An investigation into the effect of clearance aspect ratio on the performance of a variable geometry vaned diffuser for automotive turbocharger application

Lee Gibson, Stephen Spence, Sung in Kim, Charles Stuart, Martin Schwitzke, Andre Starke, Dietmar Filsinger

Research output: Chapter in Book/Report/Conference proceedingConference contribution


The current state-of-the-art in radial compressor design for automotive turbocharger applications utilize impellers with a high trailing edge backsweep angle and a vaneless diffuser to provide a high boost pressure over a wide operating range. A unique feature of this type of design is that the peak efficiency island is typically located near the choke side of the compressor map. As such, the compressor efficiency is generally satisfactory when the engine is operating at high speed, such as the rated power condition. However, at low speeds the engine operating line is located close to the compressor surge line where the efficiency is generally modest. Thus, there is a need to improve the compressor efficiency at low engine speeds without compromising performance near the choke side of the map or the overall map width.
Variable geometry devices have shown good potential to improve the compressor performance without a compromise in map width. In general, variability is achieved by moving walls or rotating vanes to best suit the flow conditions for a given mass flow rate. In order for this to be practically realised, a clearance or gap is required between the stationary and moving parts. This ultimately gives rise to leakage flows within the compressor stage and generally results in a lower achievable efficiency relative to the fixed geometry configuration.
A study by the authors on an on/off type variable geometry vaned diffuser identified significant loss mechanisms due to the clearances required for the vanes to slide in to and out of the main flow path. Moreover, the endwall position of the clearance was found to have a marked impact on the compressor stability and peak efficiency. This paper assesses the effect of the clearance depth to width ratio (or aspect ratio) at different endwall positions with the aim of identifying an appropriate geometry and position to approach an optimised design.
Steady-state Reynolds-Averaged Navier-Stokes (RANS) simulations were performed using ANSYS CFX at three operating speeds to obtain a broad sense of the effect of the clearance aspect ratio on the compressor performance. It was found that a high value of aspect ratio enabled the formation of large vortical structures in the vaned diffuser. The mixing between the core flow and the vortical structures resulted in significant losses in the vaned diffuser and affected the compressor map width differently depending on the endwall position.
Original languageEnglish
Title of host publicationASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition
PublisherThe American Society of Mechanical Engineers (ASME)
Number of pages17
ISBN (Electronic)9780791884102
ISBN (Print)978-0-7918-8410-2
Publication statusPublished - 2020
EventASME International Gas Turbine Institute Turbomachinery Technical Conference & Exposition - Virtual Conference, Online
Duration: 21 Sep 202025 Sep 2020

Publication series

NameProceedings of the ASME Turbo Expo


ConferenceASME International Gas Turbine Institute Turbomachinery Technical Conference & Exposition
Abbreviated titleASME Turbo Expo 2020
Internet address

Bibliographical note

Funding Information:
This research was funded by IHI Charging Systems International GmbH, which is gratefully acknowledged. The authors would like to thank ANSYS Inc. for the use of their CFD software in this research. Finally, the authors would like to thank the technical staff of the Kelvin HPC cluster at Queen’s University Belfast for commissioning and maintaining HPC resources.

Publisher Copyright:
Copyright © 2020 ASME.

Copyright 2021 Elsevier B.V., All rights reserved.

ASJC Scopus subject areas

  • Engineering(all)


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