Dropwise and liquid-jet laminar flow of subcooled water falling over horizontal tube banks

Falling-film heat exchangers are widely used to promote heat and mass transport within industrial environments. In contemporary condensers, for example, liquid films continuously form over a tube row and impact multiple tubes beneath, resulting in a rich variety of two-phase flow phenomena, including the classical Rayleigh-Plateau instability. It is widely known that altering the contact angle of the tubes influences the heat exchangers’ heat and mass transfer characteristics by changing the falling films’ flow behaviour (i.e. dropwise or jet). Therefore, resolving interfacial flow characteristics, such as the Rayleigh-Plateau instability, is crucial for falling film studies as this instability may change the state of the falling film from being jet flow to dropwise/jet flow or even just dropwise. To explore this aspect, three-dimensional direct numerical simulations are used in the present work to replicate the jet instability behaviour. By employing certain mesh refinement strategies demonstrated from various case studies, the interfacial flow behaviour and jet instability have been captured, showing good agreement with experimental and theoretical data previously published. This is determined from the increased jet bifurcation and a maximum error of 2.2% of the film thickness in comparison to Nusselt's result within the bulk region of the tube. In addition, the average and local heat transfer coefficients across the first tube were compared with numerical solutions to give an average error of 3.2 and 14 %. Jet instability is found to be directly related to the amplitude of the axial pressure variations which is augmented by the growth rate of the capillary wave that travels upstream. Specifically, for contact angles of 30 and 60°, the flow is in a dropwise and dropwise/jet state due to the emergence of the Rayleigh-Plateau instability. Notably, the additional satellite droplets that materialize from the unstable jet lead to an increase in the local heat transfer coefficient of the film compared to without the Rayleigh instability improvement.