Experimental and numerical investigation of the precessing helical vortex in a conical diffuser, with rotor-stator interaction
Journal article, 2016
The flow unsteadiness generated in a swirling apparatus is investigated experimentally and numerically. The swirl apparatus has two parts: a swirl generator and a test section. The swirl generator includes two blade rows, one stationary and one rotating, is designed such that the emanating flow resembles that of a Francis hydro turbine operated at partial discharge. The test section consists of a conical diffuser similar to the draft tube cone of a Francis turbine. A new control method based on a magneto rheological brake is employed in the rotating section, runner, in order to produce several swirling flow regimes. The LDV measurements are performed along three survey axes in the test section. The measured mean velocity components and its fluctuating parts are used to validate the results of unsteady numerical simulations, conducted using the FOAM-extend-3.0 CFD code. A dynamic mesh is used together with the sliding General Grid Interfaces (GGI) to mimic the effect of the rotating runner. The delayed detached eddy simulation method, conjugated with the Spalart-Allmaras turbulence model (DDES-SA), is applied to achieve a deep insight about the ability of this advanced modeling technique and the physics of the flow. The RNG k-epsilon model is also used to represent state-of-the art of industrial turbulence modeling. Both models predict the mean velocity reasonably well while DDES-SA presents more realistic flow features at the highest and lowest rotational speeds. The highest level of turbulence occurs at the highest and lowest rotational speeds which DDES-SA is able to predict well in the conical diffuser. The special shape of the blade plays more prominent role at lower rotational speeds and creates coherent structures with opposite sign of vorticity. The vortex rope is captured by both turbulence models while DDES-SA presents more realistic one at higher rotational speeds.