Hybrid outflow boundary condition for the pseudopotential LBM simulation of flow boiling

Abstract

The pseudopotential lattice Boltzmann method (LBM) has been successful in simulating the pool boiling with a density ratio up to 1000. However, the flow boiling simulation of using the same LBM is rare and the density ratio is limited to about 17. The current study investigated the challenge of flow boiling by first simulating the isothermal bubble outflow, where large pressure deviation from the initial saturation pressure is observed due to perturbation of the bubble outflow. Pressure drifts continuously as there is no constraint at both inlet and outlet when using the usual convective outflow boundary condition (OBC). The subsequent flow boiling simulation shows that boiling will be suppressed by the drifting/increasing pressure. A hybrid OBC is proposed to stabilize the pressure by imposing a fixed pressure either on the whole outlet when the bubbles are still far away from the outlet, or on a part of the outlet when the bubbles are crossing the other part of the outlet where the convective OBC is still used. With the improvement by using the hybrid OBC, the heat transfer coefficient of saturated nucleate boiling in a channel flow is studied and the results are in good agreement with the experiments. More realistic problems with structured wall surface, various surface properties and density ratios are simulated to show the algorithm stability and robustness. The current study has extended the density ratio to about 125, which is much larger than the previous record of about 17.