抄録
An experimental study has been carried out to clarify the characteristics of boiling during downward flow in a vertical tube of an FBR steam generator, especially the relationship between the flow aspect and the heat transfer mode. The experiment was done using purified water under the following conditions: initial pressures of 11.2-13.4 MPa, an initial water temperature of 200°C, and water flow rates of 0.021-1.67 kg/s. The tube was heated by high-frequency induction current, where the time averaged heat flux can be estimated using an inverse solution from the measured temperatures at two points on three different positions along the tube wall and was then confirmed to agree with the measured heat flux on the outer surface within 20% accuracy. The surface temperature and heat flux on the inner tube are also estimated correctly using the inverse solution. It is found that the characteristics of heat transfer strongly depend on flow rate. At a high mass flow rate, a stable nucleate boiling occurs on a vertical tube wall with a clockwise cycle on the q-ΔT diagram for changes in heat flux in dq/dΔT>0 at any time. At a low mass flow rate, boiling on the wall induces changes from dq/dΔT>0 to dq/dΔT<0 during heating and returns from dq/dΔT<0 to dq/dΔT>0 during the reduction in heat. The counterclockwise cycle always appears in the transition boiling region in dq/dΔT<0, where nucleate boiling and film boiling coexist and the ratios of boiling areas are changing temporally and spatially.
