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The onset of thermal instability in a dielectric rotating nanofluid layer saturating a porous medium with vertical AC electric field is investigated by employing Darcy model for porous medium. The rheology of the nanofluid is described by Walters' (model B') for calculating the shear stresses from the velocity gradients. The employed model incorporates the combined effects of movement of the molecules of the fluid striking the nanoparticles, thermophoresis and electrophoresis due to the embedded particles. The boundaries are considered to be stress free. It is assumed that nanoparticle flux is zero on the boundaries. The eigen-value problem is solved analytically using the first approximation of Galerkin method. The Darcy Rayleigh number for onset of nonoscillatory (stationary) modes is obtained. The effects of the modified Taylor number, the AC electric Rayleigh number, the Lewis number, the modified diffusivity ratio, nanoparticles Rayleigh number and medium porosity have been discussed. The kinematic viscoelasticity accounting for rheology of the nanofluid has no effect on the stationary convection for Walters' (model B') nanofluids and behaves like an ordinary Newtonian nanofluid. Oscillatory convection has been ruled out under the considered boundary conditions.

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For the last few years, thermal instability of non-Newtonian nanofluids becomes a prominent field of research because it has various applications in automotive industries, energy-saving, nuclear reactors, transportation, electronics etc. and suspensions of nanoparticles are being developed in medical applications including cancer therapy. In this paper, a free electrothermo-convective instability in a dielectric nanofluid layer in a porous medium is studied. An Oldroyd’s constitutive equation is used to describe the behaviour of nanofluid and for porous medium, the Darcy model is employed. The equation of conservation of momentum of fluid is stimulated due to the presence of an AC electric field, stress-relaxation parameter and strain-retardation parameter. The stability of the system is discussed in stationary and oscillatory convections for free–free boundaries. For the case stationary convection, it is found that the Oldroydian Nanofluid behaves like an ordinary nanofluid as the stationary Rayleigh number is independent of the stress-relaxation parameter, the strain-retardation parameter and Vadasz number. The effect of stress-relaxation-time parameter, strain-retardation-time parameter, Vadasz number, nanoparticles Rayleigh number, modified diffusivity ratio, medium porosity, Lewis number and electric Rayleigh number examined numerically and graphs have been plotted to analyse the stability of the system. It is observed that the electrical Rayleigh number has destabilizing influence whereas nanoparticles Rayleigh number, porosity and modified diffusivity ratio have stabilizing effect on the system. The oscillatory convection is possible for the values of the stress-relaxation parameter less than the strain-retardation parameter for both top-heavy/bottom-heavy distributions of nanoparticles.

In this study, the influence of suspended particles on thermal convection in a porous layer saturating a Jeffrey fluid is examined. Linear stability theory based on normal modes is employed to derive a mathematical theorem on thermal convection in a porous layer saturating a Jeffrey fluid which states that the viscoelastic thermal convection at marginal state cannot manifest as stationary convection if the thermal Rayleigh number R, the medium permeability parameter Pl, the Jeffrey parameter λ_3 and suspended particles parameter B, satisfy the inequality