Computational Fluid Dynamic Studies of Improved Circular Secondary Clarifier

Abstract

This dissertation work is to evaluate the sensitivity of the flow and concentration field of improved circular secondary using Computational fluid dynamics analysis. The phases present are water, the continuous medium, and activated sludge, the dispersed phase. The flow field is considered to be isothermal, incompressible and without phase change. The model is based on the Mixture model where Eulerian conservation equations are used for the mixture mass and momentum together with a convection diffusion equation for modelling the distribution of the dispersed phase. Constitutive relationships are used to model the relative motion between the phases. The rheological effects of the dispersed phase on the mixture, of an accumulated settled bed of the dispersed phase, is not considered. Double exponential law is used to describe the dependence of the settling velocity on volume fraction of solid where as particle size distribution is taken uniformly. CFD is intended to include the key mechanisms of importance to predict accurate flow and other characteristics for design, scale-up and optimization. 2D axis symmetric geometry was considered to find the parameter all along the clarifier using k - turbulent model is to account the turbulent flow. Sludge removal mechanism is simulated by using the negative source term. For the present simulation work the grid dependency test is carried out as to find the grid sensitivity. The performance of the model is assessed by applying it two validation cases using activated sludge suspensions as the working medium. Comparison between the experimentally measured and predicted values for small scale clarifier shows good agreement between the measured and predicted values. The results were encouraging with formation of plum above the enlarge inlet and dispersed phase concentration being generally well predicted throughout.