The intertube flow regimes and hydrodynamic behavior of the liquid profile have a significant impact on the functioning of falling-film evaporators. Column flow is one of the fundamental flow modes, which is actively observed in many industrial evaporators. A high-speed digital photographic device is utilized to visualize the complete progression of column flow phases for intertube distances of 10/20/30/40 mm and Reynolds number (Re) values ranging from 41 to 583. The Sobel edge detection algorithm is implemented to quantify the flow parameters for each frame of a sequence. The research results indicated that the column flow regime between the inline horizontal tubes can be broadly classified into the following phases: columnar droplet flow, periodic column flow, satellite drops column flow, drift column flow, and mixing liquid film column flow. The findings show that the liquid jet diameters between the tubes range from 1.6 to 8.46 mm, an increase in the liquid jet diameter with Re and a decrease with tube-to-tube distance. The axial coverage of the liquid film profile beneath the tube differed from 2.07 to 3.99 mm and over the tube surface ranged from 1.14 to 2.68 mm. The formation of dry spots and flooding is a major problem in the operation of falling-film heat exchangers. The shape and size of the liquid jet have a direct impact on the interfacial area; understanding the entire progression of column flow regimes, as well as their characteristics, can help designers avoid the aforementioned problem.

The coverage of the liquid film over the horizontal tubes, particularly the wetting ratio, is important for gravity-driven evaporators and absorbers to achieve a better heat transfer mechanism. A 2D, two-phase CFD model was developed to examine the falling-film hydrodynamics and transient flow mechanism for Reynolds numbers ranging from 100-500, static contact angles spanning from 0°/30°/60°/90°, and tube-to-tube distance of 10 mm. The VOF method is used in this article to capture the liquid-gas interface. The findings showed that the complete spreading of the liquid film is difficult at low Reynolds numbers and high contact angles. The formation of dry regions on the tube wall as a result of insufficient liquid supply, liquid film breakage, and liquid film shrinkage. Furthermore, as the Reynolds number increases and contact angle decrease, the wetting ratio over the tube surface increases. It is worth noting that each contact angle must have a minimum Reynolds number in order to keep the surface completely wet. The research also revealed that for higher Reynolds numbers, the influence of contact angle on wettability of tube wall can be ignored. For the same Reynolds number, the liquid propagation time required to wet the tube surface increases as the contact angle value increases. Fouling over the tubes can be aided by the formation of air voids near the lower stagnation zone.

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