Because of its importance in industrial processes, overall natural convective heat transfer from horizontal cylinders has been the subject of many experimental and analytical studies. However, because of the dispersions in these results, there is still uncertainty about the value of the Nusselt number Nu at a particular Rayleigh number Ra, especially for Ra 103. The values of Nu obtained from the most commonly used correlations do not reduce this uncertainty because of their own dispersion. To avoid the effect of higher Prandtl numbers Pr on the heat transfer, a review is made of the results from 34 experimental studies in air (Pr = 0.7) in the range 10 ≤ Ra ≤ 107. A review is also made of 23 analytical and numerical studies. It is concluded that the mean values of Nu within each of the two groups agree to within 4% in the range 10 ≤ Ra ≤ 104. At higher Ra, the mean of the experimental values of Nu is higher than the mean of the analytical and numerical values, the difference increasing from 7.86% at Ra = 105to 11.3% at Ra = 107. Possible reasons for this discrepancy are discussed
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