Onset of flow instability during subcooled flow boiling of seawater in a vertical annulus

The present study investigates the onset of flow instabilities during subcooled flow boiling of 3.5 wt% artificial seawater in a vertical annulus. All flow instability experiments are conducted by decreasing mass flux gradually under a specific condition of inlet temperature and heat flux. The results reveal that, comparing with de-ionized water, the artificial seawater is more susceptible to the flow instability and demonstrates 12–79% larger mass fluxes for onset of flow instability (OFI) due to the unique two-phase flow pattern of densely bubbly flow or bubble foam flow during the subcooled flow boiling of artificial seawater. After OFI, unstable bubble foam with slug bubbles resulting from foam ruptures is revealed in artificial seawater resulting in significant pressure drop oscillations at lower inlet temperature of 55 °C, 65 °C and 75 °C. The existing criteria or correlations, developed based on the data of de-ionized water, may not predict the OFI for artificial seawater. A new empirical correlation to predict the mass flux at OFI is proposed. A two-phase flow instability map is established on the plane of phase change number versus subcooling number.