Boiling explosive

Temperature determines whether or not the liquid in a vessel will boil when depressurized. The liquid will not boil if its temperature is below the boiling point at ambient pressure. If the liquid s temperature is above the superheat-limit temperature Tj] (Tsi = 0.897 ), it will boil explosively (BLEVE) when depressurized. Between these temperatures, the liquid will boil violently, but probably not rapidly enough to generate significant blast waves. However, this is not certain, so it is conservative to t sume that explosive boiling will occur (see Section 6.3.2). 

The concepts of boiling in micro-channels and comparison to conventional size channels are considered in Chap. 6. The mechanism of the onset of nucleate boiling is treated. Specific problems such as explosive boiling in parallel micro-channels, drag reduction and heat transfer in surfactant solutions are also considered. 

Onset of Nucleate Boiling in Parallel Micro-Channels 6.2.1 Physical Model of the Explosive Boiling... 

A physical model of ONB for the explosive boiling and dryout, was suggested. In order to understand why dryout occurred even at a low value of vapor quality x, it is important to keep in mind that the liquid film does not cover the entire heated surface of the micro-channel, and two-phase flow is characterized by an unsteady cyclic behavior. The following assumptions are made in the development of the model ... 

Fig. 6.18 Scheme of explosive boiling J micro-channel, 2 main area of visual observation, 3 ONB point, 4 elongated cylindrical bubble, 5 liquid in front of the bubble, 6 vapor, 7 liquid droplets and clusters. Reprinted from Hetsroni et al. (2005) with permission... 

Explosive Boiling of Water in Parallel Micro-Channels 309... 

In the study by Hetsroni et al. (2006b) the test module was made from a squareshaped silicon substrate 15 x 15 mm, 530 pm thick, and utilized a Pyrex cover, 500 pm thick, which served as both an insulator and a transparent cover through which flow in the micro-channels could be observed. The Pyrex cover was anod-ically bonded to the silicon chip, in order to seal the channels. In the silicon substrate parallel micro-channels were etched, the cross-section of each channel was an isosceles triangle. The main parameters that affect the explosive boiling oscillations (EBO) in an individual channel of the heat sink such as hydraulic diameter, mass flux, and heat flux were studied. During EBO the pressure drop oscillations were always accompanied by wall temperature oscillations. The period of these oscillations was very short and the oscillation amplitude increased with an increase in heat input. This type of oscillation was found to occur at low vapor quality. 

The last problem of this series concerns femtosecond laser ablation from gold nanoparticles [87]. In this process, solid material transforms into a volatile phase initiated by rapid deposition of energy. This ablation is nonthermal in nature. Material ejection is induced by the enhancement of the electric field close to the curved nanoparticle surface. This ablation is achievable for laser excitation powers far below the onset of general catastrophic material deterioration, such as plasma formation or laser-induced explosive boiling. Anisotropy in the ablation pattern was observed. It coincides with a reduction of the surface barrier from water vaporization and particle melting. This effect limits any high-power manipulation of nanostructured surfaces such as surface-enhanced Raman measurements or plasmonics with femtosecond pulses. 

Hazard, i.e. the potential of the material to cause injury under certain conditions (flammability, explosion limits in air, ignition and autoignition temperatures, static electricity (explosions have occurred during drying due to static electricity), dust explosion, boiling point, fire protection (specification of extinguishers, compounds formed when firing), R S (nature of special risk and safety precautions). Table 5.2-5 lists hazards associated with typical chemical reactions. 

Accidental slow addition of water to a mixture of the anhydride and acetic acid (85 15) led to a violent, large scale explosion. This was simulated closely in the laboratory, again in the absence of mineral-acid catalyst [1]. If unmoderated, the rate of acid-catalysed hydrolysis of (water insoluble) acetic anhydride can accelerate to explosive boiling [2], Essentially the same accident, fortunately with no injuries or fatalities this time, was repeated in 1990. 

Daniel A. Crowl, Ph.D. Professor of Chemical Engineering, Michigan Technological University Fellow, American Institute of Chemical Engineers (Section Editor, Process Safety Introduction, Combustion and Flammability Hazards, Gas Explosions, Vapor Cloud Explosions, Boiling-Liquid Expanding-Vapor Explosions)... 

A Boiling Liquid Expanding Vapour Explosion, or BLEVE, is an industrial event related to the laboratory bump occasioned when the inadequately mixed bottom of a vessel of liquid becomes superheated, then explosively boils. In the industrial version, rupture of a pressurised container is usually involved. Although strictly speaking a non-reactive physical hazard, chemical fires and explosions, with fatalities, often follow. Means of estimating risk and prevention, with a fist of incidents are given[l], A more ferocious version, the Boiling Liquid Compressed Bubble... 

In a number of industrial operations, there exists the possibility of contacting two different liquids—one hot and relatively nonvolatile and the other cold and more volatile. Should such an event occur, normally the more volatile liquid vaporizes and, thereby, cools the hotter liquid. In certain circumstances, however, the course of events changes dramatically, and vaporization occurs in such a brief period of time as to resemble an explosion. This explosive boiling may initiate shock waves that can damage equipment and injure personnel in the vicinity of the blast. 

Events of this nature have been described by various terms, e.g., rapid phase transitions (RPTs), vapor explosions, explosive boiling, thermal explosions, and fuel-coolant interactions (FCIs). They have been reported in a number of industrial operations, e.g., when water contacts molten metal, molten salts, or cryogenic liquids such as liquefied natural gas (LNG). In the first two examples noted above, water is the more volatile liquid and explosively boils whereas, in the last example, the cryogenic liquid plays the role of the volatile boiling liquid and water is then the hot fluid. 

Because of the hazards caused by such explosive boiling incidents, industry has supported research programs seeking answers to several basic questions ... 

What is the mechanism of explosive boiling, i.e., in a liquid-liquid accident, when might one expect simple boiling and when/how does simple boiling evolve to explosive boiling ... 

Definitive answers to these questions are not yet available. Only recently has there been the opportunity to coUect and compare explosive boiling incidents from different industries. Also, most experiments that have been conducted to explore RPT mechanisms have been limited to relatively small-scale tests. Evidence now exists to suggest that the small-scale results may only be indicative of a trigger or initiating step, and other mechanisms need to be introduced to explain large-scale boiling explosions. 

Laboratory investigations into the mechanism of smelt-water explosive boiling events have been primarily of value in delineating the effect of smelt composition on the sensitivity of the salt in producing RPTs. For example, pure molten sodium carbonate has never led to explosive boiling. Addition of either (or both) sodium chloride or sodium sulfide lead to smelts which are more prone to explosive boiling. Investigators experimented with many additives both to the smelt and to the water in an attempt to obtain less sensitivity. Most had little or no effect. 

Experimental test results for molten aluminum-water RPTs are described in Section V. Also shown is a tabulation of most documented aluminum-water explosive boiling incidents (see Table XIV). In many accidents, the quantity of water was quite small, e.g., some resulted when wet aluminum ingots were loaded into melting furnaces containing molten aluminum. In contrast, one notes that few, if any, serious events have ever been obtained when small quantities of aluminum were contacted with a large mass of water. Since laboratory tests were often carried out in the latter fashion, most of these have produced negative results. 

Explosive boiling is certainly not the normal event to occur when liquids are heated. Thus, the very rapid vaporization process must be explained by theories other than standard equilibrium models. For example, if two liquids are brought into contact, and one is relatively nonvolatile but at a temperature significantly above the boiling point of the second liquid, an explosive rapid-phase transition sometimes results. Various models have been proposed to describe such transitions. None has been... 

LNG spills on hydrocarbons. Explosive boiling was obtained with spills of LNG on pentane or hexane (pure or layered on water). No pressure measurements were given. 

A few larger-scale tests were planned wherein a larger mass of water would be driven into a mass of molten NaCl. All tests were negative and photography showed that a few leading drops of water would always contact the salt, explosively boil, and drive the descending water column back so as to prevent it from contacting the salt ... 

A thermal explosion is the metal industry s term for explosive boiling or rapid-phase transition. ... 

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