This paper presents a numerical simulation of stall flow phenomenon inside a turbocharger centrifugal compressor with vaneless diffuser. Three-dimensional Reynolds averaged compressible Navier-Stokes equations were solved with k-ε turbulence model using computational fluid dynamics software CFX. Entire geometry of the compressor, including the impeller, vaneless diffuser, and volute housing were included in the simulation. The stage performance curve was obtained using steady and unsteady calculations and compared with experimental results. The unsteady flow simulation was performed at reduced mass flowrates (compressor stall region) with a plenum chamber model added at the compressor housing outlet. The results show that there is a distinct stall frequency at the given compressor speed. The amplitude of the static pressure oscillation at this frequency in the diffuser is increased with reduction in compressor mass flow. The stall frequency is only marginally affected by the compressor mass flowrate. Time varying flow fields in the diffuser and in the volute indicate that there is a circumferential standing wave inside the volute. It is proposed that the volute behaves like a tuned pipe resonator and this dictates the stall frequency. The stall is also characterized by a periodically reversing flow at the centre of the volute discharge cross-section.
A three-dimensional (3D) model is presented to study the occurrence of weak rotating waves in vaneless diffusers of centrifugal compressors. The model is an extension of the 2D one developed by Moore. 3D incompressible linearized Euler equations are cast on a rotating frame of reference travelling at the same circumferential speeds as the waves and the viscous effects are ignored. The diffuser is assumed to have two parallel walls and discharge into a large plenum. Solutions to the equations are obtained by a finite difference method and the singular value decomposition technique. Disturbances along the axial direction are found under zero undisturbed axial velocity. Resonant disturbances in the diffuser flow regardless of the compressor characteristics are also found as in the 2D cases found by Moore. Computational results show that both the critical flow angle and the propagation velocity of the wave are affected by the departure from the axial uniform distribution of the undisturbed radial velocity at the diffuser inlet, but the angle is less affected than the wave speed. The velocity distribution that satisfies Fj0rtoft's necessary conditions for flow instability is found slightly less stable and is more affected by the departure than those that do not. Shorter diffusers are affected more by the departure than the longer ones. The critical angle is shown to be increased non-linearly with the wave number and this helps to explain why wave numbers 2 to 4 are commonly observed in experiments. Finally, comparison with the experimental results in the open literature is made and a good agreement is shown.
In this paper, a three-dimensional body force model for predicting compressor performance and stability is implemented in the Ansys CFX. The influence of the blade rows on the flow field is represented by the source terms of CFX-solver equation. At first, a high-speed and high-pressure-ratio transonic compressor with the clean inlet is investigated. The overall performance and the flow fields are in agreement well with those of the experimental date, so the model is reliable and correct. Then, the effects of the circumferential distortions in the inlet total pressure and the total temperature on the compressor performance and flow field are also illustrated, respectively. In summary, the proposed body force model is suitable to investigate the flow field of the compressor with the inlet distortions.
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