Interference effect of nearby nucleation sites on bubble release frequency in subcooled flow boiling

Along with the nucleation site density and bubble departure diameter, the bubble release frequency is one of the most important fundamental parameters required in detailed modeling of subcooled flow boiling. When many nucleation sites are activated and the distance to the nearest nucleation site is less than about the bubble departure radius, the local wall temperature at the nucleation site of interest would decrease due to the bubble nucleation not only at its own location but also at its nearby sites to cause reduction of the bubble release frequency. This interference effect of the nearby sites is however rarely taken into consideration in available correlations. In this work, the bubble nucleation process in subcooled flow boiling was explored experimentally using a high-speed camera and an infrared camera synchronized with each other. When the bubble nucleation was not interfered by the other sites, the bubble release frequency was expressed as a decreasing function of the bubble departure diameter as reported by previous investigators. While, under the presence of the interference effect, the bubble release frequency was noticeably lower. This trend was pronounced with an increase in the interference time per unit time. Although the nucleation site density was rather low in the present experiments to enable clear observation of the bubble ebullition cycle, the bubble release frequency was almost halved by the interference effect. Including the interference effect was hence shown to be of critical importance for accurate estimation of the bubble release frequency in subcooled flow boiling, especially under high heat flux conditions where many nucleation sites are activated.