VARIABLE VISCOSITY EFFECTS ON THE VORTEX INSTABILITY OF FREE-CONVECTION BOUNDARY-LAYER FLOW OVER A HORIZONTAL SURFACE IN A POROUS-MEDIUM

The role of temperature-dependent viscosity is studied in the flow and vortex instability of a heated horizontal free convection boundary layer flow in a saturated porous medium. For an isothermal surface, similarity solutions are found to exist for viscosity variation expressed as a general function of temperature. For exponential variation of viscosity with temperature, the numerical results for Nusselt number, critical Rayleigh number and associated wave number at the onset of vortex instability are presented over a wide range of wall to ambient viscosity ratio parameters. It is found that the variable viscosity effect enhances the heat transfer rate and destabilizes the flow for liquid heating, while the opposite trend is true for gas heating.