In this paper, the geysering in a natural circulation loop with 3.5 wt% artificial seawater and de-ionized water are investigated experimentally. The startup of the natural circulation loop is from the quiescent state with zero heat input at atmospheric pressure. Two-phase instabilities occur during the startup process. Exclusively, the geysering instability is identified, visualized, characterized, and discussed. The operational range of geysering of seawater is within the heat flux of 27-60 kW/m². In contrast to the bubbly flow and slug flow due to bubble coalescence in de-ionized water, a significant and unique boiling two-phase flow pattern during geysering in seawater is revealed. It includes dispersed foam flow, bubble foam, and its ruptures. With the increasing heat flux and temperature of fluid at the outlet of the heated section, geysering evolves and the rupture region is moving further downstream and eventually disappears.

A spike of pressure drop of 12.7 kPa over the heated section corresponding to the initiations of geysering in seawater is revealed while it is 11.9 kPa in de-ionized water. The oscillating magnitude of pressure drop in the heated section increases with increasing power and then decreases because of the appearances of wavy slugs in both fluids. The magnitude of pressure drop oscillations in the riser in seawater decreases approximately linearly with increase in heating power, while it increases monotonically in de-ionized water. The heat transfer coefficients during geysering for seawater decrease from 7.9 kW/m²K to 5.6 kW/m²K and is lower than that in de-ionized water at low powers. It then jumps to a level of 6.7 kW/m²K due to the strong turbulence caused by foam ruptures in seawater and is higher than that in de-ionized water. Eventually, heat transfer coefficients coincide for two fluids, as geysering is suppressed by the weakening condensation effect in the riser at high powers.