Radial and mixed flow turbines have reached a level of maturity where iterative performance improvements are very small, with real performance benefits coming from better matching to a given application as opposed to improvements in technology. One ubiquitous design feature of radial and mixed flow turbines is the radial fibre constraint, wherein material at all points on the blade is stacked radially outward. This ensures that blade fillet stresses are not amplified by any additional bending load, caused by misalignment of the blade COG (centre of gravity) with the blade root. While this constraint yields a mechanical benefit, it constrains the aerodynamic design significantly, with the blade shape defined by one camberline. In essence this forces a quasi 2D design.

One potential means of realizing a true performance improvement is the use of 3D blading, wherein the blade is no longer constrained to a radially fibred structure. In such a design, the blade shape could be controlled at multiple spanwise locations as a means of controlling blade loading and or secondary flows. This study investigated the viability of such 3D blading, through a multi-point numerical optimization exercise on a state-of-the-art mixed flow turbine, where blade shape was defined by four discrete camberlines at equally spaced spanwise locations. This paper details the optimization process, experimental validation of performance predictions, and a physical explanation of why the 3D blading facilitated a performance benefit based on the flow field and losses.

The paper includes details on the optimization process used, with a key difference from previous work being the inclusion of a constraint driven geometry modification tool. Through the use of this geometry modification tool, which utilized aerodynamically insensitive areas of the blade to control blade fillet stresses, it was shown that 3D blading can be used to gain a performance benefit without compromising mechanical integrity. Additionally, this approach facilitated a reduction in dimensionality, and thus computational cost.

The optimization process identified a performance improvement across the entire turbocharger operating line. This predicted performance improvement was experimentally validated through a test campaign at QUB, wherein the experimental measurement data was in good agreement with numerical predictions. Finally, a detailed entropy based investigation into how the physical changes to blade geometry influenced loss generation was carried out. Qualitative and quantitative insights from this are detailed in the paper, which can be broadly summarised as reduced tip leakage losses, and both reduced loss and improved pressure recovery within the conical exhaust diffuser. Finally, design guidelines for future designs are derived from these findings.


Performance Improvement of a Mixed Flow Turbine Using 3D Blading

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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