On the effect of various momentum transfer coefficient models on bubble dynamics in a rectangular gas fluidized bed

Abstract

In this paper a computational study of the effect of various gas—solid momentum transfer coefficient models on the bubble formation in a gas fluidized bed is presented. The numerical scheme developed is implicit in the gas—solid momentum transfer terms. The bed is modeled to be composed of gas and particulate phases. Beyond the minimum fluidization conditions the excess gas passes through the bed as bubbles. In addition to the gravitational effects on the solid particles, the overall behavior is characterized by the interphase momentum transfer and the pressure and viscosity of the solid phase. The momentum transfer coefficient is modeled for the flow around spherical particles and the solids pressure is modeled based on kinetic theory. The solid-phase viscosity is considered to be constant and a value of 1 Pa s is used as reported in the literature. The computations are performed for a two-dimensional rectangular bed of particles 500 μm in diameter. The bed has a single air jet in the middle at the bottom while the secondary air at minimum fluidization velocity comes in uniformly through the rest of the distributor. A jet velocity to distributed velocity ratio of 40:1 is used in the computations. The computations are started from minimum fluidization conditions and show the formation, growth, detachment, rise and subsequent bursting of the first bubble in the freeboard region. The results are presented in terms of void fraction contours. The computations are carried out for longer real times and the subsequent bubble dynamics is discussed. For different momentum transfer coefficients the bed was observed to demonstrate quite different behaviors after the collapse of the first bubble. For the prescribed flow rate only the first bubble was seen to be well defined and the gas tended to channel through a slender jet thereafter. A lower jet velocity case as well as uniformly fluidized case are also discussed for comparison purposes.