The Impact of Volute Aspect Ratio on the Performance of a Mixed Flow Turbine

Current trends in the automotive industry towards engine downsizing means turbocharging now plays a vital role in engine performance. A turbocharger increases charge air density using a turbine to extract waste energy from the exhaust gas to drive a compressor. Most turbocharger applications employ a radial inflow turbine. However, mixed flow turbines can offer non-zero blade angles, reducing leading edge (LE) separation at low velocity ratios. The current paper investigates the performance of a mixed flow turbine with three different volute aspect ratio (AR) designs (AR=0.5, 1 and 2). With constant A/r (ratio of volute area to centroid radius), the AR=0.5 volute design produced a 4.3% increase in cycle averaged mass flow parameter (MFP) compared to the AR=2 design. For the purpose of performance comparison, it was necessary to manipulate the volute A/r’s to ensure constant MFP for aerodynamic similarity. With the volute A/r’s manipulated to ensure constant MFP for aerodynamic similarity, the maximum variation of cycle averaged normalized efficiency measured between the designs was 1.47%. Purely in the rotor region, the variation in normalized cycle averaged efficiency was 1%. The smallest tested volute aspect ratio showed a significant increase in volute loss while the AR’s of 1 and 2 showed similar levels of loss. The smallest AR volute showed significant secondary flow development in the volute. The resulting variation in LE incidence was found to vary as a result.