To date, more than twenty PWRs have been affected by axial offset anomaly (AOA) or crud-induced power shift (CIPS), an unexpected deviation in the core axial power distribution from the predicted curve during operation. AOA is a current major consideration for reactors operating at increased power levels and is becoming immediate threat to nuclear power’s competitiveness in the market [1]. Despite much effort focusing on this topic, a comprehensive understanding is far from being developed. However, previous research indicates that a close connection exists between subcooled nucleate boiling occurring in core region and the formation of crud, which directly results in AOA phenomena. It is well established that deposition is greater, and sometimes much greater, on heated than on unheated surfaces. [2] A number of researchers have suggested that boiling promotes deposition, and several observed increased deposition in the subcooled boiling region [2]. Limited detailed information is available on the interaction between heat and mass transfer in subcooled nucleate boiling (SNB) flow. It is speculated that direct prediction of the AOA from SNB is difficult. Moreover, bubbles formed in SNB region play an important role in helping the formation of crud. Therefore, we are encouraged to get a better understanding of SNB phenomena and the behavior of the bubbles in SNB. This research examines bubble behavior under SNB condition from the dynamic point of view, using a high fidelity digital imaging apparatus. Freon R-134a is chosen as a simulant fluid due to its merit of having smaller surface tension and lower boiling temperature. The apparatus is operated at “reduced” pressure. Series of images at frame rates up to 4000 frames/s were obtained, showing different characteristics of bubble behavior with varying experimental parameters e.g. flow velocity, fluid subcooled level, etc. Analyses that combine the experimental results with analytical result on flow field in velocity boundary layer are considered. A tentative suggestion is that a rolling movement of a bubble accompanies its sliding along the heating surface in the flow channel. Numerical computations using FLUENT v5.5 are performed to support this conclusion.

This content is only available via PDF.
You do not currently have access to this content.