We report on experimental study of flow and pressure pulsations in draft-tube of a laboratory air model of Francis-99 hydro-turbine operating over a broad range of regimes employing a rapid prototyping of the swirl generators and computerized measurements. In total, 867 operating regimes were examined corresponding to different combinations of the runner rotation speed and flowrates. The velocity measured by a computer-automated laser-Doppler anemometer and the pressure recording by wall-mounted acoustic sensors for a selection of operating conditions reveal a variety of patterns of the flow and vortex structures, with clearly identifiable regimes with the maximum coherent flow pulsations at non-optimal operating conditions. The regimes with distinct precessing vortex cores show notable rearrangement of the velocity and vorticity fields, accompanied by a sharp increase in the amplitude of the coherent pressure pulsations, as also confirmed by the peak cross-correlation of the spectra of pressures signals from two diametrally placed wall-mounted microphones. The paper closes with a scrutiny of the swirl number, its distribution and the relation of zero-swirl with the isogonal best-efficiency loci in a broad range of operating conditions. For the nominal best efficiency regime, the integral swirl number appears to have a small positive value of about S = 0.11, but the tangential velocity distribution reveals two concentric counter-rotating cylindrical rod-like streams with a negative swirl (relative to the runner rotation direction) in the inner region and positive in the outer. In the regimes with the PVC formation, as illustrated for the maximum pressure pulsations regime at S > 0.5, the axial velocity appears to be almost stagnant in the core (though positive apart from the cowl wake), the flow being pushed into the peripheral annular region.

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The work reports on experimental study of flow and pressure pulsations in draft-tube of a laboratory air model of a Francis hydro-turbine operating over a broad range of regimes corresponding to different combinations of the runner rotation speed and flowrates. The velocity measurements using a computer-automated laser-Doppler anemometer and the pressure recording by wall-mounted acoustic sensors for a selection of operating conditions reveal different patterns of the swirling flow, with clearly identifiable regimes with the maximum flow pulsations at non-optimal operating conditions. The regimes with distinct precessing vortex cores show notable rearrangement of the velocity fields, accompanied by an increase in the amplitude of the pressure pulsations, as confirmed by the peaks of the spectra of pressure signals from wall-mounted microphones. @en

We report on the examination of several approaches to simulate computationally the unstable regime of a model Kaplan turbine operating at off-design load. Numerical simulations complemented by laboratory experiments have been performed for a 60:1 scaled-down laboratory turbine model using two Reynolds-averaged Navier–Stokes (RANS) models (linear eddy viscosity model (LEVM), and a Reynolds stress model (RSM), including realizable k-ε, k-ω SST, and LRR), detached eddy simulation model (DES), and large eddy simulation model (LES). Unlike the LEVM, the RSM, DES, and LES reproduced the mean velocity components and the intensities of their fluctuations and pressure pulsations well. The underperformance of the LEVM is attributed to the high eddy viscosity as a consequence of an excessive production of the turbulent kinetic energy due to the models’ inability to account for the turbulent stress anisotropy and the stress-stain phase lag, both naturally accounted for by the RSM. This led to a much larger modelled and a smaller resolved turbulent kinetic energy compared to those in the RSM.

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We report on parallel experimental studies of twin vortex ropes in laboratory models of an elbow and a conical hydroturbine draft tube focusing on the unstable cyclic switch-overs between the single and twin helices. The measurements involve high-speed visualization, pressure pulsation recordings and laser-beam detecting of the precessing helices. The Fourier transform of the pressure probe signals accompanied by optical detection provided the frequencies and amplitudes of the detected single and twin ropes, which were compared between the two draft tubes in a range of Reynolds number from 10⁵ to 5 × 10⁵. The experiments showed constant, though different, Strouhal numbers both for the single and twin ropes. The single-rope Strouhal numbers normalized with the swirl number for the two draft tubes collapse onto a unique value of 1.1 independent of Reynolds number. The ratio of the helix frequencies in the twin- and single rope regimes was 1.15 and 1.4, respectively, for the elbow and the conical model.

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