A simple expression is developed for the natural convection heat transfer from three-dimensional bodies of arbitrary shape immersed in an extensive fluid. The expression applies to both laminar and turbulent regimes and requires the calculation of purely geometric properties of the bodies. Experiments were performed with air, covering a Rayleigh number (Ra) range of from 10 to 108, on different body shapes oriented in various directions: for example, circular or square disks, a short circular cylinder of height equal to diameter and a similar cylinder but with hemispherical ends, prolate and oblate spheroids of various aspect ratio, and an “apple core” shape. Comparison between the predictions of the expression and the experimental results of this work and those gathered from several other sources ranging up to Ra = 1014 showed very good agreement, with an average rms difference of 3.5 percent for Ra < 108 and 22 percent for 108 < Ra < 1014.

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