AbstractExperiments have been performed to provide basic heat transfer data for thermal convection in a horizontal concentric annulus partially filled with water. A real-time holographic interferometry was employed to map the temperature distribution and a laser shadowgraph system was used to measure local heat transfer coefficients along the heated inner cylinder. Experimental results demonstrate that a minimum average Nusselt number for the water-wetted portion of the inner cylinder occurs at the air-water interface situated flush with the top of the inner cylinder. Corresponding numerical simulations were conducted for the cases of either fully or half-filled with water and the predictions agree well with the experimental results.