Oil-water flows characteristics in pipes of different diameter

The flow patterns during oil-water flows were investigated in two acrylic pipes that had different diameters, 38 mm ID and 14 mm ID respectively. Water (ρ = 1000 kgm-3, μ = 0.001 kgm-1s-1) and Exxsol D140 oil (ρ = 830 kgm-3, μ = 0.0055 kgm-1s-1) were used as test fluids, while the patterns were recorded with a high speed camera. Dispersed and rivulet flows were seen in the small pipe but not in the large one at the conditions studied. The dual continuous pattern appeared in both pipes but, with different characteristics. Larger drop sizes and smoother interface were seen in the smaller pipe. Double-wire conductance probes were used to collect time-series records of the oil-water interface variation during stratified flow in the pipes. Data was collected at 256 Hz (or 512 Hz) for up to 4 min. Following de-trending and normalisation, the signal was found to be stationary and normal. Power spectra were computed via the auto-correlation function. Low-frequency contributions (due partly to the pumps) were seen in both pipes. In the large pipe, contributions were in the range 20 – 50 Hz and their normalised power increased with the mixture velocity. These higher frequencies were attributed to the fluctuations and roughness of the interface, notorious in the large-diameter pipe during stratified flow. In contrast, the interface in the small pipe was smooth.