Falling film thickness distribution around a horizontal tube under countercurrent air flow

Abstract

Falling film heat exchangers are commonly used in various industrial applications. Although air stream–liquid interactions often occur in these exchangers, film thickness prediction models typically ignore their effects. In this study, a horizontal tube falling film experiment was performed with countercurrent airflow, and the behavior of the film was observed and analyzed. The film flow rate, nozzle height, countercurrent air velocity, heat flux, and drip temperature were varied. In the absence of forced convection, the minimum film thickness primarily occurs between 90° and 120° around the tube circumference. The film flow rate had the strongest effect on the film thickness. Countercurrent air greatly reduced the film thickness in the strong air stream–liquid interaction zone, further reducing the thickness of the thinnest sections. Boundary layer separation occurred and caused the location of minimum film thickness to move upwards on the tube to approximately 60°–80° These results highlight the need for practitioners to ensure that film ruptures and dry spots, which negatively affect heat transfer performance, do not occur in this region. A revised correlation equation for determining the film thickness and applicable in scenarios with or without counterflow air was developed and found to result in errors smaller than ±15 % for most experimental results.