MIXED FLOW TURBINE HOUSING DEVELOPMENT FOR A FIXED GEOMETRY TURBOCHARGER APPLICATION

This thesis investigates the impact of volute design on the performance of a mixed flow turbine. Both computational and experimental methods were used to assess performance. All computational work as conducted in CFX under both steady state and unsteady pulsating conditions with the models including inlet, volute, rotor and outlet volumes. Both the mixing plane and sliding mesh approaches were implemented and the results compared. The k-w SST turbulence model was implemented throughout this thesis with the exceptions of chapters 8 and 9 where the SAS SST model was used in an attempt to accurately capture secondary flows. Further SBES simulations were included for flow validation.

It was found that pulse shape had a significant impact on the instantaneous performance while the cycle averaged performance remained largely insensitive to the changes. Further thorough analysis showed, under a range of pulse frequencies, loads and amplitudes, significant variations in LE incidence over the pulse cycle. Furthermore, the spanwise distribution of the incidence also changed considerably over the pulse due to volute secondary flow development.

As result of the initial analysis both volute tilt and aspect ratio (and a combination of the two) were assessed. A new tilted volute was introduced which resulted in a performance improvement of up to 2.356% in cycle averaged rotor efficiency and 2.171% improvement in cycle average stage efficiency. This improvement reduced when volute A/r was reduced. The impact of volute aspect ratio showed that the MFP varied by up to 4.3%. Furthermore, volute secondary flows were significantly impacted by aspect ratio with smaller aspect ratios resulting in strong vortices that persisted around the volute. Increasing the aspect ratio removed these vortices. However, the span-wise distribution of LE incidence was only slightly improved with increasing aspect ratio. The maximum efficiency improvement measured over the aspect ratio range was 1.47% for the turbine stage. Combining both tilt and aspect ratio showed a maximum performance variation between the worst performing design, radial AR=0.5 and the best performing design tilted AR=2 of 3.00% in the rotor region and 2.87% over the entire stage.

Extensive experimental testing under steady state and pulsating flows was conducted at Imperial College, London to validate the computational work. It was observed that the tilted volute resulted in pulsating efficiency improvements at 48krpm and 56krpm. This trend was found to increase as pulse frequency increased. However, steady state testing only showed efficiency improvements at 30krpm for the tilted volute.