A liquid crystal based experimental investigation of the heat transfer enhancement supplied by ribs on a vertical plate, has been presented. Since the ribs were adiabatic, they did not work as extended heat transfer surfaces but by redirecting the flow, were used to enhance the heat transfer. Four different rib heights (H = 10 20, 30, 40 mm) and five different angles of inclination (theta = 0degrees, 10degrees, 20degrees, 30degrees, 45degrees) are considered. Local heat transfer coefficients and the mean Nusselt numbers for the considered experimental cases are compared to those of the flat plate without ribs. The enhanced flow is always found to have higher heat transfer values. On the other hand, both the rib height and the angle of inclination affect the magnitude of the local and total heat transfer coefficients. To reach a reasonable correlation between the geometrical parameters and the heat transfer values, three-dimensional numerical simulations of the natural convection flow along a vertical plate with ribs were performed. A reasonable agreement appears to exist between the measured and the predicted local and mean quantities. (C) 2003 Elsevier Ltd. All rights reserved.