Investigation on outlet boundary conditions in lattice Boltzmann simulations of flow boiling heat transfer

Pseudopotential lattice Boltzmann (LB) models have been applied to simulate flow boiling heat transfer in recent years. Outlet boundary condition is one of the main challenges to carry out the flow boiling LB simulations. In order to clarify the issues involved in the outlet boundary implementation for flow boiling simulations, the numerical performances of two LB outlet boundary schemes are investigated. One is the convective boundary scheme, and the other is the pressure boundary scheme. Numerical results show that the convective boundary does not control the pressure at the outlet, leading to the continuously rising of the system pressure, while the pressure boundary does not allow bubbles to flow out. Hence, a novel artificial condensation zone is designed before the pressure outlet boundary to avoid two-phase outflow. The effective artificial condensation of vapor is achieved by tuning the equation of state and the latent heat source term in the zone. By employing pressure boundary condition along with the novel artificial condensation zone, the pressure in flow boiling simulation is successfully controlled, and the two-phase flow can be simulated continuously. Finally, simulation of flow boiling in a microchannel is implemented using the proposed outlet boundary treatment. Flow regime transition from the bubbly flow to the slug flow is well captured.