Natural Convection Heat Transfer From Long Horizontal Isothermal Cylinders

A new set of measurements is reported on natural convection heat transfer in air from isothermal long horizontal cylinders of noncircular cross section at various orientations, covering the Rayleigh number (Ra) range from about 10³ to about 10⁹. The data are correlated reasonably well by a conduction layer model with a constant value (i.e., the same for all body shapes and orientations) of 5.42 for the Churchill-Usagi coefficient blending the laminar and turbulent asymptotes. The resulting correlation equation normally requires only the geometric specification of the body height and perimeter. This model is also tested against data in the literature on the subject problem, and found to be generally predictive, to within about ±10 percent. A new set of data covering the same Ra range is also reported for the circular cross-section case, i.e., the long horizontal isothermal circular cylinder. Comparison of this data with the several existing correlations for this well-known problems shows that the Kuehn and Goldstein equation predicts the data best, although the Raithby and Hollands equation also predicts the data very well, but only after a revision to the blending coefficient.