Investigation of nucleate boiling and bubble dynamics at multiple nucleation sites in subcooled boiling flow

Accurate prediction of nucleate boiling heat transfer and phase change is crucial for subcooled boiling flow modeling. Among the nucleate boiling parameters, departure diameter, departure frequency, and active nucleation site density are considered the most important. To fulfill the modeling requirements, the present study established a comprehensive experimental database with simultaneous measurement of departure diameter, departure frequency, and active nucleation site density at multiple nucleation sites. The experimental data were collected in a vertical annular test section using a microscopic high-speed imaging system. Twenty-two experiments were performed at a system pressure of 150 kPa, liquid flow rate of 0.5 m/s, local subcooling ranging from 10 to 30 °C, and heat flux varying from 250 to 700 kW/m2. Stochastic characteristics of departure diameter and departure frequency at single and multiple nucleation sites were investigated. A correction approach was developed to minimize the impact of upstream bubbles on departure frequency measurement. The sample size of single-site measurement was determined based on the analysis of data dispersions. Large variations between nucleation sites were observed, indicating that single-site measurement is insufficient to characterize all nucleation bubbles. A parametric analysis of nucleate boiling parameters over operating conditions was performed based on the multi-site data. The experimental database generated in this study will be utilized for model evaluation and development in future research.