Sensitivity Analysis of Condensation Model Constants on Calculated Liquid Film Motion in Radial Turbines

In many technical processes, a mixture of gas and steam is used as the working fluid in radial turbines. When condensation occurs during expansion, a portion of the liquid droplets can hit the rotor blades and form a water film, which can move in a radial direction and even against the flow direction. Then, the liquid film separates in the rotor tip clearance or at the leading edge of the rotor and forms coarse water droplets. The presence of coarse water droplets in the gap between stator and rotor can cause damage to the turbine rotor. To design a radial turbine which works under condensation conditions, it is essential to know where and when condensation and film formation occur. With this information, it is possible to take action to remove the liquid or to adjust the required maintenance intervals. To examine the details of condensation and film motion, an existing flow solver is extended to capture condensation effects. Models describing nucleation and droplet growth are added to a particle-tracking algorithm. Droplets impinging on the rotor blades form a liquid film. The motion of this liquid film is calculated with a newly developed thin film solver. The calculation tool is validated against third party test rig experiments as well as numerical experiments. For many parameters, the agreement between the calculation tool and the experiments is quite satisfactory. Some results, however, show larger deviations. One of these parameters is the droplet diameter. The numerical results are generally reliable, but an experimental validation is necessary for detailed understanding of the mechanism. Before expensive experiments are conducted, it is recommended to perform a sensitivity study to emphasize important parameters. This sensitivity study is performed concerning a radial turbine for an operating point at which the liquid film travels into the tip clearance. In this paper it will be shown how the thickness and movement of the liquid film change with variation in influencing parameters. Finally, model constants that have the strongest influence on the calculated film motion are highlighted.