Vortex ropes in draft tube of a laboratory Kaplan hydro turbine at low load

Application Area 6: Turbomachinery Internal Flow

Application Challenge AC6-15

Abstract

This test case deals with flow unsteadiness in the draft tube of a laboratory model of a Kaplan turbine operating at an off-design load with only 39% of the nominal flow rate, studied experimentally and by computational modelling and simulations in the Institute of Thermal Physics in Novosibirsk, Russia (Minakov A. et al., 2017). The experiments were carried out in a 60:1 scaled-down laboratory model (see figure below), in which the turbine was mimicked by a set of fixed and rotating swirlers, designed to generate the draft-tube entry flow conditions as in a real turbine. The complementing computational studies were performed using several RANS models: the realizable k-ε and k-ω-SST linear eddy-viscosity models (LEVM), and the basic (LRR) Reynolds-stress model (RSM), then DES (detached-eddy simulations) and LES (large-eddy simulations). The RANS and DES computations were done on numerical grids with 2 and 6 M (million) cells, and LES on 6 and 19.3M grids, the latter serving as the reference fine-grid simulations. The flow patterns, vortical structure and turbulence statistics in the turbine draft tube (DT), their effect on flow stability and pressure pulsations at a low load, appear to be governed by the conspicuous unsteady twin helix ropes. All these features are well reproduced by the LRR RANS model, DES and LES, but to a large extent remained intractable to the considered LEVMs.