Evaluation AC6-14
Swirling flow in a conical diffuser generated with rotor-stator interaction
Application Challenge AC6-14 © copyright ERCOFTAC 2024
Comparison of Test Data and CFD
The mean velocity field is determined by time averaging over five complete runner revolutions for the hybrid models and eight complete runner revolutions for the URANS models to filter out all unsteadiness. The survey axes, \textit{S*}, at sections W0-W2 (see Fig. \ref{Test_rig}), are normalized by the throat radius, $R_{throat}$=0.05m, and the velocity is normalized by the bulk velocity at the throat, $W_{throat}$. The axial axis is downward and the runner rotates in the positive direction according to the right-hand rule.
Figure \ref{fig6} compares the axial and tangential velocity distributions of the
high-Reynolds number turbulence models, standard $k-\epsilon$, SST $k-\omega$,
realizable $k-\epsilon$ and RNG $k-\epsilon$, with the experimental results at W1 and W2.
It is shown that the turbulence models predict the mean velocity very similarly.
It is known that a high level of swirl leads to an on-axis recirculation region due to the
high centrifugal force~\cite{Javadi2015}.
The models overestimate the size of that region, although the RNG $k-\epsilon$ and
standard $k-\epsilon$ models predict the recirculation region more realistically than
the other models.
Javadi et al.~\cite{Javadietal2014} used the RNG $k-\epsilon$ model in the swirl generator
over a wide range of runner rotational speeds, from part load to full load.
They concluded that the applicability of the RNG $k-\epsilon$ model is restricted to a
range close to the best efficiency point.
As mentioned before, the current flow field resembles the Francis turbine at 70\% load,
which is far from the best efficiency point.
The disintegration process of the vortex rope is highly unsteady and needs to be resolved
to capture its realistic representation.
The turbulence models reproduce the width of the central stagnant region reasonably well
at W2.
Figure \ref{fig7} compares the axial and tangential velocity distributions of the low-Reynolds number turbulence models, Launder-Sharma and Lien-cubic $k-\epsilon$, with the experimental results at W1. The results show that Lien-cubic $k-\epsilon$ underestimates the main character of the flow, the on-axis recirculation region, considerably. It can be seen that Launder-Sharma $k-\epsilon$ model, on the other hand, overestimates the recirculation region. The LS model was initially proposed for predicting swirling flows. The model, as a modified version of the standard $k-\epsilon$ model, is one of the earliest and most widely used models for resolving the near-wall flow behavior. Although some damping functions are added in the LS model to account for the viscous and wall effects, the improved near-wall behavior does not significantly increase the quality of the mean velocity profiles in the draft tube.
Contributed by: A. Javadi, A. Bosioc, H Nilsson, S. Muntean, R. Susan-Resiga — Chalmers University of Technology
© copyright ERCOFTAC 2024