Experimental evaluation of steady and pulsating flow performance of an asymmetric double entry turbine for an automotive turbocharger

Abstract

Variable Geometry Turbines are widely used in High Pressure Loop Exhaust Gas Recirculation to enhance positive pressure gradient between turbine inlet and compressor outlet. Multiple-entry turbines (twin or double) can achieve an enhanced pulse energy extraction when compared with single entry devices. However, when EGR takes place, an imbalance of mass flow can lead to significant energy loss. This thesis presents the design and experimental assessment of a novel asymmetric double entry turbine that aims to achieve pulse energy recovery even in the face of high EGR. The asymmetric double entry volute was developed with two different scroll lengths: a small scroll that feeds the rotor between azimuth angle of 0° and 160°, beyond which rotor is fed by the large scroll. A pivoting action is designed to enable an optimisation of the optimum angle in each scroll sector. Extensive testing was carried out to characterise the novel turbine under various steady and unsteady flow conditions (full, partial, unequal, in phase and out of phase).

The peak recorded efficiency for the full admission nozzleless condition was found to be 78%. When nozzle was present in the small scroll, the efficiency had a similar magnitude (77.6%); while when nozzles were present in both scrolls a lower efficiency was found (74.4%). Considering nozzles only on the small sector: as the vane angle opening changes from most open position (51°) to mid open position (71°) an increase in stage efficiency was observed. Under partial admission, the nozzleless unit showed a very significant the deficit in peak efficiency: 17 points with large scroll flowing and 25 points with small scroll flowing. When nozzle vanes are introduced in the small scroll sector a similar drop was found; but when vanes were present in both scrolls the deficit during partial reduced (13.9% for the best performing configuration). During unequal admission the nozzleless and small nozzle setting displayed a symmetrical efficiency characteristic.

During unsteady flow admission at 30k RPM for all frequencies showed that the nozzle configurations recorded higher cycle average efficiency than the base line nozzleless unit. Test results at 20 Hz and 40 Hz have shown higher cycle average efficiency than 60 Hz. Interestingly when nozzles were present on both scrolls the cycle average efficiency increased by almost 13 percentage point higher than nozzleless indicating the value of introducing nozzles.