An experimental study was carried out to examine the flow patterns and the stability of rimming flows of two liquids in partially-filled horizontally rotating cylinders. Pure water and oil were considered as non-miscible liquids; the third phase in the cylinder was normal air. The effect of cylinder aspect ratio (length to diameter) was investigated, too. Significant mixing and de-mixing phenomena were observed between both liquids, and interesting transition processes occurred in the rotating multi-phase flow. The liquid configuration and flow characteristics were systematically visualized, and the resulting vibration spectra and Campbell diagrams were also obtained. It was found that the existence of a special sub-synchronous vibration was intimately connected with the transition to the rimming flow state. In contrast to the simple expectation, the experimental results indicated that non-trivial turbulent flow was present in the liquid domain even in case of the rimming flow state for high running speed. This is fully in contrast to the common assumption of a primary rigid-body motion of the liquid at this running speed level.
