Natural circulation of a fluid in a thermosiphon slightly inclined to the horizontal

A pronounced stratification of the temperature (density) of the fluid over the cross section of the heating zone occurs in a thermosyphon that is slightly inclined to the horizontal and has a high degree of water filling, which leads to the onset of natural circulation of the medium along the length of the thermosiphon. In this case, there is an upward flow of water or a steam–water mixture in the upper part of the thermosiphon and a downward flow in the lower part. In experiments with a full-scale thermosiphon, the natural circulation in question increased the axial heat transfer through nonboiling water along all thermosiphon zones by three to seven times as compared to the heat transfer in a vertical thermosiphon. The estimated mass flow rate of natural circulation along the heating zone ranged from $25$ to $105$ kg/(m$^2$ s). The heat-transfer coefficient and friction factor are estimated at the boundary of countercurrent water flows induced by natural circulation. The conditions of poorer cooling of the thermosiphon heating zone due to steam separation at its upper generatrix are examined. A decrease in the mass filling of the thermosiphon, i.e., an increase in the average void fraction of the medium in it, successively leads to the onset of bubbling steam condensation within the transport section and then in the cooling zone, with a decrease in the length of the upper section of nonboiling water. A transition to film condensation of steam occurs at the limit along the entire length of the cooling zone with a countercurrent steam flow and a near-wall film of its condensate above the level of the steam–water mixture in the thermosiphon.