Visualisation of gas-liquid bubbly flows in a large diameter pipe with 90 degree bend

Two-phase gas-liquid flows are prevalent in many industries and understanding their behaviour would have significant impact on the efficiency of the systems in which they occur. However, information on two-phase gas-liquid flows in 90 degree bends is limited in the literature and their flow behaviour is not fully understood. One technique that could assist researchers in exploring flow behaviour is visualisation. Accordingly, in this study a two-phase flow experimental investigation was carried out in a large pipe of diameter 150 mm, using water and air at different superficial velocities in order to visualise the effect of 90 degree bend on two-phase flow behaviour. As optical methods are not suitable for visualising dense bubbly flows due to overlapping of bubbles, in this study, bubble size distribution and void fraction results were obtained using wire-mesh sensors before and after the bend. The results were then post-processed to visualise the flow field. The instantaneous visualisation of flow shows that gas hold-up migrates from the bottom to top wall of the pipe at the bend when the liquid superficial velocity increases for a fixed superficial gas velocity. An increase in superficial gas velocity shows insignificant influence on the gas hold-up at locations beyond the bend for the investigated conditions. This may be due to the centrifugal force imparted by the bend and hence needs further investigation. Bubble size distribution results before and after the bend indicate that the bend has influence on bubble breakup and coalescence.