The Onset of Thermal Instability in a Triply Diffusive Three Dimensional Fluid Layer in Porous Medium

An investigation is made on the onset of thermal instability in a triply diffusive three dimensional fluid layer in porous medium , since there are important fluid mechanical systems in which the density depends on three or more stratifying agencies having different diffusivities, in which the onset of convective instability in isothermal ternary and non-isothermal ternary fluids is termed as triple diffusion. Special attention is given to systems with k₁ >>k₂ , k₃ and p₁ >>K₂/K₃ where K_i is the molecular diffusivity of the ith component and Pis the molecular diffusivity of the ith component and p₁ is the thermal Prandtl number. The perturbation equations are analyzed in terms of normal modes after linearizing the relevant set of hydrodynamic equations to derive a characteristic equation. For the stationary convection, the stable solute gradients are found to have stabilizing effects on the system, due to which the onset of instability is postponed and the medium permeability parameter has a destabilizing effect on the system..The presence of stable solute gradients and medium permeability parameter also introduce oscillatory modes which were nonexistent in their absence. The case of overstability (with increasing amplitude of perturbations) is also considered and the sufficient conditions for the non existence of stable solutions are derivedr. These analytical results are confirmed numerically and the effects of various parameters on the stability analysis for both the cases of stationary convection and overstability are depicted graphically using the software Mathematica version 2007.