Choking velocity

For compressible fluids one must be careful that when sonic or choking velocity is reached, further decreases in downstream pressure do not produce additional flow. This occurs at an upstream to downstream absolute pressure ratio of about 2 1. Critical flow due to sonic velocity has practically no application to liquids. The speed of sound in liquids is very liigh. See Sonic Velocity later in this chapter. 

The boundary between the turbulent and the fast fluidization regimes has been of some dispute in the fluidization field. However, the choking velocity (Uch) appears to be a practical lower-velocity boundary for this regime (Karri and Knowlton, 1991 Takeuchi et al., 1986). 

Figure 22. The variation of choking velocity with pressure. (Knowlton.)...

Punwani, D. V., Modi, M. U., and Tarman, P. B., A Generalized Correlation for Estimating Choking Velocity in Vertical Solids Transport, Proc. International Powder and Bulk Solids Handling and Processing Conference, organized by the Powder Advisory Centre and the IIT Research Institute, Chicago (1976)... 

Finally at the highest gas velocities we exceed what is called the choking velocity. Above this the bed is in pneumatic transport. This transition velocity depends on the solid flow rate, and according to Bi and Fan (1991) occurs at... 

Although the phenomena are not clearcut, partial settling out of solids from the gas stream and other instabilities may develop below certain linear velocities of the gas called choking velocities. Normal pneumatic transport of solids accordingly is conducted above such a calculated rate by a factor of 2 or more because the best correlations are not more accurate. Above choking velocities the process is called dilute phase transport and, below, dense phase transport. 

What appears to be the best correlation of choking velocities is due to Yang AlChE J. 21, 1013-1015 (1975)], supplemented by Punwani et al. and Yang (cited by Teo and Leung, 1984, pp. 520-521). The choking velocity UKC and voidage ec are found by simultaneous solution of the equations... 

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. 

Briens, C. L., and Bergougnou, M. A. New model to calculate the choking velocity to monosize and multisize solids in vertical pneumatic transport lines, Can. J. of Chem. Eng. 64, 196 (1986). 

Knowlton, T. M and Bachovchin, D. M. The determination of gas-solids pressure drop and choking velocity as a function of gas density in a vertical pneumatic conveying line, International Conference on Fluidization, Pacific Grove, June 15-20, p. 253 (1975). 

Gas-Solid Transfer 119 Choking Velocity 119 Pressure Drop 119... 

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