Swirling flow in a conical diffuser generated with rotor-stator interaction

Application Challenge AC6-14 © copyright ERCOFTAC 2024

Overview of Tests

A closed loop experimental test rig was developed and measuresd at the Politehnica University of Timi\c soara, Romania, ~\cite{Resiga2007} for investigating decelerated swirling flows in a conical diffuser and for assessing various flow control methods. The test rig includes the following (see Fig. \ref{circuit}): (i) a main hydraulic circuit, marked blue; (ii) an auxiliary hydraulic circuit, marked red that is implemented in order to assess different control methods ~\cite{Tanasa2013,Bosioc2012}; (iii) a lower reservoir with a volume of 4$\textrm{m}^3$; (iv) a main pump that is able to provide a maximum flow rate of 40 l/s; (v) an upper reservoir equipped with a honeycomb section to provide an uniform flow at the inlet of the swirl test section, and (vi) a test section, marked magenta.

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Figure 2: Photo and schematic view of experimental closed loop test rig from UPT

The test section contains two main parts: i) the swirl generator with 13 fixed guide vanes and a runner with 10 blades, see Fig.~\ref{Swirl_Generator}, and ii) the convergent-divergent test section \cite{Bosioc2008}. The divergent part of the test section, with total angle of $17\,^{\circ}$ ($2\times8.5^{\circ}$) is designed to generate a flow similar to that in a Francis turbine draft tube. The hub and shroud diameters of the swirl generator are $D_{hub}$=0.09m and $D_{shroud}$=0.15m, respectively. The guide vanes create a tangential velocity component, yielding a pressure increase from the hub to shroud at all operating conditions. The purpose of the runner is to redistribute the total pressure by inducing an excess in the axial velocity near the shroud and a corresponding deficit near the hub, like a Francis turbine operating at partial discharge. The runner thus acts as a turbine near the hub and as a pump near the shroud. There is a gap between the blade tip and the shroud of about 0.4mm. Although the effect of tip-clearance is not investigated, the velocity profiles measured further downstream did not reveal significant leakage flows effects. The numerical simulations without and with the blade tip gap further support this conclusion.

All experimental data are gathered in single phase (non-cavitating) conditions by keeping the test rig pressurized at a large enough pressure level, since a cavitating vortex rope brings additional complexity to the flow phenomenon and the experimental methodology. The experimental velocity is obtained with a Dantec Dynamics two-component LDA. The LDA system consists of an argon-ion laser of 300mW power, and an optical probe with focal length 159.6mm, beam diameter 2.2mm and beam spacing 39.2mm. A three-dimensional traversing system is used for the probe positioning, with a 0.01mm accuracy on each axis. The measurements are performed along each survey axis with a step size of 1mm. Silver coated hollow glass particles are added into the water for efficient light back-scattering. The average particle diameter is 10$\mu$m, and the relative density is $\rho_{particles} / \rho_{water}$ = 1.402. For each experimental point there are between 20000 and 50000 particles crossing the measuring volume, during the acquisition time of 30 seconds.