Two-phase measurement of flow boiling in a vertical narrow annular channel

The study of boiling flow is crucial for designing safety features of chemical and nuclear plants. The boiling behavior and bubble dynamics depend on various system parameters such as pressure, subcooling, mass flux, heat flux, and flow geometry. While flow boiling has been widely investigated, comprehensive experimental datasets covering a broad range of operating conditions remain limited. A new dataset of upward flow boiling is therefore collected using an annular test section with a heated inner rod. The test section has an inner diameter of 25.4 mm and an outer diameter of 38.10 mm, with a heated inner rod of 3 m length. Newly developed five-sensor conductivity probes by the authors are adopted to measure void fraction, gas velocity, and interfacial area concentration following a two-group description. Their dedicated sensor pattern allows the probes to traverse across the narrow channel with minimized near-wall blind zones. In addition, high-speed visualization is conducted recording axial flow evolution, based on which the Onset of Nucleate Boiling (ONB) and Point of Net Vapor Generation (PNVG) are identified. Parametric studies are also presented investigating the flow field dependence on systematic boundary conditions. This work presents valuable experimental data on flow boiling and two-phase flow measurements.