to atmospheres of different humidity. 

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