Bubble development

Boiling in the bulk of the fluid generally takes place at submicron nucleation sites as impurities, crystals, or ions. When there is a shortage of nucleation sites in the bulk of the liquid, its boiling point can be exceeded without boiling then the liquid is superheated. There is, however, a limit at a given pressure above which a liquid cannot be superheated, and when this limit is reached, microscopic vapor bubbles develop spontaneously in the pure liquid (without nucleation sites). 

In WT boilers operating at very high firing rates, the risk exists of steam bubbles developing in downcomers. Where this occurs, it causes a temporary halt in the natural steam-water circulation and instantly leads to surging or priming followed by carryover. 

A second problem in these studies concerns cavitation dynamics on the nanometer length scale [86]. If sufficiently energetic, the ultrafast laser excitation of a gold nanoparticle causes strong nonequilibrium heating of the particle lattice and of the water shell close to the particle surface. Above a threshold in the laser power, which defines the onset of homogeneous nucleation, nanoscale water bubbles develop around the particles, expand, and collapse again within the first nanosecond after excitation (Fig. 9). The size of the bubbles may be examined in this way. 

Figure 8. Estimation of transport disengaging height (TDH) according to Ref 41, umb = fluidization velocity at which bubble development begins...

A major factor in fluidized bed behavior is the interaction between the gas flow from individual orifices and the particle and gas mixture within the bed. The jet penetration and the subsequent bubble formation have an important influence upon solids and gas mixing and, ultimately, upon the usefulness of the bed for reactor purposes. While flow visualization data are available at ambient pressures and temperatures, the natures of jet penetration and bubble development at high pressures and temperatures are not easily measured. Typical data on bubble size and bubble velocity at ambient conditions are shown, represented by the small size symbols, in Figure 2. It is well known that bubble volume can be correlated as a function of gas volumetric flow rate ( ) and that bubble velocity is related to the size of the bubble radius ( ). Such semi-empirical correlations are indicated as solid lines in that figure. 

In many cases, bubbles of gas and other contaminants are already present in the liquid or polymer solution, and these serve as sites into which the gas may diffuse. The number and size of these gas bubbles may be another important factor in bubble development. 

As seen in the sequence of still pictures presented, a spherical bubble with a finite contact area on the surface develops as soon as the boundary surface between air and water passes the orifice. The spreading of air phase on the membrane surface and the development of the bulk of a bubble occur simultaneously. However, a constant base is established in the very early stage, and the major portion of bubble development occurs using the fixed contact base. The base area differs depending on the interfacial tension between water and surface. 

In the bubble formation from a horizontal surface, the bubble development and the bubble detachment are coupled. When the buoyancy of a developing bubble overcomes the bubble attachment force due to the interfacial tension, the bubble detaches from the surface and completes the process of the bubble formation. A higher flow rate of air in the low flow rate regime (e.g., 0.2-30 seem) simply increases the frequency of the bubble formation but does not change the volume of bubble [1]. 

In the bubble formation from an inclined surface, however, the bubble development and the bubble detachment processes are decoupled because a developing bubble could drift out of the orifice due to the component of the buoyancy parallel to the inclined surface. Once a sessile bubble drift out of the orifice, the bubble development ceases because no air is fed into a sliding bubble. Since the bubble development and detachment are decoupled, the flow rate of air becomes an important factor, which controls the frequency of sliding bubble... 

The significance of sessile bubble development shown above could be rephrased as follows When a surface is immersed in water, the surface is wetted regardless of the contact angle of water or the wettability of the surface. When air is injected... 

Striking evidence of phenomena that are clearly dependent on the formation of a superficial layer across the crystal surface is provided [67] by a microscopic examination of the changes that occur during the dehydration of cleaved single crystals of a-NiS04.6H20. DSC observations (4 K min ) between 400 and 415 K detected a series of small endotherms, identified from observations as bubble development with swelling of a relatively impermeable and elastic surface layer. 

A hydrogen gas bubble developing from a nuclear reactor accident is a highly unusual event and is an example of the particular environment that is required for hydrogen to explode. 

Observation (a) A lively bubbling development of gas begins, the solution in the test tube gets hot, the piece of magnesium decreases in size and is completely dissolved. The gas in the second test tube reacts near the flame with a special pop sound hydrogen. Crystals of white salt crystallize from the solution after the evaporation of the water magnesium sulfate, (b) A small amount of gas can be observed, the gas has a peculiar smell hydrogen sulfide. 

The main postulates of the K-L model (Kunii and Levenspiel, 1968a, b 1991) are sketched in Figure CSS. lb. The bubble develops a cloud of particles around it as it moves upward at a velocity... 

In either case, the platinum-containing mass is spread uniformly on the glass (or porcelain) and carefully heat-dried so that no bubbles develop. The coated surface is then heated to a dull red heat in a muffle furnace or in a sulfur-free blowtorch flame. 

Instrumentation. A miniaturized cell used for studies of metal-deposition dynamics has been described [287]. Results pertaining mostly to metal-deposition processes have revealed that zinc can deposited on hydrogen bubbles developed at sufficiently negative electrode potentials, thus explaining common defects in zinc plating. 

A theoretical explanation of the close correlation between pick-off annihilation and the surface tension is given by the bubble model . According to this model, a small bubble develops around the Ps atom, owing to the repulsion potentials arising between the molecules of the medium and the neutral Ps atom formed in liquids. The surface tension acting on the surface of the bubble tends to decrease the surface area, and thus the resulting radius of the bubble will depend on the equilibrium of the opposing forces. 

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