Two-phase interfacial area characteristics and transport under the transition from subcooled boiling to saturated boiling flows

This study focuses on the analysis of the transition from the subcooled boiling to saturated boiling flow. During this process, vapor generates drastically in a short developing length, which makes it difficult to predict the interfacial characteristics of two-phase flow. To investigate the characteristics of this process, experiments were performed in a vertical annulus with 19.1 mm inner diameter and 38.1 mm outer diameter. The two-phase interfacial parameters including time-averaged local vapor fraction, interfacial area concentration, and bubble interfacial velocity are obtained using high-temperature miniature four-sensor electrical conductivity probes. In this study, the conductivity probe signal is processed using the newly-developed signal processing algorithm developed for small spherical bubbles by Shen and Nakamura (2014) , instead of the algorithm used in the previous subcooled boiling experimental studies Ozar (2009) and Kumar et al. (2019), [3] for large bubbles developed by Kataoka et al. (1986). The flow conditions cover a wide range of inlet subcooling temperature at 12.9-21.2 degrees C, heat flux at 57.9-197.3 kW/m(2), and inlet flow rate at 0.265-0.536 m/s. The vapor fractions are cross-validated with impedance void meter measurements. The state-of-art two-group interfacial area transport equations (IATE) [6], (Ozar, 2009) and (Brooks and Hibiki, 2016) is evaluated with the experimental data. The result shows that the two-group IATE gives fairly good predictions on the transition phenomena. Besides, a defect was identified in the condensation model (Park et al., 2007) used in IATE, which is the bubble boundary diameter classifying the two condensation mechanisms: heat-transfer-controlled and inertia-controlled condensation. A rigorous approach to estimate this boundary diameter is established in this study.