A Phenomenological Investigation on Near-Wall Bubble Behavior Close to Chf in Flow Boiling

The insight information of subcooled flow boiling is very important for the development of boiling crisis mechanistic model. In this paper, visual experiment is carried out in a vertical square channel with a heating rod with diameter of 9.5 mm. Based on the near-wall bubble behavior captured by a high-speed camera, combined with advanced image post-processing technique developed in-house, the qualitative and quantitative analysis of flow boiling behavior near the heating wall are presented when approaching the CHF (Critical Heat Flux). According to the visual experimental study, a detailed discussion on the phenomena before CHF and during the CHF is presented. With high heat fluxes, periodic large vapor clot passes with relatively constant frequency. Then periodic wavy vapor layer and liquid film clearly appear in the near-wall region when the working condition is extremely close to the CHF. It is found that high thermal-hydraulic parameters, including system pressure, mass flux and inlet subcooling, prevent the formation of thick vapor layer, and this is a significant precursor to trigger boiling crisis. This implies that the possibility of triggering boiling crisis is reduced due to the difficulty in forming thicker vapor layer, and lead to higher CHF value. Additionally, a prediction of wavelength between two-phase layers is proposed by combining visual observations and instability theory. The predicted results show good agreement compared with experimental data. Those results can provide predictions of key interfacial feature parameters for developing CHF mechanistic model