Free convection heat transfer from the outer surface of vertically oriented helical coils in glycerol-water solution

Steady state natural convection heat transfer from vertical helical coiled tubes, in glycerol-water solution 57% (g/w) by mass, is studied experimentally. Average heat transfer coefficients were obtained for laminar and transition to turbulent natural convection. The experiments have been carried out for three coil diameter to tube diameter ratios, D/d(o), and for five and ten coil turns, N. Effects of Rayleigh number, D/d(o) ratios, and N on the heat transfer behavior of the coils are investigated. Correlations are presented to calculate the average Nusselt number in terms of Rayleigh number, D/d(o) ratios, and N. The results show that the heat transfer coefficient is enhanced either by reducing the diameter ratio or the number of coil turns. The overall correlations covering all the data points using the coil length as a characteristic length are also presented.