Experimental investigation on active nucleation site density and bubble departure frequency in subcooled flow boiling by using bubble tracking algorithm

The nucleate subcooled boiling is an efficient heat transfer form and plays an important role in many cooling applications. In nucleate subcooled boiling, nucleation site density (NSD) and bubble departure frequency (BDF) are two key parameters, which determine the amount of heat taken away by vapor bubbles from the heating surface. In the present work, an experiment is carried out to closely observe the boiling behaviors on horizontal aluminum heating surface at system pressures from 121 to 312 kPa, flow velocities from 0.5 to 2.0 m/s and subcooling degrees from 5 to 15 K using high-speed photography. A bubble tracking algorithm is developed, which efficiently determines the locations of active nucleation sites. Results show that values of active NSD and BDF are greater in conditions of elevated wall superheat degrees and system pressures. Comparatively, liquid subcooling and flow velocity have little effects on the active NSD and BDF. Regarding the inconsistencies between experimental data of the current study and predicted data of existing models, revised models are proposed. The newly proposed empirical formula of active NSD conforms well to the experimental data with a mean relative error of 25.27%. The modified model for BDF based on the dimensionless BDF and dimensionless heat flux agrees well with experimental data with a mean relative error of 21.25%. (C) 2019 Elsevier Ltd. All rights reserved.