Visualization Study of Boiling Crisis on Single Rod Surface

The study of visualized bubble phenomena during critical heat flux (CHF) and CHF mechanistic models has been a topic of discussion. CHF visualization experiments are needed to explore the CHF phenomena and bubble behavior cocurrently, especially with high spatialresolution near the heated surface area. The visualization experiments can also provide input for CHF mechanistic models. In this study, the boiling crisis phenomenon on the surface of a single heated rod and the influence of grid on bubble behavior were explored by visualized method with R134a as fluid. The main purpose of this study is to visualize flow conditions near or at CHF and the effect of mixingvanes on bubble behavior by a highspeed camera. The comparison was made between the CHF data with and without mixingvanes spacer grid under the same flow condition. The square of the test section was 19 mm×19 mm, and a uniform heating rod with a diameter of 95 mm was placed in the center. The channel box was unheated and well insulated to provide an approximately adiabatic boundary condition on the outer wall of the test section. Two grids with mixingvanes were placed in the channel to form 522 mm spans to reduce vibration and ensure channel geometry. In the experiment, the pressure was in the range of 1827 MPa, the mass flux was 6002 100 kg/(m2·s), and the inlet subcooling was 1040 ℃. A highspeed camera was used to observe the phenomena during boiling crisis and the effect of mixingvanes on bubble behavior. The comparison was made between the CHF values spacer under different grids (with and without mixingvanes). The experimental results show that when boiling crisis occurs, the vapor film formed by local film boiling expands circumferentially until stable film boiling is formed on the rod surface. The transition was completed from nuclear boiling stage to film boiling stage. The effect of grid on bubble behavior is mainly reflected in tearing bubbles by strip and spring, and swirl flow caused by mixingvanes. This effect increases the turbulent mixing between the bubble layer and the core, and the mixing-vanes promote the separation of bubbles and enhance the heat transfer between the heated wall and the fluid, thus CHF increases. The turbulent mixing rate will damp along the flow direction after the strengthening of the mixingvanes. The turbulent mixing rate damps faster at low mass flux condition. Thus, the enhancement of CHF is not obvious at low mass flux condition.