Natural convection heat transfer in vertical ribbed channels, using water as working fluid, has been experimentally studied. The investigation encompassed a large range of the channel aspect ratio, defined as the ratio between channel spacing and channel height, while the wall-to-fluid temperature difference was kept fixed. The measurement of local heat transfer coefficient was facilitated by a non-intrusive diagnostic tool, the schlieren technique, whose use for the quantitative study of liquid flows is rarely documented in the literature. Results provided an insight into the nature of free convection heat transfer from ribbed channels, whose geometry is significant in such several engineering devices as electronic equipment. It was found that a general reduction of heat transfer performance, relative to that of a flat vertical surface or a smooth vertical channel, was induced by the presence of ribs, within the range of the parameters investigated. Local and heat transfer characteristics were sensitive to changes in inter plate spacing for small channel aspect ratios. Experimental data, recast in dimensionless form, were in excellent agreement with those obtained by this author in a previous research performed for air-cooled channels, using the same experimental technique, the same geometric parameters of the ribbed surface, and a similar Rayleigh number. (C) 2017 Elsevier Ltd. All rights reserved.