R134a vapour-liquid two-phase flow patterns were studied in vertical small diameter tubes. The observed flow patterns include bubbly, dispersed bubble, slug, churn, annular and mist flow. Six integrated flow pattern maps, derived for two internal diameters (2.01 and 4.26 mm) and three different pressures (6.0, 10.0, 14.0 bar), are presented. Some transition boundaries, such as slug-churn and churn-annular, were found to be very sensitive to diameter and pressure. On the contrary, the boundaries of dispersed bubble-churn and bubbly-slug are less affected. The transition boundaries are compared with the existing models for normal size tubes showing significant differences
