An investigation of advanced design techniques for improved mixed flow turbine efficiency

Abstract

With ever increasing pressure for decarbonisation of the internal combustion engine, engine brake thermal efficiency is of great importance. Turbocharging has been used for many years to facilitate an increase in specific power, and subsequent engine downsizing, which in turn is used to increase the efficiency of the engine installation. Considering the decarbonisation goal, it is imperative that efficiency is at the forefront of each component within the system. This includes the turbocharger.

Considering the maturity of turbocharger turbines, the drive for increased efficiency is challenging. This thesis details two research studies which aimed to facilitate a step change in efficiency. The first study detailed an evaluation of truly three dimensional rotor blading. In this study, the ubiquitous radial fibre constraint was relaxed, with a numerical optimisation exercise used to explore a truly three dimensional design space. A new design approach was detailed, which leveraged the aerodynamically insensitive hub region to automatically adapt to 3D geometries and mitigate against the effects of non-radially fibred blading. Through the novel design approach, a new 3D geometry was identified which offered a performance improvement across the entire turbocharger operating line of 1.1% as predicted by CFD simulations without compromising mechanical integrity. This was then experimentally validated at 0.6%. A loss analysis identified that whilst much of the loss within the stage (e.g. endwall and profile) remained unchanged by the implementation of 3D blading, there was a substantial reduction in tip leakage loss, along with improved pressure recovery downstream of the turbine rotor. Further investigation showed that the 3D design afforded a reduction in near tip loading, which facilitated this reduction in tip leakage loss.

A second study is then detailed which investigated the aerodynamic impact of turbine scalloping. Used for its mechanical advantages such as reduced bore stress and inertia, turbine scalloping is known to cause a reduction in efficiency, however the reasons for this are not well documented in literature. This study involved a flow field analysis of a scalloped and unscalloped wheel, with the key loss structure from scalloping found to be a scallop vortex. This vortex entered the passage from the back disc cavity near the suction surface of the blade, migrated up the suction surface and interacted with the tip leakage flow, resulting in an average measured efficiency penalty of 1.4% across the turbocharger operating line. A multiphysics optimisation study identified that the scallop design could be aerodynamically improved without compromising the mechanical performance. Through this process, a new scallop design was developed which reduced the scalloping efficiency penalty by 18% through better control of the scallop vortex.

Thesis embargoed until 31 July 2023.

Date of Award	Jul 2022
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	ABB Turbo Systems Ltd & Northern Ireland Department for the Economy
Supervisor	Stephen Spence (Supervisor), Marco Geron (Supervisor) & Geoffrey McCullough (Supervisor)