Multiphase flow introduces many challenges in turbomachines analysis and operation, as each of the phases responds differently to the forces in the hydraulic channel, and the mechanical and thermal interaction among the phases has to be taken into account too. Especially when very high gaseous fractions need to be covered, due to the concurrent physics involved and transient phenomena, a proper machine characterization cannot be limited to an overall description of the performance, but it needs to rely on advanced analysis tools which can reveal the local phenomena responsible for performance degradation and instabilities.

The flow regimes vary from a homogeneous distribution of fine bubbles, evenly dispersed and carried away by the main flow, to more complex flow patterns, especially if bubbles coalesce and adhere to a wide portion of the channel wall.

Tests are performed on a multiphase pump facility recently designed by the authors, which allows a complete optical access to the pump channels and fine adjustments in the inlet configuration and the tip clearance gap.

The investigation focuses on the effect of the mixture inlet pressure and machine rotational speed on performance and stability. Increasing values of the inlet pressure reduce the density ratio between the phases, thus making them become more coupled; this results in a milder performance degradation and a wider stable operating range.

Accurate flow visualization through the pump transparent casing complements the study, allowing an immediate and detailed local phenomena description.

The results are presented under performance and surging curves, showing the gas-handling capability dependence on the actual conditions.

These analyses fully characterize the machine behavior, define the operating zones which should be avoided, and will serve later to implement a control strategy to keep the machine in a safe regime.

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