Oil volume fraction and velocity profiles in vertical, bubbly oil-in-water flows

A series of experiments was carried out using a dual-sensor conductance probe to measure the local axial oil velocity distribution and the local oil volume fraction distribution in vertical, oil-in-water bubbly flows in an 80 mm diameter vertical pipe. Values of the water superficial velocity were in the range 0.276 m s−1 to 0.417 m s−1, values of the oil superficial velocity were in the range 0.025 m s−1 to 0.083 m s−1 and values of the mean oil volume fraction were in the range 0.047–0.205. For all of the flow conditions investigated it was found that the axial velocity profile of the oil droplets had a ‘power law’ shape which was very similar to the shape of the air velocity distributions previously observed for air–water bubbly flows at similar flow conditions. It was also found that the shape of the local oil volume fraction distribution was highly dependent upon the value of the mean oil volume fraction. For values of the mean oil volume fraction β̄ref less than about 0.08, the local oil volume fraction distribution had a power law shape. For values of β̄ref between about 0.08 and 0.15 the local oil volume fraction distribution was essentially flat, apart from within a bubble sub-layer close to the pipe wall. For values of β̄ref greater than about 0.15 the local oil volume fraction distribution had an ‘intermediate peak’ shape. Mathematical modelling showed that the shapes of the observed local oil fraction distributions were a result of diffusion and of hydrodynamic forces acting upon the oil droplets. For β̄ref<0.08 the net hydrodynamic force on the droplets was towards the pipe centre whilst for β̄ref>0.15 the net hydrodynamic force on the droplets was biased towards the pipe wall. The nature, and relative strength, of each of the hydrodynamic forces acting on the oil droplets is discussed.