Influence of Unsteadiness on the Control of a Hub-Corner Separation Within a Radial Vaned Diffuser

Abstract

The present work aims at evaluating the effect of the impeller-diffuser interaction on the control of a hub corner separation, which develops in the radial vaned diffuser of a centrifugal compressor designed and built by Turbomeca, Safran group. Unsteady numerical simulations of the flow in the aspirated centrifugal compressor are then performed. Numerical results are validated by comparison with the available experimental results. The analysis of the numerical flow field shows that the hub-corner separation is not completely removed by the suction, on the contrary to the steady-state results that were obtained in previous work. The boundary layer separation is only translated downstream. Its location is explained by the scrolling of the pressure waves generated by the impeller-diffuser interaction, which strengthen when crossing the diffuser throat. This result highlights the major role played by the impeller-diffuser interaction, which should be taken into account for developing control strategies in radial vaned diffusers, and stresses the shortcoming of the steady-state numerical model when suction is applied.

Active Flow Control in a Radial Vaned Diffuser for Surge Margin Improvement: A Multislot Suction Strategy

“…Figure 2 shows the two suction strategies that are compared here. The single-slot strategy is the one that has already been studied and did not reach any improvement of the surge margin according to the time-dependent numerical simulations [30]. The three-slot strategy is the one which is under the scope of the present paper, the slots being located at the maxima of the instantaneous adverse pressure gradient.…”

“…According to steady-state numerical calculations relying on the mixing-plane approach, the suction led to a significant increase of the stable operating range of the compressor. But these promising results were challenged by unsteady numerical simulations [30]. The detailed analysis of the flow in the diffuser with suction has shown that the hub-corner separation was not completely removed by the suction, contrary to the prediction of the steady-state numerical model: a corner separation still existed in the diffuser downstream of the location of the initial boundarylayer separation, and the stable operating range was not extended.…”

“…This compressor is installed in a 400 kW test rig of the DAEP Laboratory at the Institut Supérieur de l' Aéronautique et de l'Espace (ISAE), Toulouse, France. A complete description of the experimental test rig is available in [30].…”

Numerical analysis of Separate Vortex in the Diffuser

“…The averaged static pressure was specified at outlet boundary. The compressor is simulated under two rotating speeds: 500r/s (Mu=0.953) and 700r/s (Mu=1.33).In Ref [5,6], because of the limitation of experiment, the test compressor did install a volute. So in order to simulation flow in compressor under the same condition, no volute is in the model of Case 2.…”

“…There were some explanations about the inducement of instability in centrifugal compressor, which were boundary layer separation, core flow separation, or even the mutual interaction [5] . Marsaner found a boundary layer separation remains downstream of the suction slot and is due to unsteadiness, namely the scrolling of the high pressure waves which generates intense instantaneous adverse pressure gradients by unsteady simulation [6] . Recently with continuous increases in computational power and advances in CCD sensor technology, DPIV provides near real-time flow field measurements.…”