Rotor flow-field timescale and unsteady effects on pulsed-flow turbocharger turbine

Abstract

The turbocharged piston-driven engines are widely used in high altitude long endurance unmanned aerial vehicles (HALE UAVs). Repeated actions of engine pistons and valves give rise to engine pulsations resulting in intensive unsteady flow structures in the turbine rotor. The latter are, however, neglected in previous analytical studies leading to deviations in the prediction of turbine performance. The present work, for the first time, quantitatively investigates unsteady rotor flow-field timescale in a highly loaded mixed flow turbine using validated 3-D unsteady computational fluid dynamics (CFD) carried out by ANSYS-CFX. Results show that rotor flow-field timescale is in the same order as the rotation period of the rotor. In addition, the effects of rotor flow-field timescale on turbine performance are illustrated via an analogy of a pitching motion airfoil with the reduced frequency introduced as the criteria to measure the level of unsteadiness. With the increase of rotor unsteadiness, an evident inertial effect can be observed on the turbine power outputs. Rotor flow-field timescale should therefore be considered for improved turbine design and turbocharger-engine matching.