Falling film mode transitions on horizontal enhanced tubes with two-dimensional integral fins: Effect of tube spacing and fin structures

•Falling film flow mode transition on 2D finned tubes is studied experimentally.•Large tube spacing delayed the appearance of sheet mode under increasing flow rate.•Large tube spacing prolonged the sheet mode under decreasing flow rate.•The hysteresis in mode transition was enhanced by the increase in h/(p − t).•New correlations for flow mode transition on 2D finned tubes were proposed. This study proposed a correlation of Reynolds number (Re) for falling film mode transitions on horizontal enhanced tubes with 2D integral fins on the basis of an analysis of experimental data from current and previous studies considering the effect of tube spacing and fin structures. Transitional Re for flow mode transitions are analyzed on the basis of observing water falling film flow on four types of 2D finned tubes (ranges of fin height h, fin pitch p, and fin tip t of these tubes are 0.503–1.050 mm, 1.04–2.80 mm, and 0.31–1.27 mm) and smooth tubes under various tube spacings (s, including 6, 10, and 15 mm). Results show that large tube spacing delayed the appearance of sheet mode under increasing flow rate but prolonged the sheet mode under decreasing flow rate. This hysteresis phenomenon was enhanced by the increase in h/(p − t). The proposed correlation, which linked the transitional Re with three dimensionless groups (i.e., Galileo number (Ga), s/ξ and h/(p − t)) and a constant, predicted seven groups of transitional Re for the column–sheet and sheet within ±14.7%. This correlation could be applicable to the 2D finned tubes with 19 ≤ Ga1/4 ≤ 407, 2.19 ≤ s/ξ ≤ 5.49, and 0.33 ≤ h/(p − t) ≤ 27.67.