Experimental and numerical study of counter-current flow in vertical pipe

Counter current flow is encountered in a wide variety of industrial applications ranging from flows in nuclear reactors to process flows in chemical reactors. This paper describes experimental and numerical results obtained in a circular column of 240 mm inner diameter with two inner pipes. The counter current flow studied concerns an upward flow of air and a downward flow of water at ambient temperature and pressure. The following range of operating conditions is analyzed: superficial air velocities up to 0.25 m/s and superficial water velocities up to 0.04 m/s, corresponding to global void fractions up to 27.7%. The experimental investigations concerns (i) flow visualization, (ii) local data from a double optical probe and (iii) global void fraction data. Images obtained from an optical camera are used to observe the general flow pattern and to support the boundary conditions of the numerical simulations, in terms of average bubble diameter. Data obtained from the double optical probe are used to study local flow characteristics. The gas disengagement technique is used to obtain the global void fraction over a range of superficial air velocities for the validation of the numerical method. Numerical calculations are performed with an Eulerian two-fluid model using the commercial code ANSYS Fluent Release 14.5.7 and results are compared with experimental data. The effects of bubble diameter and various interfacial drag coefficients are studied. The formulation of the drag coefficient is found to have significant effects on the global void fraction predictions. However, using merely the drag law, numerical results are inaccurate.