Investigation on the Impact of Air Admission in a Prototype Francis Turbine at Low-Load Operation

Abstract

Due to significant changes in the energy system, hydraulic turbines are required to operate over a wide power range. In particular, older turbines which are not designed for these environments will suffer under off-design conditions. In order to evaluate whether or not such a turbine could fulfill the new requirements of the energy market, a study about the behavior of a prototype plant in low-load operation is presented. Therefore, prototype site measurements are performed to determine the most damaging operating point by means of acceleration sensors and pressure transducers. Moreover, unsteady computational fluid dynamics (CFD) simulations considering two-phase flow and two hybrid turbulence models are used to analyze the flow conditions inside the turbine. The resulting pressure pulsations are mapped onto the runner blade to obtain stress and further calculate damage factors. Accordingly, the stresses are compared to those obtained by the strain gauge measurement. Moreover, the influence of active flow control by means of air injection on plant behavior and runner lifetime is discussed as well.

Fatigue Strength Analysis of a Prototype Francis Turbine in a Multilevel Lifetime Assessment Procedure Part II: Method Application and Numerical Investigation

“…The position and the value of the highest occurring stress at the runner depend on the respective considered operating point of the plant. In [35,40,42] it has already been shown that the highest dynamic stresses are expected near the positions of R1 and S2 in the partial load range at P/P RP = 44%. However, the simulations aimed not only to find the critical operating point but also to numerically investigate the plant's entire operating range concerning occurring stress conditions and damage effects on the runner structure.…”

“…For example, grid sizes of about 6 million cells were used for the investigation of transient processes in the low-load region [30] for hydropower plant 2. In the case of transient processes (e.g., STU), a mesh with approximately 13 million cells [34] was created and for the investigations of air injection and vortex shedding at the trailing edge of the runner, the existing mesh had to be refined again to approximately 37.5 million cells [35].…”

“…The general procedure and setup for model construction and meshing has already been published in [28,35,41] and was carried out analogously. The mesh consists of quadratic tetrahedral elements with about 0.6 million nodes.…”

“…9 and correlating them with the results of Table 10 one notices a resonance problem caused by vortex shedding of the runner. This investigation has been published by [35]. The position and the value of the highest occurring stress at the runner depend on the respective considered operating point of the plant.…”

Influence of the Reshaped Elbow on the Unsteady Pressure Field in a Simplified Geometry of the Draft Tube

“…These phenomena may produce failures on the mechanical elements of the machine [44][45][46] diminishing the lifetime of the hydraulic turbines [47]. Different technical solutions have been proposed to mitigate their effects (e.g., fins [48][49][50], air injection/admission [51,52], water injection [53], air-water injection [54] and so on [55]).…”