Steady and pulsating performance of a variable geometry mixed flow turbocharger turbine
Creators: name: Rajoo, Srithar
Publisher: name: Imperial College London
Year: 2006

description: Variable Geometry Turbochargers (VGT) are widely used to improve engine-turbocharger matching and currently common in diesel engines. VGT has proven to provide air boost for wide engine speed range as well as reduce turbo-lag. This thesis presents the design and experimental evaluation of a variable geometry mixed flow turbocharger turbine. The mixed flow rotor used in this study consists of 12 blades with a constant inlet blade angle of +20°, a cone angle of 50° and a tip diameter of 95.2mm. A variable geometry stator has been designed within this work, consists of 15 vanes fitted into a ring mechanism with a pivoting range between 40° and 80°. A novel nozzle vane was designed to have 40° lean stacking (from the axial direction). This geometrically achieves 3-dimensional match with the mixed flow rotor and aims to improve the turbine stage performance. A conventional straight nozzle vane was also constructed in order to have a comparative design to assess the benefits of the new lean vane. The steady flow performance results are presented for vane angle settings of 40°, 50°, 60°, 65° and 70° over a non-dimensional speed range of 0.833 - 1.667. The tests have been carried out with a permanent magnet eddy current dynamometer within a velocity ratio range of 0.47 to 1.09. The optimum efficiency of the variable geometry turbine was found to be approximately 5 percentage points higher than the baseline nozzleless unit. The peak efficiency of the variable geometry turbine corresponds to vane angle settings between 60° and 65°, for both the lean and straight vanes. The maximum total-to-static efficiency of the turbine with lean vanes configuration was measured to be 79.8% at a velocity ratio of 0.675. The equivalent value with straight vanes configuration is 80.4 % at a velocity ratio of 0.673. The swallowing capacity of the turbine was shown to increase with the lean vanes, as much as 17% at 70° vane angle and pressure ratio of 1.7. The turbine pulsating flow performance is presented for 50% and 80% equivalent speed conditions and a pulse frequency range of 20 - 80 Hz, these frequencies correspond to an engine speed range of 800 - 3200 RPM respectively. The turbine was observed to go through a period of choking within a pulse for vane angle settings between 60° - 70°. The unsteady efficiency of a nozzled turbine was found to exhibit larger deviation from the quasi-steady curve compared to a nozzlesless turbine, by as much as -19.4 percentage points. This behaviour was found to be more pronounced towards the close nozzle settings, where the blockage effect is dominant. The nozzle ring was also shown to act as a “restrictor” which shields the turbine rotor from being completely exposed to the unsteadiness of the flow. This coupled with the phase shifting ambiguity was shown to result in the inaccuracy of the point-by-point instantaneous efficiency; where as much as 25% of a cycle exhibits instantaneous efficiency above unity. Finally the turbine was tested by adapting to the pulsating flow (20 - 60 Hz) by cyclic variation in the opening and closing of the nozzle vanes, called Active Control Turbocharger (A.C.T.). The nozzle vane operating schedules for each pulse period were evaluated experimentally in two general modes; natural oscillating opening/closing of the nozzle vanes due to the pulsating flow and the forced sinusoidal oscillation of the vanes to match the incoming pulsating flow. The spring stiffness was found to be a dominant factor in the effectiveness of the natural oscillation mode. In the best setting, the turbine energy extraction was shown to improve by 6.1% over a cycle for the 20 Hz flow condition. In overall it was demonstrated an optimum A.C.T. operating condition could be achieved by allowing the nozzle ring to oscillate naturally in pulsating flow, against an external spring pre-load, which eliminates the use of complex mechanism and external drive. However, the current result suggest the benefits of A.C.T. are best realised in large low speed engines.