Visualization experiments on the geyser boiling-induced instability in vertical circular tube at low-pressures

The geyser boiling in a vertical circular tube with an inner diameter of 32 mm is experimentally investigated with water under system pressure, in which the heat flux and system pressures are in the ranges of 0-7.96 W cm−2 and 0.1-0.4 MPa, respectively. The test channels consist of three parts: a heated channel section installed downstream, a nonheated middle channel section and an elevated water tank installed upstream. The process of the geyser boiling is observed by visualization, which can be characterized by a quiet phase, an explosive eruption of vapor and refilling of subcooled liquid, including four typical flow patterns, namely, bubbly, slug, churn and annular flow. The results show that the geyser boiling occurs obviously below 0.12 MPa, but the intensity of the geyser eruption becomes lower, when the system pressure is increased, and the fluid temperature is affected by system pressure. The intermittent flow and boiling disappear when the system pressure is raised beyond the threshold, which may be helpful to suppress the intermittent flow instability during the water injection in nuclear reactor at accidents.