Falling film flow mode transitions on an array of horizontal tubes under nonuniform liquid distribution conditions

This study investigated the effect of nonuniform liquid distribution on the evolution of falling film flow mode on an array of horizontal tubes. A falling film flow mode observation system with water as the working fluid was designed and evaluated. In the test system, six liquid distributors, namely, DIS1, DIS2, DIS3, DIS4, DIS5 and DIS6, were designed to realise different liquid distribution conditions. The test tube array was composed of smooth copper tubes in which their outer diameter, tube length and tube spacing were 19.05, 280 and 10.0 mm, respectively. Results showed that the falling film flow mode transitions were strongly affected by liquid distribution conditions, and the falling film flow mode transitions under nonuniform liquid distribution conditions could not be predicted well by using the classic models that have been established under uniform liquid distribution conditions. For the droplet to droplet-acolumn (D -> DC), droplet-column to column (DC -> C), column to column-sheet (C -> CS) and column-sheet to sheet (CS -> S) flow mode transitions, the transitional Reynolds number (Retr) for uniform condition (Retr,u) were 105, 145, 352 and 436, and the differences between Retr for the six nonuniform and uniform conditions were 50.4%, 38.7%, 18.1%, 5.93%, 23.4% and 37.1%; 22.1%, 37.0%, 23.3%, 55.7%, 51.9% and 55.3%; 10.7%, 3.53%, 10.7%, 12.4%, 16.7% and 29.6% and 94.1%, 58.2%, 13.7%, 26.2%, 29.6% and 96.8%, respectively. A semiempirical model was established to predict the evolution of falling film flow mode under nonuniform liquid distribution conditions. This study serves as reference for improving the flow pattern determination criteria of falling condensate on horizontal tube bundles.