Choking phenomenon

Yang, W. C. (1975). A Mathematical Definition of Choking Phenomenon and a Mathematical Model for Predicting Choking Velocity and Choking Voidage. AIChE J., 21,1013. 

Yang WC. A mathematical definition of choking phenomenon and a mathematical model for predicting choking velocity and choking voidage. AIChE J 21 1013-1021, 1975. 

The phenomenon of choked flow may also be observed in the viscous flow situation. It plays a part when venting and evacuating a vacuum vessel and where there are leaks. 

Choking is a phenomenon that occurs in high speed compressible flow (e.g. in relief systems). It occurs because, as the pressure falls along a pipe or through a nozzle, the fluid density decreases. This, means that the volumetric flow rate and, hence, the velocity increases (because the mass flow is constant). Choking occurs when the downstream pressure is reduced to the point where the velocity cannot increase any more. This effectively limits the maximum velocity and, hence, flow rate of the fluid. 

Liquid-velocity correlations are generally based on one of two concepts, and it is not always clear which one is the basis of a given correlation. One concept relies on the principle that vapour should not be entrained to the tray below. The other is grounded on the phenomenon of downcomer-inlet-choking caused by the inability of the low-density froth... 

Non-uniformity of flow profile in the radial cross-sections of a tubular reactor results either in the break-down phenomenon, when at a constant flow rate of reagents the reaction front is expelled from the reactor, or in choking , when at a constant pressure drop along the reactor length its rupturing is possible. 

Values of R, the relative importance of pore choking, are presented for selected temperatures and coke levels in Table 1. At the lowest temperature, R first increases with C. This would imply that pore choking is the predominant phenomenon of coking at low temperatures and low coke levels. However, as the coke level increases, R decreases. Hence at high coke levels, site suppression predominates. At the higher temperature, R decreases with increasing C at all coke levels, to approximately 0.07 at the 4 percent coke level. Hence, at high temperatures, Bite suppression is the predominant mode at all coke levels. 

All NiO catalysts used here showed the phenomenon of wildness described by Reilly (16). This state of the catalyst is characterized by a sharp drop in bed temperature, a large increase in product gas, and considerable laydown of carbon. The selectivity of butene dehydrogenation to butadiene falls effectively to zero. Several experiments were performed in an attempt to find the cause of wildness. There can be little doubt that this condition is due to reduction of nickel oxide to nickel metal, and it is the latter which disrupts the carbon-carbon bonds. X-ray analysis of a wild NiO catalyst showed the presence of approximately 50% metallic nickel, and reduction would probably go to completion if the reactor did not choke with carbon. 

Cavitation, the phenomenon that causes liquids to rupture and to form vaporous/gas cavities when subjected to sufficiently low pressures, can occur in any machine handling liquid when requisite hydrodynamic conditions develop (Fig. 1). Cavitation, in many cases, is an undesirable phenomenon in hydraulic machinery that can Umit performance, lower efficiency, introduce sever structural vibration, generate acoustic noise, choke flow, and cause catastrophic damage [1]. The pernicious effects of hydrodynamic cavitation on conventional fluid machinery have been recognized and actively researched in the last century. Present knowledge (experimental and analytical) of cavitation has contributed immensely toward improving the design of conventional-scale fluid machinery. 

Bilicki, Z. Kestin, J. "Physical Aspects of the Relaxation Model in Two-Phase Flow," submitted to Proceedings of the Royal Society, London. Bilicki, Z., Kestin, J. 1983. "Two-phase flow in a vertical pipe and the phenomenon of choking homogeneous diffusion model-I. 

The ability to model the spread of gases considerably denser than air is particularly interesting to chlorine processors. Relative degrees of atmospheric stability have less effect on the dispersion of heavy gases. Some of the specific characteristics of chlorine also modify the phenomenon of dispersion [73]. Ice tends to form near the release point, for example. There is also the possibility of entrainment of liquid chlorine by the formation of mist or by a choking flow that produces a two-phase mixture. The latter leads to rainout and revaporization of liquid at some distance from the source. 

In both downcomer back up and choke cases, downcomer liquid inventory increases and downcomer liquid backs up until the downcomer froth level reaches the tray above H > Hs). This phenomenon is called downcomer flood. When downcomer flood occurs to any tray, the whole tower will be flooded very quickly. A tower under downcomer flood provides virtually no distillation. In contrast, under tray flood, liquid can still leave the tower and the tower could stiU operate if the control system allows it although distillation efficiency suffers. Downcomer flood can be prevented in design by providing adequate downcomer area and clearance underneath the downcomer and minimizing tray pressure drop. Reducing reflux rate in operation could be effective in avoiding downcomer flood in operation. 

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