Analysis of viscous deformations and phase segregation in helical tubular flocculators and their relationship to the curvature ratio

Name: Maurício Sartori
Type: PhD thesis
Publication date: 02/12/2015

Namesort descending Role
Edmilson Costa Teixeira Advisor *

Examining board:

Namesort descending Role
Daniel Rigo Internal Examiner *
Edmilson Costa Teixeira Advisor *
Eduardo Cleto Pires External Examiner *
Julio Tomás Aquije Chacaltana Internal Examiner *
William Bonino Rauen External Examiner *

Summary: Helical tubular flocculators (FTHs) when compared with the currently employed hydraulic flocculators in water and wastewater treatment plants have demonstrated high efficiency in the floc formation with low hydraulic retention time (Tdh) and high power dissipation levels, challenging the current flocculation paradigm (high Tdh and low energy dissipation levels). However, the mechanisms that enable these flocculators operating satisfactorily in these operational conditions have not yet been elucidated. It is known that the fluid viscous deformations present a great influence on the collision opportunity and, consequently, on the flocculation. Another factor that also has relevance in the flocculation is the particles’ concentration which, in the current models, it is assumed uniform throughout the reactor. In this context, aiming at enhance the understanding of the flocculation in those flocculators, this work presents an evaluation of the influence of the FTHs’ curvature ratio (d/D, WHERE d is the tube diameter and D is the coil diameter) on the fluid elements strain rate, which has been assumed to be the main collision mechanism between particles. It is also shown that d/D influences the phase segregation, a characteristic of two-phase flows in curved pipes. For this purpose, we evaluated 5 FTHs configurations with curvature ratio of 0.0091, 0.0182, 0.0364, 0.0729 and 0.1458, with the support of computational fluid dynamic simulations (monophasic and biphasic), with and without considering the gravitational field, by adopting horizontal and vertical coil axes, respectively. The results demonstrate the importance of taking into account the linear strains, neglected in some collision models, besides the direct relationship of the curvature ratio with both angular and linear strains, and, consequently, with the shear strain rate. Regarding the phases segregation, there was an inverse relationship with the curvature ratio. However, this segregation is influenced by the reactor's position relative to the gravitational field. In horizontal coil axis reactors, at each turn, there is a cycle of segregation and mixing. In vertical axis FTHs, there is a significant increase of particles’ concentration in the reactor region close to the inner wall, due to the combined effect of the secondary flow drag and gravitational attraction.

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