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Increasing oil and gas recovery of reservoirs by boosting unprocessed hydrocarbons require multiphase machinery capable of pumping mixtures with gas volume fractions (GVF) from 0-100%. Unwanted flow regimes can cause pump instability and may lead to severe deterioration of the pump performance. Stable operation is essential for avoiding mechanical damage and maintaining steady production. A multiphase test rig at NTNU has been build in order to document flow regimes and flow mechanisms in a prototype pump. The pump is one stage of the MultiBooster, which is one of the multistage pumps of Aker Solutions designed for multiphase applications. The compression cell consists of a semi-axial impeller and a vaned diffuser, with a transparent shroud to get visual access into the pump channels. The impeller is a semi-open application, which introduces leakage losses over the impeller blade tips. The objectives of this thesis has been to establish flow regimes in the impeller inlet tip area, document secondary flow phenomena in the impeller, and document interactions between the inlet, the impeller and the diffuser. Both single- and two-phase conditions have been documented, with special emphasis on instabilities, and the effect of reduced clearance. Documentation of such flow phenomena gives a basis for understanding the flow mechanisms causing instabilities, and may be a foundation for improvements of existing pump design. Flow phenomena related to instabilities in single-phase operation are well established both for compressors and pumps, but the documentation of the flow mechanisms leading to unstable operation in two-phase condition is limited. Flow regimes and flow behavior at various operation conditions have been documented by filming with high-speed camera. Phase separation with gas accumulation to the suction side towards the shroud was observed at the inlet of the impeller channels at Q/Qnom = 1. As the flow rate was reduced, the tendency of phase separation diminished due to increased leakage flow over the impeller tips. Tip leakage vortex (TLV) was investigated both in single- and two-phase operation. The TLV was visualized as cavitation in single-phase, while gas bubbles made the TLV visible in two-phase operation. Analysis of the MultiBooster performance has been conducted to see the effect of reduced clearance on the performance. The performance characteristics of the MultiBooster revealed high sensitivity to the clearance level. The pressure production over the impeller stage increased, while the pressure recovery in the diffuser was reduced when the clearance level was reduced. Analyzes of dynamic pressure at the impeller inlet and at the impeller-diffuser interface has been conducted, both in the time and frequency domain. The frequency analysis revealed rotating instability in frequencies below the blade passing frequency, and can be related to rotating stall in the impeller and in the diffuser. Separation point and recirculation zone in the diffuser has been documented, showing recirculation zone at the suction side present at nominal flow towards closed valve. Rotating stall in the diffuser has been detected at Q/Qnom = 0.2, as the diffuser channel alternated between fully and partly blockage by recirculation. Problems with overheated bearings have caused limited access to the laboratory test rig during this semester. This resulted in time limitations and some of the results are therefore not corresponding to each other. The outcome of this thesis is however of great value for further investigations of the MultiBooster.