The rate of slugs passing through a pipeline 410mm in diameter and 630m in length was gradually decreased from 552 slugs per hour at 80m to a 43% level of 240 per hour at 500m, with the novel dredged mud conveyance system using compressed air. The aim of this study was to clarify the mechanism of the decrease in the slug passage rate by establishing a slug flow deformation model. Previous studies of air-liquid two-phase flows have mainly focused on liquid of low viscosity in short pipelines of narrow diameter. The flow deformation phenomena of slugs in a pipeline such as the decrease with distance of slug passage rate have been neglected. However, in the large-diameter, long-distance pipeline used at a dredging site, the flow deformation phenomena must be considered, otherwise the conveyance efficiency might not be evaluated accurately. The slug flow deformation model has been established from the viewpoint of the flow deformation phenomena: generation, development and breakdown of slugs. Slugs of various length are generated due to the development of surface waves with various wavelengths. The generated slug develops until a critical point when a balance is reached between the kinematic energy and the energy loss from friction with the pipe wall and the slug's inner energy loss. The developed slug breaks down due to the friction and the inner energy loss. Finally, the critical development length is obtained in view of the energy loss due to the friction caused by viscosity and the energy loss due to the normal stress.
