The natural convection heat transfer coefficient for the helicoidal coils are experimentally measured and compared with those for horizontal cylinders. It is found that when the coil pitch to pipe diameter ratio is less than around 1.2, the heat transfer coefficient of the coils approaches that of a horizontal cylinder having a diameter equal to the coil diameter. For the P/d ratio greater than 1.4, the heat transfer coefficient approaches that for horizontal cylinder of diameter d. A modified characteristic, length which incorporates the tube diameter, the coil diameter, and the coil spacing, is used to define Nusselt and Rayleigh numbers. Using the proposed characteristic length, it is shown that the Nusselt number for horizontal helicoidal pipes can be determined using the available Nusselt and Rayleigh number correlation for straight horizontal cylinders in the range of the experimental and available data Ra ≤ 10{sup}5, within a maximum deviation of 15.4%.
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