(409f) Evaporative Suppression of Film Instability in Pure and Binary Mixtures

The interface between a liquid lying above its own vapor is well-known to be unstable due to the Rayleigh-Taylor instability. We study this heavy-over-light configuration in the presence of evaporation. This study is motivated by the boiling crisis phenomenon observed in a heat pipe, wherein a vapor film is stably sustained below the heavier liquid resulting in an unfavorable dryout of the heat pipe and lowering of heat flux.

A model configuration of a liquid lying above its vapor confined between two at plates is chosen. The system is heated from the vapor side. A weighted residual- integral boundary layer (WRIBL) model based on the long-wavelength approximation is developed. Both liquid and vapor phases are considered active and the effects of thermal and momentum inertia are retained. We show that strong evaporation (large ΔT) can linearly stabilize Rayleigh-Taylor instability and the interface remains flat. Interestingly, when evaporation is weak (low ΔT) and the system is linearly unstable, it can still evolve nonlinearly to a steady interface configuration that sustains a stable layer of vapor. It is shown that momentum inertia slows down the rate at which the interface reaches steady configuration. It also results in a non-monotonic (oscillatory) evolution of the interface to the steady state. The thermal inertia is shown to have a destabilizing role. On further weakening of evaporation (negligible ΔT), the interface approaches the bottom plate. Under such conditions, the system exhibits features of pure Rayleigh-Taylor instability and exhibits hydrodynamic sliding, wherein the interface translates parallel to the plate.

The study is extended to the case of a binary mixture, where solutal Marangoni effect is also incorporated. We study a specific choice of binary mixture, wherein the less volatile component has higher surface tension. It is shown that the solutal Marangoni effect associated with such a binary mixture renders the system more unstable compared to pure component system, but not unstable enough that a binary system can cause the interface to rupture while a pure system might cause it to be stably suspended.

Supported by NASA-CASIS GA-2015-218, NASA NNX17AL27G and NSF 0968313