Showing papers by "Sreenivas Jayanti published in 2005"

Experimental and Numerical Study of a Rotating Wheel Air Classifier

Abstract: Rotating wheel air classifiers are often used in many process industry applications. The internal geometry of these equipment is quite complicated and has not been investigated in detail. In the present study, the flow field inside a rotating wheel air classifier has been calculated using CFD techniques, taking full account of the internal geometrical features. The predicted overall flow rate and the flow pattern are in good agreement with measurements and flow visualization studies. The calculations show that the induced flow depends strongly on geometric parameters such as the location of the inlet and outlet ports and the type of shutters used. Trajectory calculations of single particles show that the particle motion is influenced principally by centrifugal force, air drag, and wall-rebound characteristics. The wall rebound is possibly one of the means of how large particles enter the fines stream, leading to low efficiency at high speeds or large particles. Experiments of the classification using angular and radial shutter vanes show distinct range of operability of each type. These results have been interpreted coherently in the light of the flow pattern and particle trajectory calculations. © 2005 American Institute of Chemical Engineers AIChE J, 51:776–790, 2005

Effect of Taylor vortices on mass transfer from a rotating cylinder

Abstract: Mass transfer from solids, which has important applications in a number of chemical and pharmaceutical industries, has been studied experimentally and semiempirically under turbulent flow conditions, and correlations are available in the literature to calculate the mass-transfer coefficients from pellets, rotating cylinders and disks etc. However, mass transfer under laminar flow has not been sufficiently addressed. One of the difficulties here is the strong Reynolds number dependence of the flow pattern, for example, due to the onset of Taylor vortices for the case of a rotating cylinder. This problem is circumvented by using a computational fluid dynamics (CFD)-based solution of the governing equations for the case of a cylinder rotating inside a stationary cylindrical outer vessel filled with liquid. The parameters cover a range of Reynolds number (based on the cylinder diameter, and the tangential speed of the cylinder), Schmidt number and the ratio of the outer to inner cylinder diameters. The results confirm that the circumferential velocity profile is a strong function of the Reynolds number and varies from a nearly Couette-type flow at very low Reynolds numbers to a boundary layer-like profile at high Reynolds numbers. The onset of Taylor vortices has a strong effect on the flow field and the mass-transfer mode. The calculations show that the Sherwood number has a linear dependence on the Reynolds number in the Couette-flow regime, and roughly square-root dependence after the onset of Taylor vortices. Correlations have been proposed to calculate the Sherwood number taking account of these effects.

Power Number Calculations for a Taylor-Couette Flow

Abstract: The classical problem of Taylor-Couette flow between rotating concentric cylinders under laminar flow conditions is studied using computational fluid dynamics (CFD) techniques. The flow between concentric cylinders with the inner cylinder rotating and the outer cylinder stationary for a Newtonian fluid has been calculated for a range of Reynolds numbers, radius ratios, length to diameter ratios using the CFD code CFX. The predicted tangential velocity profiles have been found to agree well with analytical profiles in the Couette flow regime and the transition from Couette flow to Taylor vortex flow has also been correctly predicted. Results for the power number show that it varies inversely with Reynolds number in the Couette flow regime and as Re -n where n ~ 0.7 in the Taylor vortex flow. These results agree well with the experimental data of Sinevic et al (1). A new correlation is proposed to calculate the power number for the Couette flow and the Taylor vortex flow regimes.

On axial coherence of interfacial waves in countercurrent flow

Abstract: Flooding is the limiting condition of stable countercurrent gas-liquid flow in columns and has been the subject of extensive research over a number of decades. An exhaustive review of the subject, as pertaining to unpacked columns, has been given by Bankoff and Lee.1 While a number of correlations exist in the literature to predict the onset of flooding, the topic of the mechanism of flooding has been debated and discussed widely in the recent literature. Briefly, there are two schools of thought on the flooding mechanism: (1) the formation and upward transport of large waves from the bottom of the column near the gas injection/liquid withdrawal point, and (2) the destabilization of the liquid film near the liquid inlet resulting in the formation of large waves. Direct visual evidence of formation of large waves at the liquid exit under flooding conditions has been reported by a number of researchers.2,3,4 However, this idea was opposed by Zabaras and Dukler.5 They measured the local instantaneous wall shear stress using an electrochemical method just below the liquid feed, as well as just above the liquid withdrawal point. They found that in all instances of countercurrent flow, including conditions along the flooding curve, the time-averaged wall shear stress was directed upward indicating that the velocity near the wall was downward. They also measured the instantaneous film thickness from two conductance probes located 0.053 m apart. Cross-correlation of these signals showed a well-defined peak at a measurable time delay even under flooding conditions. Zabaras and Dukler, therefore, concluded that waves were travelling downward even under flooding conditions and that they did not reverse direction at the flooding point. They explained the photographic observation of upward-travelling waves by McQuillan et al.3 as a transient phenomenon generated by the small depressurization used by McQuillan et al. to induce flooding. Thus, Zabaras and Dukler argued that the process of flooding was controlled by conditions that existed at the or just below the feed rather than at the liquid exit. Jayanti et al.6 attributed the existence of different mechanisms of flooding to the effect of tube diameter. In small diameter tubes, a coherent, ring-like wave could be formed around the inside wall of the tube which could then be transported upward by the gas. A flooding model, based on the formation of a large standing wave at the flooding point was developed by Shearer and Davidson.7 Jayanti et al. argued that such a wave would be very unstable in large diameter tubes and, hence, could not travel upward for long distances. Vijayan et al.8 conducted flooding experiments in tubes of 25, 67 and 99 mm inner dia. and confirmed visually the existence of upward travelling waves in the 25 mm dia. tube under flooding conditions. In the larger diameter tubes, the waves were observed to move only a short distance before collapsing. Similar behavior was also observed by Biage et al.9 in a duct of rectangular cross-section and by Watson and Hewitt10 in a tube of 82 mm internal dia. While there is, thus, a convergent view on the flooding mechanism, the careful experimental results of Zabaras and Dukler5 remain unexplained. Specifically, their observations that the wall shear stress always indicated a downward moving flow and that the film thickness measurements recorded no upward-moving waves need to be explained in a manner consistent with the recent visual observations of flooding mechanisms. Toward this end, film thickness measurements have been conducted under conditions in which different flooding mechanisms prevailed. The details of these studies and the results obtained are reported here.

On Axial Coherence of Interfacial Waves in Countercurrent Flow (R&D Note)

Abstract: Flooding is the limiting condition of stable countercurrent gas-liquid flow in columns and has been the subject of extensive research over a number of decades. An exhaustive review of the subject, as pertaining to unpacked columns, has been given by Bankoff and Lee.1 While a number of correlations exist in the literature to predict the onset of flooding, the topic of the mechanism of flooding has been debated and discussed widely in the recent literature. Briefly, there are two schools of thought on the flooding mechanism: (1) the formation and upward transport of large waves from the bottom of the column near the gas injection/liquid withdrawal point, and (2) the destabilization of the liquid film near the liquid inlet resulting in the formation of large waves. Direct visual evidence of formation of large waves at the liquid exit under flooding conditions has been reported by a number of researchers.2,3,4 However, this idea was opposed by Zabaras and Dukler.5 They measured the local instantaneous wall shear stress using an electrochemical method just below the liquid feed, as well as just above the liquid withdrawal point. They found that in all instances of countercurrent flow, including conditions along the flooding curve, the time-averaged wall shear stress was directed upward indicating that the velocity near the wall was downward. They also measured the instantaneous film thickness from two conductance probes located 0.053 m apart. Cross-correlation of these signals showed a well-defined peak at a measurable time delay even under flooding conditions. Zabaras and Dukler, therefore, concluded that waves were travelling downward even under flooding conditions and that they did not reverse direction at the flooding point. They explained the photographic observation of upward-travelling waves by McQuillan et al.3 as a transient phenomenon generated by the small depressurization used by McQuillan et al. to induce flooding. Thus, Zabaras and Dukler argued that the process of flooding was controlled by conditions that existed at the or just below the feed rather than at the liquid exit. Jayanti et al.6 attributed the existence of different mechanisms of flooding to the effect of tube diameter. In small diameter tubes, a coherent, ring-like wave could be formed around the inside wall of the tube which could then be transported upward by the gas. A flooding model, based on the formation of a large standing wave at the flooding point was developed by Shearer and Davidson.7 Jayanti et al. argued that such a wave would be very unstable in large diameter tubes and, hence, could not travel upward for long distances. Vijayan et al.8 conducted flooding experiments in tubes of 25, 67 and 99 mm inner dia. and confirmed visually the existence of upward travelling waves in the 25 mm dia. tube under flooding conditions. In the larger diameter tubes, the waves were observed to move only a short distance before collapsing. Similar behavior was also observed by Biage et al.9 in a duct of rectangular cross-section and by Watson and Hewitt10 in a tube of 82 mm internal dia. While there is, thus, a convergent view on the flooding mechanism, the careful experimental results of Zabaras and Dukler5 remain unexplained. Specifically, their observations that the wall shear stress always indicated a downward moving flow and that the film thickness measurements recorded no upward-moving waves need to be explained in a manner consistent with the recent visual observations of flooding mechanisms. Toward this end, film thickness measurements have been conducted under conditions in which different flooding mechanisms prevailed. The details of these studies and the results obtained are reported here.