Pressure fluctuations and mixing characteristics in turbulent fluidized beds

Abstract

Hydrodynamic characteristics such as the transition velocity to the turbulent flow regime, the rise velocity of voids and the bed expansion in turbulent fluidized beds of glass beads ($d_p$ = 0.362 mm) and coal particles ($d_p$ = 0.507, 0.987, 1.147 mm) have been studied from the pressure fluctuations in a 0.1 m -ID × 3.0 m high Plexiglas column. In addition, the mixing characteristics of gas and solids phases in the slugging and turbulent flow regimes have been determined. The transition velocities from the slugging to turbulent flow regimes nave been determined from the statistical properties of pressure fluctuations in the bed such as mean amplitude, fluctuation interval, standard deviation, skewness and flatness. Variation of the velocity ratio between the transition velocity to the turbulent flow regime and the terminal velocity of an individual particle can be quantitatively demarcated by the criterion between group A and B of Geldart``s classification. The column size has no appreciable influence on the transition velocity to the turbulent flow regime. A correlation of the transition velocity to the turbulent flow regime has been derived based on the data of the present and previous studies in terms of the Reynolds and Archimedes numbers as: $Re_c=0.700Ar^{0.485}$ An attempt has been made to develop a model of slug breakdown in the turbulent flow regime based on the bubble stability in terms of the Froude number criterion in which the slug breakdown in the turbulent flow regime is mainly caused by the instability of a maximum stable slug which comes from the inertial force exceeding the gravitational force of solids refluxing in the bed. This Froude number criterion provides the transition velocity to the turbulent flow regime as: $Re_c=[(33.7)^2+0.0408Ar]^{1/2}+0.598Ar^{1/2}-33.7$ A flow regime map has been proposed to demarcate flow regimes quantitatively from the bubbling to dilute phase flows in fluidized beds in terms of the Reynolds...