Numerical investigation on characteristics of fully developed nucleate boiling (FDB) in petal-shape fuel assembly channel

Fully developed nucleate boiling (FDB) distinguishes between single-phase flow and two-phase flow. In downstream of FDB, two-phase flow occurs. FDB is important for subcooled boiling. In this paper, a numerical method combining the Eulerian two-fluid model and the RPI wall boiling model was adopted to numerically investigate the heat transfer characteristics of coolant flow boiling in a 3 × 3 petal-shaped fuel assembly channel and studied on the characteristics of FDB. If the diameter of vapor bubble exceed the departing bubble diameter, the bubble can depart from the wall, FDB is determined. The results showed that the heating power had a minor effect on the characteristic of FDB. As the inlet subcooling increased, the temperature of mainstream on the cross section of FDB decreased and wall superheat increased. As the inlet mass flow rate increased, the mainstream temperature on the section of FDB increased and the wall superheat increased. With the pressure increase, the change trend of mainstream temperature and wall superheat was the same as mass flow rate increased. Studied the suitability of existing prediction correlations, and the result showed that the error of Unal’s expression prediction of FDB was the minimum. To decrease the error, a factor was introduced to modify the prediction expression. Comparing with simulation result, the error of modified expression was less than 15 %. The wall temperature at the inner concave arc was much higher than that at the outer convex arc, and the intensity of the coolant subcooled boiling was sharply at the inner concave arc.