Effect of diameter and axial location on upward gas–liquid two-phase flow patterns in intermediate-scale vertical tubes

•A vertical two-phase flow system is manufactured to study flow behavior adiabatically.•Two test sections are studied with inner diameters of 40mm and 70mm at two locations.•Flow pattern maps are presented for both tubes.•Effects of tube diameter and heights on pattern transition boundaries are investigated.•Three sub-patterns bubbly flow and two types of slug pattern are recognized.