Distribution characteristics of falling film thickness around a horizontal tube

An experimental investigation of a liquid film falling around a horizontal tube was performed using a displacement micrometer to determine the distribution characteristics of the film thickness. Measurements were carried out for circumferential angles ranging from 15° to 165°, outside diameters varying from 20 to 32 mm, intertube spacings ranging from 10 to 40 mm, and film Reynolds numbers varying from 150 to 800. It is found that the distribution characteristics of the film thickness are mainly affected by circumferential angle, intertube spacing and film Reynolds number. However, the outside diameter has little influence on the film thickness. The experimental results also validate the Nusselt's theory giving a relatively reasonable prediction of the film thickness around the upper perimeter of the tube but a poor prediction around the lower perimeter. The minimal values of the film thickness locate at different circumferential angles in the range of 90°–115° rather than at the single point of β = 90° predicted by Nusselt. Moreover, the liquid film gets thinner around the scaled tube, and the thickness distributions for pure water agree well with those for seawater. Based on the experimental data, a new correlation has been suggested to predict the film thickness.

► The film thickness presents an asymmetric circumferential distribution.
► The minimal film thickness locates at different angles in the range of 90°–115°.
► Effects of the intertube spacing and film Reynolds number were investigated.
► Comparisons of the thickness on two kinds of tubes and for two fluids were performed.
► A new correlation was suggested to predict the film thickness.