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Abstract: Empirical stochastic multi-variable models for prediction of treatment efficiency of wetlands are presented in this article. Wetlands of seven different shapes are visualized using tracer studies. Two different variants of experiments are carried out. Numerous flow rate variations are performed keeping surface area of the wetland constant. The experiment is also carried out with a variation in volume of the wetland which helps to study the effect of flow height on the hydrodynamics within the wetland. A multi-variable model for treatment efficiency in terms of change in tracer concentration as a function of shape, volumetric height of water within the wetland, time, and mass flow rate is considered. Further, another set of experiments is performed studying the treatment efficiency in terms of electro-kinetic parameters. This involves measuring the pH, turbidity, temperature, electrical conductivity, total dissolved salts at inlet and outlet and residence time with varying flow rate, and height of water for the seven different wetland models under study. The electro-kinetic parameters changes due to difference in concentration of the tracer dye which simulates impurities. In this case, treatment efficiency is expressed as a function of the above-discussed electro-kinetic variables, time variation, water height, as well as variation in the mass flow rate. The stochastic multi-parameter models, thus, empirically derived in the above two cases have high coefficient of determination. The models thus derived may be used as a tool for quick analysis of treatment efficiency of any shape and size of a three-dimensional wetland. © 2014 Balaban Desalination Publications. All rights reserved.

World University root node

Department of Mechanical Engineering

Department of Computer Science & Engineering

Desalination and Water Treatment

Multi-variable approach to determine treatment efficiency of wetland: size effect and electro-kinetic effects

Formation of freckles during directional solidification of hypereutectic aqueous ammonium- chloride in a bottom cooled cavity is studied numerically. The system studied is thermally stable but solutally unstable, which causes plume type convection and formation of channels in the growing solid mush. Solidification of hypereutectic solutions is usually characterized by detached and drifting solid crystals, thus resulting in multiphase convection. The numerical simulation is performed using a fixed grid single domain approach with single-phase and multiphase convection phenomena. For the multiphase convection modeling in the solidifying system under consideration, the mushy region is assumed to consist of an immobile coherent zone containing packed equiaxed crystals and a mobile noncoherent zone where the solid crystals are able to move. The two zones are demarcated by a critical solid fraction criterion, referred to as the coherency point. The overall effects of drifting solid phase on the freckles formation are compared with the results from conventional single-phase convection model. © 2013 by ASME.

Journal of Thermal Science and Engineering Applications

Study of freckles formation during directional solidification under the influence of single-phase and multiphase convection

Coalescence of secondary dendrite arms in terms of the evolution of specific surface area for a dendritic envelope with respect to the fraction solid during directional solidification is studied analytically in 2-D and 3-D frame of reference. For 2D case study, the dendritic growth has been captured by gradual penetration of a parabola with fixed tip radius (ρ) inside a square domain, while for 3D case study, we replace the parabola and square domain for 2-D case with a paraboloid of revolution and cubic domain, respectively. The dimensions of the enclosing volumes are characterized by the secondary dendrite arm-spacing. Coalescence obtained by the analytical solution is compared with existing dendrite growth simulation results obtained by phase-field method.

Transactions of the Indian Institute of Metals

Evolution of specific surface area with solid volume fraction during coalescence with parabolic shape assumption for dendrite

A generalized enthalpy update scheme is presented for evaluating solid and liquid fractions during the solidification of binary alloys, taking solid movement into consideration. A fixed-grid, enthalpy-based method is developed such that the scheme accounts for equilibrium as well as for nonequilibrium solidification phenomena, along with solid phase movement. The effect of solid movement on the solidification interface shape and macrosegregation is highlighted. © The Minerals, Metals & Materials Society and ASM International 2011.

Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science

Communication a generalized enthalpy update scheme for solidification of a binary alloy with solid phase movement

Purpose - The purpose of this paper is to present a new numerical approach for modeling the multi-phase flowduring an alloy solidification process. Inmany solidification processes, advection of solid may have a dramatic effect on bulk convection field as well as on the solid front growth and hence on the macro-segregation pattern. In the present work, a numerical model is developed to simulate directional solidification in presence of melt convection as well as solid advection in the form of sedimentation. A 2D cavity filled with hyper-eutectic aqueous ammonium chloride solution (25 wt.% of ammonium chloride) being chilled fromone of the sidewalls has been chosen as themodel problemfor the numerical simulation. Design/methodology/approach - A fixed grid volume averaging technique has been used for solving mass, momentum, energy, and species equation while taking into account the solid phase advection and local re-melting. Two different criteria have been identified for the solid particles in the mushy zone to be mobile. These two criteria are represented by a critical solid fraction, and a critical velocity. Based on these two criteria, the mushy zone has been subdivided into two different regions namely, an immobile coherent zone consisting of packed equiaxed crystals and a mobile non-coherent zone where the solid crystals are able to move. Findings - The numerical results are compared with corresponding experimental observations. Originality/value - The solid advection velocity and source terms dealing with solid velocity have been calculated using an explicit scheme, whereas the main conservation equations are solved using an implicit scheme. © Emerald Group Publishing Limited.