An experimental and numerical smoothed particle hydrodynamics (SPH) analysis was performed for the convective flow arising from a horizontal, thin cylindrical heat source enclosed in a glycerin-filled, slender enclosure at low Rayleigh numbers (). Both the experiments and the SPH calculations were performed for positive ( K) and negative ( K) temperature differences between the source and the surrounding fluid. In all cases, a pair of steady, counter-rotating vortices is formed, accompanied by a plume of vertically ascending flow just above the source for and a vertically descending flow just below the source for . The maximum flow velocities always occur within the ascending/descending plumes. The SPH predictions are found to match the experimental observations acceptably well with root-mean-square errors (RMSE) in the velocity profiles of the order of m s−1. The fact that the SPH method is able to reveal the detailed features of the flow phenomenon demonstrates the correctness of the approach.