Dynamics of bubble-enhanced natural convection in a superheated water cavity

In this paper, we report on direct numerical simulations of bubble-enhanced natural convection in a cavity filled with water. The bottom wall is superheated, resulting in the formation of bubbles, while the free surface remains subcooled. The governing equations of the liquid are solved in the Eulerian framework and those of the bubbles in the Lagrangian one, employing two-way coupling between the two phases. We consider cases at Rayleigh numbers Ra=3.3×105 and Ra=3.3×106. Our study focuses on the effect of superheat. At very low superheat, the bubbles condense rapidly without impacting the flow dynamics. However, when the superheat is sufficiently high for the bubbles to reach the bulk of the domain, they induce a reorganization of the large-scale circulation and increase the vertical velocity of the liquid. Furthermore, they tend to homogenize the temperature distribution in the bulk and shift it toward the saturation temperature. Our simulations further confirm that the hydrodynamic interactions between the bubbles and the liquid promote significantly thermal mixing and convective heat transport. More specifically, at high superheat levels, the Nusselt number can be up to three times greater than that computed in the corresponding single-phase flow.