Experimental investigation of variability in departure diameter and frequency between nucleation sites in boiling flow

Wall nucleation experiments have been performed in a vertical channel for investigation of the bubble departure diameter and bubble departure frequency. The experimental data in forced convective subcooled boiling flow of water is presented at two pressures, 143kPa and 419kPa. The uniqueness of this dataset is the measurement of nucleation characteristics from many nucleation sites simultaneously, providing distribution of departure diameter and frequency from active sites under the same system conditions. The conditions, one with 11 nucleation sites and the other with 7 nucleation sites within the field of view, are recorded with a high-speed imaging system. Up to one hundred departures from each site are measured manually to characterize the site departure diameter and departure frequency. Current models for departure diameter showed good prediction of the condition-average departure size. The distributions of departure diameters are similar between sites of a given condition but change with increasing pressure. The bubble departure frequency data highlights an issue in the current definitions of nucleation site characteristics. Difficulty arises in the definition of an ‘active’ nucleation site and that site’s bubble departure frequency. The data shows that a site may have periodic ‘inactivity’, which if not accounted for in the active nucleation site density, will result in a large over prediction of bubbles generated at the wall.