[en] We numerically study the effect of an externally-evanescent magnetic field on total entropy generation in conducting and non-reactive fluid enclosed in a square cavity. The horizontal walls of the enclosure are assumed to be insulated while the vertical walls are kept isothermal. A control volume finite element method is used to solve the conservation equations at Prandtl number of 0.71. The values of relaxation time of the magnetic field are chosen, so that the Lorentz force acts only in the transient state of entropy generation in natural convection. The total entropy generation was calculated for fixed value of irreversibility distribution ratio, different relaxation time varying from 0 to 1/5 and Grashof number equal to 10⁵

[en] This paper numerically investigates the effect of an externally evanescent magnetic field on flow patterns and heat transfer of fluid in a square cavity. The horizontal walls of the enclosure are assumed to be insulated while the vertical walls are kept isothermal. A control volume finite element method is used to solve the conservation equations at Prandtl number of 0.71. The effect of constant Hartman number on Nusselt number was studied. Validation tests with existing data demonstrate the aptitude of the present method to produce accurate results. The effects of magnetic field inclination angle from 0 degree to 90 degree on streamlines distributions are shown for different values of Hartman number. For Grashof number equal to 10⁵, the values of relaxation time of the magnetic field are chosen, so that the Lorentz force acts only in the transient state of Nusselt number in natural convection. The Nusselt number was calculated for different values of the inverse relaxation time varying from 0 to + ∞. The magnitude and the number of oscillations of the Nusselt number were observed. It has been found that no oscillation was seen at relaxation time equal to 20

[en] An analysis of thermodynamic irreversible principle (TPI), through determination of entropy generation rate, during double-diffusive convection in an octagonal shape, filled with saturated fluid in a porous enclosure, is numerically investigated in this work, by studying the influence of thermodiffusion effect on entropy generation variations. According to the fluid flow evolution under consideration, the influence of mass flux due to temperature gradient is incorporated into the governing equations of the problem, which are solved numerically, using the COMSOL software. Appropriated thermal and solutal Dirichlet boundary conditions are used under the Boussinesq approximation. For the porous medium, the Darcy–Brinkman model is assumed in coupling with energy and mass transfer balance equations. The flow model is described in terms of mass flux due to temperature gradient, buoyancy ratio, thermal Rayleigh number and porosity of the medium. Results of the variations of heat transfer and entropy generation in the studied enclosure are graphically illustrated and are basically discussed, through various physical aspects of the problem. The numerical computations are presented for various values of thermal Rayleigh number (Ra_T), thermal diffusion parameter (Kt), Darcy number (Da), buoyancy ratio (N) and porosity of the medium (ε). In addition, the total entropy generation due to thermal, species and mixed diffusion gradients; Darcy–Brinkman dissipation; and fluid friction are studied and discussed. It is found that the entropy generation increases strongly when passing from the cooperative case of buoyancy forces to the opposite case. A similar behaviour is obtained with increasing the thermal diffusion ratio, at a constant buoyancy ratio. Also, an accentuated increase in entropy generation is observed when the Darcy number exceeds the value 10⁻⁶. Moreover, the results show that the augmentation of the thermodiffusion effect induces an increase in total entropy generation, at fixed value of the porosity.