Dipole-to-monopole-flow transition in bubbly thermal convection

The hydrodynamic and thermal interactions between neighboring vapor bubbles on hot surfaces play a crucial role in heat transport and flow characteristics. To investigate these interactions, we conducted numerical simulations of saturated vapor bubbles in a two-dimensional square enclosure filled with liquid water. The water was heated at the bottom and cooled at the top to replicate boiling at 100^{∘}C and normal atmospheric pressure. In our simulations, we varied the Jakob number (Ja), which measures sensible heat to latent heat, and the separation distance (d) between the bubbles. We observed that the flow zone between the bubble seeds is thermally active and stable for d less than half the height of the enclosure, and becomes oscillatory for d greater than half the height. The stable state leads to a dipole mode, while the oscillatory state triggers a tripole mode. When Ja exceeds a critical value (approximately 2.8), hydrodynamic instabilities are induced in both the core bulk region and the boundary layers. At high Ja values, bubbles frequently move away from the wall, leading to intense hydrodynamic fluctuations and a monopole mode. Our findings indicate that at low Ja, the phase-change-related heat transport dominates, while at high Ja, the convective terms predominate. Furthermore, we observed that overall heat transport in bubbly convection is consistently greater than in classical thermal convection.