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978-3-8439-0759-0, Reihe Strömungsmechanik
Christoph K. Natkaniec
Sekundärströmungen in Turboladerturbinen mit variabler Turbinengeometrie
163 Seiten, Dissertation Universität Hannover (2012), Softcover, A5
Today turbocharging is used more commonly than ever on internal combustion engines. Most diesel engines of all types and sizes and increasingly gasoline engines are turbocharged. Radial turbines with variable geometry nozzle are widely used in turbochargers, especially for automotive applications. The applications range from small passenger cars to heavy duty vehicles. Real radial turbines with variable geometry nozzle possess system specific geometry features, which assure the proper functionality of nozzle vane movement at any time. The mentioned system specific geometry features are nozzle vane endwall clearances, spacers and spindles. These geometric features generate additional secondary flow structures inside the variable nozzle, which lead to higher losses and therefore to lower efficiencies of the radial turbine stage.
For the analysis of the influence of the single geometry features on the formation, interaction, and propagation of secondary flow structures inside the variable nozzle, a systematic variation of these geometric features is carried out. Thus, eight variations are created where the geometry features are systematically either accounted for or disregarded. For this purpose, a complete turbine stage of a commercial vehicle turbocharger is modeled. The numerical model includes the entire 360° rotor and nozzle geometry, in order to account for the circumferential non-uniformity of the flow. Unsteady CFD and spatially highly resolved simulations are carried out in order to account for the complex three dimensional nature of the flow in radial turbomachinery.
In this thesis, a method using the Q-Criterion, relative helicity and helicity to detect and identify secondary flow structures is developed. The analysis using fields of helicity shows that the system specific geometry features contribute to the formation of additional secondary flow structures. These secondary vortices are the endwall clearance vortex, the spacer vortex and the spindle vortex. The spacer vortex and the spindle vortex are identified in this thesis for the first time ever.
The analysis of losses shows that additional losses are generated by the additional secondary vortices, which are induced by the system specific geometry features. The endwall clearances show the highest contribution to the losses, followed by the spacers. The influence of the spindles is negligible.