A Study on the Flow Around the Scallops of a Mixed-Flow Turbine and Its Effect on Efficiency

Abstract

Automotive turbocharger radial inflow turbines are required to have both a high efficiency and a low inertia. In many cases this favours the use of a mixed-flow design. The inertia is minimised by scalloping the back-face, which removes a significant amount of material from the highest radius region of the hub. However, there is a concern that, for a mixed-flow geometry, the presence of a scallop introduces a forward-facing step to the inducer flow. This gives rise to the potential for an additional loss mechanism in the impeller. Two variants of a mixed-flow design, one having scallops and the other without, have been tested in a steady-state rig. The performance deltas are compared against the CFD representations. In reality there is the additional complexity of pulsing inlet conditions due to the operation of the exhaust valves of the piston engine. This causes severe variations in the inducer flow field. To assess whether the presence of a scallop might impair performance in an engine installation, a number of off-design CFD comparisons have been carried out using boundary conditions from a transient model covering a pulsing cycle.