Thermohydraulics and suppression of nucleate boiling in upward two-phase annular flow : probing multiscale physics by innovative diagnostics

Abstract

In the fuel assemblies of a boiling water reactor (BWR) the steam quality increases along the assembly's length as heat is transferred from the fuel rods to the water coolant. Nucleate boiling is the dominant heat transfer mechanism at low and intermediate steam qualities (typical of the bubbly and slug/churn flow regimes), while forced convective evaporation dominates at higher steam quality in the annular flow regime. The transition of the heat transfer mechanism, also called suppression of nucleate boiling (SNB), affects the local heat transfer coefficient (HTC), the stability of the liquid film, and the entrainment dynamics. To support the efficient design and safe operation of future BWRs with higher power density, a thorough understanding of the thermohydraulic mechanisms and an accurate prediction of the transition conditions for SNB in annular flow is quite desirable. An innovative diagnostic technique combining synchronized infrared thermography and an electrical conductance-based liquid film thickness sensor was utilized here to investigate the details of the SNB phenomena with high spatial and temporal resolutions. The main control parameters of the tests included: the mass flux from 700 to 1400 kg-m⁻²-s⁻¹, steam quality from 0.01 to 0.08, and heat flux from 100 to 2000 kW-m⁻². The system pressure was held close to atmospheric pressure. At each set of conditions, the local distributions of the 2D surface temperature, 2D heat flux, and quasi-2D liquid film thickness were measured. From the measured data, the SNB heat flux, the SNB wall superheat, and the hydrodynamic properties of the disturbance waves were extracted. The experimental observations show for the first time the multiscale interaction of the extremely thin film and small nucleation cavities (on the scale of 10 micron), with the large disturbance waves and their associated temperature oscillations (with wavelengths of ~10 cm). A first of a kind 1D mechanistic model was developed to accurately capture this unique transient effect of the disturbance waves on the local heat transfer. The experimental results also suggest a strong dependency of the SNB heat flux and wall superheat on steam quality, with a second-order, weaker dependency on total mass flux. The same dependency is also found for the disturbance wave properties. A complete set semi-empirical correlations was proposed for predicting the time-averaged film thickness and SNB thermal conditions. Good agreement is found between the semi-empirical correlations and the experimental results. The database generated in this project can be further used for development and validation of CFD models of SNB and two-phase heat transfer in annular flow.