Natural convection in thermal plumes emerging from a single heat source

• We consider the onset of thermal plumes emerging from a small discrete heat source.
• We perform 2D and 3D numerical simulations considering the heat source at the bottom of a slender cavity.
• We examine the effect of Prandtl number, Rayleigh number and aspect ratio of the cavity on both the heat transfer and the plume dynamics.

We report numerical simulations of confined natural convection from a single heat source, leading to the evolution of thermal plumes in two and three dimensions. Thermal plumes are driven through a single heat source mounted flush at the bottom of a slender cavity where vertical and top walls are isothermal heat sinks. Velocity and temperature fields were obtained for two Prandtl numbers, P=0.025,0.71P=0.025,0.71 at three different values of the Rayleigh number, R=104,5×104,105R=104,5×104,105 and for different box aspect ratios. Two kind of flow solutions were found: (i) Steady states corresponding to stable thermal plumes characterized by a well defined flow circulation inside the cavity and (ii) periodic states where both the flow and thermal fields oscillate in time. Unsteadiness of fluid and thermal flows is favored by choosing low Prandtl number fluids, working at high Rayleigh numbers inside high aspect ratio cavities. Instabilities are characterized by a periodic and propagative motion of the thermal plume in both transverse and vertical direction. It can be attributed to destabilizing shear stresses between ascending and descending fluid layers.