Influence of operating parameters on electrocoagulation of C.I. disperse yellow 3

This work deals with the electrocoagulation (EC) process for an organic dye removal. The chosen organic dye is C.I. disperse yellow 3 (DY) which is used in textile industry. Experiments were performed in batch mode using Al electrodes and for comparison purposes Fe electrodes. The experimental set-up was composed of 1 L beaker, two identical electrodes which are separated 2 cm from each other. The main operating parameters influencing EC process were examined such as pH, supporting electrolyte concentration CNaCl, current density i, and DY concen­tration. High performance EC process was shown during 45 min for 200 mg/L dye concentration at i = 350 A m-2 (applied voltage 12 V) and CNaCl = 1 g L-1 reaching 98 % for pHs 3 and 10 and 99 % for pH 6. After 10 min, DY was also efficiently removed (86 %) showing that EC process may be conveniently applied for textile industry wastewater treatment. EC using Fe electrodes exhibited slightly lower performance comparing EC using Al electrodes

Numerical nonlinear analysis of subcritical Rayleigh-B\ue9nard convection in a horizontal confined enclosure filled with non-Newtonian fluids

Non-Newtonian subcritical convection in a Rayleigh-B\ue9nard configuration was investigated numerically, using rigid boundary conditions. The flow configuration consisted of a finite aspect ratio enclosure subject to a vertical temperature gradient, which was established by heating and cooling the lower and the upper walls by either a constant heat flux or by applying constant temperatures. The non-Newtonian fluid viscosity was modeled using the Carreau-Yasuda model. The convective flow was governed by the conservation equations, which were solved numerically using a finite difference method with a time-accurate scheme. For a shallow enclosure, when the walls were subject to constant heat fluxes, an asymptotic solution was derived assuming a parallel flow behavior. A comparison between the numerical and asymptotic solutions was performed. The effects of the controlling parameters, namely, the Rayleigh and the Prandtl numbers, and the fluid rheological parameters on the onset of subcritical convective flow were investigated. The threshold for the onset of subcritical convection was found to be well below the threshold of stationary convection and decreased considerably (with the fluid rheological parameters variation) as the fluid became more and more shear thinning. Depending on the governing parameters, steady and unsteady-periodic flow solutions were possible. Within a square enclosure with slip or no-slip vertical walls, maintaining the active walls at a constant heat flux or at constant temperature, the rheological subcritical flow behavior remained qualitatively the same, but led to different subcritical convection thresholds.Peer reviewed: YesNRC publication: Ye

Revisiting the stability of circular Couette flow of shear-thinning fluids

International audienceThree-dimensional linear stability analysis of Couette flow between two coaxial cylinders for shear-thinning fluids with and without yield stress is performed. The outer cylinder is fixed and the inner one is rotated. Three rheological models are used: Bingham, Carreau and power-law models. Wide range of rheological, geometrical and dynamical parameters is explored. New data for the critical conditions are provided for Carreau fluid. In the axisymmetric case, it is shown that when the Reynolds number is defined using the inner-wall shear-viscosity, the shear-thinning delays the appearance of Taylor vortices, for all the fluids considered. It is shown that this delay is due to reduction in the energy exchange between the base and the perturbation and not to the modification of the viscous dissipation. In the non axisymmetric case, contrary to Caton[1], we have not found any instability

Energy growth in Hagen-Poiseuille flow of Herschel-Bulkley fluid

International audienceLinear stability of Poiseuille flow of Herschel-Bulkley fluid in a cylindrical pipe is studied using modal and non-modal approaches. The first part of the present study thus deals with the classical normal mode approach in which the resulting eigen-value problem is solved using a Chebyshev collocation method. Within the considered range of parameters, the modal-linear theory predicts that perturbations are dumped exponentially. In the second part, the effect of the rheological behaviour of the fluid on the pseudospectra and the most amplified perturbations is investigated. At very low Herschel-Bulkley number (Hb << 1), the optimal perturbation consists of almost streamwise vortices, and the amplification of the kinetic energy is provided by the lift-up mechanism. In contrast, for sufficiently large values of Hb, the optimal perturbation is axisymmetric and the growth of the kinetic energy is provided by the Orr-mechanism. For intermediate values of Hb, the optimal perturbation is oblique. The amplification of such perturbation is due to a synergy between Orr and lift-up mechanisms. In the last part of the study, the maximal value of the Reynolds number, Re cE , below which the perturbation energy decreases monotoni-cally with time is computed for a large range of Hb. Asymptotic behaviors of Re cE for Hb << 1 and Hb >> 1 are established. The influence of the terms arising from the viscosity perturbation is highlighted throughout this study