Modelling of the flow at the last stage blade tenon in a geothermal turbine using renormalization group theory turbulence model

摘要

A bi-dimensional modelling investigation of the flow in the last stage of a 110 MW geothermal turbine has been conducted. The study was based upon a Renormalization Group Theory turbulence model. The results confirmed the existence of flow conditions which may play a main role in the erosion of the L-0 stage blade tenon, which had been detected in periodic overhauls. According to predicted results the relationship between erosion and flow patterns might exist due to: (1) a vapour jet hitting directly on tenon surface at velocities around 65 m/s; (2) a low-pressure region identified with recirculating flow, which may be causing cavitation on the damaged surface. Afterwards, the flow was simulated with changes on the geometry and grid. These changes are, indeed, practically feasible of being implemented. The simulations showed that it is possible to reduce the erosion process by enlarging a flat region close to the L-0 rotor stage. Namely, this change of geometry produces a flow pattern that diminishes the strength of recirculation flow making it possible to reduce both the flow rate through tenon region and its velocity on tenon surface. The pressure drop diminishes as well, clearly reducing a risk of cavitation.