Nucleate boiling theory

Thus, the BLEVE theory predicts that, when the temperature of a superheated liquid is below T, liquid flashing cannot give rise to a blast wave. This theory is based on the solid foundations of kinetic gas theory and experimental observations of homogeneous nucleation boiling. It is also supported by the experiments of BASF and British Gas. However, because no systematic study has been conducted, there is no proof that the process described actually governs the type of flashing that causes strong blast waves. Furthermore, rapid vaporization of a superheated liquid below its superheat limit temperature can also produce a blast wave, albeit a weak... 

Evaporation-of-microlayer theory. A later hypothesis for the mechanism of nucleate boiling considers the vaporization of a micro layer of water underneath the bubble. This was first suggested by Moore and Mesler (1961), who measured... 

As indicated in Sec. IIB, ordinary nucleate boiling is a two-step process. First, nuclei must appear. Second, the nuclei must grow into bubbles large enough to move away from the nucleation sites. The rate of heat absorption by the liquid may be controlled by either one or both of these two processes. The growth of a nucleus (tiny bubble) into ordinary bubbles has received attention recently. The theoretical attack of Forster and Zuber was discussed in Sec. IIB2. Inasmuch as the theory of Zwick and Plesset (P3, P4, Zl, Z2) represents another attempt to obtain exact expressions for bubble growth, and since the theory fits well with the few data for steam bubbles in superheated water, their theoretical method is summarized below. 

Two types of condensation, drop-wise and film-wise, have been known for many years. As soon as the two types of boiling, nucleate and film, were described, certain similarities to condensation became evident. Nucleate boiling and drop-wise condensation were seen to be analogous. This is of little practical value, because no good theory of drop-wise condensation exists. However the analogy between film boiling and film condensation is fruitful, because a good theory of film condensation exists. 

The second RPT criterion relates to the temperature of the hot liquid. That is, this temperature must exceed a threshold value before an RPT is possible. From one theory of RPTs, the superheated-liquid model (described later), this criterion arises naturally, and the threshold hot-liquid temperature is then equal to the homogeneous nucleation temperature of the colder liquid T. This temperature is a characteristic value for any pure liquid or liquid mixture and can be measured in independent experiments or estimated from theory. From alternate RPT theories, the threshold temperature may be equated, approximately, to the hot fluid temperature at the onset of stable film boiling. 

A few of the many contributors to the classical rate theory of boiling nucleation are Volmer (VI), Becker and Doring (B2), Frenkel (F7), Fisher (F3), and Bernath (B4). All agree that a prime requirement for nucleation to occur in a liquid is that the liquid must be superheated. The bubbles formed are cooler than the liquid therefore nucleation is strictly irreversible. Because of the superheat, a temperature driving force exists between liquid and bubble. However, because surface tension forces are immense for tiny bubbles, a collapsing tendency exists which may counteract the tendency of a bubble to grow by absorbing heat. One problem faced by any theory of nucleation is to explain the formation of a bubble which will not collapse. 

In spite of these difficulties, thermodynamics and the reaction-rate theory give a picture of nucleation which is reasonably consistent with experimental evidence. Researchers studying crystallization, condensation, and other nucleation phenomena have accumulated experimental values that show that this theoretical approach is a defensible one. The application of this theory to boiling has received scant attention it is clear that the science of boiling will progress rapidly as the attention to nucleation theory expands. 

Generation Spontaneous generation of gas bubbles within a homogeneous liquid is theoretically impossible (Bikerman, Foams Theory and Industrial Applications, Reinhold, New York, 1953, p. 10). The appearance of a bubble requires a gas nucleus as a void in the liquid. The nucleus may be in the form of a small bubble or of a solid carrying adsorbed gas, examples of the latter being dust particles, boiling chips, and a solid wall. A void can result from cavitation, mechanically or acoustically induced. Blander and Katz [AlChE 21, 833 (1975)] have thoroughly reviewed bubble nucleation in liquids. 

The explanation for the existence of boiling at much lower superheats than predicted by homogeneous nucleation theory is that bubbles are initiated from cavities on the heat transfer surface. Gas or vapor is trapped in these cavities as shown in Fig. IS.lb-d. Once boiling is initiated, these cavities may remain vapor-filled and continue to be active sources for the initiation and growth of bubbles from the surface. The growth process from a conical cavity whose mouth radius is rc is illustrated in Fig. 15.8. 

Z. Bilicki, The Relation Between the Experiment and Theory for Nucleate Forced Boiling, in Proc. 4th World Conf on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Brussels, Belgium, vol. 2, pp. 571-578, June 1997. 

Nucleation plays a fundamental role whenever condensation, precipitation, crystallization, sublimation, boiling, or freezing occur. A transformation of a phase a, say, a vapor, to a phase p, say, a liquid, does not occur the instant the free energy of p is lower than that of a. Rather, small nuclei of p must form initially in the a phase. This first step in the phase transformation, the nucleation of clusters of the new phase, can actually be very slow. For example, at a relative humidity of 200% at 20°C (293 K), far above any relative humidity achieved in the ambient atmosphere, the rate at which water droplets nucleate homogeneously is about 10 54 droplets per cm3 per second. Stated differently, it would take about 1054 s (1 year is 3 x 107 s) for one droplet to appear in 1 cm3 of air. Yet, we know that droplets are readily formed in air at relative humidities only slightly over 100%. This is a result of the fact that water nucleates on foreign particles much more readily than it does on its own. Once the initial nucleation step has occurred, the nuclei of the new phase tend to grow rather rapidly. Nucleation theory attempts to describe the rate at which the first step in the phase transformation process occurs—the rate at which the initial very small nuclei appear. Whereas nucleation can occur from a liquid phase to a solid phase (crystallization) or from a liquid phase to a vapor phase (bubble formation), our interest will be in nucleation of trace substances and water from the vapor phase (air) to the liquid (droplet) or solid phase. 

Nucleation and growth theories constitute a very attractive methodology in solid state physics to describe phenomena related to phase transitions. In this view, a new phase is pictured as growing out from a small finite-sized droplet. Typical examples are liquid bubbles formed from a gaseous phase (e.g., rain droplets), gas bubbles forming under close-to-critical liquids (e.g., boiling water), or crystallization in amorphous liquids (e.g., in silica-based semiconductors or organic polymers) [69]. 

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