Under constant heat flux conditions

an experimental investigation was conducted on heat transfer to carbon dioxide flowing upward through a helically coiled tube with inner diameter 9 mm

coil diameter 283 mm and coil pitch 32 mm.The operating pressure was maintained at 8 MPa for all the tests. The mass flux and heat flux were in the range of 0-650 kg·m-2·s-1 and 0-50 kW·m-2

respectively.It is found that the inner wall temperatures and heat transfer coefficients are determined by the combined effects of physical property variation

buoyancy force and centrifugal force.The Buoyancy number can qualitatively stand for the relative importance of physical property variation and buoyancy effect.When Bo>8×10-7

the natural convection becomes predominant

and heat transfer rate is obviously enhanced because of the buoyancy induced secondary flow.Due to the combined effects of buoyancy force and centrifugal force

the site with the lowest inner wall temperature and the highest heat transfer coefficient is located in the region 90°<ø<180°.When the buoyancy force takes advantage over the centrifugal force

the heat transfer coefficient in the bottom region is larger than that in the outer region. When the centrifugal force becomes dominant

the heat transfer coefficient in the bottom region is smaller than that in the outer region.Based on the 2346 data obtained in the present experimental study

two correlations of Nusselt number suitable for different temperature regions is proposed.It is verified that at least ninety percent of experimental data fall within 20% deviation predicted by the proposed correlations.