Steam chimney

As the scale continues to develop on the surface the conditions will be modified. For instance, if the deposit layer is porous or contains fissures, it is possible for steam to be generated within the deposit, sometimes referred to as wick boiling, although the phenomenon may be more associated with corrosion deposits. Macbeth [1971] observed steam issuing from cracks in a deposit which he termed "steam chimneys". A schematic diagram is presented as Fig. 8.16. Such a condition would give rise to a different pattern of scale formation. 

The character of the bubbles (i.e. shape and size) is also likely to be affected by the presence of the deposit. For instance the so-called wick boiling mechanism mentioned earlier, is likely to play an important role in the heat transfer process. The evaporation may be considered to take place at the bottom of the steam chimney or on the walls of that channel. If the steam chimneys are absent as might be the case with small pore size or without interconnecting chaimels, heat transfer is only possible by conduction. It could also be possible to consider that the liquid film was directly on the heating surface. Mass transfer rather than heat transfer might also form the basis of a mathematical model. 

The mixture may not be well pressed so if a large quantity of steam has been generated in the pre-mixing phase and a steam chimney exists above, then the energy of a wave can rapidly decay. 

Fig. 4 Vapor volvime fraction at time 0.3 s in RUNl showing steam chimney.

In RUN4A the subcooled boiling model, surface entrainment model, and surface tracking were used. The melt distribution at time of bottom contact for this case is shown in Fig. 5, where it is apparent that the overall mixture region is considerably larger than in RUNl. The time of bottom contact is also later, 0.4s vs 0.3s. A steam chimney is still present, but steam is now being generated near the main melt mass at the tank bottom, as seen in Fig. 6. 

Fig. 6 Vapor volxjone fraction at time 0.4 s In RXJN4A showing steam chimney.

Fig. 8 Vapor voliome fraction at time 0.31 s in RUNl showing collapse of steam chimney.

It should be noted that many of the parameters available from IFCI calculations are difficult to observe experimentally for instance, the actual water/melt mass ratio in the mixture region requires determination of the spatial distribution of the three-phase system. Another example is the calculation of a steam chimney forming in the mixture region, which has not been observed directly in experiments, although its presence has been suggested. It appears important to somehow measure the spatial variation of the three-phase mixture experimentally, both to verify the calculational results and to accurately determine the characteristics of the actual mixture region. 

Incandescent particles from incinerators, flarestacks, chimneys Vehicle exhaust Steam pipes... 

The pressure vessel is 79 ft high with an upper diameter of 23 ft and lower diameter of 20 ft. This height is key to establishing natural circulation core flow by providing a chimney in the space between the top of the core and the steam separator assembly. This large top diameter increases the water inventory above the core (no accumulators needed), and the smaller lower diameter reduces the volume of water needed to be replaced to provide core cooling. 

In Britain, Leblanc pollution went uncontrolled for decades. A visitor outside Liverpool, a major Leblanc factory center, described in 1846 a sordid ugly town. The sky is a low-hanging roof of smeary smoke. The atmosphere is a blend of railway tunnel, hospital ward, gas works and open sewer. The features of the place are chimneys, furnaces, steam jets, smoke clouds and coal mines. The products are pills, coal, glass, chemicals, cripples, millionaires and paupers. An estimated 40,000 men, women, and children— many of them Irish escaping the potato famine—worked in British Leblanc factories. Until 1875, workers stirred batches of chemicals in a cloud of hydrochloric acid gas. Their teeth decayed, and their clothing burned. Inhaling deeply could make them faint and vomit. 

Top spray systems During top-spray cooling of an overheated core, the wall temperature is usually higher than the Leidenfrost temperature, which causes water to be sputtered away from the wall by violent vapor formation and then pushed upward by the chimney effect of the steam flow generated at lower elevations (as shown in Fig. 4.25). A spray-cooling heat transfer test with BWR bundles was reported by Riedle et al. (1976). They found the dryout heat flux to be a function of spray rate and system pressure. The collapsed level required to keep the bundle at saturation for various pressures compared reasonably well with that in the literature (Duncan and Leonard, 1971 Ogasawara et al., 1973). 

In order to avoid explosion, the gases should be cooled and diluted by injection of water or steam before release into the chimney. A water-injection device is shown schematically in Fig. 7.2-10. It consists of a water tank (1), a Venturi device (2) in the discharge line (3), nozzles (4) to inject the water into the gas stream, a dip-tube (5), and a line (6) which connects the water tank and the inlet of the Venturi device. The water tank is equipped with a... 

State-of-the-art first law efficiencies of boilers are typically between 85% and 90%, that is, the fraction of fuel energy value (heat of combustion) captured in steam. It is not surprising that the efficiency is not 100%, since the flue gas that exits the boiler is much hotter than the temperature of the environment and therefore takes with it some of the energy ("heat goes up the chimney"). Typical conditions in a steam cycle are as given in Table 9.6 [3,10]. In this table, we have not included the exergies of the fuel, since we will... 

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