This paper presents a combined experimental and numerical analysis of rotating stall in a transonic centrifugal compressor impeller for automotive turbochargers. Stall characteristics of the compressor were examined by two high-response pressure transducers mounted on the casing wall near the impeller inlet. The pressure traces were analyzed by wavelet transforms to estimate the disturbance waves quantitatively. Three-dimensional unsteady internal flow fields were simulated numerically by Detached Eddy Simulation (DES) coupled LES-RANS approach. The analysis results show good agreements on both compressor performance characteristics and the unsteady flow features at the rotating stall. At stall inception, spiral-type breakdown of the full-blade tip leakage vortex was found out at some passages and the brokendown regions propagated against the impeller rotation. This phenomenon changed with throttling, and tornado-type separation vortex caused by the full-blade leading edge separation dominated the flow field at developed stall condition. It is similar to the flow model of short-length scale rotating stall established in an axial compressor rotor.

Recently, the application of turbochargers is increasing because they are effective in improving fuel consumption of engines. One of the most important turbocharger characteristics is compressor operating range, since it has been used in various driving patterns with the advent of variable geometry turbochargers. Owing to the complicated phenomena, such as rotating stall occurring at low flow rate condition, flow analysis is very difficult and details of flow structure have not been fully understood for a long time since the early 1970s. In this study, two compressors with different operating range width were investigated with experimental and computational flow analysis. In the compressor with narrow operating range, the amplitude of blade passing pressure fluctuation decreases rapidly and rotating stall occurs near surging. On the other hand, in the compressor with wide operating range, the blockage by the tip leakage vortex breakdown play a role in stabilizing the flow field and keeping up a high performance even at low flow rates.

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In an internal combustion engine, the centrifugal compressor is placed upstream of the inlet manifold and therefore, it is exposed an unsteady flow regime caused by the inlet valves of the cylinder arrangement. This valve motion sets a pulsating state at the compressor exit, having greater influence when the operation is near the surge margin of the compressor. This paper presents the experimental results of the evaluation of the surge dynamics on a compressor with induced downstream pulsating flow. Different pulsation levels are achieved by the variation of three different parameters on the induced pulse: pulse frequency, amplitude, and system storage volume (plenum). Each pulse parameter was evaluated independently in order to assess its effect on the compressor stability limit. The main effect on the surge margin of the compressor was found to be due to the presence of a storage volume in the system for all cases (steady/pulsating condition) and at all frequencies. It was found that the magnitude of the pulse frequency determines the hysteresis behavior of the system that leads to a phase difference between the convected terms and the acoustic dominated terms, and therefore this affects the onset of flow instability, surge, in the compression system under study.

A transonic centrifugal compressor for turbocharger applications has been redesigned by means of a multidisciplinary multipoint optimization system composed of: a 3D Navier-Stokes solver, a Finite Element stress Analyzer, a Genetic Algorithm and an Artificial Neural Network. The latter makes use of a database, containing the geometry and corresponding performance of previously analyzed impellers and allows a considerable reduction in computational effort. The performance of every new geometry is verified by a 3D Navier-Stokes solver. A Finite Element Analysis verifies the mechanical integrity of the impeller. The geometrical description of the impeller has been extended to better adapt the inducer part of the impeller to transonic flows. The splitters are no longer copies of the full blades but specially designed for minimum losses and equal mass flow on both sides. The blade thickness and number of blades are unchanged because defined by robustness and inertia considerations. The operating range is guaranteed by a two-step optimization procedure. The first one provides information allowing a modification of the inlet section to guarantee the required choking mass flow and a more accurate prediction of the boundary conditions for the Navier-Stokes analysis of the modified impeller. The second one predicts the performance curve of the new geometry for which the choking mass flow is known. It is shown how these extensions of the optimization method have led to a considerable improvement of the efficiency and corresponding pressure ratio, while respecting the surge and choking limits without increase of the stress level.