This study relates in general to accident scenarios in a closed cycle, nuclear powered, three-shaft, helium gas turbine, and in particular to finding explanations for the high energy dissipation rates at certain compressor operating modes on a four quadrant compressor map. A four quadrant compressor map allows the presentation of all combinations of positive and negative pressure rise and flow direction for positive or negative direction of rotation.

The paper presents measured velocity profiles between blade rows, and computed particle path lines in blade passages. They reveal radially oriented vortices between the blades in a blade row when operating at low positive and negative flow rates. These vortices almost completely block the flow, and flow passing through the blade passage has to follow a helical path from casing to hub or vice versa around the vortex. The flow paths through these vortices are linked to the flow paths around circumferential ring vortices near the hub or near the casing in the blade free passages between blades rows. When operating as a turbine under high flow rates the vortices associated with negative incidence stall may be sheltered by the stator blade concave surfaces and deflect the flow in addition to blocking it.

The vortex structures appear to be fundamental in nature as they were evident in three quadrants and in two different compressors. The vortices play a role in the high energy dissipation rates in axial flow compressors at very low flow rates, where they operate effectively as flow mixers and not as compressors in possible accident scenarios. They also explain the poor performance as turbine in the fourth quadrant.

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