Homogeneous Nucleate Boiling

Instantaneous boiling takes place only if the temperature of a liquid is higher than its supeiheat-limit temperature (also called the homogeneous-nucleation temperature), in which case, boiling occurs throughout the bulk of the liquid. This temperature is only weakly dependent on the initial pressure of the liquid and the pressure to which it depressurizes. As stated in Section 6.1., T has a value of about 0.89T,., where is the (absolute) critical temperature of the fluid. 

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... 

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 common theme developed in this article is that many RPTs can be explained by stipulating that the colder, more volatile liquid is heated, without boiling, to its homogeneous nucleation temperature where prompt vaporization occurs. This sequence then leads to a sharp, but localized, shock. Laboratory-scale studies are concerned primarily with conditions affecting this initial event. Yet there are undoubtedly other mechanisms which could produce similar end results, and some of these alternatives are described. 

These results stimulated a number of studies, both in industry (Conoco, Esso, Shell Pipeline) and in academia (University of Maryland, M.I.T.). The objective was, primarily, to delineate the mechanism that led to these explosive events. The results of many small-scale experiments, primarily conducted by Shell Pipeline Corporation and M.I.T., led to the hypothesis that the apparent explosion was, in fact, a very rapid vaporization of superheated LNG. Contact of LNG, of an appropriate composition, with water led to the heating of a thin film of the LNG well above its expected boiling temperature. If the temperature reached a value where homogeneous nucleation was possible, then prompt, essentially explosive vaporization resulted. This sequence of events has been termed a rapid phase transition (RPT), although in the earlier literature it was often described by the less appropriate title of vapor explosion. 

In this article, we suggest that a modified superheated-liquid model could explain many facts, but the basic premise of the model has never been established in clearly delineated experiments. The simple superheated-liquid model, developed for LNG and water explosions (see Section III), assumes the cold liquid is prevented from boiling on the hot liquid surface and may heat to its limit-of-superheat temperature. At this temperature, homogeneous nucleation results with significant local vaporization in a few microseconds. Such a mechanism has been rejected for molten metal-water interactions since the temperatures of most molten metals studied are above the critical point of water. In such cases, it would be expected that a steam film would encapsulate the water to... 

Several investigators studied R-12. Holt and Muenker (1972) and Rausch and Levine (1973) made simple spills of this cryogen into water. The highest water temperature used by both teams was —342 K and weak explosions were noted. From Table XVI, it can be seen that this water temperature was barely within the range of the superheat-limit temperature, so no or only minor explosions might have been expected. Henry et al. (1974) spilled R-12 on top of a hot mineral oil. For oil temperatures less than about 409 K, there was little interaction except rapid boiling. Above 409 K, explosions resulted. Henry et al. state that this oil temperature would lead to an interface temperature [see Eq. (1)] close to the expected homogeneous nucleation temperature (—345 K) so that the explosions were to be expected. 

Increasing the temperature or lowering the pressure on a superheated liquid will increase the probability of nucleation. Also, the presence of solid surfaces enhances the probability because it is often easier to form a critical-sized embryo at a solid-liquid interface than in the bulk of the liquid. Nucleation in the bulk is referred to as homogeneous nucleation whereas if the critical-sized embryo forms at a solid-liquid (or liquid-liquid) interface, it is termed heterogeneous nucleation. Normal boiling processes wherein heat transfer occurs through the container wall to the liquid always occur by heterogeneous nucleation. 

The nucleation temperature, which exceeds the boiling point of the species, is the temperature at which bubbles spontaneously appear in the liquid. Bubble nucleation is a rate process, and its description on the basis of a nucleation temperature is a simplification. Homogeneous nucleation temperatures are substantially above the boiling point heterogeneous nucleation—aided, for example, by impurities like dust—may occur at somewhat lower temperatures that nevertheless still exceed the boiling point. 

The liquid state is limited by two absolute limit spinodals, located well above the boiling temperature and below P < 0. They are the loci of the absolute stability limit of the homogeneous nucleation. The significance of this issue showed studies on unusual properties of water where the estimation of the spinodal within the negative pressures domain is the fundamental checkpoint for theoretical models. The significance of this issue taken into account in the novel pressure counterpart of the VFT equation recently proposed, namely " ... 

Figure 9. Temperature ranges of states of stable, superheated and supercooled water at atmospheric pressure. Stationary homogeneous nucleation rate during crystallization (1) and boiling-up (2). Inverse isothermal compressibility for stable and metastable states of water (3) in the absence of the spinodal in a supercooled liquid (3 ) and in the case of its presence according to (3 % T - the temperature of the spinodal of a superheated liquid.

Indicated in Fig. 9 are temperature ranges of supercooled, stable and superheated water at atmospherie pressure. Ibidem one can see curves representing the temperature dependenee of the logarithm of the homogeneous nucleation rate for crystallization (curve 1) and boiling-up (curve 2). The maximum rate of formation of vapor nuclei is attained at the approach of the spinodal determined by condition (3). Fig. 9 also shows how the inverse isothermal eompressibility =-v(5p/5v) changes with temperature (curve 3). An arrow shows the temperature of the spinodal of superheated water. 

The bulk fluid at its boiling point but agitation is good and few bubbles form on the metal surfaces. Instead, homogeneous nucleation occurs near the surface of the liquid where the temperature exceeds the boiling point. The liquid head suppresses boiling deep within the vessel, and the bubble volume is typically a few percent of the liquid volume. This form of heat transfer also scales as... 

For < > = 0, /(< >) = 1 and the onset of boiling will occur at a superheat identical to that for homogeneous nucleation. On the other hand, if < > = 180° (its maximum value), then /(<)>) = 0 and boiling will be initiated at the surface as soon as the fluid reaches the saturation temperature. However, real contact angles are normally less than 90° Shakir and Thome [19] report values ranging from 86° for water on a copper surface down to 8° for n-propanol on a brass surface. 

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. 

A model of explosive boiling making use of the idea of intensive homogeneous nucleation allows us not only to give a qualitative explanation to effects observed in two-phase nonequilibrium flows but also to make trustworthy quantitative evaluations (for example of critical flow rates throu short channels) which cannot be obtained with the aid of traditional schemes of heterogeneous media mechanics. The field of applicability of the model is outlined quite definitely. This model is a usefxil addition to all other models of fluid mechanics. 

---