Estudo experimental e numérico da dinâmica de partículas granulares em um tambor rotatório

Abstract

Since the industrial processes efficiency depends on the granular flow regime established under given operating conditions on the rotary drum, the ability to predict the particle motion inside this equipment, including the particle properties effect, is of primary importance. So, in this work an experimental and numerical study was carried out in order to investigate: the transition phenomenon between different flow regimes, the mixture and segregation phenomenon and the particle dynamic behavior inside an unbaffled rotary drum, under different operating conditions, using particles of different physical properties. A modification of the Blumberg and Schlünder model equation for rolling-cascading transition was proposed by the introduction of the particle shape effect, represented here by the sphericity. It was observed, for the first time, the hysteresis phenomenon in the transition between cataractingcentrifuging regimes which was shown to be dependent on the physical properties of the particles such as sphericity, density and particle-wall friction coefficient. A new expression relating the critical rotation speed for centrifuging as a function of the filling degree, which takes into consideration the particle properties and the hysteresis effects, was proposed. Regarding the segregation phenomenon, radial segregation due to particle diameter and density differences was observed in all systems studied after a few drum rotations. Size induced axial segregation (banding) was observed, as expected. However density differences alone did not induce axial segregation. As regards the numerical investigation, two different approaches were used for the granular flow prediction inside a rotary drum: Eulerian and Lagrangian. For the Eulerian approach, the results indicated that the kinetic model, which has been successfully applied in many dilute granular flow simulations, may also be applied in the dense granular flow treatment present in rotary drums. It was also observed that, the drag force can be neglected in the case of a rotating drum operated in the rolling regime where there is no fluid entering or leaving the system. Taking the computational efforts into account, this force can be set to zero in the entire calculation domain. For the Lagrangian model calibration, a sensitivity analysis of the numerical dynamic angle of repose due to variations in the friccional coefficient (μf) and the damping ratio (β), both varying from 0.149 to 0.701, was assessed using a Central Composite Design. The smallest deviation from the experimental data when using rice grains was obtained in the simulation whose parameters values were μf = 0.425 and β = 0.149 with an error of about 2.9%. As regards the glass beads, the smallest deviation between experiment and simulation was found using a friction coefficient μf = 0.701 and damping ratio β = 0.425 with an error of about 3.4%. For the case of the rotary drum using rice grains, which are characterized by irregular shapes, the calibrated model was affected by neither the filling degree nor the drum rotation speed. On the other hand, in the case of rounded particles (glass beads), the Lagrangian model parameters should be calibrated to specific conditions of rotation speed and filling degree and cannot be generalized.