MICROGRAVITY AND POOL BOILING - MODELING OF COUNTERBALANCING EFFECTS IN THE HEAT FLUXES

On the basis of the existing experimental results as well as on various models given in the literature [1-4] the authors show that an important effect observed in evaporation under constraint of non-equilibrium vapour pressure or overheating of the evaporating liquid could play an important role in pool boiling under microgravity. This effect is the so-called ''vapour recoil instability'' [5-8]. In evaporation under constraint, indeed, that boundary layer instability is responsible for a violent motion of the liquid underneath the evaporating surface, accompanying a dramatic increase of the flux of matter through the surface. In the experiment of the evaporation of superheated drops by direct contact with an immiscible liquid of the same density, it is responsible for the ''explosion'' of the bubble droplet system [9, 10]. In pool boiling, it can induce an increase of heat flux of the same order of magnitude as that induced by the drift flow. A consequence of the replacement of the drift effect by the vapour recoil under low gravity could explain the rather small difference of maximum heat flux observed in the Nukiyama curve (heat flux versus T(w) - T(b)) under normal or reduced gravity. Moreover, as the vapour recoil effect favours the formation of a gas film on the plate, one should expect a shift of the transition point between nucleate boiling and film boiling towards the small values of overheating when the gravity is reduced.