Position and velocity of a large particle in a gas/solid riser using the radioactive particle tracking technique

Abstract

The flow behavior of the solids phase in the fully developed region of a laboratory-scale circulating fluidized bed riser was studied using an assembly of sixteen NaI detectors to determine the position of a 500 pm radioactive particle, 100 timeds. The particle location was inferred from the number of y-rays recorded by the assembly. The knowledge of the instantaneous positions enabled the determination of the instantaneous and mean velocity fields. Tests were conducted in a 0.082 m diameter, 7 m tall riser using 150 p silica sand particles. Data were obtained at a gas superficial velocity of 4 m/s and solids mass fluxes from 23 to 75 kg/m2.s. Radial profiles of axial particle velocity showed that the solids velocity decreased with increasing solids circulation rates. Correspondingly, turbulent particle velocities and solids dispersion coefficient in the longitudinal direction were found to decrease as the solids circulation rate increased. The crosssectional area where, on average, solids downflow took place, increased with increasing solids circulation rate.On a etudie le comportement en koulement de la phase des solides dans la region pleinement developpee d'une colonne montante a lit fluidise circulant a I'echelle de laboratoire en utilisant un assemblage de seize detecteurs NaI afin de determiner la position d'une particule radioactive de 500 pm, a une frhuence de 100 foids. La localisation de la particule a ete estimee a partir du nombre de rayons y et la connaissance des positions instantanees a permis de determiner les champs de vitesse instantane et moyen. Nous avons utilid une colonne de laboratoire montante de 0,082 m de diametre et 7 m de hauteur avec des particules de sable de d i c e de 150 p. Des donnees ont ete obtenues a une vitesse de gaz superficielle de 4 m/s et a des taux de circulation de solides de 23 a 75 kg/m2.s. Des profils radiaux de la vitesse axiale des particules montrent que la vitesse des solides diminue avec I'augmentation des taux de circulation des solides. De la mBme maniere, on a trouve que les vitesses de particules turbulentes et le coefficient de dispersion des solides dans la direction longitudinale diminuaient avec I'augmentation du taux de circulation des solides. La region transversale a la paroi oh, en moyenne, se produit I'ecoulement de solides descendant, augmente avec la circulation des solides. Quantification of the flow patterns and backmixing is essential for proper design and scale-up of CFB risers. Existing models often suffer from a lack of pertinent experimental data for model validation. The gadsolid flow in the CFB riser is characterized by a strong heterogeneity. Yet, experimental investigations have demonstrated that the solids distribution is nonhomogeneous, in both axial and radial directions in the CFB riser. For example, the nonhomogeneous character of the solids distribution in the axial direction was attributed to the acceleration of particles entering with low velocities at the bottom of the riser. Harris et al. (1994) and Grace (1996) have also sh...

Three-component solids velocity measurements in the middle section of a riser

“…This is to be expected as the turbulence in riser flows is proved to be anisotropic [3]. The anisotropy could be attributed to the length to diameter ratio of the riser that is much larger than unity, implying that the axial a) b) measurements in riser flows [3][4]8]. This observation implies that the instantaneous radial and azimuthal particle velocities exhibit a rather broad distribution of slightly positive and slightly negative values, which result in very low mean values upon averaging.…”

“…The development of theoretical models and empirical equations to simulate the riser flow suffer from this lack of relevant experimental data, e.g. in view of model validation and refinement [3][4]. Furthermore, the majority of the available experimental data for two-phase riser flow provides at its best two simultaneously measured solids velocity components.…”

Computation of gas and solid dispersion coefficients in turbulent risers and bubbling beds

“…For the radial autocorrelation, the profile dips below zero, then oscillates to a stationary value of zero due to the wall limitation of x direction. For the direction of flow, the autocorrelation coefficient simply decayed exponentially, corresponding to Roy et al (2005) in a liquid-solid system and Godfroy et al (1999) in a gas-solid riser. Fig.…”

Computation of turbulence and dispersion of cork in the NETL riser

“…The dispersion coefficients in the radial and axial directions are expressed as in Hinze (1959) and Godfroy et al (1999), as follows:…”