Bubble crowding

B) The DNB in a medium- or low-subcooling bubbly flow is caused by near-wall bubble crowding and vapor blanketing. 

Near-wall bubble crowding and vapor blanketing... 

Since wet foams contain approximately spherical bubbles, their viscosities can be estimated by the same means that are used to predict emulsion viscosities. In this case the foam viscosity is described in terms of the viscosity of the continuous liquid phase (tjo) and the amount of dispersed gas (4>). In dry foams, where the internal phase has a high volume fraction the foam viscosity increases strongly due to bubble crowding, or structural viscosity, becomes non-Newtonian, and frequently exhibits a yield stress. As is the case for emulsions, the maximum volume fraction possible for an internal phase made up of uniform, incompressible spheres is 74%, but since the gas bubbles are very deformable and compressible, foams with an internal vol-... 

In dry foams, where the internal phase has a high volume fraction, the foam viscosity increases strongly, because of bubble crowding or... 

Bubble crowding at the heated surface. In this postulated mechanism, bubbles (or vapor stems in the macrolayer) coalesce, leading to a reduction in the amount of liquid in contact with the wall and, hence, in the overall heat transfer rate, which begins to decrease with increasing wall superheat when this coalescence process begins. 

Bubble Crowding at Heated Surface. In this mechanism, close packing of bubbles (or... 

Near-Wall Bubble Crowding and Vapor Blanketing. Here, a layer of vapor bubbles builds up near the wall and this prevents the ingress of liquid to the tube surface, leading to a decrease in efficiency of cooling and to the critical phenomenon. 

As a result, this equation is usually the only one needed for liquid or solid aerosols. Figure 6.18 shows several sets of experimental data compared with the Einstein equation. In practice once cp reaches between 0.1 and 0.5, dispersion viscosity increases significantly and can also become non-Newtonian (due to particle/droplet/bubble crowding or structural viscosity). The maximum volume fraction possible for an internal phase made up of uniform, incompressible spheres is 0.74, although emulsions and foams with an internal volume fraction of over 0.99 can exist as a consequence of droplet/bubble distortion. Figure 6.18 and Equation 6.33 illustrate why volume fraction is such a theoretically and experimentally favoured concentration unit in rheology. In the simplest case, a colloidal system can be considered Einsteinian, but in most cases the viscosity dependence is more complicated. 

The behavior of large numbers of bubbles crowded together is different from that of isolated bubbles. Rising velocities are smaller because of crowding, and bubble diameter may be altered by liquid turbulence, which causes bubble breakup, and by coalescence of colliding bubbles. 

The foam of everyday experience, though different in several ways from the foam that occurs and is used in reservoirs, is worthy of some examination. Such everyday foam is a two-phase mixture of gas and liquid, in which the liquid is the continuous fluid and the gas is held in separate cells. See also the discussions in Chapters 1—3 of this book.) To display the distinctive foamlike characteristics, the volume fraction of the discontinuous phase must be greater than about 70%. At this high gas volume fraction (the so-called quality), the bubbles of gas are closely crowded together so that they cannot move independently. They also change in shape, and the walls of the cells become approximately planer, polygonal surfaces that are called lamellae or bubble-films. 

The latter form is a good approximation for any 0> Oq and h/R 1. In most foams, the effect is expected to be minimal, as the bubbles tend to be relatively large. For emulsions of small drop size, however, the effect may be considerable and the peculiar properties resulting from extreme crowding may commence at an apparent volume fraction that is considerably smaller than one would expect for zero film thickness. For example, in an emulsion with droplets of 2/ - 1 um and A = 50 nm, the effective volume fraction already reaches a value of 0.74 at an apparent volume fraction of only about 0.64 The finite film thickness... 

Then one day walking round Tavistock Square I made up, as I sometimes make up my books, To the Lighthouse in a great, apparently involuntary rush. . . Blowing bubbles out of a pipe gives the feeling of the rapid crowd of ideas and scenes which blew out of my mind, so that my lips seemed syllabling of their own accord as I walked. What blew the bubbles. Why then I have no notion. ( A Sketch of the Past , MB, 1976, p. 94)... 

About 60 miles North of Jeddah, Saudi Arabia, where the desert meets the Red Sea is a hyper-modem metropolis in the making. Constmction cranes crowd the sky. Hundreds of workers labor under the sun, turning sand and dirt into palm-lined promenades, bubbling fountains, and glass covered skyscrapers. The first phase of the project, which involves the construction of a seaport, an industrial zone, and a residential city district, is close to completion. When fully finished, in 2025, King Abdullah Economic City (KAEC http //207.5.46.159/en/Home/index.html) will stretch over 70 square miles, house some 2 million people, and offer one of the most competitive economic investment destinations in the world. 

Mr. Broughton (same year 1866) recalls the following fact, well known to him, that, in a soap bubble, the portions which, seen from a certain distance, appear of a uifiform color, show, when they are examined more closely, a crowd of small bands... 

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