A Comparison of flow control devices for variable geometry turbocharger a pplication

Abstract

In this paper the aerodynamic performance of two common variable geometry inlet flow control devices for usein turbocharger turbines is investigated, namely: the pivoting vane and sliding wall flow restrictors, as well a s acombination of the two mechanisms at the inlet to the turbine rotor, acting as coupled Active ControlTurbocharger (ACT)/ Variable Geometry Turbine (VGT) mechanisms in series (one mechanism providinginstantaneous area flow control for ACTan instantaneous exhaust energy recovery capability; the otherproviding optimum mean nozzle position in conventional VGT mode), using computational fluid dynamics(CFD) techniques. The latter coupled study was carried out with the purpose to explore a more optimalapplication of turbine inlet flow control to an advanced ACT system. Numerical models of the stator passagesfor the different mechanisms were developed and a study of the flow unsteadiness based on the Strouhal numberwas performed. The latter found that the flow was quasi-steady allowing transient conditions to be modelled bysuperposition of steady state scenarios. The numerical models were subsequently validated using experimentaldata. An investigation of the NACA profile thickness of the pivoting vane s was also undertaken, the findings ofwhich indicated that a NACA thickness of 0018 constituted the optimum compromise between increasedvelocity and incurred losses. The results for the pivoting vane simulations demonstrated the effect of lossmechanisms such as leakage and flow separation. It was established that the 65° vane angle delivered the highestvelocity to the rotor, corroborating the earlier research findings. In the case of the coupled mechanisms, the mainloss generating flow structure was found to be the large wake produced by the sliding wall, which significantlyincreased the inefficiencies through the stator. In comparison, the pivoting vane mechanism exhibitedsubstantially lower levels of pressure losses and delivered higher velocities to the rotor.