Natural convection heat transfer from enclosed helical coils

Experiments were conducted to investigate natural convection heat transfer from enclosed helical coils in a vertical orientation. Coils ranging in mean diameter from 26.8 to 88.7 mm were tested in combinations of two, three and four coils, and as single coils enclosed in a shell of 100 mm inner diameter. A model was developed to determine the distribution of the forced convection flow rate in the tubes. A dimensional analysis was also carried out to determine the dimensionless parameters that are relevant to the natural convection heat transfer on the shell side of the enclosure. Heated propylene glycol was pumped inside the coils, while water circulated naturally on the shell-side of the heat exchanger. Validation tests were conducted using a straight tube of the same diameter as that used for the coils in the same shell diameter. Eleven configurations, including single foils, were tested at power settings between 200 W and 3000 W, and tube-side flow rates between 0.010 and 0.035 kg/s.The measured flow rate distribution and pressure drop were in good agreement with the theoretical predictions. The Nusselt number for the shell-side heat transfer, based on the hydraulic diameter of the enclosure, was found to depend on a modified Rayleigh number and the dimensionless flow space, which is a function of the shell-to-coil diameter ratio. The Nusselt number is also a function of the number of coils and the flow number, when the latter is less than unity. The function for the experimental Nusselt number was in general agreement with that predicted by the dimensional analysis. Correlations are presented covering the range of modified Rayleigh number, flow number and flow space of interest for the coils tested. A general correlation covering all flow numbers, and flow space less than 1.5 is NuFor any configuration of coils and their enclosure, an equivalent annulus exists, which is obtained by using an equivalent diameter of the coil(s) as the diameter of the imaginary inner solid cylinder. The resulting annular space is the flow space and the dimensionless form is the dimensionless flow space S...