  The buoyancy-induced parallel flow in a vertical cylindrical porous layer is
analysed. A radial thermal gradient caused by a uniformly distributed heat
source is assumed to induce the buoyant flow. The layer boundaries are modelled
as isothermal and permeable to an external fluid reservoir. The onset of the
convective instability is analysed by linearising the governing equations for
the perturbations. The governing parameters driving the instability are the
heat-source Rayleigh number and the ratio between the internal radius and the
external radius. Neutral stability curves and the critical values of the
Rayleigh number, the perturbation wave number and the angular frequency are
computed numerically. It is shown that axisymmetric modes form the most
dangerous mode of instability.
