  The heat transfer mechanism in Rayleigh-Benard convection in a liquid with a
mean temperature close to its boiling point is studied through numerical
simulations with point-like vapor bubbles, which are allowed to grow or shrink
through evaporation and condensation and which act back on the flow both
thermally and mechanically. It is shown that the effect of the bubbles is
strongly dependent on the ratio of the sensible heat to the latent heat as
embodied in the Jacob number Ja. For very small Ja the bubbles stabilize the
flow by absorbing heat in the warmer regions and releasing it in the colder
regions. With an increase in Ja, the added buoyancy due to the bubble growth
destabilizes the flow with respect to single-phase convection and considerably
increases the Nusselt number.
