Evaluation of thermal fatigue induced by dryout oscillation in printed circuit steam generator

Abstract

To enhance the compactness and inherent safety of a Pressurized Water Reactor (PWR) type Small Modular Reactor (SMR), a printed circuit steam generator (PCSG) is recently being considered. Due to the characteristics of the once-through steam generator, a PCSG is designed to produce superheated steam through the multiple boiling regimes including nucleate boiling, dryout, and dispersed flow film boiling regimes. The dryout occurs where the liquid film in contact with the heated wall disappears and it induces a large axial wall temperature variation due to deterioration of heat transfer. The dryout front fluctuates due to the nature of boiling process, which can potentially cause thermal fatigue fracture due to cyclic thermal stresses in the heated wall. To investigate the thermal fatigue issue of a PCSG, a reference PCSG was designed in this study using in-house PCSG design tool. For the stress analysis, a finite element method (FEM) analysis model was developed to obtain temperature and stress fields of the designed PCSG. Fatigue assessment was conducted based on ASME Boiler and Pressure Vessel Code to identify the major parameters influencing the fatigue life time originating from the dryout front oscillation. As a result of this study, the limit on temperature difference between hot side and cold side fluids is obtained. Furthermore, it is found that heat transfer coefficient of nucleate and film boiling regimes play essential role in the fatigue life cycle as well as determining the hot cold temperature difference limit. Since there are limited studies for measuring the dryout frequency under mini-sized channel, experiments were performed with the single semi-circular flow channel having diameters of 2 mm, 3 mm, and 4 mm respectively. The key parameter affecting the dryout frequency was found to be the channel diameter. The dryout frequency was measured between 0.3 and 0.9 Hz, which are similar in orders of magnitude with the macro-sized experimental results. Additionally, heat transfer coefficients for nucleate boiling and dispersed flow film boiling were obtained and compared with existing heat transfer correlations. Overall, it is concluded that a PCSG having smaller size flow channel diameter is preferred for PWR-type SMR when considering dryout fatigue fracture.