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. In the present research, a two-phase flow system is designed, manufactured, assembled and adjusted to study two-phase flow behavior isothermally. Test sections are tubes standing in vertical position and are made of transparent acrylic with inner diameters of 40mm and 70mm. Two axial locations of 1.73m and 3.22m are chosen for data acquisition. Flow pattern maps are presented for both tubes. Effects of tube diameter and axial location on pattern transition boundaries are investigated. Air and water are chosen as working fluids. The range of air and water superficial velocities are 0.054–9.654m/s and 0.015–0.877m/s for the 40mm diameter tube, but these values are 0.038–20.44m/s and 0.036–1.530m/s for 70mm diameter tube. The results show that for both tubes, increasing axial location does not affect flow transition boundaries significantly. However, slug pattern region shrinks considerably by changing tube diameter from 40mm to 70mm. Using image processing techniques, recorded high speed movies were investigated accurately. As a result, bubbly flow in the 40mm tube can be divided into three sub-patterns as dispersed, agitated and agglomerated bubbly. Also, two types of slug pattern are also recognized in the same tube diameter which are called small and large slugs. Semi-annular flow is observed as an independent pattern in the 70mm tube that does not behave as known churn or annular patterns.