Subcooled flow boiling of seawater in a vertical annulus

The present study investigates experimentally subcooled flow boiling heat transfer characteristics of 3.5 wt% artificial seawater and de-ionized water in an annulus. Boiling behaviors through a high-speed video camera reveals significantly different two-phase flow patterns in the heated annulus between artificial seawater and de-ionized water. Bubbles in de-ionized water tend to merge each other to form large bubbles or slug bubbles, while bubble coalescence is absent in seawater and bubble foam may eventually be formed. Subcooled boiling results in the quick rise of void fraction in the flow direction downstream of the net vapor generation point, as evident through flow visualizations and the evaluation of Saha and Zuber's model and Pan's model. Fluid temperature measurements at the inlet and exit of the annulus enable the evaluation of qualities and corresponding void fractions at the exit of the annulus through energy balance. Calculated void fraction as high as 50–70% is demonstrated and is consistent with the prediction of models. Correspondingly, the pressure drop rises very quickly after the heat flux exceeding the initiation of boiling two-phase flow and reaches a plateau of constant pressure drop in seawater, indicating the appearance of bubble foam. Generally, the pressure drops of artificial seawater under the same operating condition are much higher than those of de-ionized water in two-phase boiling region due to its unique bubbly flow pattern. The subcooled flow boiling heat transfer coefficients with the mass flux of 564, 874, and 1200 kg/m2s in artificial seawater are slightly higher than those in de-ionized water, possibly due to strong bubble agitation. The heat transfer coefficients obtained in the present study agree well with the predictions of the correlation of Papel in the early literature.