Recently, a variable flow capacity turbine has become important for automotive turbochargers to satisfy the demands of engine performance and emissions. Especially, a variable nozzle turbine is an indispensable component for diesel engines of both passenger and commercial vehicles to realize a high boosting from the low end to the rated engine condition.
On the other hand, since a variable nozzle turbine has several gaps and clearances around movable nozzle vanes to ensure the smooth operation under the hot gas condition, therefore there is a possibility that these complicated geometries may cause the additional loss and the turbine efficiency decrement.
The authors conducted detailed CFD calculations of a variable nozzle turbine taking account of nozzle vane clearance, strut pins, spindles. As most realized calculation, a full-domain analysis which includes a flow leakage through a nozzle link assembly, and these complicated geometries' effects on a turbine performance are revealed when nozzle opening is varied from the closed to the fully-opened condition.
CFD results show that the nozzle vane clearance dominates the turbine performance at the nozzle closed condition and strut pins have a relatively large influence at the fully-opened condition. In a full-domain analysis, it is confirmed that the predicted turbine efficiency becomes close to the experimental result and CFD modeling considering actual gaps and leakage is necessary for the aerodynamic design of a variable nozzle turbine and its improvement.
