MHD double-diffusive mixed convection and entropy generation of nanofluid in a trapezoidal cavity

•Entropy generations due to heat and mass transfer increases highly with the increment of aspect ratios. So low aspect ratio cavity is preferable in many engineering applications to minimize entropy generation.•Forced convection flow reduces entropy generations than natural convection and increases the efficiency of any system in which double diffusive MHD flow is occuring.•To enhance the efficiency rectangular cavity is preferable and the strength of magnetic field should be low.•For sprinkle of heat and mass throughout the whole of the cavity source of heat and mass at middle portion of the lower wall is highly acceptable.•Nusselt and Sherwood numbers increase with aspect ratio as well as Richardson number. To prevent this the strength of magnetic field should be low. [Display omitted] Double diffusive, Magnetohydrodynamics (MHD), mixed convection flow of Al2O3-water nanofluid inside a trapezoidal enclosure with discrete heat and mass source at the bottom wall of the cavity has been discussed numerically. Together with these entropy generations due to fluid friction, heat transfer, mass transfer and magnetic field are discussed. The upper wall of the cavity is considered to move with a fixed velocity U0 towards positive X-direction. Different inclination angles and different aspect ratios are considered in the present study. Second and fourth order finite difference approximations are used to solve the governing equations by using biconjugate gradient stabilized method (BiCGStab). It is observed that, entropy generation mostly depends on heat transfer and lower values of Richardson number (Ri=0.01) reduces the entropy generation. The highest quantitative value of |Sθ|max is 21,664 for Ri=100,Re=100andϕ=450 and lowest quantative value of |Sθ|max is 5,824 for Ri=0.01,Re=1000andϕ=600 throughout the present study. Also the highest quantitative values of average Nusselt and Sherwood numbers are 0.2597 and 0.4208 for Ri=10,Re=100andϕ=45∘. The main objective of our present study is to minimize entropy generations due to combined effects of fluid flow, heat transfer, mass transfer and the effect of magnetic field so that the loss of energy can be reduced. This study observed that mixed convection flow, low magnetic field and low aspect ratio cavity of rectangular shape is always preferable to reduce total entropy generation and the middle part of the lower wall of the cavity is highly acceptable for heat and mass source for sprinkle of heat and mass throughout t