Condensation heat transfer of pure steam and steam from gas–steam mixture in tubes of AES-2006 PHRS SG heat exchanger

Results of experimental determination of the average heat transfer coefficient upon condensation of pure steam α_c and steam from air–steam mixture α_as.m in tubes of a large-scale model of the emergency cooling heat exchanger in the system of passive heat removal through steam generators of AES-2006 project at Leningrad II NPP are presented. The model contained 16 parallel tubes with a diameter of 16 × 2 mm and a length of 2.9 m connected to the upper steam distributing and lower condensate gathering horizontal collectors; the distance between their axes was 2.28 m. The tube segments were vertical, horizontal, or inclined. The internal diameter of the collectors was 40 or 60 mm. The model was placed in the lower part of a tank with a height of 6.5 m and a volume of 5.85 m³ filled with boiling water at atmospheric pressure. The experimental parameters were as follows: pressure range 0.43–7.77 MPa, condensate Reynolds number Re_f = (0.87–9.3) × 10³, and average air volume fraction at the segment with air–steam mixture 0.18–0.85. The studies showed that nonuniformity of static pressure distribution along the steam-distributing collector strongly influences the reduction of α_c value (ejecting effect). The agreement between experimental and calculated according to statutory guidelines values of α_c for vertical tubes is achieved if the dynamic head of the steam flow at the input of the steam-distributing collector does not exceed 1 kPa. Equations for calculation of the diffusion heat transfer coefficient at steam condensation from the air–steam mixture α_as.m on the internal tube surface are proposed. In the considered conditions, air is completely displaced by steam flow from the upper to the lower part of the tubes. The boundary between these regions is characterized by an average reduced steam velocity through this cross section of 1.6 ± 0.4 m/s. Above the boundary cross section, it is recommended to calculate α_c. according to [1].