In the paper the behaviour of the five-stage experimental steam turbine 10 MW is described during the expansion transition from superheated steam to wet steam as well as the influence of wet steam on the flow path efficiency. The influence of wet steam occurrence on one to four last stages is given by setting the inlet steam parameters. The wetness of steam behind the last stage is calculated using two methods, first by enthalpy drop with the output measured by water brake and second by an optical probe. The results of both methods are compared. The impact of steam wetness increasing on a decrease in turbine efficiency is observed. In the paper the influence of steam wetness on the individual stage efficiency is also described and wetness loss coefficient is determined. Using the optical probe, assessment of poly-disperse structure of fine droplets is carried out, which is presented here by Sauter mean diameter. A pneumatic probe is used to define the distribution of velocities and pressures along the blade length after the last stage.

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