Visualization of flow patterns of water in open-ended vertical annulus during natural convection flow

Experiments have been carried out to understand the fl ow patt erns encountered during natural convection subcooled boiling flow of water in an open-ended closed-loop internally heated vertical annulus at diff erent heat fluxes. The experiments were performed on an in-house experimental setup. The flow visualization was accomplished by using high-speed area scan cameras (Basler). The boiling process was found to be periodic in nature. Thus, the flow repeats between single-phase flow, nucleate boiling, and slug flow to single-phase flow and large slug flow patt erns. When the flow rate reduces due to the limited size of exit, then backflow phenomenon was also observed.