The steady-state natural convection from heat sinks with parallel arrangement of rectangular cross section vertical plate fins on a vertical base are numerically investigated in order to obtain a validated model that is used for investigating inclined orientations of a heat sink. Taking a previous experimental study as a basis, aluminum heat sinks with two different practical lengths are modeled. The models and the simulation approach are validated by comparing the flat plate heat sink results with the available correlations, and by comparing the finned heat sink results with the experimental data. Natural convection and radiation heat transfer rates from the fronts of the heat sinks heated from the back with a heater are obtained from finite volume computational fluid dynamics simulations. The sensitivities of the heat transfer rates to the geometric parameters are determined. A set of dimensionless correlations for the convective heat transfer rate is suggested. The validated model is used for several upward and downward inclination angles by varying the direction of gravitational acceleration. At small inclinations, it is observed that convection heat transfer rate stays almost the same, even increases slightly for the downward inclinations. At larger angles, the phenomenon is investigated for the purpose of determining the flow structures forming around the heat sink. For the inclination angles of +/- 40 degrees, +/- 10 degrees, +/- 20 degrees, +/- 30 degrees, +/- 45 degrees, +/- 60 degrees, +/- 75 degrees, +/- 80 degrees, +/- 85 degrees, +/- 90 degrees from the vertical, the extent of validity of the obtained vertical case correlation is investigated by modifying the Grashof number with the cosine of the inclination angle. It is observed that the correlation is valid in a very wide range, from -60 degrees (upward) to +80 degrees (downward). It is also observed that the flow separation inside the fin channels of the heat sink is an important phenomenon and determines the validity range of the modified correlation. It is further shown that the correlations are also applicable to all available inclined case data in the literature, verifying both our results and correlations. Since the investigated ranges of parameters are suitable for electronic device cooling, the suggested correlations have a practical use in electronics cooling applications. (C) 2012 Elsevier Ltd. All rights reserved.