The study of multiphase flow is more difficult and complex in undulating pipelines due to flow interaction between differentrnpipe sections and influence of the specific angles being studied. While previous studies (Zheng et al., 1995; Al-Safran et al.,rn2005) are low angles of hilly pipeline, there is no adequate information on flow characteristics for high angles. Therncomplexity in sand transportation in multiphase flow is more aggravated in undulating pipelinernThis paper presents experimental investigations to understand the multiphase flow characteristics, conditions for slugrninitiation mechanism that would enhance sand transport in undulating pipes and assist pipeline design. The investigationsrnwere conducted using a 2-inch (ID=50.24 mm) dip facility which consists of a downhill pipeline (-24o), a lower elbow (dip)rnand an uphill pipeline (+24o).rnThe slug initiation mechanisms phenomena were investigated. New flow regimes were identified for different pipe sectionsrnusing both visual observation and statistical parameters. Sand transport characteristics in water and air-water were studied atrndifferent sand concentration. Also, flow condition to transport sand bed deposit at the dip was determined.rnThree slug behaviours were observed at the dip: complete stratified flow downhill with slug initiation at dip; stratified flowrn(with energetic ripple) downhill with slug initiation and slug growth at the dip; and aerated slug downhill and slug growth atrnthe dip. This behaviour is different from existing works on this subject with low angle of inclination.
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