A comparison of flow control devices for variable geometry turbine application

Abstract

In this paper the aerodynamic performance of two common variable geometry inlet flow control devices for use in turbocharger turbines is investigated, namely: the pivoting vane and sliding wall flow restrictors, as well as a combination of the two mechanisms at the inlet to the turbine rotor, acting as coupled Active Control Turbocharger (ACT)/ Variable Geometry Turbine (VGT) mechanisms in series (one mechanism providing instantaneous area flow control for ACT – an instantaneous exhaust energy recovery capability; the other providing 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 optimal application of turbine inlet flow control to an advanced ACT system. Numerical models of the stator passages for the different mechanisms were developed and a study of the flow unsteadiness based on the Strouhal number was performed. The latter found that the flow was quasi-steady allowing transient conditions to be modelled by superposition of steady state scenarios. The numerical models were subsequently validated using experimental data. An investigation of the NACA profile thickness of the pivoting vanes was also undertaken, the findings of which indicated that a NACA thickness of 0018 constituted the optimum compromise between increased velocity and incurred losses. The results for the pivoting vane simulations demonstrated the effect of loss mechanisms such as leakage and flow separation. It was established that the 65° vane angle delivered the highest velocity to the rotor, corroborating the earlier research findings. In the case of the coupled mechanisms, the main loss generating flow structure was found to be the large wake produced by the sliding wall, which significantly increased the inefficiencies through the stator. In comparison, the pivoting vane mechanism exhibited substantially lower levels of pressure losses and delivered higher velocities to the rotor.