Numerical investigation of thermal-hydraulic parameter distribution characteristics during dryout evolution in the helically coiled once-through steam generator

A two-fluid three-flow-field model that considered interactions among the droplets, steam and liquid film during dryout evolution in the helically coiled once-through steam generator was introduced to numerically study the distribution of the thermal-hydraulic parameters. Simulation results showed that the slip ratio first increased and then decreased in the flow boiling process. The slip ratio was relatively small due to the secondary flow in the tube, and the maximum value was only 1.065. Because the steam had a relatively strong entrainment effect on the droplets, the frequency of droplets hitting the wall was high, resulting in only a small level of deviation from thermodynamic equilibrium. When dryout occurred, the local wall temperature at different circumferential angles, as well as the average wall temperature, reached a maximum and then decreased. The dryout incipience occurred at a distance of 2.8 m from the inlet of the helically coiled tube. The circumferential angle at the position of dryout incipience was 100–140° while the dryout region at the outlet extended to 70–270°. The circumferential wall temperature was non-uniformly distributed and its maximum value was first increased and then decreased.