Rod bundle geometry keeping a compact structure is widely utilized in the nuclear reactor core to remove the heat efficiently. The complex rod bundle geometry poses many challenges in knowing and modeling the heat transfer and flow under two-phase flow conditions. It is of importance to make local measurements, establish flow parameter database and understand flow characteristics of the two-phase flows in the rod bundle geometry, which will be the base to model the flows and further to improve the reactor performance and safety. In the past studies, the two-phase flows in the rod bundle channels were mainly measured by using differential pressure cells and Gamma-/X-ray densitometries for the local volume-averaged, area-averaged and line-averaged void fractions. However, the local measurements of double- and four-sensor probe measuring technique, which can obtain not only the local void fraction but also the other local important parameters such as interfacial area concentration (IAC), bubble diameter and interfacial velocity, are necessary for understanding the flows in the rod bundle geometry. In the present paper, the four-sensor optical probe has been used to perform local measurements at 16 measuring points in the octant triangular measuring region of a cross-section in a 6×6 rod bundle flow channel under 16 flow conditions. The local parameters of void fraction, IAC, bubble diameter and bubble velocity vector were obtained in the bubbly flows and a corresponding database was established for the rod bundle flow channel. The measured local void fraction and IAC show their radial distributions with both core-peaking and wall-peaking shapes closely linked with the superficial velocity of two phases. By increasing the superficial liquid velocity or decreasing the superficial gas velocity, the distribution shapes tend to change from the core-peaking to the wall-peaking. The measured local bubble diameters increase with the decrease of superficial liquid velocity and the increase of superficial gas velocity. The bubbles move along the rod bundle flow channel with their velocity component in the main flow direction showing a typical radial core-peaking profile and their velocity components in the cross-section showing a significant movement from the central region to the channel box wall region especially under high superficial liquid velocities. The cross-sectional area-averaged values of the local parameters were obtained by adopting a measuring-point-based graphical integration. The resultant area-averaged void fraction and IAC have been used to evaluate and analyze two drift-flux model correlations and three IAC correlations respectively in this paper.

 

Keywords: Rod bundle flow channel; Four-sensor probe; Void fraction; Interfacial area concentration; Bubble diameter; 3D bubble velocity


Flow Characteristics of Upward Two-Phase Flows in a Rod Bundle Geometry

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

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