Influence of the snubber on temperature distribution at last stage blade exit of a steam turbine during low volume flow operations

Highlights: • Detailed traverse measurements on last stage blades with/without snubber. • Validation of the results for both set of rear stages via CFD. • Analysis and comparison of the results. • Detailed discussion of tip cooling mechanism for snubber baldes at low load. • Advantages for power plant operations at low load and full speed no load. -- Abstract: Flow field and temperature distribution at last stage moving blades outlet for freestanding and snubber blades have been investigated experimentally and numerically. The same airfoil geometry of a steam turbine last stage blades, with and without snubber, has been tested in a four-stage scaled steam turbine and computed using ANSYS CFX 16.0. Initially, measurements comparison of the main flow quantities such as velocity components, pressure and temperature distribution at the exit of the last stage blades is carried out. Afterwards, the CFD results are validated against measurements. Finally, a detailed discussion of the flow field of the two geometries and its influence on maximum temperature is presented. The local cooling effect at the blade tip experienced by snubber blades enables a further reduction of the sprayed water during low volume flow operations with a consequent reduction in trailing edge erosion. Snubber blades, for a given maximum flow tip temperature, can run at even reduced volumetric flow compared to equivalent freestanding blades. The capability of assessing the maximum temperature achieved by the last stage moving blades (LSMB) along with the influence of the coupling elements usually required to stiff long last stage blades, is a key aspect for the optimization of the power plant design for low volume flow operations and lifetime improvement. Furthermore, the flow physics of the freestanding and snubber last stage moving blades during low volume flow (LVF) operations is presented.