Coarse Droplet Measurement at the Last Stage Stator Exit of a Four-Stage Scaled Steam Turbine Using an Optical Backscatter Probe

Abstract

Abstract Size and velocity distributions in polydisperse coarse droplets are important to estimate and reduce erosion damage and wetness losses in wet steam turbines. Diameters and velocities of coarse droplets are measured by an optical backscatter probe downstream of the last stage stator in a four-stage model steam turbine at the rated condition. The droplet diameters distribute in the range from 30 to 90 micrometers. The Sauter mean droplet diameter is about 57 micrometers and the maximum droplet diameter is 88 micrometers. The corresponding Weber numbers are 11.4 and 17.6 respectively in the Sauter mean and maximum diameters. The Weber numbers are generally consistent with calculated values by both Kelvin-Helmholtz and Rayleigh-Taylor instability models. The mass flow rate and number of the coarse droplets is larger in the wake region from the stator, because the coarse droplets might be mainly generated by atomization of water films and rivulets on the stator. The larger droplets in diameter over 60 micrometers tend to axially deflect from steam flows. The Sauter mean diameters are slightly larger downstream from suction surface of the stator. The droplet velocities distribute in the range up to 40 m/s. The smaller droplets have the higher velocities as expected. Mean velocities are also consistent with predictions by the instability models. The measured number density distributions of coarse droplet diameters and velocities with the spatial resolution could improve the accuracy in models for blade erosion and wetness loss from models based on mean or maximum droplet diameter.