Study on the Accuracy of RANS Modelling of the Turbulent Flow Developed in a Kaplan Turbine Operated at BEP. Part 1 - Velocity Field

Abstract

This paper investigates the accuracy of Reynolds-averaged Navier-Stokes (RANS) turbulence modelling applied to complex industrial applications. In the context of the increasing instability of the energy market, hydropower plants are frequently working at off-design parameters. Such operation conditions have a strong impact on the efficiency and life span of hydraulic turbines. Therefore, research is currently focused on improving the design and increasing the operating range of the turbines. Numerical simulations represent an accessible and cost efficient alternative to model testing. The presented test case is the Porjus U9 Kaplan turbine model operated at best efficiency point (BEP). Both steady and unsteady numerical simulations are carried out using different turbulence models: k-epsilon, RNG k-epsilon and k-omega Shear Stress Transport (SST). The curvature correction method applied to the SST turbulence model is also evaluated showing nearly no sensitivity to the different values of the production correction coefficient Cscale. The simulations are validated against measurements performed in the turbine runner and draft tube. The numerical results are in good agreement with the experimental time-dependent velocity profiles. The advantages and limitations of RANS modelling are discussed. The most accurate results were provided by the simulations using the kepsilon and the SST-CC turbulence models but very small differences were obtained between the different tested models. The precision of the numerical simulations decreased towards the outlet of the computational domain. In a companion paper, the pressure profiles obtained numerically are investigated and compared to experimental data.

Transient-state analysis of hydropower plants with cross-flow turbines

“…Namely, the unit hill charts given in Figure 4 cover only the regimes expected in normal that is, designed steadystate operation of the turbine. However, as noticed by Iova˘nel et al, 23 due to the increasing instability of the energy market, hydropower plants frequently work at off-design parameters, even in steady-state regimes. When it comes to transient-state operation, for example, after sudden load rejection, the turbine always passes (albeit briefly) through the regimes that are outside the designed operating range.…”

A Case Study: Sediment Erosion in Francis Turbines Operated at the San Francisco Hydropower Plant in Ecuador

“…A realizable k-turbulence model was selected for its ability to correctly capture the turbulent nature of the flow in Francis turbines [20][21][22]. This model was selected for its robustness and its improved boundary-layer-solving capacity under strong pressure gradients and flow separation compared to the standard k-model [23].…”

Numerical study on the performance characteristics of a high-head pump turbine based on the GEKO model

“…The shear stress transport model (SST k-ω) is an excellent RANS model and one of the best performing eddy viscosity assumption models currently in wide use. It cleverly combines the advantages of k-ω, k-ε and J-K models, and can better handle the transport of turbulent shear stress in adverse pressure gradients and separated boundary layers [5]. With its high precision, good robustness and wide applicability, it has become one of the most widely used turbulence models in the field of fluid machinery.…”