Showing papers in "Multiphase Science and Technology in 1986"

A critical review of the flooding literature

Abstract: Countercurrent flow of a gas and a liquid in direct contact with each other is, of necessity, gravity dominated. That is, in the absence of electromagnetic force fields, thermocapillary effects, or concentration-capillary effects, countercurrent flow can be sustained only as a result of the difference in the gravitational force per unit volume on the gas and on the liquid. If the gas and liquid are simultaneously introduced into a porous medium or into a vertical or inclined pipe, the gas tends to rise relative to the liquid. If conditions allow complete separation, it is possible to maintain steady countercurrent flow in which the liquid discharges at the bottom while the gas flows out from the top. The countercurrent flow is opposed by interfacial friction between the phases, which always seems to increase monotonically as the relative countercurrent mean velocity of the phases increases. Hence, for a given geometry and liquid-gas pair, there is a maximum relative velocity that can be sustained in countercurrent flow. This point is known as the onset of flooding. Further increases in gas or liquid input ratas result in only partial delivery of the liquid out of the bottom. Eventually, if the gas velocity becomes sufficiently high, none of the liquid is delivered at the bottom, and fully cocurrent upward flow is established. If the liquid is being introduced from an upper plenum, none will penetrate into the pipe or porous medium when this second critical gas velocity is reached.

Flow pattern transitions in gas-liquid systems: measurement and modeling

Abstract: The need for reliable design methods has been the driving force behind a very large research effort in two-phase gas-liquid flow over the past 25 years. This work has been carried out at universities, national laboratories, and at industrial research and design organizations in many countries of the world. The result of this effort has been an extraordinary number of publications on the subject. Over 7500 papers, theses, and reports have appeared in these 25 years. Furthermore, the rate of publication has been increasing in recent years. Most predictive correlations that have been proposed as a result of all this work have been based largely on experiment and are valid only under conditions near that of the experiment. It is precisely for this reason that so many publications result. Each time a new condition is to be investigated new experiments are necessary.

A Comprehensive Examination of Heat Transfer Correlations Suitable for Reactor Safety Analysis

Abstract: Fuel sheath temperatures in water-cooled nuclear reactors are usually near the saturation temperature of water. However, during an accidental increase in power, or a decrease in flow and pressure, deterioration in heat transfer is possible. It occurs when the surface temperature increases to such a high level that the heated surface can no longer support continuous liquid contact. This phenomenon is usually referred to as the boiling crisis (or dryout) and the corresponding heat flux as the critical heat flux (or CHF). The boiling crisis is characterized by either a sudden rise in surface temperature, caused by the heated surface being covered by a stable vapor film (film boiling), or by small surface temperature spikes, corresponding to the appearance and disappearance of dry patches (transition boiling).

Flow of Gas-Solid Mixtures through Standpipes and Valves

Abstract: The subject matter of this chapter is the flow of a gas-solid mixture in a pipe and through an orifice: the flow of the granular solid will be downward, while the flow of the gas may be cocurrent or countercurrent to the solid Such a pipe, with a downflow of particulate solid, is often referred to as a standpipe It can be a vertical pipe connected to the bottom opening of a hopper, a cyclone dipleg, or a solids transfer line for transferring solids out of a fluidized bed Examples of standpipes can be found in the hydrocarbon cracking process, the Fischer-Tropsch process, the coal gasification and liquefaction processes, and other processes involving the downflow of solids in a pipe A number of such processes have been described by Zenz and Othmer (1960) and by Kunii (1980) The flow of solids through an orifice is often associated with the downflow of solids through a pipe where a valve (ie, an orifice) is present at one or both ends of a standpipe