Natural convection heat transfer and fluid flow characteristics in horizontal and vertical narrow enclosures with heated rectangular finned base plate have been experimentally investigated at a wide range of Rayleigh number (Ra) for different fin spacings and fin lengths. Quantitative comparisons of finned surface effectiveness (epsilon) and heat transfer rate between horizontal and vertical enclosures have been reported. In comparison with enclosure of a bare base plate, insertion of heat conducting fins always enhances heat transfer rate. Optimization of fin-array geometry has been addressed. The results gave an optimum fin spacing at which Nusselt number (NUH) and finned surface effectiveness (e) are maximum. It has been found that: (1) increasing fin length increases NUH and epsilon; (2) increasing Ra increasesNuH for any fin-array geometries and (3) for any fin-array geometry and at Ra > 10,000, increasing Ra decreases - while for fin-array geometries of large fin spacing and at Ra < 10,000, increasing Ra increases e. Useful design guidelines have been suggested. Correlations of NUH have been developed for horizontal and vertical enclosures. Correlations predictions have been compared with previous data and good agreement was found. (c) 2006 Elsevier Ltd. All rights reserved.