Based on the moment consistency principle, the bubble threshold was determined by using the traversing method. A novel approach was proposed to identify bubbles and agglomerates from transient signals registered in a cold mode fluidized bed dealing with different particles. Bubble and agglomerate hydrodynamics including volume fraction, solid holdup, immigration velocity, and frequency were investigated. Adding fine powder promotes the formation of particle agglomerates at low superficial gas velocity and leads to a great bubble volume fraction at high superficial gas velocity. Bubble frequency increases significantly during the flow pattern transition from bubbling to turbulence bed. Compared with FCC particles, the mixed particles have a narrow bubble chord length distribution. The net velocity of agglomerates is positive in the center and close to zero or even negative in the vicinity of the wall. The agglomerate frequency in the mixed particle bed is significantly lower than that in the FCC particle bed.
Transient
signals of an optical probe were registered in a gas–solid
fluidized bed dealing with four kinds of particles, respectively.
Particle agglomerates were identified, and the diameters were investigated.
The force balance model and the energy balance model are analyzed,
but the predictions underestimate the agglomerate diameter because
of unsuitable assumptions for FCC particle-formed agglomerates. A
modified force balance model is proposed on the basis of imperfect
elastic collision, and the restitution coefficient is considered and
calculated when calculating the collision force. The predictions are
in great agreement with experimental results. A criterion of agglomerate
coalescence and breakup is proposed. The effect of the ratio of the
diameters of two collision agglomerates is discussed, and the critical
ratio for agglomerate coalescence is calculated. The agglomerate strength
and tensile stress are calculated, and the agglomerate strength is
always greater than the tensile stress, which indicates that agglomerates
are almost impossible to break up in collision.
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