On the performance of a cascade of turbine rotor tip section blading in wet steam. Part 5: theoretical treatment

During the course of expansion of steam in turbines, the fluid first supercools and then nucleates to become a two-phase mixture. The formation and behaviour of the liquid create problems that lower the performance of turbine wet stages and the mechanisms underlying these are insufficiently understood. Steam turbines play a dominant role in the generation of the main electrical power supply, and the economic returns on improved performance are substantial. This article is the last of a set and describes the theoretical part of an investigation into the performance of a turbine rotor tip section profile in wet steam. The experimental results are described in the earlier parts of the paper. To describe the behaviour of the flow theoretically, the conservation equations describing the main flow field are combined with equations describing droplet behaviour and the set is treated by the time-marching method. Comparisons are carried out with the experimental results presented in the earlier parts of the paper and the agreement obtained is good. When the droplets present in the steam are 0.15 mu m in radius at inlet to the cascade, there is considerable secondary nucleation. With droplets of 0.05 mu m radius, secondary nucleation is suppressed, but at high pressure ratios, the thermodynamic loss though reduced is not eliminated.