Coupled Calculations of Bubble Departure Diameter and Frequency from Mechanistic Principles for Nucleate Boiling Applications

Nucleate boiling is trusted as an efficient heat transfer mechanism in a wide range of engineering applications. However, the entire physical process of boiling is extremely difficult to predict with accuracy, and engineers have mostly relied on empirical models and correlations for this purpose. In recent years, however, significant advances in the development of more mechanistic approaches have been made. Developments are driven by the benefits in safety and efficiency that are achievable with a more accurate estimation of boiling heat transfer and a reduced operational safety margin on the critical heat flux. This paper further develops a bubble departure diameter mechanistic model based on the forces that impact bubble growth and departure. The heat transfer model includes contributions from the microlayer beneath the bubble, the superheated liquid layer around the bubble surface and condensation when the bubble cap is surrounded by subcooled liquid. Improvements in the modelling of the contribution of condensation are implemented and successfully tested. The model is validated against a large set of measurements that includes saturated and subcooled flow boiling and a new database for forced convection boiling in cross-flow conditions. This database is used to validate the model for the coupled calculation of bubble departure diameter and bubble departure frequency. Although the model predicts the bubble growth time with accuracy, improvements are still required in the modelling of the waiting time after bubble departure. Models of this kind can be used as a basis for the prediction of boiling beyond nucleate boiling conditions, as well as for implementation in wall boiling routines of computational fluid dynamic multiphase flow models