Abstract
An experiment study was conducted on convection heat transfer process in vertical heating tube on natural circulation conditions. The investigation result shows that the convection heat transfer process on natural circulation conditions has a violent characteristic of mixed convection heat transfer. On low flow-rate conditions, the natural convection process in heating tube induced by the intense temperature difference between inner surface of heating tube and bulk of cooling water dominates the whole heat transfer process inside heating tube, and heat transfer coefficients in this region are distinctly higher than calculation values of forced circulation formula, due to the existence of secondary flows induced by natural convection process. And on high flow rate conditions, the convection heat transfer process in heating tube is strongly influenced by the heating-surface thermal buoyancy, which leads to the laminarization of the fluid in the boundary layer. Therefore, the convection heat transfer coefficients in high flow-rate region are lower than the calculation results. The primary causations leading the above-mentioned characteristics of convection heat transfer are the relatively high heating power compared with low natural circulation flow rate and the self-adaption coupling between circulations parameters. Geometrical structure of heating tube also affects convection heat transfer significantly, namely the convection heat transfer coefficients of natural circulation decrease with the diminution of the tube caliber. Finally, an empirical correlation for calculation of heat transfer coefficients in heating tubes on natural circulation conditions is putted forward, by using the non-dimensional parameter Gr/Re^2 to modifier the Gnielinski Formula. The fitting result shows good agreements with experiment results.
