With turbines, operation conditions far from the design range are possible. With gas turbines, in nuclear and solar energy plants in the case of a fault, flow in reverse directions may even occur. For extreme operations insufficient knowledge was available, so measurements were carried out at first on a single-stage turbine and then on a seven-stage turbine, with both the mass flow and the rotor speed systematically varied in the positive (normal direction) and negative ranges (four-quadrant characteristics). The measurement on the seven-stage turbine basically tally with the single-stage measurements. Stage performance curves can be approximately calculated also for four quadrants from the cascade characteristics of the middle section. The calculated curves are compared to the results of the single-stage measurements. During operation far from the design point, the individual stages of a multi-stage turbine work at a wide variety of operating points, so the characteristics of such a turbine have to be determined by the superimposition of the corresponding stage performance curves on each other.

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