Abstract
An experimental investigation of a steady state natural convection from uniformly heated helical coiled tubes oriented horizontally in air has been made. Average heat transfer coefficients are obtained for laminar natural convection. The experiments have been carried out for four coils and for various values of heat fluxes of 500–5000 W m
−2. The data are correlated with Rayleigh number using the coil tube diameter as the characteristic length. It is obtained that average heat transfer coefficient decreases with increasing the number of coil turns up to (or close to) the middle turn then a transition to turbulent begins. Correlation for all heat fluxes for all coils using the horizontal coil axis distance as a characteristics length is also presented.