On the stability of inclined liquid films with confined counter-current gas*

Thin liquid films flowing in inclined channels in the presence of a strongly confined counter-current laminar gas flow are employed in several industrial equipments, such as heat exchangers. In these devices, the intensification of the inter-phase transfer can be achieved by increasing the gas flow rate which promotes interfacial waves. However, the channel might flood in the presence of large-amplitude waves, thus it is crucial to characterize the effect of the gas on the interfacial instability.
To this purpose, we focus on a particular region of parameter space, small inclination angles and strong confinement. We show that a laminar counter-current gas flow stabilizes the liquid film at very strong confinement.
Interestingly, a sufficiently high gas flow rate leads to the fully suppression of the Kapitza surface instability in these conditions.
We have investigated the dependence on various parameters of the confinement threshold at which the gas reverses the trend and destabilizes the liquid film.