Bubble Shape and Size

The refractory used for metal refining processes and continuous casting molds is commonly poorly wetted by the molten metal [39], Water model experiments have shown that bubbles ascending near a wall of poor wettability sometimes attach to the wall [32] which differs markedly from bubbles ascending near a wall of good wettability. Although the interaction between a wall and bubbles has been widely studied, focus has been on wetted surfaces [40-44], In contrast, there is little information on the attachment and detachment of bubbles from a wall of poor wettability [32]. 

Due to the typically high melting points of molten metals (often 1,000°C) and their opacity, direct observation of bubbles in the molten metals is quite difficult. An X-ray fluoroscope is a powerful tool [21] that can be used, but the spatial and temporal resolutions of bubble images are not sufficiently high. Thus a number of cold model experiments have been carried out by using water and silicone oils to simulate molten metals [48,49]. Such model liquids are easy to handle, cheap and transparent. In most of the previous model experiments, the wall of the vessel is usually wetted by the liquid, which is a major departure from reality. 

In this section, the behavior of a bubble colliding with an acrylic flat plate coated with paraffin is considered. The shape and size of a bubble in contact with the plate are also investigated in detail. 

---

Macroscopic effects at boiling are associated with changes in the intrinsic characteristics of the process (e.g., bubble shape and sizes, nucleation frequency, etc.). Let s discuss the existing experimental data in more detail. 

Bubble Shape and Size and Critical Volume Using Laplace and Potential Methods... 

Modelling of the interactions between water and air in MBR systems is important as aeration plays a key role in the mass transfer, particle size distribution and back transport of particles from the membrane surface. In addition, the behaviour of bubbles (shape and size. Fig. 15.8) is recognised to be affected by the injection method, sparger type and aeration rate. Table 15.2 summarises the models for the evaluation of effects of air bubbling in MBRs. 

His serious interest molecular biology began about 1935. He was intrigned by the question of how protein molecules were constructed. As a professor at the California Institute of Technology, he was known forgiving "baby toy lectures because he made models of molecules out of string, rod- and-ball structures, and plastic bubbles in different colors, shapes, and sizes. One day, working with paper, he sketched atoms and chemical bonds and folded them in different ways and discovered the basic structure of the protein molecule,... 

The rate of foam drainage is determined not only by the hydrodynamic characteristics of the foam (border shape and size, liquid phase viscosity, pressure gradient, mobility of the Iiquid/air interface, etc.) but also by the rate of internal foam (foam films and borders) collapse and the breakdown of the foam column. The decrease in the average foam dispersity (respectively the volume) leads the liberation of excess liquid which delays the establishment of hydrostatic equilibrium. However, liquid drainage causes an increase in the capillary and disjoining pressure, both of which accelerate further bubble coalescence and foam column breakdown. 

The character of the bubbles (i.e. shape and size) is also likely to be affected by the presence of the deposit. For instance the so-called wick boiling mechanism mentioned earlier, is likely to play an important role in the heat transfer process. The evaporation may be considered to take place at the bottom of the steam chimney or on the walls of that channel. If the steam chimneys are absent as might be the case with small pore size or without interconnecting chaimels, heat transfer is only possible by conduction. It could also be possible to consider that the liquid film was directly on the heating surface. Mass transfer rather than heat transfer might also form the basis of a mathematical model. 

The Linde surface consists of a thin layer of porous metal bonded to the heat transfer substrate. (See Figure 7-15.) Bubble nucleation and growth are promoted within a porous layer that provides a large number of stable nucleation sites of a predesigned shape and size. 

There exists only one coherent structure of turbulence in an air-water bubbling jet. Turbulence is produced mainly by ejection everywhere in the bubbling jet. The difference in the turbulence structures in the two types of bubbling jets is attributable to the shape and size of bubbles. Bubbles in the He-Wood s metal bubbling jet are of the skirted type, while those in an air-water bubbling jet adopted for comparison can be classified into the wobbling type. 

---