Role of bubble dynamics in heat and mass transfer in annular flows

Though bubbles are abundantly formed during the nucleate boiling in annular flows and affect the industrial performance during nucleation, departure, and bursting, the contribution of bubble dynamics in the heat and mass transfer in annular flows still needs to be quantified. Here, we used the semi-analytical film flow model to simulate the annular flow boiling in a vertical tube. We found the liquid film significantly affects the bubble departure diameter at wide working conditions when its thickness is smaller than the bubble's free departure size. A model for the departure diameter of the bubble considering the confinement of the thin liquid film was proposed, based on which theoretical models for the bubble-induced heat flux due to bubble formation and the bubble-induced droplet entrainment rate due to bubble bursting ejection were proposed. The contributions of the bubble-induced heat flux to the total (wall-to-liquid) heat transfer and the bubble-induced droplet entrainment rate to the total (liquid film-to-vapor core) mass transfer of annular flows can be up to 61% and 38% in the studied parameter range, respectively. The contribution of the bubble-induced heat flux to the total heat transfer increases with decreasing inlet mass flux and increasing tube diameter, wall heat flux, and pressure. The contribution of the bubble-induced droplet entrainment rate to the total mass transfer increases with decreasing inlet mass flux and increasing tube diameter. The proposed models of bubble-induced heat and mass transfer in annular flows may offer the first step to precisely control industrial performance by governing the bubble dynamics.