Large eddy simulation of upward co-current annular boiling flow using an interface tracking method

Towards the full understanding of the mechanism of boiling in annular flow regime, a computational fluid dynamics method with interface tracking has been developed. Smagorinsky model is used for the sub-grid scale turbulence, the constant turbulent Prandtl number being used to model the turbulent thermal conductivity. A sharp-interface phase change model is used to simulate heat and mass transfer phenomenon at the liquid-vapor interface. The developed numerical method is applied to a simulation of vertical upward co-current boiling flow in the annular flow regime. The experiment of Barbosa et al. is selected for validation: the mass flow rate is 30 kg/m2·s with the heat flux 159 kW/m2 under the system pressure of 1.9 bar. The computed results clearly show how the disturbance waves are generated: the waves are caused from inhomogeneous shear stress acting on the liquid-vapor interface, which is amplified by flow separation in the vapor phase. Subsequently, the interaction between the disturbance waves, the mass transfer and the temperature distribution over the heat-transfer surface is investigated. The temperature gradient in the normal direction to the heat-transfer surface is larger in the thin liquid film than that in the disturbance wave, and consequently, mass transfer rate is also higher there. The higher temperature region is observed underneath the disturbance waves resulting from lower temperature gradient (i.e. heat flux). The computed results imply that the bubble nucleation observed in the experiment may be caused by the higher temperature underneath the disturbance wave on the heat-transfer surface.