Studies on free surface flow of concentrated suspension in open channels and rotating cylinders
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2011
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Abstract
Free surface flow of concentrated suspension has been studied in open channel and rotating cylinders at low Reynolds number flow. In this work, the wall slip was characterized through flow visualization experiments. The conventional rheometers and Couette cells has been used for determining slip velocity for suspension of smaller particles. However, characterization of wall slip for suspensions of larger particles such as debris flow, concrete mixtures etc. cannot be performed in conventional rheometers. For such systems we have provided the technique of determining wall slip from the velocity profile measurements in open channel flow. To measure the velocity profiles in open channel flow, we have used the method of particle image velocimetry (PIV). Experiments have been carried out under the conditions of slip and no-slip conditions and velocity and surface corrugation patterns have been analyzed. Our study showed that the wall slip only changes the velocity profile and it has no apparent effect on the surface corrugation. This is because the slip layer affects are confined near the wall, the bulk shape of the surface remains unchanged. To measure the height of corrugation structures we have performed measurements in velocity-vorticity plane. The interface was seeded with hollow glass balloons which floated on the surface and their location was determined using an edge detection technique. Position of maximum gradient of image intensity determines the interface height. By comparing the interface profile between two consecutive image frames we determined the vertical velocity of the interface. It is observed that interface fluctuation increases linearly with increase in shear rate. The perturbation of the interface in the velocity-vorticity plane increases with increase in particle concentration until an optimum particle concentration, and thereafter the height of the free surface corrugation decreases continuously. These results also support the findings from the analysis of spectra of refracted light from the free surface. It is observed that the interface fluctuation increases with increase in particle size for a given suspending fluid whereas it decreases with increase in the viscosity of suspending fluid for suspension of given particle size. In another study we have, investigated the band formation during free surface flow of concentrated suspension of bi-dispersed particles in rotating cylinders. In this work, we have studied the behavior of bi-dispersed neutrally and non-neutrally buoyant suspension of non-colloidal particles in a horizontally rotating cylinder separately. Experiments were carried out at various filling fractions and angular speed of the cylinder. A number of distinct patterns of particle size segregation were observed for various conditions. At low rotation speeds, the gravity force on the particles dominates and we observe that alternate bands of larger and smaller particles completely fill the tube. These bands also move axially and merge to form bigger bands. The number of bands decreases with rotation speed whereas the band width increases. At higher speeds the bands of smaller particles are disturbed by the bigger particles and no stable axial segregation is observed. On the other hand, at much higher speed when the centrifugal force on the both smaller and larger particles dominates, the particle bands segregate from the suspending fluid..
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Supervisor: Anugrah Singh
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CHEMICAL ENGINEERING