SEMI-EMPIRICAL ANALYSIS OF THE CRITICAL TRANSITION FROM ANNULAR TO WAVY-STRATIFIED OIL-WATER ADIABATIC FLOW WITHIN HORIZONTAL AND SLIGHTLY INCLINED PIPES

The main subject of this paper concerns the critical transition from annular to wavy-stratified oil-water adiabatic flow within horizontal and slightly inclined pipes. The transition is referred to as critical because it occurs suddenly, giving rise to a sharp and strong increase in the pressure drop due to the contact of the high-viscosity oil with the pipe wall. This could be a dangerous accident, e.g. in pipelines operation, because the peak-wise increase in the pressure drop might give rise to a “chugging” phenomenon in the flow. Experimental runs were performed on eight test sections of both Plexiglas® and Pyrex® pipes with internal diameters ranging from 21.5 mm to 50 mm, using tap water and viscous oil (ρo=890 kg/m^3, μo=0.9 Pa s at 20°C). Inclination angles from 0 to 6 deg upward were tested. Water and oil superficial velocities were varied in their proper range and the pressure drop was measured for several combinations of these quantities. Flow pattern visualizations were also taken by a CCD photo-video camera in order to draw flow pattern maps. Based on these findings, the transition boundary between annular and wavy-stratified flow was analytically determined and compared with the data reported in the existing flow pattern maps.