The main aim of this work is to investigate the effect of vertical to horizontal 90° bends on the structure and the frequency of the flow. The paper describes how the flow structures in the vertical section develop as it moves downstream the mixing section. It also highlights the effect of centrifugal force on slugs passing around the bend. Finally the flow development downstream the bend and how it alters as it goes around the bend predominantly forming stratified and slug flow is discussed.
Most of the process industries have long pipe lines to transport the process fluids from one place to another. These pipelines have bends to change the orientation of the pipes as warranted by the layout of the process equipment and vessels. Single phase flows in such pipelines have been studied and a good understanding of the flow behaviour has been established making hydraulic designs robust and trustworthy. However, two phase flows that occur in transportation of oil and gas from wells to the production platforms or boiler tubes in power generation are complex and the development of such flows around bends were not well understood. The complexity of the flow behaviour is increased due to the effects of centrifugal force induced by curvature of the bend and the existence of the two phases with significantly different densities. As the fluid passes through curved pipes or bends, a considerable centrifugal force is applied to the mixture forcing it to separate. Separation is affected by the orientation of the bend giving rise to complications such as void fraction fluctuation and formation of dryout zones.
The effects of the bends on the two phase flow is a common issue in a wide range of industrial applications, nevertheless low number of studies is available on this subject compared to single phase flow around bends. Several correlation were proposed to estimate the pressure drop around bends for single phase flows including the work of Ito for turbulent  and laminar flow . Several attempts to predict the additional pressure drop due to the two phase flow around bends have been reported, specially the work carried out by Chisholm in a series of papers [3–6]. Azzi et al. critically evaluated the available correlation used to predict the pressure drop in bends for two phase flows with Newtonian liquid phase. They emphasised the inconstancy of most of the equations especially at zero bend angle and zero mass flow rate where the pressure drop should be equal to straight pipe and approaching zero velocity respectively. Ultimately B-type correlation given by Chisholm  was considered the most suitable correlation for two phase flows.