Sergey Shtork

Comparative analysis of twin vortex ropes in laboratory models of two hydro-turbine draft-tubes

ABSTRACT 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.

Study of the velocity distribution influence upon the pressure pulsations in draft tube model of hydro-turbine

One of the mechanisms of generation of powerful pressure pulsations in the circuit of the turbine is a precessing vortex core, formed behind the runner at the operation points with partial or forced loads, when the flow has significant residual swirl. To study periodic pressure pulsations behind the runner the authors of this paper use approaches of experimental modeling and methods of computational fluid dynamics. The influence of velocity distributions at the output of the hydro turbine runner on pressure pulsations was studied based on analysis of the existing and possible velocity distributions in hydraulic turbines and selection of the distribution in the extended range. Preliminary numerical calculations have showed that the velocity distribution can be modeled without reproduction of the entire geometry of the circuit, using a combination of two blade cascades of the rotor and stator. Experimental verification of numerical results was carried out in an air bench, using the method of 3D-printing for fabrication of the blade cascades and the geometry of the draft tube of hydraulic turbine. Measurements of the velocity field at the input to a draft tube cone and registration of pressure pulsations due to precessing vortex core have allowed building correlations between the velocity distribution character and the amplitude-frequency characteristics of the pulsations.

Vortex ropes in draft tube of a laboratory Kaplan hydroturbine at low load: an experimental and LES scrutiny of RANS and DES computational models

ABSTRACT 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-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.

Experimental investigation of a twin vortex breakdown on the draft tube models

This work is devoted to the experimental study of the flow structure in the hydro turbine draft tube models. A precessing helical vortex rope was formed at the flow with high swirling degree. Effect of transition between single- and double-vortex rope was revealed in different laboratory models. Frequency characteristics of the precessing structures were measured in the range Re = 5 · 104 - 5 · 105. From the analysis of high-speed camera records, the scenario of full transition between single and double vortex was proposed. The study of this phenomenon is of particular interest for the design and safe operation of hydraulic turbine equipment, because powerful pressure pulsations with unexpected frequency and amplitude may be generated in the flow.

Application of particle image velocimetry technique for study of reacting jet flows

Present work is devoted to experimental study of reacting jet flow in a model open flame burner. The work was inspired by the problem of extension of the range for stable and effective combustion which basically determined by the turbulent flow structure in reaction zone. To approach this target spatial distributions of the mean velocity, turbulent kinetic energy were measured and the role of organized vortical structures have been studied.The main technique used is a laser based stereo Particle Image Velocimetry (PIV) system. The measurements were performed in a central section of the jet flame. Regular swirl parameter of the flow was varied from 0 to 1.0. Reynolds number was changed between 1000 and 8000. Equivalence ratio variation band was between 0.5 and 4. Effect of the nozzle geometry was studied by using nozzles with different exit diameters.