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In the present research, two-phase flow is studied adiabatically in vertical plexigalss tubes with inner diameters of 40 mm and 70 mm in heights of 1.73 m and 3.22 m. Flow pattern maps are presented for both tubes and effect of diameter and height on the transition curves between flow patterns is investigated. Air and water are used as working fluids. Superficial velocities of air and water for 40 mm tube are 0.054-9.654 m/s and 0.015-0.877 m/s; and for 70 mm tube are 0.038-20.44 m/s and 0.036-1.530 m/s, respectively. By changing the tube diameter from 40 mm to 70 mm, slug pattern region shrinks considerably. Inlet is designed to be "annular" for which bubbly flow in 70 mm tube is not observed in low water superficial velocities. However, this pattern is observed in higher water and lower air superficial velocities. For both tubes, the main flow regimes observed are bubbly, slug, churn and annular. The results obtained using image processing technique show that bubbly regime in 40 mm can be divided into three sub-patterns called dispersed, agitated and agglomerated bubbly. In addition, two sub-patterns are recognized in slug regime as large slug and small slug. Also semi-annular pattern is observed as an independent flow pattern in tube with inner diameter of 70 mm which has not been analyzed accurately up to now.

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Supercritical fluids have substituted non-super critical fluids in some areas of industry because of their unique characteristics and have been the subject of numerous experimental, numerical and analytic studies since their discovery. In this study laminar natural convection between a hot vertical tube with constant temperature and supercritical carbon dioxide with uniform temperature at inlet is simulated by utilizing a numerical model. The simulation is a two-dimensional, pseudo-transient numerical model based on finite volume method. The main objective of this study is to investigate and analyze the effect of severe property variations of supercritical carbon dioxide on the flow and temperature field of natural convection that ultimately affect heat transfer rates with respect to non-critical natural convection. Numerical simulations have been carried out for temperature and pressure ranges of 305K to 312K and 7.5MPa to 9MPa respectively. Span and Wanger’s multi-parameter equation of state have been used directly to determine carbon dioxide properties around pseudo critical temperature for the first time. Results indicate an increased rate of total heat transfer up to 160% near pseudo-critical temperature and 118% in other temperatures for supercritical natural convection with respect to ideal gas assumption.