Bubble lift-off diameter and nucleation frequency in vertical subcooled boiling flow

A series of experiments were carried out to investigate phenomena from bubble nucleation to lift-off for a subcooled boiling flow in a vertical annulus channel. A high-speed digital video camera was used to capture the bubble dynamics. The bubble lift-off diameter and bubble nucleation frequency were evaluated in terms of heat flux, mass flux, and degree of subcooling. The fundamental features of the lift-off diameter and nucleation frequency (i.e., the variations across nucleation sites and the dependence on the flow and heat flux conditions) were addressed based on the present observation. A database for the bubble lift-off diameter was built by gathering and summarizing the data of Prodanovic et al., Situ et al., and the present experiments. We evaluated the predictive capabilities of Unal's model, Situ et al.'s model, and Prodanovic et al.'s correlation against the database. We obtained the best prediction results by modifying the wall superheat correlation in Unal's model. In addition, we suggested a new correlation for a combined parameter of the bubble nucleation frequency and bubble lift-off diameter. (author)