Thermal-hydrodynamic modeling of bubbly flows with heat and mass transfer

A three-dimensional (3-D) two-fluid model coupled with the population balance equation is used to predict bubbly flows with the presence of heat and mass transfer processes, particularly on subcooled boiling flows at low pressures. To account for the nonuniform bubble distribution in the bulk liquid at subcooled conditions, the CFX MUSIG (MUltiple-Size Group) model is further developed by incorporating the wall nucleation at the heated wall and condensation in the flow regime. Model predictions, covering a wide range of diffrent mass and heat fluxes and inlet subcooling temperatures, are compared against local and axial measurements. For the local case, the bubble size predictions include the comparison of the empirical bubble diameter relationships adopted in the CFX boiling model. The study shows that good agreement is better achieved by the present model with respect to the radial distributions of the bubble Sauter diameter, void fraction, interfacial area concentration, and liquid velocity profiles. However, significant weakness prevails over the vapor velocity distribution. Work is in progress to circumvent the deficiency by considering additional momentum equations or developing an algebraic slip model to account for bubble separation. For the axial case, good agreement is also achieved for the axial distributions of the mean bubble Sauter diameter, void fraction, and interfacial area concentration profiles. Here, the model correctly represents the plateau at the initial boiling stages at upstream, typically found in low-pressure subcooled boiling flows, followed by the significant increase of the void fraction at downstream.